Book of abstracts, "Jaszowiec" 2011 - Instytut Fizyki PAN
Book of abstracts, "Jaszowiec" 2011 - Instytut Fizyki PAN
Book of abstracts, "Jaszowiec" 2011 - Instytut Fizyki PAN
You also want an ePaper? Increase the reach of your titles
YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.
�����������������������<br />
������������������������������������<br />
���������������������������������<br />
�<br />
����������������������<br />
������� ��� �������� �� �������<br />
�������������������������������������������������<br />
�������������������������������������������������������������<br />
��������������������������������������������������������<br />
���������������������������������������������������������������<br />
��������������������������������������������������<br />
�������������������������<br />
�������������������������������������������<br />
������������������������������������������<br />
�<br />
������������������������������������<br />
�<br />
������������
This work relates to Department <strong>of</strong> the Navy Grant N62909-11-1-1076 issued by Office <strong>of</strong><br />
Naval Research Global. The United States Government has a royalty-free license throughout<br />
the world in all copyrightable material contained herein.<br />
�<br />
�<br />
�<br />
�<br />
�<br />
�<br />
�<br />
�<br />
�<br />
�<br />
�<br />
�<br />
�<br />
�<br />
�<br />
�<br />
�<br />
�<br />
�<br />
�<br />
�<br />
�<br />
�<br />
�<br />
�<br />
�<br />
�<br />
�<br />
�<br />
�<br />
�<br />
�<br />
�<br />
�<br />
�<br />
�<br />
Skład własny (Ł. Cywi�ski, podzi�kowania dla T. Słupi�skiego)<br />
Druk i oprawa: Drukarnia Zalesie, 05-501 Piaseczno, ul. Norwida 10<br />
�
PREFACE<br />
The 40 th “Jaszowiec” International School and Conference on the Physics <strong>of</strong><br />
Semiconductors continues the tradition <strong>of</strong> annual meetings <strong>of</strong> the international and Polish<br />
semiconductor communities. For the third time the event will be held in the mountain resort<br />
Krynica-Zdrój on June 25 th – July 1 st , <strong>2011</strong>. The first thirty seven meetings took place in the<br />
village Jaszowiec in another part <strong>of</strong> Polish mountains. For years, the name “Jaszowiec” has<br />
become the symbolic trademark <strong>of</strong> the conference and, therefore, it is still kept.<br />
The scope <strong>of</strong> the conference covers essentially all research fields in semiconductor<br />
physics, including technology, experiment, theory and modeling. During the present<br />
conference particular attention will be paid to the physics <strong>of</strong> semiconductor devices, and a<br />
special day-long symposium “Physics and modeling <strong>of</strong> devices” has been organized.<br />
The Conference is preceded by the School (tutorial session), addressed mainly to<br />
undergraduate and graduate students, and young scientists. The School will be held on June<br />
25 th -26 th . During the School outstanding scientists will give four lectures providing an<br />
introduction to important fields in semiconductor physics. The scientific program <strong>of</strong> the<br />
school and conference consists <strong>of</strong> 25 invited lectures, and 194 contributed papers (18 chosen<br />
for oral presentation and 176 posters). The School and Conference will be attended by<br />
approximately 250 participants.<br />
This booklet contains the detailed program and <strong>abstracts</strong> <strong>of</strong> all presentations that will<br />
be given during the event. The invited and contributed papers will be published in a special<br />
volume <strong>of</strong> Acta Physica Polonica A.<br />
The International School and Conference is organized by two institutes <strong>of</strong> the Polish<br />
Academy <strong>of</strong> Sciences: Institute <strong>of</strong> Physics and Institute <strong>of</strong> High Pressure Physics; two<br />
institutes <strong>of</strong> the Faculty <strong>of</strong> Physics, University <strong>of</strong> Warsaw: Institute <strong>of</strong> Experimental Physics<br />
and Institute <strong>of</strong> Theoretical Physics; Foundation “Pro Physica”, and Committee on Physics <strong>of</strong><br />
the Polish Academy <strong>of</strong> Sciences. The financial support <strong>of</strong> the organizing institutions as well<br />
as <strong>of</strong> the U.S. Army Forward Element Command-Atlantic, Research Division (USARFEC-A)<br />
and the U.S. Navy Office <strong>of</strong> Naval Research Global is gratefully acknowledged.<br />
Current information can be found on the conference web page:<br />
www.ifpan.edu.pl/jaszowiec .<br />
I would like to thank the members <strong>of</strong> the International Advisory Committee and the<br />
Program Committee for their contribution to the scientific program <strong>of</strong> the School and<br />
Conference. I would like also to express my gratitude to Łukasz Cywi�ski, the Secretary <strong>of</strong><br />
the Conference, to Jacek Szczytko, the Chairman <strong>of</strong> the School, as well as other members <strong>of</strong><br />
the Organizing Committee, Agnieszka J�drzejewska and Maciek Zaj�czkowski for their<br />
enthusiasm and excellent work.<br />
I wish all the participants <strong>of</strong> the meeting scientifically inspiring days and joyful social<br />
life. Have a pleasant stay in Krynica.<br />
Chairman <strong>of</strong> the Conference<br />
Czesław Skierbiszewski
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Saturday, June 25 th , <strong>2011</strong><br />
9:20 -- 9:30 Jacek Szczytko – School opening address<br />
INVITED LECTURES (SaI1, SaI2)<br />
9:30 – 12:30 Detlef Hommel (University <strong>of</strong> Bremen, Germany)<br />
Modern technologies for semiconductor epitaxy and nano-processing<br />
15:00 – 18:00 Aldo Di Carlo (University <strong>of</strong> Rome “Tor Vergata”, Italy)<br />
Physical simulation <strong>of</strong> electronic devices<br />
19:00 Barbecue (on the terrace <strong>of</strong> the ‘Geovita’ hotel)<br />
Sunday, June 26 th , <strong>2011</strong><br />
INVITED LECTURES (SuI1, SuI2)<br />
9:30 – 12:30 Gerhard Abstreiter (Walter Schottky Institut and Institute for Advanced<br />
Study TU München, Germany)<br />
Physics and Technology <strong>of</strong> Arsenide Based Quantum Dots and Nanowires<br />
15:00 – 18:00 Debdeep Jena (University <strong>of</strong> Notre Dame, USA)<br />
Tutorial: Engineering <strong>of</strong> Electric Fields in Nitride-Based Semiconductors<br />
20:30 – 21:30 Concert – chamber music performed by young artists,<br />
laureates <strong>of</strong> international music competitions<br />
Angelika Kumi�ga (violin)<br />
Agata B�k (cello)<br />
Michał Kubarski (accordion)<br />
21:30 Welcoming glass <strong>of</strong> wine<br />
5
_______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Monday, June 27 th , <strong>2011</strong><br />
8:50 -- 9:00 Czesław Skierbiszewski – Conference opening address<br />
INVITED TALKS (MoI1 … MoI4)<br />
9:00 – 10:00 Joerg Wrachtrup (University <strong>of</strong> Stuttgart, Germany)<br />
Coherent optical control <strong>of</strong> the NV center in diamond<br />
10:00 – 10:10 Break<br />
10:10 – 11:10 Tomasz Dietl (Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences)<br />
Understanding and exploiting magnetism <strong>of</strong> semiconductors<br />
11:10 – 11:40 C<strong>of</strong>fee break<br />
11:40 – 12:40 Łukasz Kłopotowski (Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences)<br />
Charging Effects in Self Assembled CdTe Quantum Dots<br />
12:40 – 12:50 Break<br />
12:50 – 13:50 Grzegorz S�k (Wrocław University <strong>of</strong> Technology, Poland)<br />
Optical properties <strong>of</strong> strongly in-plane asymmetric epitaxial<br />
nanostructures<br />
CONTRIBUTED TALKS (MoO1 … MoO6)<br />
19:15 – 19:30 P. Utko, R. Ferone, I.V. Krive, R.I. Shekhter, M. Jonson, M. Monthioux,<br />
L. Noe, J. Nygård<br />
Coupling between electronic and vibrational excitations in carbon<br />
nanotubes filled with C60 fullerenes<br />
19:30 – 19:45 A. Urba�czyk, F.W M. van Otten, R. Nötzel<br />
Solid-state self assembly: a route to hybrid metal-semiconductor epitaxial<br />
nanostructures<br />
19:45 – 20:00 M. Galicka, P. Kacman, R. Buczko<br />
First-Principles Study <strong>of</strong> Doped III–V nanowires<br />
20:00 – 20:10 Break<br />
20:10 – 20:25 F. Schuster, P. Kopyt, P. Lukasik, W. Gwarek, D. Coquillat, F. Teppe,<br />
B. Giffard, W. Knap<br />
Terahertz Detectors Based on Low Cost 130 nm Silicon Field Effect<br />
Transistors<br />
20:25 – 20:40 P. Wojnar, E. Janik, S. Kret, A. Petrouchik, M. Goryca, T. Kazimierczuk,<br />
P. Kossacki, G. Karczewski, T. Wojtowicz<br />
Growth <strong>of</strong> optically active CdTe quantum dots in ZnTe nanowires<br />
20:40 – 20:55 J.M. Schneider, B.A. Piot, I. Sheikin, G.A. Goncharuk, P. Vasek,<br />
P. Svoboda, Z. Vyborny, L. Smrcka, M. Orlita, M. Potemski, D.K. Maude<br />
Electronic properties <strong>of</strong> graphite<br />
21:05 – 23:00 MONDAY POSTER SESSION (MoP1 … MoP60)<br />
6
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Tuesday, June 28 th , <strong>2011</strong><br />
INVITED TALKS (TuI1 … TuI4)<br />
9:00 – 10:00�������Alex Greilich���(Technische Universität Dortmund, Germany)<br />
Optical control <strong>of</strong> one and two spins in interacting quantum dots<br />
10:00 – 10:10 Break<br />
10:10 – 11:10 Shaffique Adam (National Institute <strong>of</strong> Standards and Technology, USA)<br />
Graphene Transport Properties<br />
11:10 – 11:40 C<strong>of</strong>fee Break<br />
11:40 – 12:40 Arkadiusz Wójs (Wrocław University <strong>of</strong> Technology, Poland)<br />
Theoretical search for non-Abelian statistics in fractional quantum Hall<br />
states<br />
12:40 – 12:50 Break<br />
12:50 – 13:50 Dmitry Krizhanovskii (University <strong>of</strong> Sheffield, UK)<br />
Polariton condensation in dynamic acoustic lattices<br />
CONTRIBUTED TALKS (TuO1 … TuO6)<br />
15:30 – 15:45 K. Nogajewski, K. Karpierz, M. Grynberg, W. Knap, R. Gaska, J. Yang,<br />
M.S. Shur, J. Łusakowski<br />
Resonant Terahertz Absorption by Magnetoplasmons in Grating-Gate<br />
GaN/AlGaN-based Field-Effect Transistors<br />
15:45 – 16:00 K. Milowska, M. Birowska, J.A. Majewski<br />
Structural and electronic properties <strong>of</strong> functionalized graphene<br />
16:00 – 16:15 O. Proselkov, W. Stefanowicz, S. Dobkowska, J. Sadowski, T. Dietl,<br />
M. Sawicki<br />
Analysis <strong>of</strong> the magnetic anisotropy in ultrathin GaMnAs<br />
16:15 – 16:25 Break<br />
16:25 – 16:40 M. Goryca, P. Plochocka, P. Wojnar, T. Kazimierczuk, M. Potemski,<br />
P. Kossacki<br />
Spin-lattice relaxation <strong>of</strong> a single Mn 2+ ion in a CdTe quantum dot<br />
16:40 – 16:55 T. Kazimierczuk, M. Goryca, P. Wojnar, A. Golnik, P. Kossacki<br />
Magnetophotoluminescence <strong>of</strong> CdTe/ZnTe Quantum Dots: G-factor and<br />
Diamagnetic Shift Variation in a Single Dot<br />
16:55 – 17:10 T. Jakubczyk, W. Pacuski, A. Golnik, C. Kruse, D. Hommel, H. Hilmer,<br />
R. Schmidt-Grund, J.A. Gaj<br />
Control over CdTe Quantum Dots Emission using ZnTe-based Micropillar<br />
Cavities<br />
17:30 – 19:15 TUESDAY POSTER SESSION (TuP1 … TuP58)<br />
20:00 Conference Banquet<br />
7
_______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Wednesday, June 29 th , <strong>2011</strong><br />
INVITED TALKS (WeI1 … WeI4)<br />
9:00 – 10:00 Tomas Jungwirth (Institute <strong>of</strong> Physics, Academy <strong>of</strong> Sciences <strong>of</strong> the Czech<br />
Republic)<br />
Spintronics with antiferromagnetic materials<br />
10:00 – 10:10 Break<br />
10:10 – 11:10 Ewelina M. Hankiewicz (Universitaet Wuerzburg, Germany)<br />
Transport in topological insulators<br />
11:10 – 11:40 C<strong>of</strong>fee Break<br />
11:40 – 12:40 James Speck (University <strong>of</strong> California, Santa Barbara)<br />
Progress in Nonpolar and Semipolar GaN Materials and Devices<br />
12:40 – 12:50 Break<br />
12:50 – 13:50 Yasushi Nanishi (Ritsumeikan University, Japan, and Seoul National<br />
University, Korea)<br />
Recent Progress <strong>of</strong> DERI process for growth <strong>of</strong> InN and Related Alloys<br />
CONTRIBUTED TALKS (WeO1 … WeO6)<br />
19:15 – 19:30 N. Gonzalez Szwacki, J. A. Majewski, T. Dietl<br />
Properties <strong>of</strong> TM pairs in the bulk and at the surface <strong>of</strong> GaN with and<br />
without Si or Mg codoping<br />
19:30 – 19:45 A. Bonanni, W. Stefanowicz, M. Sawicki, T. Devillers, B. Faina, T. Li,<br />
T.E. Winkler, D. Sztenkiel, A. Navarro-Quezada, M. Rovezzi, R. Jakieła,<br />
A. Meingast, G. Kothleitner, T. Dietl<br />
Experimental Probing <strong>of</strong> Exchange Interactions Between Localized Spins<br />
in a Dilute Magnetic Insulator (Ga,Mn)N<br />
19:45 – 20:00 M. Birowska, C. �liwa, K. Milowska, J.A. Majewski, T. Dietl<br />
Origin <strong>of</strong> uniaxial magnetic anisotropy in (Ga,Mn)As<br />
20:00 – 20:10 Break<br />
20:10 – 20:25 E. Calleja, A. Bengoechea-Encabo, S. Albert, M.A. Sanchez-García,<br />
F. Barbagini, E. Luna, A. Trampert, U. Jahn, P. Lefebvre<br />
Understanding the Selective Area Nucleation and Growth <strong>of</strong> GaN<br />
20:25 – 21:40 S.P. Łepkowski, I. Gorczyca<br />
Influence <strong>of</strong> composition and atomic arrangement on elastic properties <strong>of</strong><br />
wurtzite InGaN and InAlN alloys<br />
20:40 – 20:55 H. Turski, M. Siekacz, M. Sawicka, G. Cywi�ski, C. Cheze, S. Grzanka,<br />
Z. Wasilewski, P. Perlin, G. Muzioł, J. Pawłowska, I. Grzegory,<br />
C. Skierbiszewski<br />
InGaN laser diodes operating at 450-460 nm grown by RF-Plasma MBE<br />
21:05 – 23:00 WEDNESDAY POSTER SESSION (WeP1 … WeP58)<br />
8
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Thursday, June 30 th , 2010<br />
SYMPOSIUM<br />
"Physics and modeling <strong>of</strong> devices "<br />
INVITED TALKS (ThI1 …ThI9)<br />
9:00 – 9:30 Maciej Bugajski (Institute <strong>of</strong> Elektron Technology, Warsaw, Poland)<br />
Quantum Cascade Lasers<br />
9:30 – 10:00 Wladek Walukiewicz (Lawrence Berkeley National Laboratory,USA)<br />
New Concepts and Materials for Solar Power Conversion Devices<br />
10:00 – 10:10 Break<br />
10:10 – 10:40 Danek Elbaum (Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences)<br />
ZnO biosensing<br />
10:40 – 11:10 Katarzyna Holc (Institute <strong>of</strong> High Pressure Physics, Warsaw, Poland)<br />
Nitride laser diodes<br />
11:10 – 11:30 C<strong>of</strong>fee break<br />
11:30 – 12:00 Detlef Hommel (University <strong>of</strong> Bremen, Germany)<br />
Application <strong>of</strong> CdSe quantum dots for single photon emitters at room<br />
temperature<br />
12:00 – 12:30 Wojciech Knap (University <strong>of</strong> Montpellier, France)<br />
THz emitters based on GaN/AlGaN HEMTs<br />
12:30 -- 13:00 Tomasz Stobiecki (AGH University <strong>of</strong> Science and Technology, Kraków,<br />
Poland)<br />
Spin transfer torque in TMR and GMR nanostructures for spintronic<br />
devices<br />
15:00 – 15:30 Jan Dziuban (Wrocław University <strong>of</strong> Technology, Poland)<br />
Si-based MEMS devices<br />
15:30 – 16:00 Zbigniew Ku�nicki (Ecole Nationale Superiere de Physique, Illkirch-<br />
Graffenstaden)<br />
Giant photoconversion on silicon drived materials<br />
16:00 Czesław Skierbiszewski – Closing address<br />
19:00 Farewell dinner<br />
9
_______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
MONDAY POSTER SESSION (MoP1 …MoP60)<br />
1. K.A. Kolwas, G. Grabecki, S. Trushkin, Ł. Cywi�ski, M. Aleszkiewicz, T. Dietl, G.<br />
Springholz, G. Bauer<br />
Nonlocal transport in PbTe/PbEuTe microstructures<br />
2. M. Szot, L. Kowalczyk, E. Smajek, B. Taliashvili, P. Dziawa, W. Kn<strong>of</strong>f, A. Reszka, V.<br />
Domukhovski, E. Łusakowska, P. Dłu�ewski, M. Wiater, T. Wojtowicz, T. Story, M.<br />
Bukała, R. Buczko, P. Kacman<br />
Optical transitions in PbTe/CdTe quantum wells grown by molecular beam epitaxy on<br />
GaAs (001) and BaF2 (111) substrates<br />
3. A. Woło�, A. Drabinska, M. Kaminska, G. Strzelecka, A. Hruban, A. Materna, M. Piersa,<br />
Z. Wilamowski<br />
Properties <strong>of</strong> three-dimensional topological insulators studied by microwave resonance<br />
spectroscopy<br />
4. A. Duzynska, A. Kaminska, H. Teisseyre, E. Przezdziecka, D. Dobosz, Z.R. Zytkiewicz,<br />
A. Kozanecki, J.D. Fidelus, A. Durygin, V. Drozd, R. Hrubiak, A. Suchocki<br />
Analysis <strong>of</strong> Optical Properties and Pressure Dependence <strong>of</strong> the Energy Gap <strong>of</strong> ZnO<br />
Layers, Bulk and Nano-Powders<br />
5. A. Radzvilavicius, E. Anisimovas<br />
Defect structure <strong>of</strong> two-dimensional Wigner crystals<br />
6. A. Korbecka, J.A. Majewski<br />
Gauge invariant computational scheme for heterostructures in magnetic field<br />
7. M. Bukała, P. Sankowski, R. Buczko, P. Kacman<br />
Modeling PbTe-based low dimensional structures<br />
8. A. Hruban, A. Materna, W. Dalecki, G. Strzelecka, M. Piersa, E. Jurkiewicz-Wegner, R.<br />
Diduszko, M. Romaniec, W. Orłowski<br />
Topological insulators – materials for fundamental researches and perspective<br />
applications<br />
9. M. Chwastyk, P.T. Ró�a�ski, M. Zieli�ski<br />
Atomistic calculation <strong>of</strong> screened Coulomb interactions in semiconductor<br />
nanostructures<br />
10. M. Birowska, K. Milowska, J.A. Majewski<br />
Van der Waals Density Functionals in Materials Science<br />
11. T. Gro�, E. Malicka, A.W. Pacyna, H. Duda, J. Krok-Kowalski<br />
Hopkinson-Like Effect in Single-Crystalline CdCr2Se4 Semiconductor<br />
12. Ł. Wachnicki, B.S. Witkowski, S. Gierałtowska, E. Janik, M. Godlewski, E. Guziewicz<br />
Optical and structural characterization <strong>of</strong> zinc oxide nanostructures obtained by<br />
Atomic Layer Deposition method<br />
10
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
13. V.V. Khomyak, M.I. Ilaschuk, O.A. Parfenyuk, V.V. Brus, Z.D. Kovalyuk<br />
Electrical properties <strong>of</strong> surface-barrier structures n-Zn1-xCdxO/ p-CdTe<br />
14. E.A. Wolska, D. Sibera, B.S. Witkowski, S.A. Yatsunenko, I. Pełech, U. Narkiewicz, M.<br />
Godlewski<br />
Photoluminescence and chromaticity properties <strong>of</strong> ZnO nanopowders obtained by a<br />
microwave solwothermal method<br />
15. T. Š�epka, D. Gregušová, R. Kúdela, Š. Gaži, V. Cambel<br />
Technology and testing the mechanical properties <strong>of</strong> the InGaP/GaAs/InGaP<br />
microcantilevers<br />
16. M.A. Borysiewicz, E. Dynowska, V. Kolkovsky, J. Dyczewski, E. Kami�ska, A.<br />
Piotrowska<br />
ZnO thin films <strong>of</strong> different crystalline structures grown on Si (100) substrates by<br />
reactive DC sputter deposition<br />
17. W. Kn<strong>of</strong>f, M.A. Pietrzyk, B.A. Orłowski, B. Taliashvili, T. Story, R.L. Johnson<br />
Comparison <strong>of</strong> the valence band <strong>of</strong> amorphous and crystalline GeTe and (Ge,Mn)Te<br />
layers<br />
18. A. Pietnoczka, R. Bacewicz, T. Słupi�ski, S.H. Wei, M. Jie, J. Antonowicz, T. Drobiazg<br />
Local Structure Study <strong>of</strong> GaAs:Te Using Te K-edge X-ray Absorption Fine Structure<br />
19. A. Taube, K. Korwin-Mikke, R. Mroczy�ski, S. Gierałtowska, A. Łaszcz, I. Pasternak,<br />
M. Sochacki, M. Zychowska, J. Dyczewski, M. Zdrojek, E. Dynowska, A. Piotrowska<br />
Thermal Stability <strong>of</strong> HfO2/SiO2 Gate Stack on Silicon Substrate<br />
20. M. Iwi�ska, D. Pier�ci�ska, K. Pier�ci�ski, A. Szerling, P. Karbownik, M. Bugajski<br />
Investigation <strong>of</strong> thermal properties <strong>of</strong> AlGaAs/GaAs Quantum Cascade Lasers by<br />
thermoreflectance spectroscopy<br />
21. P. Łach, M.G. Brik, I. Sildos, A. Kami�ska, A. Suchocki<br />
Luminescence properties <strong>of</strong> Sm 3+ in different phases <strong>of</strong> TiO2<br />
22. L.A. Karachevtseva, S.Ya. Kuchmii, O.A. Lytvynenko, F.F. Sizov, O.J. Stronska, A.L.<br />
Stroyuk<br />
Investigation <strong>of</strong> electro-optical properties <strong>of</strong> 2D macroporous silicon<br />
23. G.V. Lashkarev, A.M. Yaremko, V.A. Karpyna<br />
Investigation <strong>of</strong> multi-phonon excitations in ZnO textured crystalline films by Raman<br />
spectroscopy<br />
24. M.L. Peres, V.A. Chitta, D.K. Maude, N.F. Oliveira Jr., P.H. Rappl, A.Y. Ueta, E.<br />
Abram<strong>of</strong><br />
Rashba effect in n-type PbTe/Pb1-xEuxTe Quantum Wells<br />
11
_______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
25. I.I. Shtepliuk, G.V. Lashkarev, O.Yu. Khyzhun, V.V. Khomyak, V.I. Lazorenko, I.I.<br />
Tim<strong>of</strong>eeva, L.A. Klochkov, B. Kowalski, A. Reszka<br />
Enhancement the intensity <strong>of</strong> ultraviolet luminescence <strong>of</strong> ZnO film upon doping<br />
isovalent impurity <strong>of</strong> cadmium<br />
26. V.V. Brus, Z.D. Kovalyuk, P.D. Maryanchuk<br />
Optical Properties <strong>of</strong> TiO2 – MnO2 Thin Films Prepared by the Electron-Beam<br />
Evaporation Technique<br />
27. S. Gierałtowska, Ł. Wachnicki, B.S. Witkowski, T.A. Krajewski, M. Godlewski, E.<br />
Guziewicz<br />
Properties <strong>of</strong> high-k oxides grown by Atomic Layer Deposition method for transparent<br />
electronics<br />
28. E. Maci��ek, T. Gro�, A.W. Pacyna, T. Mydlarz, J. Krok-Kowalski<br />
High Spin-Low Spin Transitions in Cu0.2Co0.76Cr1.83Se4 Semiconductor<br />
29. D. Shevchenko, J. Mickevi�ius, G. Tamulaitis, N. Starzhinskiy, K. Katrunov, V.<br />
Ryzhikov<br />
Photoluminescence Study <strong>of</strong> ZnSe Scintillating Crystals Doped with Isovalent<br />
Tellurium and Oxygen<br />
30. K. Olender, T. Wosi�ski, A. M�kosa, P. Dłu�ewski, V. Kolkovsky, G. Karczewski<br />
Native Deep-Level Defects in MBE-Grown p-Type CdTe<br />
31. P. Urbanowicz, E. Tomaszewicz, T. Gro�, H. Duda, Z. Kukuła<br />
Residual Paramagnetism at High-Temperature CuEu2W2O10 and Cu3Eu2W4O18<br />
Semiconductors<br />
32. M.V. Radchenko, G.V. Lashakrev, M.E. Bugaiova, V.I. Sichkovskyi, V.I. Lazorenko,<br />
L.A. Krushynskaya, Y.A. Stelmakh, W. Kn<strong>of</strong>f, T. Story<br />
Ferromagnetic nanocomposite Co-Al2O3 as a spintronic material with engineered<br />
magnetic properties<br />
33. D.M. Bercha, K.E. Glukhov, M. Sznajder, S.A. Bercha<br />
The role <strong>of</strong> electron subsystem in the phase transition process in a layered CuInP2S6<br />
crystal<br />
34. A..I. Dmitriev<br />
Van der Waals surface <strong>of</strong> InSe as a standard nanorelief in the metrology <strong>of</strong><br />
nanoobjects<br />
35. A. Kaminska, C.G. Ma, M.G. Brik, A. Kozanecki, M. Bo�kowski, E. Alves, A. Suchocki<br />
Crystal field analysis <strong>of</strong> the Yb 3+ energy level scheme in III-V semiconductors<br />
36. J. Weszka, M. Doma�ski, J. Jurusik, B. Hajduk, M. Chwastek, H. Bednarski<br />
Studies <strong>of</strong> Electric Transport Properties <strong>of</strong> Single Layer Devices Based on the<br />
Polyazomethine Thin Films<br />
12
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
37. R. Jarimavi�iu-te.-Žvalioniene., I. Prosy�evas, Ž. Kaminskiene., S. Lapinskas<br />
Optical properties <strong>of</strong> black silicon with precipitated silver and gold nanoparticles<br />
38. I.P. Koziarskyi, P.D. Marianchuk, E.V. Maistruk, D.P. Koziarskyi<br />
Optical Properties <strong>of</strong> Crystals (3HgSe)0.5(In2Se3)0.5, (3HgS)0.5(In2S3)0.5,<br />
(3HgS)0.5(Al2S3)0.5, (3HgSe)0.5(Al2Se3)0.5, Doped with Mn or Fe<br />
39. N.S. Yurtsenyuk<br />
Self-Compensation Mechanism in Semi-Insulating CdMnTe:Sn Crystals Intended for<br />
X/�-Ray Detectors<br />
40. J. Krok-Kowalski, G. Władarz, T. Gro�, H. Duda, A.W. Pacyna, K. Nikiforov, P. Rduch<br />
Influence <strong>of</strong> Cu, Ga and Au Dopants and Technology Conditions on the Magnetic<br />
Interactions in HgCr2Se4 Single Crystals<br />
41. A.I. Ievtushenko, G.V. Lashkarev, V.I. Lazorenko, Z.J. Horvath, M.G. Dusheyko<br />
The Photodetectors with Vertical Integration Based on ZnO Films<br />
42. A.I. Ievtushenko, G.V. Lashkarev, V.I. Lazorenko, O.Y. Khyzhun, L.O. Klochkov, O.I.<br />
Bykov, V.M. Tkach, V.A. Baturin, A.Y. Karpenko<br />
Features <strong>of</strong> the Properties for Nitrogen Doping and Al-N Codoping <strong>of</strong> ZnO Films<br />
43. A.I. Ievtushenko, G.V. Lashkarev, V.I. Lazorenko, L.O. Klochkov, O.I. Bykov, O.M.<br />
Kutsay, S.P. Starik, V.A. Baturin, A.Y. Karpenko<br />
Influence <strong>of</strong> Oxygen Pressure on the Properties <strong>of</strong> AZO Films<br />
44. H. Duda, E. Malicka, T. Gro�, A. G�gor, R. Sitko, J. Krok-Kowalski, P. Rduch<br />
Thermoelectric Power <strong>of</strong> CuCrxVySe4 p-Type Spinel Semiconductors<br />
45. H. Duda, P. Rduch, E. Malicka, T. Gro�, A. G�gor<br />
Critical Behaviour <strong>of</strong> the 3D-Heisenberg Ferromagnetic<br />
46. J.M. Sajkowski, M.A. Pietrzyk, D. Dobosz, M. Stachowicz, A. Droba, E. Przezdziecka,<br />
A. Wierzbicka, A. Kozanecki<br />
Optical properties <strong>of</strong> ZnO/ZnMgO single quantum wells grown by molecular beam<br />
epitaxy<br />
47. B.A. Orłowski, S.P. Dziawa, A. Reszka, K. Gas, S. Mickievicius, S. Thiess, W. Drube<br />
Electronic structure <strong>of</strong> CdTe/PbEuTe/CdTe<br />
48. D. Ziółkowska, K.P. Korona, M. Kami�ska, S.H. Wu, M.S. Chen<br />
Raman Spectroscopy <strong>of</strong> LiFePO4 and Li3V2(PO4)3 Cathode Materials for Lithium-Ion<br />
Battery Applications<br />
49. M. Hosatte, M.Ł. Basta, B.S. Witkowski, Z.T. Ku�nicki, M. Godlewski<br />
Multi-interface layered P-doped silicon structures for third generation photovoltaics<br />
50. M. Stachowicz, E. Przezdziecka, D. Dobosz, M.A. Pietrzyk, J.M. Sajkowski, A. Droba,<br />
A. Wierzbicka, A. Kozanecki<br />
Optical Properties <strong>of</strong> Thin ZnO Layers Grown by MBE<br />
13
_______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
51. K. Gas, E. Dynowska, E. Janik, A. Kami�ska, S. Kret, J.F. Morhange, I. Pasternak, M.<br />
Wiater, W. Zaleszczyk, R. Hołyst, E. Kami�ska, T. Wojtowicz, W. Szuszkiewicz<br />
ZnO-based Nanotubes Obtained by the Oxidation <strong>of</strong> ZnTe and ZnTe/Zn Nanowires<br />
52. D. �ak, W. Nakwaski<br />
Composition-dependent thermal resistance <strong>of</strong> multilayered structures taking into<br />
account phonon reflection, scattering and tunneling<br />
53. A. Reszka, B.A. Orłowski, M.A. Pietrzyk, A. Szczerbakow, S. Mickievicius, S.<br />
Balakauskas, R.L. Johnson<br />
Pb1-xGexTe surface with Sm 2+ and Sm 3+ doping<br />
54. H. Bednarski, J. Weszka, M. Doma�ski, V. Cozan<br />
Studies <strong>of</strong> Optical Properties <strong>of</strong> Protonated Poliazomethine Thin Films<br />
55. A.V. Atrashchenko, V.P. Ulin, V.P. Evtikhiev<br />
The reasons <strong>of</strong> destruction <strong>of</strong> nanoporous GaAs matrix fabricated by electrochemical<br />
etching<br />
56. E. Malicka, T. Gro�, A.W. Pacyna, H. Duda, J. Krok-Kowalski<br />
Effect <strong>of</strong> Substitution <strong>of</strong> Ti for Cd in CdCr2Se4 p-Type Semiconductor<br />
57. K. Dybko , M. Szot, T. Story, G. Karczewski, S. Schreyeck, C. Schumacher, K. Brunner<br />
and L. W. Molenkamp<br />
Thermoelectric power in epitaxial Bi2Se3/Si(111) layers<br />
58. M. Zapalska and T. Doma�ski<br />
Diamagnetism <strong>of</strong> the pre-paired electronic systems: the flow equation study<br />
59. N. Gonzalez-Szwacki, J.A. Majewski<br />
Quantum Monte Carlo vs. Density Functional Methods for the prediction <strong>of</strong> relative<br />
energies <strong>of</strong> small Si-C clusters<br />
60. M. Koba and P. Szczepa�ski<br />
Analysis <strong>of</strong> a Spatial Hole Burning Effect in a Square and Triangular Lattice Photonic<br />
Crystal Laser<br />
14
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
TUESDAY POSTER SESSION (TuP1 …TuP58)<br />
1. T. Smole�ski, Ł. Cywi�ski, M. Goryca, P. Kossacki<br />
Exciton and Mn spin dynamics in a nonresonantly coupled pair <strong>of</strong> (Cd,Mn)Te<br />
quantum dots<br />
2. J. Kobak, W. Pacuski, T. Kazimierczuk, J. Suffczy�ski, T. Jakubczyk, A. Golnik,<br />
P. Kossacki, M. Nawrocki, J.A. Gaj, C. Kruse, D. Hommel<br />
Three-dimensional anisotropy studies <strong>of</strong> CdTe quantum dots<br />
3. K. Grodecki, W. Strupi�ski, A. Wysmołek, R. St�pniewski, J.M. Baranowski<br />
Probing electron concentration in epitaxial graphene using Raman spectroscopy<br />
4. M. Zieli�ski, G.W. Bryant, N. Malkova, J. Sims, W. Jaskolski, J. Aizpurua<br />
Dynamical control <strong>of</strong> excitonic fine structure with nanomechanical strain<br />
5. K. Roszak, T. Novotný<br />
Non-Markovian noise at the Fermi-edge singularity in quantum dots<br />
6. M. Czapkiewicz, J. Wróbel, V. Kolkovsky, P. Nowicki, M. Aleszkiewicz, M. Wiater,<br />
T. Wojtowicz<br />
Transport and Spin Properties <strong>of</strong> CdTe/CdMgTe Quantum Point Contacts<br />
7. M. Kozub, P. Machnikowski<br />
The Role <strong>of</strong> Strong Coupling in the Superradiance <strong>of</strong> Ensembles <strong>of</strong> Quantum Dots<br />
8. M. Szymura, Ł. Kłopotowski, P. Wojnar, K. Fronc, T. Kazimierczuk, G. Karczewski,<br />
T. Wojtowicz<br />
Photoluminescence Linewidth Analysis <strong>of</strong> Single CdMnTe Quantum Dots<br />
9. M. Koperski, T. Kazimierczuk, M. Goryca, A. Golnik, J.A. Gaj, M. Nawrocki,<br />
P. Wojnar, P. Kossacki<br />
Statistical Study <strong>of</strong> the Inter-Dot Excitation Transfer in CdTe/ZnTe Quantum Dots.<br />
10. Ł. Marcinowski, M. Krzy�osiak, K. Roszak, P. Machnikowski, R. Buczko, J. Mostowski<br />
Phonon influence on the weak measurement <strong>of</strong> double quantum dot spin states.<br />
11. P. Kaczmarkiewicz, A. Musiał, G. S�k, P. Podemski, J. Misiewicz, P. Machnikowski<br />
Influence <strong>of</strong> Carrier Trapping on the Optical Properties <strong>of</strong> InAs/InP Quantum Dashes<br />
12. K. Dziatkowski, D. Ratchford, T. Hartsfield, X. Li, Y. Gao, Z. Tang<br />
CdSe/ZnS Colloidal Quantum Dots with Alloyed Core/Shell Interfaces:<br />
a Photoluminescence Dynamics Study<br />
13. K. Kukli�ski, Ł. Kłopotowski, K. Fronc, P. Wojnar, T. Wojciechowski, M. Czapkiewicz,<br />
J. Kossut, G. Karczewski, T. Wojtowicz<br />
Spectroscopy <strong>of</strong> Indirect Excitons in Vertically Stacked CdTe Quantum Dot Structures<br />
15
_______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
14. W. Abdussalam, A. Sitek, P. Machnikowski<br />
Collective spontaneous emission from pairs <strong>of</strong> quantum dots: the role <strong>of</strong> coupling and<br />
system geometry<br />
15. K. Korzekwa, P. Machnikowski<br />
Tunneling transfer protocol in a quantum dots chain immune to inhomogeneity<br />
16. A. Musial, G. Sek, A. Marynski, P. Podemski, J. Andrzejewski, J. Misiewicz, A. Loeffler,<br />
S. Hoefling, S. Reitzenstein, J.P. Reithmaier, A. Forchel<br />
Thermal Quenching <strong>of</strong> Photoluminescence from epitaxial InGaAs/GaAs Quantum<br />
Dots with High Lateral Aspect Ratio<br />
17. P. Schillak, G. Czajkowski<br />
Excitonic Magnetoabsorption <strong>of</strong> Cylindrical Quantum Disks<br />
18. P. Karwat, A. Sitek, P. Machnikowski<br />
Spontaneous emission from double quantum dots: collective effects and carrier-<br />
phonon kinetics<br />
19. P. Łach, A. Reszka, G. Karczewski, P. Wojnar, T. Wojtowicz, A. Kami�ska, A. Suchocki<br />
Cathodoluminescence studies <strong>of</strong> the II – VI semiconducting quantum dots grown by<br />
molecular beam epitaxy<br />
20. E. Zielony, E. Popko, Z. Gumienny, P. Kamyczek, A. Henrykowski, J. Jacak, G.<br />
Karczewski<br />
Raman spectroscopy <strong>of</strong> CdTe/ZnTe quantum dots<br />
21. J. Bara�ski, T. Doma�ski<br />
Quantum interference in charge transport via the quantum dots coupled between<br />
the metallic and superconducting leads<br />
22. H. Bednarski, J. Spałek<br />
Bound magnetic polaron molecule in diluted magnetic semiconductors within the<br />
Heitler-London approximation<br />
23. M. Zału�ny, A. Kozłowski<br />
Intersubband polaritons in strongly pumped microcavities<br />
24. M.S. Mukhin, Y.V. Terent'ev, L.E. Golub, M.O. Nestoklon, B.Y. Meltser,<br />
A.N. Semenov, V.A. Solov'ev, A.A. Sitnikova, A.A. Toropov, S.V. Ivanov<br />
Magneto-Optical Studies <strong>of</strong> Narrow Band-Gap Heterostructures with Type II Quantum<br />
Dots InSb in an InAs Matrix<br />
25. K.H. Gawarecki, P. Machnikowski<br />
Phonon-assisted tunneling between hole states in double quantum dots<br />
26. I. Bragar, K. Gawarecki, P. Machnikowski<br />
Intermediate band formation for electrons and holes in an inhomogeneous chain <strong>of</strong><br />
quantum dots<br />
16
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
27. O. Rancova, E. Anisimovas<br />
Modelling <strong>of</strong> 1D-2D structural transitions in small Yukawa clusters<br />
28. P. Kowalski, P. Machnikowski<br />
Efficient Coulomb mixing between single- and biexciton states in InAs nanocrystals<br />
29. K. Gołasa, M. Molas, J. Borysiuk, Z.R. Wasilewski, A. Babi�ski<br />
Strongly disordered wetting layer in the InAs/GaAs self-assembled quantum dots<br />
system<br />
30. M. Molas, K. Gołasa, J. Łusakowski, T. Wojtowicz, A. Babi�ski<br />
Optical transformation <strong>of</strong> zero-dimensional confinement in the CdTe/CdMgTe multiple<br />
quantum wells<br />
31. M. Kozub, A. Musiał, G. S�k, J. Misiewicz, V. Zürbig, J.P. Reithmaier<br />
Optical Properties <strong>of</strong> InGaAs Quantum Dots on (100) GaAs Substrates Formed by<br />
Droplet Epitaxy<br />
32. M. Molas, K. Gołasa, T. Kazimierczuk, J. Łusakowski, T. Wojtowicz, A. Babi�ski<br />
Magnetooptical study <strong>of</strong> excitons confined in potential fluctuations in the<br />
CdTe/CdMgTe quantum well<br />
33. A. Ballester, J.M. Escartín, J.L. Movilla, M. Pi, J. Planelles<br />
Mixed Correlation Phases In Elongated Quantum Dots<br />
34. J. Ebeling, T. Aschenbrenner, S. Figge, D. Hommel<br />
Simulation and Realization <strong>of</strong> Photonic Crystals in Light Emitting Devices<br />
35. E. Zielony, E. Popko, Z. Gumienny, G. Karczewski<br />
Electro-optical characterization <strong>of</strong> Ti/Au-ZnTe Schottky diodes with CdTe quantum<br />
dots<br />
36. J. Jadczak, L. Bryja, A. Wójs, G. Bartsch, D.R. Yakovlev, M. Bayer, M. Potemski<br />
Magneto-photoluminescence studies <strong>of</strong> many body scattering processes in two-<br />
dimensional hole gas<br />
37. H. Videlier, N. Dyakonova, F. Teppe, C. Consejo, W. Knap, J. Lusakowski,<br />
D. Tomaszewski, J. Marczewski, P. Grabiec<br />
Spin related effect in Si-MOSFETs THz Photoresponse<br />
38. N. Dyakonova, A. ElFatimy, Y. Meziani, D. Coquillat, W. Knap, F. Teppe, P. Buzatu,<br />
L. Varani, H. Marinchio, J. Torres, P. Nouvel<br />
THz emission related to hot plasmons and plasma wave instability in field effect<br />
transistors<br />
39. M. Białek, M. Czapkiewicz, K. Fronc, J. Wróbel, V. Umansky, M. Grynberg,<br />
J. Łusakowski<br />
A Comb-Like Shape <strong>of</strong> the Cyclotron Resonance Line in GaAs/AlGaAs<br />
Heterostructure<br />
17
_______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
40. J. Przybytek, M. Gryglas-Borysiewicz, M. Baj<br />
Impurity-Related Noise in Si �-doped Single-Barrier GaAs/AlAs/GaAs Resonant<br />
Tunneling Devices<br />
41. F. Teppe, C. Consejo, J. Torres, B. Chenaud, P. Solignac, Z.R. Wasilewski, M. Zholudev,<br />
N. Dyakonova, D. Coquillat, A. El Fatimy, W. Knap<br />
Current Driven Terahertz Detection <strong>of</strong> Quantum Cascade Laser Emission by Plasma<br />
Waves in Nano-Transistors<br />
42. G. K�pisty, K. Grodecki, W. Strupi�ski, A. Wysmołek, R. St�pniewski, J.M. Baranowski<br />
Influence <strong>of</strong> SiC substrate orientation on epitaxial graphene quality studied by Raman<br />
spectroscopy<br />
43. M. Syperek, B. Bujko, J. Jadczak, M. Kubisa, L. Bryja, J. Misiewicz<br />
Fermi edge and band-to-band recombination dynamics in n-doped GaAs/AlGaAs<br />
quantum well<br />
44. M. Kozubal, M. Gryglas-Borysiewicz, J. Przybytek, J. Borysiuk, M. Baj, R. St�pniewski,<br />
A. Wysmołek, W. Strupi�ski, J. Baranowski<br />
Magnetotransport Studies <strong>of</strong> Graphene Exposed to Water Vapours<br />
45. K. Grodecki, J.A. Błaszczyk, A. Dominiak, W. Strupi�ski, A. Wysmołek,<br />
R. St�pniewski, A. Drabi�ska, M. Sochacki, J.M. Baranowski<br />
Interaction <strong>of</strong> epitaxial graphene with SiC substrates studied by Raman spectroscopy<br />
46. K. Nogajewski, H. Boukari, P. Kopyt, W. Gwarek, T. Wojtowicz, H. Mariette,<br />
M. Grynberg, J. Łusakowski<br />
Antenna-Equipped Field Effect Transistors on CdTe/CdMgTe Quantum Wells as<br />
Terahertz Detectors<br />
47. A.D. Chegodaev, D.K. Loginov<br />
Effect <strong>of</strong> homogeneous electric field on exciton dispersion in wide quantum well<br />
48. V.Ya. Roshko<br />
Theoretical Analysis <strong>of</strong> Optical Losses in CdS/CdTe Solar Cells<br />
49. M.Kubisa, K.Ryczko and J.Misiewicz<br />
Anisotropy <strong>of</strong> B=0 spin splitting <strong>of</strong> holes in symmetric GaAs/Ga(1-x)AlxAs quantum<br />
wells<br />
50. J. Szczytko, P. Arcade, E. Papis, A. Bara�ska, B. Pi�tka, J. Łusakowski<br />
Terahertz Properties <strong>of</strong> Gold Layers on GaAs<br />
51. P. Sznajder, B. Pi�tka, J. Szczytko, J. Łusakowski<br />
Towards Optically Tunable Terahertz Plasmonic Detectors<br />
52. I. Grigelionis, K. Fobelets, B. Vincent, J. Mitard, B. DeJaeger, E. Simoen, D. Jaworski,<br />
J. Łusakowski<br />
Mobility <strong>of</strong> Holes in Nanometer Ge-on-Si p-type Metal-Oxide-Semiconductor Field-<br />
Effect Transistors at Low Temperatures<br />
18
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
53. Paweł Utko, Morten H. Madsen, Trine Berthing, Sara Bonde, Claus B. Sorensen,<br />
Karen L. Martinez, Jesper Nygard<br />
Vertical InAs nanowires as biological probes<br />
54. M. �ciesiek, T. Jakubczyk, W. Pacuski, A. Golnik, P. Kossacki, C. Kruse, D. Hommel<br />
Towards better light-confinement in pillar cavities<br />
55. M. Rawski, J. �uk, A. Drozdziel, K. Pyszniak, M. Turek<br />
Investigation <strong>of</strong> ion-implanted SiC properties by means <strong>of</strong> a light absorption technique<br />
56. P. Kamyczek, E. Placzek-Popko, Łukasz Gelczuk, Maria D�browska-Szata<br />
SiC Schottky barrier diode studied by admittance spectroscopy<br />
57. P. Sitarek, K. Ryczko, J. Misiewicz, D. Reuter, A. Wieck<br />
Optical transitions between confined and unconfined states in p-type asymmetric<br />
GaAs/InGaAs/AlGaAs QW structures<br />
58. K. Tahy, W. S. Hwang, J.L. Tedesco, R.L. Myers Ward, P.M. Campbell, C.R. Eddy Jr.,<br />
D.K. Gaskill, H. Xing, A. Seabaugh, D. Jena<br />
Sub-10 nm Epitaxial Graphene Nanoribbon FETs<br />
19
_______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
WEDNESDAY POSTER SESSION (WeP1 …WeP58)<br />
1. C. �liwa, T. Dietl<br />
Thermodynamic and thermoelectric properties <strong>of</strong> (Ga,Mn)As<br />
2. M. Sobanska, K. Klosek, Z.R. Zytkiewicz, J. Borysiuk, A. Wierzbicka, A. Reszka, E.<br />
Lusakowska<br />
Plasma-assisted molecular beam epitaxy <strong>of</strong> GaN on Si (111) substrates<br />
3. J. Plaziak, J. Higersberger, J.A. Majewski<br />
Role <strong>of</strong> Interface Carrier Transport in Electrical Characteristics <strong>of</strong> Light Emitting<br />
Devices Based on Heterostructures<br />
4. A. Łusakowski, P. Bogusławski, W. Kn<strong>of</strong>f, T. Story<br />
Influence <strong>of</strong> crystal structure and hole concentration on magnetic anisotropy <strong>of</strong><br />
GeMnTe<br />
5. I. Ulfat, L. Ilver, J. Sadowski<br />
High-resolution study <strong>of</strong> valence band maximum for (GaMn)As<br />
6. J. Suffczy�ski, K. Gałkowski, P. Ka�mierczak, J. Papierska, M. Furman, W. Pacuski,<br />
A. Golnik, A.M. Witowski, J.A. Gaj, W. Stefanowicz, M. Sawicki, M. Łukasiewicz,<br />
E. Guziewicz, M. Godlewski<br />
Low temperature grown (Zn,Co)O studied in the band-gap spectral region<br />
7. J. Levrat, G. Rossbach, A. Dussaigne, H. Teisseyre, I. Grzegory, M. Bockowski,<br />
T. Suski, R. Butte, N. Grandjean<br />
Nonpolar III-nitride microcavities for polariton lasing<br />
8. W. Bardyszewski, S.P.. Łepkowski<br />
Symmetry <strong>of</strong> the top valence band states in GaN/AlGaN quantum wells and its<br />
influence on the polarization <strong>of</strong> emitted light<br />
9. D. Sztenkiel, T. Dietl<br />
Tunnelling effects in (Ga,Mn)As based heterostructures<br />
10. M. Lopuszynski, J.A. Majewski<br />
Modelling <strong>of</strong> Ordering Phenomena in Nitride Semiconducting Alloys<br />
11. M. Welna, R. Kudrawiec, M. Motyka, J. Misiewicz, R. Kucharski, M. Zaj�c, R.<br />
Doradzi�ski, R. Dwili�ski<br />
Infrared Spectroscopy <strong>of</strong> GaN Crystals Obtained by Ammonothermal Method<br />
12. M. Latkowska, R. Kudrawiec, G. S�k, J. Misiewicz, J. Ibán~ez, M. Henini,<br />
M. Hopkinson<br />
Micro-photoluminescence <strong>of</strong> GaInNAs layers: Thermal quenching <strong>of</strong> individual<br />
exciton lines<br />
13. M. Fandrich, G. Kunert, T. Aschenbrenner, S. Figge, C. Kruse, D. Hommel<br />
20
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Nitride based sensors with Ga- and N-polarity<br />
14. V.Kh. Le, K. Swiatek, M. Pawlowski, R.R. Galazka<br />
Zinc vacancy induced ferromagnetic interaction in semimagnetic semiconductor<br />
(Zn,Mn)Te<br />
15. S. Dobkowska, W. Stefanowicz, O. Proselkov, R. �uberek, J. Sadowski, T. Dietl,<br />
M. Sawicki<br />
Magnetic Properties <strong>of</strong> (Ga,Mn)As near Metal-Isolator Transition<br />
16. M. Sakowicz, N. Gauthier, R. Leonelli, C. Silva, H. Nguyen, K. Cui, S. Zhang, Z. Mi<br />
Time-Resolved Photoluminescence <strong>of</strong> InGaN/GaN Dot-in-a-Wire Heterostructures on<br />
Si(111)<br />
17. M. Bajda, F. Dybała, A. Bercha, W. Trzeciakowski, J.A. Majewski<br />
Wide range wavelength tuning <strong>of</strong> InGaAsP/InP laser diodes<br />
18. M. Gryglas-Borysiewicz, J. Przybytek, M. Baj, A. Kwiatkowski, P. Juszy�ski, D. Wasik,<br />
P. Dziawa, J. Sadowski<br />
Transport properties <strong>of</strong> GaMnAs layers<br />
19. C. Chèze, M. Sawicka, M. Siekacz, H. Turski, A. Feduniewicz-Zmuda, G. Cywinski, B.<br />
Grzywacz, S. Grzanka, I. Dziecielewski, B. Lucznik, M. Bockowski, C. Skierbiszewski<br />
Group III-nitrides growth on N-polar substrates<br />
20. J. Binder, K.P. Korona, J. Borysiuk, M. Kaminska, M. Baeumler, K. Köhler, L. Kirste<br />
Absorption and Emission Properties <strong>of</strong> Light Emitting Diode Structures Containing<br />
GaInN/GaN QWs<br />
21. B.J. Kowalski, R. Nietuby�, J. Sadowski<br />
Mn contribution to the valence band <strong>of</strong> Ga1-xMnxSb<br />
22. M. Baranowski, M. Latkowska, M. Syperek, R. Kudrawiec, J. Misiewicz, T. Sarmiento,<br />
J.S. Harris<br />
Time resolved photoluminescence studies for GaInNAsSb quantum wells emitting at<br />
1.3 µm<br />
23. A. Kafar, J. Goss, S. Sta�czyk, R. Czernecki, M. Leszczynski, T. Suski, P. Wi�niewski,<br />
P. Perlin<br />
InGaN laser diodes with passive absorber section<br />
24. W. Szuszkiewicz, F. Ott, J.Z. Domagała, E. Dynowska, J. Sadowski<br />
Evidence <strong>of</strong> a new magnetic order in short-period (Ga,Mn)As/GaAs SLs<br />
25. Z. Wi�niewski, K. Izdebska, P. Sybilski, Z.R. �ytkiewicz, M. Soba�ska, K. Kłosek,<br />
A. Reszka, B.J. Kowalski, A. Suchocki<br />
Application <strong>of</strong> n-GaN layers grown by MBE for light-induced water splitting and<br />
hydrogen generation<br />
26. M. Sarzynski, J. Goss, M. Leszczynski, A. Reszka, B. Kowalski, P. Perlin, T. Suski<br />
21
_______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Broad optical gain spectrum in III-N quantum structures<br />
27. M.I. Łukasiewicz, A. Cabaj, M. Godlewski, E. Guziewicz, A. Wittlin, M. Jaworski,<br />
A. Woło�, Z. Wilamowski<br />
Microwave techniques investigations <strong>of</strong> (Zn,Co)O films grown by Atomic Layer<br />
Deposition<br />
28. O. Yastrubchak, T. Andrearczyk, J. Sadowski, J. �uk, T. Wosi�ski<br />
Band-structure analysis from photoreflectance spectroscopy in (Ga,Mn)As<br />
29. K. Klosek, M. Sobanska, Z.R. Zytkiewicz, H. Teisseyre, E. Lusakowska, A. Wierzbicka,<br />
P. Nowakowski, L. Klopotowski<br />
Influence <strong>of</strong> nitrogen plasma parameters on growth <strong>of</strong> GaN by plasma-assisted<br />
molecular beam epitaxy<br />
30. T.A. Krajewski, P. Stallinga, E. Zielony, P. Kruszewski, K. Go�ci�ski, Ł. Wachnicki,<br />
S. Figge, D. Hommel, E. Guziewicz, M. Godlewski<br />
Deep defects in ZnO/GaN heterostructure analyzed by the admittance spectroscopy<br />
31. T.A. Krajewski, A.J. Zakrzewski, G. Łuka, Ł. Wachnicki, S. Gierałtowska,<br />
B.S. Witkowski, P. Kruszewski, E. Łusakowska, R. Jakieła, E. Guziewicz, M. Godlewski<br />
Electrical characterization <strong>of</strong> the ZnO-based Schottky diodes for possible sensor<br />
applications<br />
32. T. Zakrzewski, P. Bogusławski<br />
Ab initio calculations <strong>of</strong> transition metal impurity levels in III-V semiconductors<br />
33. P. Juszy�ski, D. Wasik, M. Baj, J. Przybytek, M. Gryglas-Borysiewicz, J. Sadowski<br />
Anisotropy <strong>of</strong> GaMnAs thin film. Planar and Anomalous Hall measurements.<br />
34. G. Staszczak, A. Khachapuridze, S. Grzanka, R. Piotrzkowski, R. Czernecki, P. Perlin,<br />
T. Suski<br />
Interplay between internal and external electric field studied by photoluminescence in<br />
InGaN/GaN light emitting diodes<br />
35. M. Baranowski, M. Latkowska, R. Kudrawiec, M. Syperek, J. Misiewicz,<br />
G.S. Karthikeyan, S. Jong-In, K.L. June<br />
Influence <strong>of</strong> antimony on the optical quality <strong>of</strong> GaInN:Sb multi quantum wells<br />
36. J. Sadowski, A. Siusys, P. Dziawa, A. Reszka, B.J. Kowalski<br />
Properties <strong>of</strong> Mn-doped GaAs Nanowires and GaAs/(Ga,Mn)As Core-Shell Nanowire<br />
Structures Grown by MBE on GaAs(111)B Substrates<br />
37. I.A. Kowalik, M.A. Nin~o, A. Locatelli, T. OnurMente�, A. Navarro-Quezada,<br />
M. Rovezzi, A. Bonanni, T. Dietl, D. Arvanitis<br />
Room temperature nano-magnetism <strong>of</strong> (Ga,Fe)N films: element specific spectroscopy<br />
and microscopy<br />
38. L. Marona, S. Grzanka, P. Wisniewski, T. Suski, M. Leszczynski, R. Czernecki,<br />
M. Bockowski, S.P. Najda, P. Perlin<br />
22
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
The non-radiative recombination rate in InGaN laser diodes during the aging<br />
39. L. Janicki, M. Gladysiewicz, R. Kudrawiec, M. Rudzi�ski, R. Kucharski, M. Zaj�c,<br />
J. Misiewicz, W. Strupinski, R. Doradzi�ski, R. Dwili�ski<br />
Electromodulation Spectroscopy <strong>of</strong> GaN/AlGaN Quantum Wells Grown on Polar and<br />
Non-polar GaN Substrates Obtained by Ammonothermal Method<br />
40. E.P. Smakman, R. vanVoornveld, J.G. Keizer, J.K. Garleff, P.M. Koenraad<br />
Spin-Polarized STM with Fe-Coated W Tips and Bulk Cr Tips<br />
41. B.S. Witkowski, Ł. Wachnicki, E. Guziewicz, M. Godlewski<br />
Cathodoluminescence measurements at liquid helium temperature <strong>of</strong> monocrystalline<br />
ZnO layers<br />
42. T. Andrearczyk, I. Krogulec, T. Wosi�ski, T. Figielski, A. M�kosa, Z. Tkaczyk,<br />
J. Wróbel, J. Sadowski<br />
Towards electrically controllable read-write devices based on ferromagnetic<br />
semiconductors<br />
43. J.B. Gosk, Z. Werner, C. Pochrybniak, M. Barlak, J. Szczytko, A. Twardowski<br />
Magnetic properties <strong>of</strong> Mn-ion implanted and plasma pulse treated Si<br />
44. G. Muzioł, M. Siekacz, H. Turski, C. Skierbiszewski<br />
Simulations <strong>of</strong> optical modes in InGaN based laser diodes operating at 455 nm<br />
45. M. Woi�ska, K. Madrak, J. Szczytko, J. Gosk, A. Majh<strong>of</strong>er, D. Pociecha, E. Górecka,<br />
A. Twardowski<br />
Monte-Carlo simulations and magnetic studies <strong>of</strong> ferromagnetic nanocomposites<br />
46. J. Szczytko, J. Szydlowska, N. Gonzalez Szwacki, K. Dziatkowski, P. Gizi�ski, A. Kaim,<br />
A. Twardowski<br />
Di-TEMPO amine for molecular spintronics – model <strong>of</strong> p-shell magnetism<br />
47. M. Baranowski, M. Latkowska, R. Kudrawiec, J. Misiewicz<br />
Hopping excitons in GaInNAs alloys: Radiative versus non-radiative recombination at<br />
various temperatures<br />
48. J.Z. Domagala, B. Lucznik, H. Teisseyre, M. Bockowski, I. Grzegory<br />
Structural characterization <strong>of</strong> the nonpolar substrate grown by multistep hydride vapor<br />
phase epitaxy.<br />
49. Ł. Gluba, O. Yastrubchak, H. Krzy�anowska, J.Z. Domagała, T. Andrearczyk, J. �uk,<br />
J. Sadowski, T. Wosi�ski<br />
Investigation <strong>of</strong> fundamental parameters <strong>of</strong> low doped (Ga,Mn)As epitaxial layers<br />
50. P. Kamyczek, E. Popko, Z. Gumienny, E. Zielony, S.Grzanka, R. Czernecki, T. Suski<br />
Admittance spectroscopy in GaN p-n junction<br />
51. Pawel Kempisty, Pawel Strak, Stanislaw Krukowski<br />
23
_______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Role <strong>of</strong> hydrogen in the ammonia based growth <strong>of</strong> GaN and InGaN - ab initio study<br />
52. S.P. Lepkowski, W. Bardyszewski, H. Teisseyre<br />
Effect <strong>of</strong> the built-in strain on the in-plane optical anisotropy <strong>of</strong> m-plane GaN/AlGaN<br />
quantum wells<br />
53. J. Papierska, J.-G. Rousset, A. Golnik, W. Pacuski, M. Nawrocki, J. A. Gaj,<br />
J. Suffczynski, I. Kowalik, W. Stefanowicz, M. Sawicki, T. Dietl, A. Navarro-Quezada,<br />
B. Faina, Tian Li, A. Bonanni<br />
Magnetooptical properties <strong>of</strong> (Ga,Fe)N layers<br />
54. Maria Ptasi�ska, Jacek Piechota, Jakub Sołtys, Stanisław Krukowski<br />
Gas Phase Reactions during GaN Growth by MOVPE Method – Ab initio Study<br />
55. Konrad Sakowski, Leszek Marcinkowski, Szymon Grzanka, Elzbieta Litwin-<br />
Staszewska, Stanislaw Krukowski<br />
Simulations <strong>of</strong> Gallium Nitride luminescent devices with extension <strong>of</strong> standard<br />
nonradiative recombination model<br />
56. Pawel Strak, Pawel Kempisty, Konrad Sakowski, Stanislaw Krukowski<br />
Density Functional Theory (DFT) simulations <strong>of</strong> the physical properties <strong>of</strong> AlN/GaN<br />
multiple quantum wells (MQWs)<br />
57. Z. R. Zytkiewicz, P. Dluzewski, J. Borysiuk, M. Sobanska, K. Klosek, B. Witkowski,<br />
P. Nowakowski, J. Dabrowski<br />
Plasma-assisted MBE growth and characterization <strong>of</strong> GaN nanocolumns on Si (111)<br />
substrates<br />
58. Zbigniew R. Zytkiewicz, Pawel Strak, Stanisław Krukowski<br />
Crossing Size Limits <strong>of</strong> Bulk III-V Crystals Feasible by Liquid Phase Electroepitaxy<br />
24
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors SaI1<br />
Modern technologies for wide-gap semiconductor epitaxy<br />
and nano-processing<br />
Detlef Hommel<br />
Institute <strong>of</strong> Solid State Physics, University <strong>of</strong> Bremen, Germany<br />
An overview will be given on the growth and application <strong>of</strong> wide bandgap<br />
semiconductors like group-III nitrides and group-II selenides and tellurides. It will be shown<br />
that different epitaxial methods have to be used for different materials (MBE for II-VI and<br />
mostly MOVPE for nitrides). In this respect homo- and heteroepitaxy will be compared as<br />
well.<br />
The growth mechanisms for CdSe and InGaN quantum dots differ significantly and<br />
both have not much common with the well known Stranski-Krastanov growth mode. This will<br />
be discussed in detail.<br />
As an important tool for nano-processing a focused ion beam (FIB) tool will be<br />
presented. Such a dual beam system with a Ga-source for nanostructuring, an electron source<br />
for imaging, an integrated e-beam lithography and sources for insulator and metal deposition<br />
on a nanometer scale allows to design novel device structures.<br />
Approaches to grow fully monolithic microcavities based on selenides, nitrides and<br />
tellurides will be presented. Using the FIB micropillars can be designed out <strong>of</strong> such planar<br />
structures with interesting physical properties. Introducing quantum dots into the cavity one<br />
can obtain efficient sources for single photon emission up to room temperature.<br />
25
SaI2 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Physical simulation <strong>of</strong> electronic devices<br />
Aldo Di Carlo<br />
Dept. <strong>of</strong> Electronics Engineering, University <strong>of</strong> Rome “Tor Vergata”, Rome, Italy<br />
Physical Simulation is a fundamental step for the design and fabrication <strong>of</strong><br />
commercially available and research developed electronic devices.[1] Since ’80 several<br />
Technology Computer Aided Design (TCAD) tools has been developed to this end, starting<br />
from the seminal work <strong>of</strong> R. Dutton at Stanford University. Nowadays, the need for faster<br />
and better performing electronic devices with higher integration density has been driving<br />
almost since the dawn <strong>of</strong> semiconductor technology a steady trend <strong>of</strong> scaling down the device<br />
dimensions. This led to an incredible increase in computing power and memory capacity,<br />
quantified by the well-known Moore’s law. Due to this downscaling, the dimensions <strong>of</strong> the<br />
active region <strong>of</strong> conventional metal–oxide–semiconductor field-effect transistors (MOSFETs)<br />
have already reached the nanometer scale. This makes device simulation very challenging and<br />
new theoretical framework should be considered. [2]<br />
Hand in hand with device scaling, new devices exploiting quantum mechanical effects for<br />
their functioning have been proposed, e.g., transistors based on quantum wires, dots, or<br />
molecules. Quantum dots and wires have also gained much attention for their use in<br />
optoelectronic devices for lasing or single photon emission or as color tunable and white-light<br />
emitters. The electronic, optical, and transport properties <strong>of</strong> the active regions <strong>of</strong> such highly<br />
scaled or new devices cannot be described by classical or semiclassical theories. A fully<br />
quantum mechanical treatment has to be used instead. [3]<br />
In this lecture, I will review the methods for electronic device simulation starting from the<br />
common picture based on drift-diffusion equation up to quantum mechanical based<br />
approaches for both continuum and atomistic models. A review <strong>of</strong> available TCAD tools will<br />
be given.<br />
Fig. 1<br />
Simulation <strong>of</strong> GaN based nanocolumn LED.<br />
Electrostatic potential and current flow lines<br />
around the intrinsic part <strong>of</strong> the column.<br />
(a) The classical results.<br />
(b)The self-consistent Quantum/driftdiffusion<br />
results, showing also the contours <strong>of</strong> the electron<br />
and hole densities at half <strong>of</strong> the mean density<br />
inside the intrinsic region. (from Ref. 2)<br />
[1] S. Selberherr, Analysis and Simulation <strong>of</strong> Semiconductor Devices, Springer, (1984).<br />
[2] M. Auf der Maur et al. IEEE Trans. Electron Devices 58, 1425 (<strong>2011</strong>)<br />
[3] A. Pecchia and A. Di Carlo, Rep. Prog. Phys., vol. 67, no. 8, pp. 1497–1561, (2004).<br />
26
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors SuI1<br />
Physics and Technology <strong>of</strong> Arsenide Based Quantum Dots and Nanowires<br />
Gerhard Abstreiter<br />
Walter Schottky Institut and Institute for Advanced Study<br />
TU München, 85748 Garching<br />
Optical control <strong>of</strong> charges, spins, excitons and photons has been achieved in recent years in<br />
semiconductor nanostructures. They are therefore excellent candidates for demonstrating<br />
basic principles <strong>of</strong> quantum information technology. I will discuss the basic optical properties<br />
and recent results on electro-optic control <strong>of</strong> single self-assembled InGaAs quantum dots.<br />
This includes results on spin storage and spin lifetimes <strong>of</strong> electrons and holes [1, 2] in dot<br />
ensembles, as well as single charge and spin control and readout [3]. In the second part <strong>of</strong> my<br />
lecture I will also discuss recent results on MBE grown GaAs and InAs based nanowire<br />
hetero-structures with new functionalities [4 - 9].<br />
[1] M. Kroutvar et al., Nature 432, 81 (2004)<br />
[2] D. Heiss et al., Phys. Rev. B 76, 24136 (2007)<br />
[3] D. Heiss et al., Phys. Rev. B 82, 245316 (2010)<br />
[4] A. Fontcuberta i Morral et al., Small 4, 899 (2008) and APL 92, 063112 (2008)<br />
[5] M. Heigoldt et al., J. Mater. Chem. 19, 840 (2009)<br />
[6] D. Spirkoska et al., Phys. Rev. B 80, 245325 (2009)<br />
[7] I. Zardo et al., Phys. Rev. B 80, 245324 (2009)<br />
[8] S. Hertenberger et al., J. Appl. Phys. 108, 114316 (2010)<br />
[9] M. Soini et al., Appl. Phys. Letters 97, 263107 (2010)<br />
27
SuI2 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Tutorial: Engineering <strong>of</strong> Electric Fields in Nitride-Based Semiconductors<br />
Debdeep Jena<br />
Associate Pr<strong>of</strong>essor <strong>of</strong> Electrical Engineering University <strong>of</strong> Notre Dame,<br />
Notre Dame, IN 46556 USA<br />
djena@nd.edu, http://www.nd.edu/~djena<br />
The built-in polarization fields in III-Nitride semiconductor heterostructures play<br />
fundamental and crucial roles in many electronic and optical phenomena exhibited by these<br />
semiconductors. In addition, they strongly influence the design and performance <strong>of</strong> electronic<br />
and optical devices fabricated from III-Nitride heterostructures. The goals <strong>of</strong> this tutorial are –<br />
1) To introduce the concept <strong>of</strong> electronic polarization in semiconductors<br />
2) To explore its influence on electronic and optical properties<br />
3) To explore details <strong>of</strong> how charge electrostatics and transport are affected<br />
4) To explore how optical transitions and emission properties are influenced,<br />
5) To apply them to electronic and optical devices <strong>of</strong> the present, and<br />
6) To discuss open problems, and what is possible in the future using polarization.<br />
Specifically, the concept <strong>of</strong> polarization will be traced down to its microscopic origin in<br />
charge dipoles in the highly polar III-Nitride semiconductor crystals. We will discuss how the<br />
macroscopic properties emerge from the microscopic picture. The effect <strong>of</strong> dipoles on charge<br />
transport, electrostatics, and optical transition matrix elements will be described. We will<br />
study how the fields emerging from the polarization can be used to design ‘traditional’ highperformance<br />
electronic and optical devices such as high electron mobility transistors<br />
(HEMTs), light-emitting diodes (LEDs) and lasers. The discussion will cover both the ‘good’<br />
and ‘bad’ effects <strong>of</strong> polarization in such devices.<br />
We will also discuss various “novel” device applications <strong>of</strong> polarization physics in III-<br />
Nitride semiconductor heterostructures. Some <strong>of</strong> these include polarization-induced Zenerinterband<br />
tunneling and p-type doping for applications in light emitting devices. The wellknown<br />
concept <strong>of</strong> “lattice-- matching” in III-V semiconductor heterostructures will be<br />
extended to include the idea <strong>of</strong> “Polarization-- matching” and the opportunities <strong>of</strong> such<br />
concepts will be discussed.<br />
28
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoI1<br />
��<br />
Coherent optical control <strong>of</strong> the NV center in diamond<br />
Jörg Wrachtrup<br />
3rd� Physics Institute and Research Center SCOPE, University <strong>of</strong> Stuttgart, D-70659<br />
Stuttgart, Germany<br />
29
MoI2 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Understanding and exploiting magnetism <strong>of</strong> semiconductors<br />
Tomasz Dietl<br />
Laboratory for Cryogenic and Spintronic Research,<br />
Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences and<br />
Institute <strong>of</strong> Theoretical Physics, University <strong>of</strong> Warsaw<br />
Triggered by a theoretical model <strong>of</strong> ferromagnetism mediated by valence band<br />
holes put forward a dozen years ago, the search for compounds combining the resources<br />
<strong>of</strong> semiconductors and ferromagnets has evolved into an important and challenging field<br />
<strong>of</strong> materials science. This endeavour has been fuelled by continual demonstrations <strong>of</strong><br />
remarkable low-temperature functionalities found for ferromagnetic structures <strong>of</strong><br />
(Ga,Mn)As, p-(Cd,Mn)Te, are related materials -- now transferred to elemental<br />
ferromagnets -- as well as by abundant experimental observations and theoretical<br />
modellings <strong>of</strong> ferromagnetic features persisting up to high temperatures in a number <strong>of</strong><br />
magnetically doped semiconductors and oxides without any valence band holes or even<br />
in materials nominally undoped with transition metals.<br />
In the talk, a broad overview <strong>of</strong> recent breakthrough experimental and theoretical<br />
developments will be given emphasizing that, from the one hand, they disentangle<br />
controversies and puzzles accumulated over the last decade and, on the other, <strong>of</strong>fer new<br />
outstanding research prospects [1]. More specifically, it will be shown how element<br />
specific and spin sensitive contemporary nanocharacterization methods help to asses the<br />
real structure <strong>of</strong> materials down to the atomic scale and, thus, to build a sensible<br />
theoretical understanding <strong>of</strong> macroscopic properties, including the origin <strong>of</strong> low- and<br />
high-temperature ferromagnetism in semiconductors. Recent experiments that<br />
demonstrate the influence <strong>of</strong> the electric field and current on magnetism and the<br />
generation <strong>of</strong> current by time-dependent magnetization will be discussed in a broad<br />
context <strong>of</strong> spin physics and spintronic applications.<br />
[1] See, T. Dietl, “A ten-year perspective on dilute magnetic semiconductors and oxides”,<br />
Nature Mater. 9, 965 (2010).<br />
30
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoI3<br />
Charging Effects in Self Assembled CdTe Quantum Dots<br />
Łukasz Kłopotowski<br />
Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences al. Lotników 32/46, Warszawa, Poland<br />
Semiconductor quantum dots (QDs) are <strong>of</strong>ten referred to as artificial atoms. This<br />
analogy stems mostly from zero dimensional density <strong>of</strong> states hindering interactions between<br />
these nanostructures and the environment. In fact, QDs are more than atom-like<br />
structures: they can be easily incorporated into semiconductor devices enabling studies<br />
<strong>of</strong> properties impossible to achieve in atom particles. These phenomena are related to<br />
a possibility <strong>of</strong> controllable feeding the dots with charge carriers one by one: an effect<br />
resulting from a Coulomb blockade. Such deterministic charging not only allows to study<br />
effects related to Coulomb interaction between carriers, but has proved to be essential<br />
in discoveries <strong>of</strong> phenomena requiring a particular QD charge state: creation <strong>of</strong> dynamic<br />
nuclear polarization, coherent qubit operations, creation <strong>of</strong> Mahan excitons, to name just<br />
a few.<br />
Most studies on charge tunability <strong>of</strong> QDs were done in InGaAs system. On the other<br />
hand, II - VI QDs possess a few interesting advantages over their better known counterparts:<br />
they can be easily doped with magnetic ions and exhibit stronger excitonic<br />
interactions. Combining these features with a possibility to prepare a II - VI QD in<br />
a given charge state can result for instance in electrical control over magnetism on a<br />
nanoscale.<br />
In this talk, we report on our recent achievements in studies <strong>of</strong> charging effects in<br />
self assembled CdTe QDs [1]. By studying QD photoluminescence (PL), we show charge<br />
tunability in dots embedded in a field-effect structure. Exploiting the vertical electric<br />
field resulting from biasing the device, we study the quantum confined Stark effect to<br />
gain insight into Coulomb interactions and changes <strong>of</strong> charge distribution upon tuning<br />
the QD occupancy. In particular, we find that the electron wave function is stiffer than the<br />
hole wave function – an effect which we subsequently confirm with analysis <strong>of</strong> PL lifetimes<br />
<strong>of</strong> different QD occupancies. We demonstrate that the confinement in our dots is far from<br />
the strong confinement limit. This in turn results in Coulomb correlations dominating<br />
over confinement. A fingerprint <strong>of</strong> this phenomenon is a universal shape <strong>of</strong> the QD PL<br />
spectrum featuring the same transition sequence – an effect entirely different from the<br />
InGaAs system, where confinement conditions totally determine the PL spectrum. We<br />
conclude with presenting an outlook for future studies involving magnetically doped dots<br />
and different approaches for extending the tunability in this system.<br />
[1] Ł. Kłopotowski et al. Phys. Rev. B 83, 155319 (<strong>2011</strong>).<br />
31
MoI4 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Optical properties <strong>of</strong> strongly in-plane asymmetric epitaxial nanostructures<br />
Grzegorz Sęk<br />
Institute <strong>of</strong> Physics, Wrocław University <strong>of</strong> Technology<br />
Wybrzeże Wyspiańskiego 27, 50-370 Wrocław, Poland<br />
Strongly laterally asymmetric nanostructures have already been proven advantageous in<br />
both laser applications, <strong>of</strong>fering higher gain and broader spectral tunability, or fundamental<br />
studies <strong>of</strong> solid state quantum electrodynamics (QED), due to enhanced exciton oscillator<br />
strength. On the other hand, such structures are believed to have selective and geometrydriven<br />
polarization properties. However, our spectroscopic measurements on self-assembled<br />
structures <strong>of</strong> two material systems but similar geometries (lateral aspect ratio <strong>of</strong> about 3-5)<br />
demonstrated significantly different properties.<br />
For InAs elongated structures (called dashes) on InP substrate the linear-polarizationresolved<br />
photoluminescence measurements have shown the degree <strong>of</strong> polarization (DOP) at<br />
the level <strong>of</strong> 30% for high excitation conditions and/or at elevated temperatures. A systematic<br />
increase <strong>of</strong> the DOP has been observed for the enlarged dash height and eventually exceeded<br />
90% for very tall structures called columnar quantum dashes. The dependence, tending to<br />
saturate for larger heights, has been reproduced theoretically by both analytical considerations<br />
assuming the heavy - light hole bands mixing as the primary factor responsible for the<br />
polarization properties and by full 8 band kp calculations including strain. Unexpectedly<br />
however, low temperature single dot study revealed exciton to biexciton lifetimes ratio <strong>of</strong><br />
about 2 and biexciton binding energy below 0.5 meV, both together characteristic for the<br />
strong confinement regime. In addition, rather small exciton fine structure splitting in the<br />
range <strong>of</strong> 60 – 200 µeV has been detected, comparable to values reported for common selfassembled<br />
dots. All these facts suggest the occurrence <strong>of</strong> an additional exciton localization<br />
within the structures, e.g. on local size or content fluctuations, causing a decreased<br />
confinement anisotropy. This has further been supported by an observed decrease <strong>of</strong> the DOP<br />
down to about 10% at low temperatures and low excitation. The calculated temperature<br />
dependence <strong>of</strong> the DOP reproduced this feature very well after taking into account a<br />
fluctuation <strong>of</strong> the dash cross-sectional size. The latter confirmed that the low temperature<br />
emission originates from excitons trapped on potential fluctuations with the localization<br />
energy <strong>of</strong> about 5 meV.<br />
Quite an opposite behavior has been found for In0.3Ga0.7As/GaAs extended quantum<br />
dots. On one hand, single dot study, QED experiments on a dot inside a pillar microresonator<br />
and time-resolved spectroscopy have shown large oscillator strengths and weakened quantum<br />
confinement manifested in a small value <strong>of</strong> the exciton to biexciton lifetime ratio (below 1)<br />
and a decreased (as compared to standard quantum dots) exciton radiative lifetime (~ 300 ps).<br />
Further, we observed no fingerprints <strong>of</strong> any localization effects in the polarization-resolved<br />
experiments versus temperature and excitation, which agrees with a very smooth morphology<br />
<strong>of</strong> these structures when compared to the InP-based ones. The DOP is only about 6% despite<br />
the shape asymmetry and similar light hole contribution to valence band states as in InP-based<br />
structures (based on 8 band kp calculations). This is caused by a shallow electron confining<br />
potential making them (and hence the entire exciton) to experience less anisotropic<br />
confinement potential. Furthermore, exciton localization within the wetting layer enhancing<br />
their radiative lifetime to almost 1 ns has been observed. This appeared to be responsible for<br />
strong temperature sensitivity <strong>of</strong> the wetting layer - quantum dot energy transfer efficiency<br />
and the appearance <strong>of</strong> a low-energy sideband emission accompanying the biexciton, being a<br />
fingerprint <strong>of</strong> its interaction with excitons from the wetting layer.<br />
32
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoO1<br />
Coupling between electronic and vibrational excitations in<br />
carbon nanotubes filled with C60 fullerenes<br />
Pawel Utko 1 , Raffaello Ferone 2,3 , Ilya V. Krive 3,4 , Robert I. Shekhter 3 , Mats<br />
Jonson 3,5,6 , Marc Monthioux 7 , Laure Noé 7 , and Jesper Nyg˚ard 1<br />
1 Niels Bohr Institute & Nano-Science Center, University <strong>of</strong> Copenhagen,<br />
DK-2100 Copenhagen, Denmark<br />
2 Department <strong>of</strong> Physics, Lancaster University, Lancaster LA1 4YB, UK<br />
3 Department <strong>of</strong> Physics, University <strong>of</strong> Gothenburg, SE-412 96 Göteborg, Sweden<br />
4 B. I. Verkin Institute for Low Temperature Physics and Engineering,<br />
61103 Kharkov, Ukraine<br />
5 School <strong>of</strong> Engineering and Physical Sciences, Heriot-Watt University,<br />
Edinburgh EH14 4AS, UK<br />
6 School <strong>of</strong> Physics, Konkuk University, Seoul 143-701, Korea<br />
7 CEMES-CNRS, B.P. 94347, F-31055 Toulouse Cedex 4, France<br />
Filling single wall carbon nanotubes with C60 fullerenes yields hybrid structures [1],<br />
the so-called C60 fullerene peapods, which can <strong>of</strong>fer enhanced functionality with respect<br />
to empty tubes. Prospective nanoelectronic applications <strong>of</strong> such composites include data<br />
storage devices, single electron transistors and spin-qubit arrays for quantum computing.<br />
However, the extent to which the electronic properties <strong>of</strong> peapods are affected by<br />
entrapped fullerenes is still unclear. Even though theoretical investigations predict dramatic<br />
changes in the electronic band structure <strong>of</strong> the nanotube, the experimental results<br />
remain equivocal.<br />
Here, we investigate the effect <strong>of</strong> the C60 fullerenes on the low-temperature electron<br />
transport via peapod quantum dots [2] operated in the regime <strong>of</strong> weak electronic coupling<br />
Γ between energy states <strong>of</strong> the dot and its electrical leads. Compared with empty nanotubes,<br />
we find an abnormal temperature dependence <strong>of</strong> Coulomb blockade oscillations,<br />
indicating the presence <strong>of</strong> a nanoelectromechanical coupling between electronic states <strong>of</strong><br />
the nanotube and mechanical vibrations <strong>of</strong> fullerenes. This provides a useful method to<br />
detect the C60 presence and to probe the interplay between electrical and mechanical<br />
excitations in peapods, which thus emerge as a new class <strong>of</strong> nanoelectromechanical systems.<br />
We note that other types <strong>of</strong> devices for which a coupling to mechanical vibrations<br />
plays a crucial role in electrical transport are suspended nanotubes, as well as molecular<br />
conductors and transistors.<br />
[1] B. W. Smith, M. Monthioux, and D. E. Luzzi, Nature 396, 323 (1998).<br />
[2] P. Utko, R. Ferone, I. V. Krive, R. I. Shekhter, M. Jonson, M. Monthioux, L. Noé,<br />
and J. Nyg˚ard, Nature Commun. 1, 37 (2010).<br />
33
MoO2 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Solid-state self assembly: a route to hybrid metal-semiconductor epitaxial<br />
nanostructures<br />
Adam Urba�czyk, Frank W.M. van Otten and Richard Nötzel<br />
COBRA Research Institute, Departament <strong>of</strong> Applied Physics, Eindhoven Univeristy <strong>of</strong><br />
Technology, The Netherlands<br />
�<br />
�<br />
Recently there has been a lot <strong>of</strong> research interest in metallic nanostructures for<br />
confining light at deeply subwavelength dimensions utilizing surface plasmons. Such<br />
plasmonic nanostructures are currently finding applications in fields <strong>of</strong> sensing and energy<br />
conversion. Combining them with optically active media such as semiconductor quantum dots<br />
(QDs) could lead to even more exciting applications in the field <strong>of</strong> nanophotnics and<br />
nanoelectronics, where the metal-QD hybrid structures could be used as ultrasmall light<br />
sources or switching elements. In order to obtain such hybrid structures in reproducible and<br />
scalable manner one has to control the metal-QD separation with single nanometer precision<br />
owing to extreme light confinement due to surface plasmon resonance (SPR). Recently our<br />
group demonstrated a promising approach to solve this problem by utilizing solid-state selfassembly.<br />
Here we report alignment <strong>of</strong> epitaxial nanocrystals <strong>of</strong> Ag and In on isolated InAs<br />
QDs and one dimensional QD arrays obtained by molecular beam epitaxy (MBE). By<br />
changing the process conditions the SPR wavelength <strong>of</strong> the metal nanocrystals can be easily<br />
tuned over wide range in order to match the emission wavelength <strong>of</strong> the QDs.<br />
Photoluminescence (PL) measurements reveal large enhancement <strong>of</strong> the emitted light<br />
intensity as compared to a reference structure without metal. What is more, in those hybrid<br />
nanostructures PL polarization is modified, evidencing electromagnetic character <strong>of</strong> the<br />
interaction between the metal nanocrystals and the QDs.<br />
�<br />
Figure 1. Atomic force microscope image <strong>of</strong> a Ag nanocrystas aligned on a single near-surface<br />
InAs QD and a reference structure with bare near surface QDs.<br />
34
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoO3<br />
First-Principles Study <strong>of</strong> Doped III–V nanowires<br />
M. Galicka 1 , P. Kacman 1 and R. Buczko 1<br />
1 Institute <strong>of</strong> Physics PAS, al. Lotników 32/46, 02-668 Warsaw, Poland<br />
GaAs and InAs nanowires (NWs) are considered as one <strong>of</strong> the most interesting structures for<br />
nanoelectronics and nanophotonics. Their potential application in novel electronic devices<br />
requires, however, controllable p-type and n-type conductivity. The specific growth conditions<br />
for NWs, their crystal structure and orientation <strong>of</strong> their side facets lead sometimes to different<br />
incorporation <strong>of</strong> dopants than known for planar layers. For example, GaAs epitaxial layers are<br />
typically p-type doped with Be and n-type doped with Si ions. In contrast, in GaAs NWs<br />
although Si doping leads sometimes to n-type, but most <strong>of</strong>ten p-type conductivity is observed<br />
in Si-doped GaAs NWs [1,2]. The problem <strong>of</strong> doping <strong>of</strong> NWs has become now a major<br />
challenge to the growers <strong>of</strong> one-dimensional III-V semiconductor structures.<br />
To explain the observed phenomena we study theoretically the properties <strong>of</strong> GaAs and<br />
InAs NWs, in which one cation/anion is substituted by a dopant ion. Since the III-V<br />
semiconductor NWs can grow in both, zinc-blende (ZB) and wurtzite (WZ) structures, we<br />
check whether the crystal structure <strong>of</strong> the wire has an impact on the doping level and the<br />
distribution <strong>of</strong> impurities in the NW and, most important, on the electronic properties <strong>of</strong> the<br />
doped NWs. The calculations show that the distribution <strong>of</strong> impurities in the wires as well as<br />
the conductivity depend crucially on the crystal structure.<br />
We consider ZB NWs oriented along (111) axis and WZ NWs along (0001) axis. These<br />
growth directions are chosen because they are the most energetically preferred for both GaAs<br />
and InAs NWs [3]. Using ab initio methods based on the density functional theory we have<br />
calculated the formation and segregation energies for GaAs and InAs NWs doped with<br />
different ions. In GaAs NWs, we consider Be atom, which substitutes a cation and should lead<br />
to p-type conductivity and Si atom which substitutes either anion (p-type) or cation (n-type).<br />
The formation energies for Si in GaAs NWs calculated for various sets <strong>of</strong> chemical potentials<br />
suggest that the energy needed for substituting the anion and the cation, and thus the type <strong>of</strong><br />
conductivity obtained by Si-doping, can be different in ZB and WZ wires. As-grown InAs<br />
NWs have usually high concentration <strong>of</strong> electrons in the conduction band. Thus, in InAs NWs<br />
we show, by comparing the formation energies, that among Be, Zn or Si impurities, which can<br />
be used to obtain p-type wires, Be is preferred.<br />
The segregation energy shows where the dopants prefer to stay – it is defined as the<br />
difference between the energy <strong>of</strong> a NW with the impurity ion in a given position in the wire<br />
and in its center. It has been obtained that in ZB structures the impurities prefer to stay at the<br />
side surfaces <strong>of</strong> the NW. In WZ NWs the positions inside the wire are preferred. For all kinds<br />
<strong>of</strong> impurities studied, the segregation energy in WZ NWs is much smaller than in ZB NWs,<br />
thus, the distribution <strong>of</strong> impurities in WZ wires should be much more homogenous.<br />
To study the electronic properties we have calculated the density <strong>of</strong> states <strong>of</strong> the doped<br />
NWs. In contrast to the formation and segregation energies, which have been calculated for<br />
wires with the surfaces not saturated by foreign ions, the density <strong>of</strong> states has been obtained<br />
for wires with side surfaces fully passivated by hydrogen atoms.<br />
[1] M. Hilse et al., Appl. Phys. Lett. 96, 193104 (2010)<br />
[2] J. Dufouleur et al., Nano Lett. 10, 1734 (2010)<br />
[3] M. Galicka, et al., J. Phys: Condens. Matter 20, 454226 (2008)<br />
Work supported by EC network SemiSpinNet (PITN-GA-2008-215368).<br />
35
MoO4 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
������������������������������������������������������������������<br />
������������<br />
�<br />
�������������� ∗��� ������������� � ��������������� � ����������������� � ������������<br />
��������� � ���������������� � ���������������� � ������������������ � �<br />
�<br />
�� �����������������������������������������������������������������������������������<br />
�������<br />
� �����������������������������������������������������������<br />
� �����������������������������������������������������������������������������������<br />
����������������������<br />
�<br />
��������������������������������������������������������������������������������������<br />
��������������������������������������������������������������������������������������������<br />
��������������������������������������������������������������������������������������������<br />
�������������������������������������������������������������������������������������������<br />
����������������������������������������������������������������������������������������������<br />
��������������������������������������������������������������������������������������������<br />
����������������������������������������������������������������������������������������<br />
���������������������������������������������������������������������������������������<br />
��������������������������������������������������������������������������������������<br />
������������������������������������������������������������������������������������������<br />
���������������������������������������������������������������������������������������������������<br />
���������������������������������������������������������������������������������������<br />
���������������������������������������������������������������������������������������������<br />
�����������������������������������������������������������������������������������������<br />
��������������������������������������������������������������������������������������<br />
�����������������������������������������������������������������������������������������<br />
�������������������������������������������������������������������������������������<br />
������������������������������������������������������������������������������������������<br />
�����������������������������������������������������������������������������������������<br />
����������������������������������������������������������������������������������������������<br />
����������������������������������������������������������������������������������<br />
��������������������������������������������������������������������������������������<br />
��������������������������������������������������� ��� ������������������������������<br />
���������������������������������������������������������������������������������<br />
�������������������������������������������������������������������������������������������<br />
��������������������������������������������������������������������������������������������������<br />
������������������������������������������������������������������������������������������<br />
������������������������������������<br />
�<br />
���� ����������������������������������������������������������������������������������������������������������<br />
���� ��������������������������������������������������������������������������������������������<br />
���� ������������������������������������������������������������������������������<br />
���� �������������������������������������������������������������������������������������������������<br />
���� ����������������������������������������������������������������������������������������������������������<br />
���� ��������������������������������������������������������������������������������������������������������<br />
���� �����������������������������������������������������������������������������������������������<br />
���� ������������������������������������������������������������������������������������������������������<br />
�<br />
���������������������������������������������������<br />
∗ �Email: franz.schuster@cea.fr<br />
36
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoO5<br />
Growth <strong>of</strong> optically active CdTe quantum dots in ZnTe nanowires<br />
�<br />
P. Wojnar 1 , E. Janik 1 , S. Kret 1 , A. Petrouchik 1 , M. Goryca 2 ,<br />
T. Kazimierczuk 2 , P. Kossacki 2 , G. Karczewski 1 and T. Wojtowicz 1<br />
�<br />
1 Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences, Al Lotników 32/46, 02-668 Warsaw, Poland<br />
2 Institute <strong>of</strong> Experimental Physics, University Warsaw, ul Ho�a 69, 00-681 Warsaw, Poland<br />
Quantum dots consisting <strong>of</strong> nanometer sized insertions <strong>of</strong> low energy gap semiconductor in large<br />
energy gap nanowire have recently risen a great interest because <strong>of</strong> their emerging applications as<br />
single photon sources at room temperature [1], biological markers, nanobarcodes [2] and also in view<br />
<strong>of</strong> electronic coupling <strong>of</strong> several quantum dots.<br />
In this work the growth and optical properties <strong>of</strong> CdTe<br />
quantum dots in ZnTe nanowires are reported. The structure is<br />
grown by molecular beam epitaxy in the vapor-liquid-solid<br />
growth mechanism induced by gold catalysts. Nanometer sized<br />
droplets <strong>of</strong> gold/gallium eutectic are formed on GaAs substrate<br />
by thermal treatment <strong>of</strong> a 1 nm thick gold layer deposited in a<br />
separate growth process. Subsequently, ZnTe nanowires with<br />
the length <strong>of</strong> about 1.5 �m are grown at relatively high<br />
substrate temperature <strong>of</strong> 420°C. For the deposition <strong>of</strong> the CdTe<br />
insertion, the temperature is decreased to 380°C, which is still<br />
a too high temperature for the epitaxial growth and, therefore,<br />
ensures that CdTe growth occurs in the axial direction <strong>of</strong> the<br />
nanowire. Deposition time <strong>of</strong> CdTe is set to 60s. The further<br />
growth <strong>of</strong> 200 nm ZnTe nanowire segment takes place at<br />
380°C.<br />
Photoluminescence measurements performed at 5 K show<br />
clearly that the presence <strong>of</strong> CdTe insertions in the ZnTe<br />
nanowires results in the appearance <strong>of</strong> a strong emission in the<br />
2.0 eV – 2.2 eV energy region. When the excitation spot<br />
Figure Low temperature micro-<br />
photoluminescence <strong>of</strong> the ZnTe<br />
nanowires with CdTe insertions –<br />
(a) “as grown” sample - several objects<br />
excited simultaneously (b) nanowires<br />
dispersed on silicon – only one<br />
emission line present, 9 K, excitation:<br />
532 nm (2.33 eV), 100 �W<br />
diameter is reduced to the size <strong>of</strong> the order <strong>of</strong> 1 �m, this broad<br />
band splits into several sharp lines with the spectral width <strong>of</strong><br />
the order <strong>of</strong> 2 meV, figure a. Those lines are related to the<br />
emission from individual quantum dots in nanowires as<br />
confirmed by our further study. In order to better resolve the<br />
individual emission lines, the nanowires are removed from the<br />
GaAs substrate in a ultrasonic methanol bath and dispersed on<br />
a silicon substrate, figure b.<br />
The emission lines exhibit a relatively large degree <strong>of</strong> linear<br />
polarization ranging from 70% to 95% depending on the particular line. This effect is due to large<br />
contrast <strong>of</strong> the dielectric constant <strong>of</strong> the nanowire and the surrounding and, thus, confirms that the<br />
emissions come from an object placed inside a nanowire. On the other hand, photon correlation<br />
measurements prove the zero dimensional character <strong>of</strong> studied objects. A clear antibunching at the zero<br />
delay time between arriving <strong>of</strong> two subsequent photons from the same emission line is observed<br />
confirming that these individual objects cannot emit simultaneously two photons at the same energy.<br />
Moreover, the power dependence <strong>of</strong> the emission and the cross-correlation measurements allow us to<br />
indentify biexcitonic emissions with binding energies ranging from 7 - 12 meV<br />
The research has been supported by the European Union within European Regional Development Fund through Innovative<br />
Economy grant (POIG.01.01.02-00-008/08) and “support for the creation <strong>of</strong> joint research infrastructure <strong>of</strong> scientific<br />
institutions.” - POIG.02.02.00-00-003/08<br />
[1] A. Tribu et al., NanoLett., 8, 4326 (2008)<br />
�������������������������������������������������������������������������������� ��� ���������� ���<br />
37
MoO6<br />
_______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Electronic properties <strong>of</strong> graphite<br />
J. M. Schneider 1,2 , B. A. Piot 2 , I. Sheikin 2 , N. A. Goncharuk 3 , P. Vaˇsek 3 , P. Svoboda 3 , Z. V´yborn´y 3 , L. Smrčka 3 ,<br />
M. Orlita 2 , M. Potemski 2 , and D. K. Maude 2<br />
1 Instituto de Física, Universidade de São Paulo, PB 66318, 05315-970 São Paulo, SP, Brazil<br />
2 Grenoble High Magnetic Field Laboratory, Centre National de la Recherche Scientifique, 38042 Grenoble, France<br />
3 Institute <strong>of</strong> Physics, Academy <strong>of</strong> Science <strong>of</strong> the Czech Republic, 162 53 Prague 6, Czech Republic<br />
Despite more than 50 years <strong>of</strong> reserach, graphite is not yet fully understood. From the viewpoint <strong>of</strong> graphene, a<br />
single layer <strong>of</strong> carbon atoms set in a honeycomb lattice, graphite can be considered as a quasi-two-dimensional (2D)<br />
system consisting <strong>of</strong> a macroscopic stack <strong>of</strong> graphene layers. A fundamental question concerns the link between<br />
the coupling <strong>of</strong> the graphene layers and the dimensionality <strong>of</strong> the electronic system in graphite. According to the<br />
Slonczewski, Weiss, and McClure (SWM) band structure calculations <strong>of</strong> graphite the weak coupling leads to the form<br />
<strong>of</strong> the in-plane dispersion depending upon the momentum kz in the direction perpendicular to the layers. The carriers<br />
occupy a region along the H-K-H edge <strong>of</strong> the hexagonal Brillouin zone. The Fermi surface <strong>of</strong> graphite reveals two<br />
extremal majority charge carriers cross sections: Electrons at the K point (kz = 0) and holes at kz = 0.35. For both<br />
types <strong>of</strong> charge carriers the in-plane dispersion is parabolic (massive fermions). Only at the H point (kz = 0.5) the<br />
in-plane dispersion is linear, similar to that <strong>of</strong> charge carriers in graphene (massless Dirac fermions). However, the<br />
extremal cross section at the H point is negligible (minority charge carriers) and does not contribute significantly to<br />
the electronic properties <strong>of</strong> graphite.<br />
This work represents a review <strong>of</strong> recent studies <strong>of</strong> the electronic properties <strong>of</strong> graphite using the Shubnikov-de Haas<br />
(SdH) and the de Haas -van Alphen (dHvA) effect at millikelvin temperatures [1, 3–5]. Because <strong>of</strong> the low temperatures<br />
used, both the SdH and the dHvA effect are much richer than previously published data. Quantum oscillations<br />
are observed for both majority electrons and holes with an orbital quantum number up to almost N = 100. The<br />
high quality <strong>of</strong> the data allows a detailed investigation <strong>of</strong> the fine structure <strong>of</strong> the Fermi surface [1]. Using both the<br />
semiclassical model and the full magnetic field Hamiltonian we show that these oscillations are fully consistent with<br />
the three-dimensional (3D) SWM bandstructure calculations <strong>of</strong> graphite. At magnetic fields B > 2 T, a significant<br />
deviation from the 1/B periodicity occurs. A self-consistent calculation <strong>of</strong> the Fermi energy shows that the deviation<br />
from the 1/B periodicity is caused by a considerable movement <strong>of</strong> the Fermi energy when the quantum limit is approached.<br />
This seriously questions the validity <strong>of</strong> using the high-field data to extract the phase <strong>of</strong> the Shubnikov-de<br />
Haas oscillations and hence the nature <strong>of</strong> the charge carriers [2, 3]. The detailed information <strong>of</strong> the band structure<br />
<strong>of</strong> graphite and the movement <strong>of</strong> the Fermi energy in magnetic field allows to examine the spin splitting <strong>of</strong> quantum<br />
features. We show that an effective g-factor value <strong>of</strong> g ∗ = 2.5 is required to explain the data [4].<br />
We performed dHvA measurements to make a complete map <strong>of</strong> the Fermi surface <strong>of</strong> graphite. For tilt angles θ < 60 ◦ ,<br />
the fundamental frequencies <strong>of</strong> both the electrons and holes follow almost exactly a 1/cos(θ) dependence (quasi-2D<br />
behavior). For θ > 60, a clear deviation from the quasi-2D behavior is observed. The deviation is a signature <strong>of</strong><br />
the dispersion along kz and unequivocally demonstrates the 3D character <strong>of</strong> the Fermi surface which arises from the<br />
coupling between the graphene layers. Furthermore, the observation <strong>of</strong> quantum oscillations at θ = 90 ◦ is a direct<br />
pro<strong>of</strong> that the Fermi surface <strong>of</strong> graphite is closed. Additionally, the θ = 90 ◦ data reveals minority holes with a Dirac<br />
like energy spectrum much like as for charge carriers in graphene [5].<br />
References<br />
[1] J. M. Schneider, Phys. Rev. Lett. 102, 166403 (2009).<br />
[2] I. A. Luk’yanchuk and Y. Kopelevich, Phys. Rev. Lett. 97, 256801 (2006).<br />
[3] J. M. Schneider, Phys. Rev. Lett. 104, 119702 (2010).<br />
[4] J. M. Schneider et al, Phys. Rev. B 81, 195204 (2010).<br />
[5] J. M. Schneider, to be submitted to Phys. Rev. Lett.<br />
38
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoP1<br />
Nonlocal transport in PbTe/PbEuTe microstructures<br />
K. A. Kolwas 1 , G. Grabecki 1,2 , S. Trushkin 1 , Ł. Cywiński 1 , M. Aleszkiewicz 1 ,<br />
T. Dietl 1,3 , G. Springholz 4 , G. Bauer 4<br />
1 <strong>Instytut</strong> <strong>Fizyki</strong> <strong>PAN</strong>, al. Lotnikow 32/46, PL-02-668 Warszawa, Poland<br />
2 Wydział Matematyczno-Przyrodniczy, Szkoła Nauk Ścisłych, UKSW, ul. Wóycickiego 1/3,<br />
PL 01-938 Warszawa, Poland<br />
3 <strong>Instytut</strong> <strong>Fizyki</strong> Teoretycznej, UW, ul.Hoża 69 PL 00-681 Warszawa, Poland<br />
4 Institut für Halbleiter-und-Festkörperphysik, Johannes Kepler University,<br />
Altenbergerstr. 69, A-4040 Linz, Austria)<br />
Heterostructures <strong>of</strong> narrow-gap semiconductors have recently received a renewed attention, due to the discovery<br />
<strong>of</strong> Quantum Spin Hall phase in HgTe/HgCdTe quantum wells [1], the fingerprint <strong>of</strong> which is the presence <strong>of</strong><br />
large quantized nonlocal voltage signals due to the edge currents [2]. However, even in the topologically trivial<br />
phase, a large nonlocal voltage signal has been seen in Hall bar structures <strong>of</strong> HgTe [3] due to the presence <strong>of</strong><br />
Spin Hall Effect (SHE) in the ballistic regime [4].<br />
Heterostructures <strong>of</strong> IV-VI compounds (Pb,Eu,Sn)Te are another attractive candidates in which one can<br />
search for physics related to the crucial influence <strong>of</strong> the spin-orbit coupling on the electronic structure <strong>of</strong><br />
relevant bands. In contrast to HgTe case, there is no inversion asymmetry (because <strong>of</strong> rock-salt crystal structure)<br />
and the Dresselhaus splitting is absent. The huge dielectric constant <strong>of</strong> PbTe (e0 > 1000) makes it an ideal<br />
material in which quantum ballistic transport effects can be observed [5].<br />
We have fabricated, by means <strong>of</strong> the e-beam<br />
lithography techniques, multi-probe quantum wires (see<br />
Fig. 1 inset) based on an MBE-grown PbTe quantum<br />
well (QW), <strong>of</strong> the thickness <strong>of</strong> 12 nm, embedded<br />
between<br />
Pb1-xEuxTe barriers, deposited on BaF2 substrates. The<br />
wires were equipped with side gates for tuning the<br />
electron concentration. The wire widths W were as<br />
narrow as 1 mm, and their lengths L = 5 mm. The electron<br />
mobility in the initial QW was 5 m 2 /Vs, which<br />
corresponds to the electron mean free path le = 3 mm.<br />
Therefore, W < le < L , and our structures were in quasiballistic<br />
regime. The application <strong>of</strong> negative voltage to<br />
the gates has led to the total depletion <strong>of</strong> the wires. The<br />
measured resistance was then above 10 7 Ohms.<br />
The low-temperature transport measurements have<br />
shown characteristic negative magnetoresistance maxima<br />
which were almost temperature independent in the range<br />
Fig. 1: Local magnetoresistance <strong>of</strong> structure T < 20 K (Fig. 1). We assign it to the suppression <strong>of</strong> the<br />
shown in the inset.<br />
electron non-specular backscattering by the wire<br />
boundaries [6].<br />
Nonlocal resistance measurements have been performed in order to search for the signatures <strong>of</strong> the SHE and<br />
the inverse SHE [3, 4]. The obtained signal at B = 0 is at least one order <strong>of</strong> magnitude larger than the value<br />
expected from the classical van der Pauw formula. However, it is suppressed by the magnetic field with a<br />
dependence distinctly correlated with the local magnetoresistance. This suggests that the nonlocal resistance may<br />
be related to non-specular boundary scattering in our wires.<br />
Acknowledgments: This work was supported by Ministry <strong>of</strong> Science (Poland) under Grant<br />
No. 1247/B/H03/2008/35 and under Iuventus Plus grant No. IP2010 006070.<br />
References<br />
[1] Markus König et al., Science 318, 766 - 770 (2007).<br />
[2] Andreas Roth et al., Science 325, 294 - 297 (2009).<br />
[3] C. Brune et al., Nature Physics 6, 448 - 454 (2010).<br />
[4] E. M. Hankiewicz et al., Phys. Rev. B 70, 241301 (2004).<br />
[5] G. Grabecki et al., Phys. Rev. B 72, 125332 (2005).<br />
[6] C. W. J Beenakker and H. van Houten, Cond-Mat/0412664 (2004).<br />
39
MoP2 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Optical transitions in PbTe/CdTe quantum wells grown by molecular beam<br />
epitaxy on GaAs (001) and BaF2 (111) substrates<br />
M. Szot,* L. Kowalczyk, E. Smajek, B. Taliashvili, P. Dziawa, W. Kn<strong>of</strong>f, A. Reszka,<br />
V. Domukhovski, E. Łusakowska, P. Dłu ewski, M. Wiater, T. Wojtowicz, T. Story,<br />
M. Bukała, R. Buczko, P. Kacman<br />
Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences, Al. Lotników 32/46, 02-668 Warsaw, Poland<br />
IV-VI narrow band gap semiconductors PbTe, PbSe, and PbS constitute materials system<br />
alternative to III-V semiconductors in the area <strong>of</strong> mid-infrared lasers and detectors. The<br />
attractiveness <strong>of</strong> PbTe/CdTe heterosystem results, inter alia, from a very strong confinement<br />
<strong>of</strong> electrons in PbTe caused by large difference in the energy gaps <strong>of</strong> these semiconductors<br />
( Eg=1.2 eV at 300 K) and from low Auger recombination rate in PbTe. Additionally, PbTe<br />
and CdTe exhibit excellent matching <strong>of</strong> their lattice parameters at room temperature, even<br />
tough they crystallize in different cubic lattices: zinc blende (a0=6.48 Å) for CdTe and rock<br />
salt (a0=6.46 Å) for PbTe. Such features are strongly desirable for efficient optoelectronic<br />
devices operating at room temperature. In this paper, we undertake experimental and<br />
theoretical studies <strong>of</strong> ground-state photoluminescence (PL) from PbTe/CdTe quantum wells<br />
grown along different crystal directions by molecular beam epitaxy (MBE).<br />
The PbTe/CdTe QWs were grown by the MBE on (001) oriented GaAs and (111) oriented<br />
BaF2 substrates with thick (1÷4 µm) high quality CdTe buffer layers. The structural quality <strong>of</strong><br />
the heterostructures was examined by x-ray diffraction (XRD) and cross-sectional TEM and<br />
SEM electron microscopy techniques. The PbTe/CdTe heterostructures with the best crystal<br />
quality were obtained for the growth at low temperatures (about 260 °C). The XRD rocking<br />
curve width parameter below 200 arcsec was found for both PbTe and CdTe. For multiple-<br />
QWs diffraction satellites up 7 th order were observed. The single- and multiple-QWs (3<br />
repetitions <strong>of</strong> a basic PbTe/CdTe bilyer) were grown with the thicknesses <strong>of</strong> PbTe quantum<br />
wells varying from 4 to 20 nm and 60 nm thick CdTe barriers.<br />
The optical properties <strong>of</strong> the heterostructures were examined by the PL measurements using<br />
1064 nm line <strong>of</strong> YAG:Nd pulsed laser for excitation. For both (001) and (111) PbTe QWs we<br />
observed relatively narrow and strong PL which we attribute to the ground-state transitions in<br />
the PbTe well. The PL energy at T=4 K shifts from 310 meV to 460 meV for (001) PbTe<br />
QWs with thickness varying from 20 nm to 4 nm, respectively. For a given quantum well<br />
thickness <strong>of</strong> 12 nm, the PL energy observed for (111) PbTe QW grown on BaF2 is about 70<br />
meV lower than for (001) PbTe well. The observed blue shift <strong>of</strong> the PL line is in good<br />
agreement with our theoretical analysis <strong>of</strong> the influence <strong>of</strong> quantum size effect on the groundstate<br />
electronic transitions in the PbTe wells. Both studied growth directions were considered<br />
theoretically. Our calculations were performed within two methods: tight-binding and<br />
effective mass approximation. The dependence on band <strong>of</strong>fsets in the conduction and valence<br />
band in the type-I band alignment at the PbTe/CdTe interface was studied. The strong<br />
effective mass anisotropy and strain induced change <strong>of</strong> the band gap in PbTe were taken into<br />
account. The latter effect results from the large difference between PbTe and CdTe<br />
temperature expansion coefficients.<br />
This work was partially supported by the European Union within the European Regional<br />
Development Fund, through the Innovative Economy grant (POIG.01.01.02-00-108/09).<br />
40
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoP3<br />
Properties <strong>of</strong> three-dimensional topological insulators<br />
studied by microwave resonance spectroscopy<br />
A. Wolos, 1,2 A. Drabinska, 1 M. Kaminska, 1 G. Strzelecka, 3 A. Hruban, 3 A. Materna, 3<br />
M. Piersa, 3 Z. Wilamowski 2<br />
1 Faculty <strong>of</strong> Physics, University <strong>of</strong> Warsaw, ul Hoza 69, 00-681 Warsaw, Poland.<br />
2 Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences, Al. Lotnikow 32/46, 02-668 Warsaw,<br />
Poland.<br />
3 Institute <strong>of</strong> Electronic Materials Technology, ul. Wolczynska 133, 01-919 Warsaw, Poland.<br />
Bismuth selenide and telluride, as well as other related materials have been recently<br />
proposed as three-dimensional topological insulators with a single Dirac cone on the surface<br />
[1]. This class <strong>of</strong> materials is predicted to show an insulating bulk gap together with gapless<br />
surface states. Occurrence <strong>of</strong> the topological surface states has been confirmed by angleresolved<br />
photoemission spectroscopy [2]. There are, however, only a few reports in literature<br />
concerning experimental observation <strong>of</strong> electric transport in the conducting surface, owing to<br />
large number <strong>of</strong> bulk carriers (either electrons or holes) originating from crystal defects. In<br />
order to override presence <strong>of</strong> highly conductive bulk states shading electric properties <strong>of</strong> the<br />
surface, we propose to apply a contactless method basing on a standard electron spin<br />
resonance (ESR) spectrometery.<br />
In this communication we discuss results <strong>of</strong> microwave resonance experiments<br />
performed on Bi2Te3, Bi2Se3, and Bi2Te2Se crystals grown by vertical Bridgman method in<br />
Institute <strong>of</strong> Electronic Materials Technology, Warsaw. The experiment was performed using<br />
Bruker ELEXSYS E580 spectrometer operating in X- and Q-bands. Semiconducting samples<br />
placed in a cavity <strong>of</strong> a standard ESR spectrometer show, in addition to spin resonance spectra,<br />
also spectral features related to sample conductivity. Using this technique, we have earlier<br />
investigated Shubnikov-de Haas oscillations, cyclotron resonance or coupled plasmoncyclotron<br />
resonance in two dimensional electron gas (2DEG) in GaN-based heterostructures<br />
[3] and in Si-based quantum wells.<br />
First results obtained for bismuth selenide and telluride samples show a broad<br />
resonance line dependent only on the component <strong>of</strong> the external magnetic field perpendicular<br />
to the sample plane. This feature is characteristic for the cyclotron-related resonances in two<br />
dimensional electron gas. The relation <strong>of</strong> the observed resonance line to the topological<br />
surface states, together with parameters <strong>of</strong> the 2DEG (concentration and mobility) derived<br />
from the resonance will be discussed.<br />
[1] H. Zhang et al. Nature Phys. 5, 438 (2009).<br />
[2] Y. Xia et al. Nature Phys. 5, 398 (2009).<br />
[3] A. Wolos et al. Phys. Rev. B 76, 045301 (2007).<br />
41
MoP4 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Analysis <strong>of</strong> Optical Properties and Pressure Dependence <strong>of</strong> the Energy Gap <strong>of</strong><br />
ZnO Layers, Bulk and Nano-Powders<br />
A. Duzynska 1,* , A. Kaminska 1 , H.Teisseyre 1,2 , E. Przezdziecka 1 , D. Dobosz 1 ,<br />
Z.R. Zytkiewicz 1 , A. Kozanecki 1 , J. D. Fidelus 1,2 , A. Durygin 3 , V. Drozd 3 , R. Hrubiak 3 ,<br />
A. Suchocki 1,4<br />
1 Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences, Al. Lotników 32/46, 02-668 Warsaw, Poland<br />
2 Institute <strong>of</strong> High Pressure Physics, Polish Academy <strong>of</strong> Sciences, Sokołowska 29/37, 01-142 Warsaw, Poland<br />
3 CeSMEC, Florida International University, University Park, Miami, FL 33199, USA<br />
4 Institute <strong>of</strong> Physics, University <strong>of</strong> Bydgoszcz, Weyssenh<strong>of</strong>fa 11, 85-072, Bydgoszcz, Poland<br />
* Corresponding author: A. Duzynska, e-mail address duzyn@ifpan.edu.pl<br />
Nowadays zinc oxide (ZnO) is a promising wide-band-gap semiconductor for<br />
optoelectronic applications, especially in the blue and ultraviolet spectral ranges. An important<br />
factor affecting mechanical, electric and optical properties <strong>of</strong> ZnO are methods/conditions <strong>of</strong><br />
growth as well as its crystal form.<br />
In this project we have studied pressure dependence <strong>of</strong> the near band gap<br />
photoluminescence (using the diamond anvil cell technique) <strong>of</strong> several various samples:<br />
- two zinc oxide thin layers grown by plasma-assisted molecular beam epitaxy (PA-MBE) on<br />
sapphire substrates (thickness <strong>of</strong> the layers: 1µm and 200 nm),<br />
- bulk ZnO crystal grown by vapor-phase deposition during industrial process <strong>of</strong> production <strong>of</strong><br />
zinc white,<br />
- two samples <strong>of</strong> hexagonal ZnO nano-powders (size <strong>of</strong> crystallite: 1µm and 65 nm) grown by<br />
plasma technique.<br />
We also investigated the stability and the volume behaviour under hydrostatic pressure by<br />
X-ray powder-diffraction <strong>of</strong> bulk crystal using synchrotron radiation. The values the bulk moduli<br />
(B0), and their pressure derivative (B0’), before and after phase transition from wurtzite (B4) to<br />
rock salt (B1) structure were obtained from these measurements. We found that for wurtzite<br />
structure B0 is equal to (156 ± 13) GPa and for rock salt phase B0 = (180 ± 59) GPa. In zinc oxide<br />
the B4-B1 transition has been earlier reported at ~ 10 GPa. However, the change <strong>of</strong> nanoscale<br />
grain size has a huge influence on the characteristic <strong>of</strong> phase transition. We supposed that in the<br />
case <strong>of</strong> nano-powders a tetragonal intermediate (iT) appears at the B4/B1 interface and a<br />
hexagonal intermediate (iH) is also showed as an effect <strong>of</strong> axial compression [1]. These results<br />
are visible in our experimental data <strong>of</strong> pressure dependence <strong>of</strong> the energy gap EG (≈ EPL),<br />
especially for nano-powder with 65 nm crystallite size. Moreover experimental band gap<br />
pressure coefficients for bulk crystal and films grown on sapphire are about 20 meV/GPa, but for<br />
nano-powders are lower, about 15 meV/GPa. This effect will be also discussed.<br />
The photoluminescence is quenched at about 10 GPa for ZnO bulk materials as results <strong>of</strong><br />
transition from direct to indirect gap in the rock salt structure. Nonetheless for smaller size nanopowder<br />
the photoluminescence vanishes at 20 GPa. The origin <strong>of</strong> this phenomenon will be<br />
analysed in this work.<br />
References:<br />
1. S. E. Boulfelfel and S. Leoni, Phys. Rev. B. 78, 125204 (2008).<br />
Acknowledgement: This work was partly supported by the European Union within European Regional Development<br />
Fund, through the Innovative Economy grants (POIG.01.01.02-00-008/0 and POIG.01.01.02-00-108/2009/2) and<br />
the project no N N202 010134 <strong>of</strong> Polish Ministry <strong>of</strong> Science and Higher Education.<br />
42
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoP5<br />
Defect structure <strong>of</strong> two-dimensional Wigner crystals<br />
A. Radzvilavičius and E. Anisimovas<br />
Department <strong>of</strong> Theoretical Physics, Vilnius University, Saul˙etekio al. 9-III, LT-10222<br />
Vilnius, Lithuania<br />
As the density and temperature <strong>of</strong> the electron gas is lowered beyond a certain critical<br />
value, the potential Coulomb energy dominates the kinetic energy and particles tend to<br />
crystallize. The lowest energy structure <strong>of</strong> a two-dimensional system thus corresponds to<br />
the perfect hexagonal Wigner lattice [1]. Finite-size clusters, however, are usually formed<br />
due to the presence <strong>of</strong> an external confinement. In many experimental realizations, such<br />
structures are trapped by a symmetric parabolic potential; these include, but are not<br />
limited to, micro-metric charged spheres in low-temperature dusty plasmas and electrons<br />
in semiconductor quantum dots [2]. The structural properties <strong>of</strong> finite two-dimensional<br />
Wigner clusters strongly depend on the system size and energetic state.<br />
In the present contribution, we report the results <strong>of</strong><br />
the numerical Monte Carlo and minima hopping [3] experiments,<br />
and analyze the structural properties <strong>of</strong> twodimensional<br />
Wigner clusters, consisting <strong>of</strong> classical point<br />
charges, confined by an external parabolic potential trap.<br />
Systems <strong>of</strong> up to N = 1000 particles are considered.<br />
Thenumber<strong>of</strong>metastableconfigurations,corresponding<br />
to the local minima <strong>of</strong> the potential energy, grows<br />
very rapidly with the number <strong>of</strong> particles. Structurally<br />
different states are realized with different probabilities.<br />
By analyzing the number and probabilities <strong>of</strong> metastable<br />
configurations we conclude that the configurational uncertainty<br />
(entropy), grows faster than exponentially – as<br />
the logarithm <strong>of</strong> the factorial <strong>of</strong> the number <strong>of</strong> particles<br />
N [4].<br />
Small crystals exhibit near-circular symmetry – shell<br />
structure, induced by the external confinement. Ground<br />
Figure 1: The triangulated<br />
structure and defects <strong>of</strong> 600particle<br />
Wigner cluster.<br />
states <strong>of</strong> larger clusters correspond to a few circular shells at the exterior and an ordered<br />
hexagonal core. Due to the necessity to satisfy Euler’s theorem and to lower strain energy<br />
<strong>of</strong> the lattice, the presence <strong>of</strong> intrinsic topological defects is unavoidable; these are mainly<br />
located in the transition region between the circular surface and ordered interior, at the<br />
six corners <strong>of</strong> a near-perfect hexagon (figure 1). The number <strong>of</strong> defective vertices in<br />
six defect groups grows non-uniformly with the system size. A few distinct species <strong>of</strong><br />
defects are distinguished: disclinations, dislocations, lengthy grain boundaries, unique<br />
triangular vacancies and interstitial sites. Large pentagonal rosette defects (see figure)<br />
are also observed in large clusters. As the energy <strong>of</strong> the metastable state grows, the<br />
orientational symmetry <strong>of</strong> the crystal is lost. Long grain boundaries in the interior, large<br />
number<strong>of</strong>dislocationsandnear-surfacecomplexestendtoappearinthehighlymetastable<br />
configurations.<br />
[1] E. Wigner, Phys. Rev. 46, 1002 (1934),<br />
[2] A. V. Filinov, M. Bonitz and Yu. E. Lozovik, Phys. Rev. Lett. 86, 3851 (2001),<br />
[3] S. Goedecker, J. Chem. Phys. 120, 9911 (2004),<br />
[4] A. Radzvilavičius and E. Anisimovas, J. Phys.: Condens. Matter 23, 075302 (<strong>2011</strong>).<br />
43
MoP6 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Gauge invariant computational scheme for heterostructures in magnetic field<br />
Anna Korbecka and Jacek A. Majewski<br />
Institute <strong>of</strong> Theoretical Physics, Faculty <strong>of</strong> Physics, University <strong>of</strong> Warsaw,<br />
ul. Hoża 69 PL-00-681, Poland<br />
Multiband k·p models together with the envelope function theory have become a standard<br />
approach for studies <strong>of</strong> the electronic structure <strong>of</strong> not only bulk semiconductors but also their<br />
heterostructures, other low-dimensional forms, and semiconductors in the translational symmetry<br />
breaking magnetic field. This continuum scheme proved to be very useful for modeling <strong>of</strong><br />
semiconductor nanostructures, albeit not free from flaws. In applications <strong>of</strong> this method for<br />
structures where the translational symmetry is broken, the corresponding component <strong>of</strong> the wave<br />
vector is substituted by differential operator. Together with the fact that parameters <strong>of</strong> the k·p<br />
Hamiltonian are position dependent, it leads to the situation where the operators representing<br />
individual components <strong>of</strong> the total matrix operator are not hermitian. Therefore, these operators are<br />
modified (so-called ‘symmetrization’ is performed) to acquire hermitian form. It solves some <strong>of</strong> the<br />
severe problems, however the ‘symmetrization’ procedure is not unique and very <strong>of</strong>ten leads to<br />
unphysical spurious solutions [1] and nonphysical outcomes for the heavy-hole subbands [2]. The<br />
problem has been disputed hotly for a long time in semiconductor physics without a final solution<br />
on the horizon. In the meantime, the new approach has emerged that is based on the so-called Burt’s<br />
exact envelope function theory [3] and formulates the whole operator matrix in a proper form<br />
instead <strong>of</strong> ‘symmetrizing’ its individual components [4], i.e., the whole matrix <strong>of</strong> operators is<br />
hermitian, however, its individual operator components are not. This procedure <strong>of</strong> B.A. Foreman is<br />
not plagued by spurious solutions problem and gives in most cases physically reasonable results,<br />
which can be, however, quite different from ones obtained within the standard approach. In<br />
addition, it has been recently proved [2] that the Foreman’s scheme is the only one that guarantees<br />
the gauge invariance <strong>of</strong> the k·p Hamiltonian for semiconductor low-dimensional structures in the<br />
constant magnetic field.<br />
We have implemented Foreman’s theory for low dimensional semiconducting systems in<br />
magnetic field into nextnano 3 simulation package [5] together with the magnetic p-d exchange<br />
interaction [6] and interface corrections for systems without mirror symmetry (e.g., no-common<br />
atom systems like InAs/GaSb). The Hamiltonian has been solved on discrete lattice. This allows us<br />
for reliable studies <strong>of</strong> the Landau level formation in the valence bands in non-magnetic<br />
heterostructures (GaAs/AlGaAs, InGaAs/GaAs, InAs/GaSb) and also heterostructures with<br />
modulated magnetization (such as (Ga,Mn)As/AlGaAs and (Ga,Mn)Sb/InAs). The scheme deals<br />
also with the doped structures, by including the electrostatic potential obtained from the Poisson’s<br />
equation self-consistently into Foreman’s k·p Hamiltonian.<br />
In particular, we have computed the electronic structure <strong>of</strong> GaAs/Al0.33Ga0.67As [001] and<br />
[311] heterostructures in the magnetic field. The obtained results help to interpret experiments on gfactor<br />
in 2D-hole gases [6] and the intrinsic photo-induced anomalous Hall effect [7].<br />
[1] E. P. Pokalitov, V. A. Fonoberov, V. M. Fomin, and J.T. Devreese, Phys. Rev. B. 64, 245328 (2001).<br />
[2] T. Andlauer, R. Morsch, P. Vogl, Phys. Rev. B. 78, 075317 (2008).<br />
[3] M. G. Burt, J. Phys. Condens. Matter 4, 6651 (1992).<br />
[4] B. A. Foreman, Phys. Rev. B. 48, 4964 (1993); ibid. 56, R12 748 (1997).<br />
[5] http://www.nextnano.de<br />
[6] D.A. Vasyukov, A.S.Plaut, M.Henini, Physica E 42, 964 (2010).<br />
[7] D.A. Vasyukov, A.S.Plaut, M.Henini, L.N.Pfeiffer, K.W.West, C.A.Nicoll, I.Farrer, and D.A. Ritchie,<br />
Physica E 42, 940 (2010).<br />
44
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoP7<br />
Modeling PbTe-based low dimensional structures<br />
M. Bukała 1 , P. Sankowski 2 , R. Buczko 1 and P. Kacman 1<br />
1 Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences, Warsaw 02-668, Poland<br />
2 Institute <strong>of</strong> Informatics, University <strong>of</strong> Warsaw, Warsaw 02-097, Poland<br />
We present a theoretical study <strong>of</strong> various PbTe low dimensional structures. First, we<br />
consider rock-salt PbTe inclusions in zinc-blende CdTe matrix as well as <strong>of</strong> zinc-blende CdTe<br />
nano-clusters in rock-salt PbTe matrix i.e. structures with possibly improved thermoelectric<br />
properties. Next, we model PbTe-based 2D heterostructures, in which quantum spin-Hall<br />
phase is expected. For these purposes a tight-binding parameterisation for PbTe has been<br />
obtained, which, in contrast to all available in literature parameterisations, leads not only to<br />
proper values <strong>of</strong> the energy gaps but also to adequate effective masses in the valence and<br />
conduction bands. Our tight-binding model includes the sp3 atomic orbitals with spin-orbit<br />
interactions and accounts for the nearest neighbour and next near neighbour interactions. We<br />
have checked on 2D PbTe/CdTe heterostructures that such tight-binding description and the<br />
effective mass approach lead to similar energy structures.<br />
The aim <strong>of</strong> the former research is to show how introducing nanostructures <strong>of</strong> different<br />
size and shape changes the electron density <strong>of</strong> states near the Fermi level and thus can be used<br />
to optimize the thermoelectric power Seeback coefficient. We have considered the Fermi level<br />
situated near the top <strong>of</strong> the valence band (in p-type) as well as at the bottom <strong>of</strong> the conduction<br />
band (in n-type). We have studied structural and electronic properties <strong>of</strong> PbTe quantum wires<br />
in the CdTe matrix, and CdTe anti-wires and CdTe anti-dots in the PbTe matrix. For structures<br />
with less than 500 atoms in the unit cell, all atomic positions in the model nanostructures, in<br />
particular at the interfaces, have been calculated using first principles methods with relaxation<br />
and re-bonding allowed. The obtained relaxed atomic positions have been further used in the<br />
calculations <strong>of</strong> electron density <strong>of</strong> states <strong>of</strong> the studied structures, which are performed within<br />
the tight-binding approximation. In larger structures the relaxation has not been taken into<br />
account.<br />
Our calculations show that all kinds <strong>of</strong> inclusions can lead to shape and size dependent<br />
changes <strong>of</strong> the density <strong>of</strong> states at the Fermi level. Thus, the electronic contribution to the<br />
Seebeck coefficient can be optimized by changing the shape, size, density as well as the<br />
spatial arrangement <strong>of</strong> the inclusions.<br />
The same tight-binding model, supported by effective mass approach, have been used to<br />
study the PbTe-based 2D heterostructures. In particular, it has been shown that states related<br />
to the interfaces can appear in PbSnTe/PbTe structure, in which the Sn content leads to band<br />
inversion. By changing the Sn content and the diameter <strong>of</strong> the heterostructure, we are looking<br />
for systems, in which such states may lead to the spin-Hall effect.<br />
The work was partially supported by the European Union within the European Regional<br />
Development Fund, through grant Innovative Economy (POIG.01.01.02-00-108/09)<br />
45
MoP8 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
! " # $ % !&<br />
' ! ' ( ) %<br />
! "#<br />
# $<br />
%&' % '!<br />
( ) # *<br />
$ # ) * # ) ) ) ) ! # $ #<br />
# $ ) )+ !<br />
, # # ! ! #<br />
- ! .<br />
/ # & / 0 *<br />
&1&2 ) # / # & / &1 * &1&2 ) ! # #<br />
# # # ! 3<br />
# $<br />
! 3 4<br />
!<br />
3 # * # #<br />
! # #<br />
5 4 ! - # #<br />
4 / 0 6#! 7<br />
# $ ≤ &1 µ * # ! #<br />
8<br />
− 9 # # )<br />
& / * &1&2 ) !& ! ( 4 # *<br />
# 0 * &1&: ) ;<br />
# # !<br />
− 9 # / & /<br />
0 * &1&2 ) ! < ( ) )<br />
# = &1&2 ) !<br />
− 3 # 4 /<br />
0 / 2 * &1&: ) ! "<br />
# # # !<br />
> !&! Φ = 0 !<br />
%&' ?# , "# , *+ @& 1A , 1&1<br />
% ' B! ?# C "# ! , @ : 112 !<br />
46
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoP9<br />
Atomistic calculation <strong>of</strong> screened Coulomb interactions in semiconductor<br />
nanostructures.<br />
M. Chwastyk, P. T. Różański, and M. Zieliński<br />
<strong>Instytut</strong> <strong>Fizyki</strong> UMK, ul. Grudziądzka 5, 87-100 Toruń, Poland<br />
Dielectric screening <strong>of</strong> Coulomb interactions affects optical and electronic properties<br />
<strong>of</strong> semiconductor nanostructures including multi-exciton generation rates in colloidal<br />
nanocrystals [1], excitonic spectra and self-energy corrections in nanopillar or graphene<br />
quantum dots [2,3]. Using tight-binding approach we compare several methods <strong>of</strong> accounting<br />
for dielectric screening in atomistic calculation including semi-classical approximation [4],<br />
Thomas-Fermi approach [5] and real-space dielectric matrix inversion [6]. We propose an<br />
efficient method in which dielectric screening can be decomposed into contribution from<br />
volume and surface-polarization terms, further screened by microscopic Thomas-Fermi like<br />
contribution. We illustrate our approach by calculating electronic and optical properties <strong>of</strong><br />
several different spherical semiconductor nanocrystals differing in size and composition. We<br />
compare results obtained with proposed approach and real-space dielectric matrix inversion<br />
and study role <strong>of</strong> different approximation used in calculation. We discuss possibility<br />
generalizing our model for a multi-million atom systems like self-assembled quantum dots.<br />
[1] Z. Lin, A. Franceschetti, and M.T. Lusk, arXiv:1009.0417<br />
[2] Y. M. Niquet et al., Phys. Rev. B 73, 165319 (2006).<br />
[3] D. Guclu, P. Potasz, and P. Hawrylak Phys. Rev. B 82, 155445 (2010).<br />
[4] A. Franceschetti and M. C. Troparevsky, Phys. Rev. B 72, 165311 (2005).<br />
[5] S. Lee et al., Phys. Rev. B 63, 195318 (2001).<br />
[6] F. Trani, D. Ninno, and G. Iadonisi, Phys. Rev. B 76, 085326 (2007).<br />
47
MoP10 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Van der Waals Density Functionals in Materials Science<br />
Magda Birowska, Karolina Milowska, and Jacek A. Majewski<br />
Faculty <strong>of</strong> Physics, University <strong>of</strong> Warsaw, ul. Hoża 69, 00-681 Warszawa, Poland<br />
The density functional theory (DFT) has revolutionized materials science making it possible<br />
to quantitatively predict numerous properties <strong>of</strong> materials. However, exact in principle, the DFT<br />
relies on various approximations to the exchange and correlation functionals that are nowadays<br />
prerequisites for practical implementations <strong>of</strong> DFT. In spite <strong>of</strong> a long list <strong>of</strong> successes, the local<br />
density approximation (LDA) or generalized gradient approximation (GGA) that are commonly<br />
used in so-called ab-initio calculations have as well many flaws. One <strong>of</strong> its weaknesses is rather<br />
poor description <strong>of</strong> Van der Waals (VdW) type <strong>of</strong> bonding in solids. The best example is the<br />
geometry unit cell geometry <strong>of</strong> graphite. Whereas the atomic distances in the carbon planes with<br />
strong covalent bonds are excellently predicted by LDA or GGA, the distances between the<br />
planes, which are determined by Van der Waals type <strong>of</strong> interactions, are not. Therefore, recently<br />
new correlation functionals have emerged that take into account polarization effects and could<br />
improve the description <strong>of</strong> Van der Waals forces [1,2].<br />
In this communication, we present the results <strong>of</strong> theoretical studies <strong>of</strong> various systems with<br />
considerable role <strong>of</strong> VdW forces where the new functionals have been employed. The results <strong>of</strong><br />
test studies for graphite are very promising, providing the solution to the long standing problem<br />
and giving the distance between the carbon layers equal to 3.349 Å (with DRSLL functional [1])<br />
or 3.345 Å (with LMKLL one [2]) in excellent agreement with experimental value <strong>of</strong> 3.36 Å<br />
(LDA and GGA lead to distances equal to 3.117 Å and 3.425 Å, respectively). However, in the<br />
studies we focus on graphene single-, bi-, and multi-layers, either free standing or bound in an<br />
incommensurable way to silicon carbide substrate. We consider also functionalized graphene<br />
layers. The correct description <strong>of</strong> the structural and electronic properties <strong>of</strong> these systems is <strong>of</strong><br />
crucial importance for the quickly developing electronics based on graphene. In our studies <strong>of</strong><br />
materials we have used the numerical package SIESTA with implemented VdW functionals and<br />
the standard LDA and GGA ones. This facilitates the direct comparison <strong>of</strong> the material<br />
properties obtained within both approaches. We find differences in the electronic structures <strong>of</strong><br />
these systems and their energetics. In particular, we find that the potential barriers in these<br />
systems calculated with VdW functional and standard approaches in many cases differ<br />
considerably. It is <strong>of</strong> crucial importance, particularly if one uses atomistic ab initio methods as a<br />
starting point for multi-scale modeling <strong>of</strong> materials and determines effective potentials on the<br />
basis <strong>of</strong> DFT calculations.<br />
This paper has been supported by the project “SiCMAT” (POIG.01.03.01-14-155/09-00).<br />
[1] M. Dion, H. Rydberg,E. Schoder, D. C. Langreth, and B. I. Lundqvist, Phys. Rev. Lett. 92,<br />
246401 (2004)<br />
[2] K. Lee, E. Murray, L. Kong, B. I. Lundqvist and D. C. Langreth, arXiv:1003.5255v1 (2010).<br />
48
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoP11<br />
Hopkinson-Like Effect in Single-Crystalline CdCr2Se4 Semiconductor<br />
T. Groń 1 , E. Malicka 2 , A.W. Pacyna 3 , H. Duda 1 and J. Krok-Kowalski 1<br />
1 University <strong>of</strong> Silesia, Institute <strong>of</strong> Physics, ul. Uniwersytecka 4, 40-007 Katowice, Poland<br />
2 University <strong>of</strong> Silesia, Institute <strong>of</strong> Chemistry, ul. Szkolna 9, 40-006 Katowice, Poland<br />
3 The Henryk Niewodniczański Institute <strong>of</strong> Nuclear Physics, Polish Academy <strong>of</strong> Sciences,<br />
ul. Radzikowskiego 152, 31-342 Kraków, Poland<br />
The magnetization <strong>of</strong> many compounds shows a peak near the Curie temperature, TC,<br />
when heating the sample in a fixed (small) magnetic field [1]. This behavior is commonly<br />
called the Hopkinson effect [2]. The accepted explanation [3] <strong>of</strong> the Hopkinson effect is based<br />
only on domain wall motion. This mechanism is obviously inapplicable to the case <strong>of</strong> singledomain<br />
particles. However, a thermomagnetic effect which is quite similar to the Hopkinson<br />
effect has been experimentally observed in most <strong>of</strong> the amorphous magnetic materials as well<br />
as in some spin glasses where the existence <strong>of</strong> multi-domain particles is questionable or even<br />
practically impossible [2]. In Nd2Fe14B-type ribbons the existence <strong>of</strong> a maximum in the<br />
thermomagnetic curves <strong>of</strong> thermally demagnetized samples in low fields was connected with<br />
the processes <strong>of</strong> irreversible rotation <strong>of</strong> magnetic moments <strong>of</strong> non-interacting uniaxial single<br />
domain particles according to the Stoner-Wohlfarth model [2,4].<br />
Magnetization, ac and dc magnetic susceptibility <strong>of</strong> the p-type CdCr2Se4 semiconductor<br />
[5] were measured in the zero-field-cooled mode using a Lake Shore 7225 dc<br />
magnetometer/ac susceptometer at 4.3 K and in applied external magnetic fields up to 60 kOe,<br />
and a Faraday type Cahn RG automatic electrobalance in the temperature range 4.5-400 K and<br />
'<br />
"<br />
at 1 kOe, respectively. The in-phase c ( T)<br />
and out-<strong>of</strong>-phase c ( T)<br />
components <strong>of</strong> the ac<br />
1<br />
fundamental susceptibility were recorded in the temperature range 4.5-160 K using an<br />
oscillating field Hac = 1 Oe with frequency <strong>of</strong> 120 Hz for external magnetic fields Hdc = 0, 100<br />
Oe, 200 Oe, 450 Oe and 1 kOe. The signals <strong>of</strong> the second (c2) and third (c3) harmonics were<br />
detected at the same temperature range, for the same amplitude and frequency as the ac c1<br />
measurements without an external static magnetic field.<br />
The dc magnetic measurements showed ferromagnetic order with a Curie temperature<br />
TC = 130 K and a saturation magnetization <strong>of</strong> 5.91 µB/f.u. at 4.5 K and at 60 kOe. The ac<br />
'<br />
magnetic measurements revealed a spectacular peak at 450 Oe and 1 kOe in the c 1(<br />
T)<br />
curve<br />
near TC, similar to the Hopkinson peak. The increasing dc magnetic field suppresses the<br />
'<br />
magnetic susceptibility intensity <strong>of</strong> c ( T)<br />
and slightly shifts the Hopkinson peak to higher<br />
1<br />
"<br />
temperatures. A small and positive value <strong>of</strong> c1( T)<br />
below TC indicating a small energy loss<br />
correlates rather with strong spin fluctuations than with spin rearrangement processes. This is<br />
consistent with zero values <strong>of</strong> second and third harmonic ac susceptibility in the temperature<br />
range <strong>of</strong> 4.5-160 K, suggesting the lack <strong>of</strong> short-range magnetic interactions. One can<br />
conclude that the peaks in the thermomagnetic curves at 450 Oe and 1 kOe can be<br />
qualitatively explained with the aid <strong>of</strong> the Hopkinson-like effect.<br />
This work is partly founded from science Grant No. N N204 145938.<br />
[1] J. Hopkinson, Proc. R. Soc. London 48, 1 (1890).<br />
[2] O. Popov, and M. Mikhov, J. Magn. Magn. Mater. 75, 135 (1988).<br />
[3] M. Kersten, Z. Angew. Phys. 8, 313 (1956).<br />
[4] E.C. Stoner, and E.P. Wohlfarth, Phil. Trans. R. Soc. A 240, 599 (1948).<br />
[5] H.W. Lehmann, Phys. Rev. 163, 488 (1967).<br />
49<br />
1
MoP12 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Optical and structural characterization <strong>of</strong> zinc oxide<br />
nanostructures obtained by Atomic Layer Deposition method<br />
Ł. Wachnicki 1 , B. S. Witkowski 1 , S. Gierałtowska 1 , E. Janik 1 , M. Godlewski 1,2 ,<br />
E. Guziewicz 1<br />
1 Polish Academy <strong>of</strong> Sciences, Institute <strong>of</strong> Physics, al. Lotników 32/46, Warszawa 02-668,<br />
Poland<br />
2 Cardinal Stefan Wyszynski University, College <strong>of</strong> Science, Department <strong>of</strong> Mathematics and<br />
Natural Sciences, Warszawa, Poland<br />
Zinc oxide has been extensively investigated in the past few years mainly<br />
because <strong>of</strong> a large variety <strong>of</strong> potential applications. For example, it can be used<br />
in light-emitting diodes and transistors, as gas sensors, and transparent<br />
conductive oxide in solar cells.<br />
Among many applications, ZnO is also a prospective material for sensor<br />
technology, where developed surface morphology is very important. In this<br />
work we present growth <strong>of</strong> ZnO nanostructures using ALD. As a substrate we<br />
used gallium arsenide with gold eutectic mixture prepared on the surface at<br />
600°C. To obtain eutectic solution we spread gold thin film on a GaAs substrate<br />
and then annealed the sample in a nitrogen gas flow. Au-Ga droplets where<br />
formed in this way. The so-prepared substrate was used for growth <strong>of</strong> ZnO<br />
nanostructures in the ALD system. We applied deionized water and zinc<br />
chloride as an oxygen and zinc precursors, respectively. The eutectic mixture<br />
plays a role <strong>of</strong> a catalyst for the ZnO nanostructures growth. Au-Ga droplets<br />
flow on the front <strong>of</strong> growth forcing the growth <strong>of</strong> ZnO nanostructures. Optical<br />
and structural characterization was investigated by photoluminescence, scanning<br />
electron microscope (SEM) and atomic force microscope (AFM). SEM images<br />
show ZnO nanorods in a form <strong>of</strong> crystallites <strong>of</strong> up to 1 m length and 100 nm<br />
diameters. It is the first demonstration <strong>of</strong> ZnO nanostructures growth by ALD<br />
using VLS (vapour-liquid-solid) approach.<br />
The research was supported by the European Union within European Regional<br />
Development Fund, through grant Innovative Economy (POIG.01.01.02-00-<br />
008/08).<br />
50
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoP13<br />
����������������������������������������������������n-�����������p������<br />
�<br />
�������������������� � �������������������� � �������������������� � ���������������� � ��<br />
����������������� ��<br />
�<br />
� ��������������������������������������������������������������������������������������������<br />
� ���������������������������������������������������������������������������������������������<br />
�����������������������������������������������������������������������<br />
�<br />
� ����������������������������������������������������������������������������������������<br />
��� �������� ��� ����� ����� ������������������������ ��������������������������������������� ����<br />
���������������� ������������ ��������� �� ������� ��� ������ ������������ ��� ���� ������������ ���<br />
���������������� ������ ��� ���������������� ����� ���� �������� ������� ����� ��� ��������������<br />
����������������������������������������������������������������������������������������<br />
� ����� ����� �������� ���� �������� ��� ��� �������������� ����� ���� ����������� ����������� ���<br />
���������� ���������������� ���������������������� ��������� ��� ����������� ��������������� �����<br />
��������������������������������������������������������������������<br />
� ������������ ����� ������ ����� ���������� ����� �������� �������� �������� ��� ���������������<br />
���������������������� ��� ���������� ��������� ��������� ����������������������������������<br />
������ �����≈����������� ���� ��������� ���������� ������� ������������� ��� ����� ��� �����<br />
���������������������������������������������������������≈��� � ����⋅���������������������������<br />
���������������������������������������������������������������������������������������������<br />
���������������������������� �� ����⋅�����<br />
� ���������������������������������������������������������������������������������������<br />
�������������������ϕκ����������������������������������������������������������������������ϕκ��<br />
����������������������������������������ϕκ������������������������������������������������<br />
��������������������������������������������������������������������������������������������<br />
����� ���� �������� ��������� ����� ���� ������ ������� ������� ��� �������� ��� ������ ���� ����������<br />
���������������������������������������������������������������������������������������������������<br />
��������������������������⋅�� � �����������������������������������������������������������������<br />
�����������������������������������������������������������������������������������������������<br />
������������������������������������������<br />
� �������������������������������������������������������������������������������������<br />
�����������������������������������������������������
MoP14 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Photoluminescence and chromaticity properties <strong>of</strong> ZnO nanopowders<br />
obtained by a microwave solwothermal method<br />
E. Wolska 1 , D. Sibera 2 , B.S. Witkowski 1 , S.A. Yatsunenko 1 , I. Pełech 2 ,<br />
U. Narkiewicz 2 , M. Godlewski 1,3<br />
1 Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences, Al. Lotników 32/46,<br />
02-668 Warsaw, Poland<br />
2 Institute <strong>of</strong> Chemical and Environment Engineering, West Pomeranian University <strong>of</strong><br />
Technology, K. Puławskiego 10, 70-322 Szczecin, Poland.<br />
3 Dept. <strong>of</strong> Mathematics and Natural Sciences College <strong>of</strong> Science,<br />
Cardinal S. Wyszy ski University, Dewajtis 5, 01-815 Warsaw, Poland<br />
ZnO is one <strong>of</strong> the most intensively studied wide band gap semiconducting material.<br />
Different applications <strong>of</strong> ZnO in transparent electronics, optoelectronics, photovoltaics<br />
and spintronics were demonstrated recently, which explains the increasing interest in this<br />
material. Moreover, new applications in medicine and biology are predicted. It is claimed<br />
that ZnO nanopowders can be used as luminescence markers for such applications.<br />
Luminescence markers are promising materials to be used for early recognition <strong>of</strong> many<br />
<strong>of</strong> diseases, including cancer.<br />
In the present work we discuss growth and properties <strong>of</strong> ZnO nanopowders obtained by a<br />
microwave solvothermal method. We compare properties <strong>of</strong> nanopowders obtained by<br />
two methods. In the first process as a zinc precursor we used zinc nitrate hexahydrate<br />
(Zn(NO3)2 6H2O) dissolved in distilled water. In the second process we used zinc nitrate<br />
hexahydrate and ethanol. For each <strong>of</strong> these two processes we performed growth at five<br />
different values <strong>of</strong> pressure in the microwave reactor during the solvothermal synthesis.<br />
The so-obtained powders were studied as-grown or after annealing at 750 o C in the air<br />
atmosphere.<br />
Each type <strong>of</strong> the samples were characterized with photoluminescence,<br />
cathodoluminescence and their size was determined from SEM and XRD investigations.<br />
The growth processes were optimized to obtain the brightness light emission and, in some<br />
cases, the best chromaticity. The latter study was motivated by possibility <strong>of</strong> white color<br />
emission <strong>of</strong> ZnO upon UV excitation.<br />
We discuss relations between synthesis parameters in the solvothermal microwave<br />
processes and optical properties <strong>of</strong> the samples obtained.<br />
The research was partially supported by the European Union within European Regional<br />
Development Fund, through grant Innovative Economy (POIG.01.01.02-00-008/08).<br />
52
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoP15<br />
Technology and testing the mechanical properties <strong>of</strong> the<br />
InGaP/GaAs/InGaP microcantilevers<br />
Tomáš Ščepka, Dagmar Gregušová, Róbert Kúdela, Štefan Gaži and Vladimír<br />
Cambel<br />
Institute <strong>of</strong> Electrical Engineering, Slovak Academy <strong>of</strong> Sciences, Dúbravská cesta 9, SK-<br />
84104 Bratislava, Slovakia<br />
Microcantilevers are at the heart <strong>of</strong> atomic force microscopes and also perspective for<br />
future chemical and biological sensing applications. Most <strong>of</strong> the commercially produced<br />
cantilevers are made <strong>of</strong> silicon or silicon nitride. However, microcantilever probes based on<br />
III-V semiconductor materials and their heterostructures can be equipped with novel<br />
functionalities through the incorporation <strong>of</strong> electronic and opto-electronic devices designed<br />
directly on them.<br />
GaAs is the most used III-V material for cantilever probe development thanks to its<br />
optoelectronic properties, high mobility <strong>of</strong> electrons, sufficient mechanical properties and<br />
high piezoresistivity (values higher than those <strong>of</strong> silicon) [1]. Application <strong>of</strong> mature<br />
deposition methods, such as metal organic chemical vapor deposition, allows for the<br />
formation <strong>of</strong> sandwich layers whose thickness can be defined with extreme precision.<br />
This work is a first step towards sensors with improved sensitivity and integrability.<br />
We focus on the fabrication and initial mechanical testing <strong>of</strong> microcantilevers based on an<br />
InGaP/GaAs/InGaP heterostructure (Fig. 1.). The heterostructure consists <strong>of</strong> a GaAs layer,<br />
that determined the cantilever thickness. The layer is embedded between two InGaP etch-stop<br />
layers [2]. All layers are lattice matched to a thinned semi-insulating GaAs (100) substrate.<br />
The front-side patterning was accomplished using optical photolithography, Ar ion milling <strong>of</strong><br />
InGaP layer, and wet chemical etching <strong>of</strong> GaAs. The substrate was at first thinned to ~250 µm<br />
and then cantilevers were released via back-side bulk micromachining in two solutions. The<br />
1H2SO4:8H2O2:1H2O mixture was used as a fast etchant<br />
which was followed by a finer 1H3PO4:2H2O2:8H2O<br />
etchant. The experimental cantilevers presented here<br />
were designed as simple rectangles, nominally 100-300<br />
µm long and 35-65 µm wide, with a thickness <strong>of</strong> 0.1, 0.2,<br />
0.5, 1, and 2 µm. We have investigated the influence <strong>of</strong><br />
geometrical dimensions, especially thickness <strong>of</strong> GaAs<br />
layer, on the fabrication process and mechanical<br />
properties <strong>of</strong> the cantilevers. To determine the resonance<br />
frequencies, a detection system <strong>of</strong> an Atomic Force<br />
Microscope (NT-MDT NTEGRA) was used. The<br />
measured resonance frequencies <strong>of</strong> the cantilevers<br />
correspond with theoretical values.<br />
This work is the result <strong>of</strong> the ESF project implementation, CENTE - 2 nd stage, ITMS code<br />
26240120019, supported by the R&D Operational Program funded by the ERDF.<br />
[1] K. Hjort et al., J. Micromech. Microeng. 4, 1-13 (1994).<br />
[2] D. Gregušová et al., J. Micromech. Microeng. 20, 097001 (2010).<br />
53<br />
Figure 1. SEM micrograph <strong>of</strong><br />
fabricated microcantilever.
MoP16 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
ZnO thin films <strong>of</strong> different crystalline structures grown on Si (100)<br />
substrates by reactive DC sputter deposition<br />
Michał A. Borysiewicz 1 , El bieta Dynowska 1,2 , Valery Kolkovsky 2 , Jan Dyczewski 2 ,<br />
Eliana Kami ska 1 , Anna Piotrowska 1<br />
1 Institute <strong>of</strong> Electron Technology, Al. Lotników 32/46, 02-668 Warsaw, Poland<br />
2 Institute <strong>of</strong> Physics, Polish Acad. <strong>of</strong> Sci., Al. Lotników 32/46, 02-668 Warsaw, Poland<br />
ZnO is considered an advantageous material in the transparent electronics,<br />
optoelectronics and photovoltaics due to its wide band-gap <strong>of</strong> 3.37 eV and high exciton<br />
binding energy <strong>of</strong> about 60 meV. Most research on ZnO thin films is performed on layers<br />
deposited on sapphire substrates. The sapphire has the advantage <strong>of</strong> having a crystalline<br />
structure matching that <strong>of</strong> the ZnO, but also a disadvantage <strong>of</strong> a large lattice mismatch <strong>of</strong><br />
~18%. Therefore, to grow high-quality ZnO thin films on sapphire substrates, buffer layers<br />
have to be introduced [1-3]. On the other hand, in order to enable the integration <strong>of</strong> electronic<br />
and optoelectronic elements on one chip and take advantage <strong>of</strong> the high quality, large wafer<br />
sizes and low price, ZnO growth on silicon (100) substrates should be studied, regardless <strong>of</strong><br />
the lattice mismatch to ZnO.<br />
This work reports on the structure <strong>of</strong> thin ZnO films on Si (100) substrates deposited<br />
using room-temperature reactive DC sputtering from a Zn target in an Ar-O2 mixture. Films<br />
grown in several deposition processes are compared. The power fed to the Zn target was kept<br />
at 75 W and the total gas pressures and amounts <strong>of</strong> oxygen in the gas mixture were changed<br />
between the processes. The structure <strong>of</strong> the films was studied by means <strong>of</strong> X-ray diffraction in<br />
the -2 geometry and scanning electron microscopy <strong>of</strong> the surface as well as the crosssections.<br />
The stoichiometry was estimated using Rutherford backscattering spectrometry.<br />
Transport properties were determined through Hall effect measurements. The<br />
characterisations were carried out after deposition and after subsequent 15 minutes-long<br />
annealing processes in an RTP furnace in O2 flow at 400 o C, 600 o C and 800 o C. We found, that<br />
by varying the parameters <strong>of</strong> the growth accordingly, films with very different structures<br />
could be produced. In particular, growth in oxygen-poor conditions resulted in porous<br />
polycrystalline structures (Fig. 1) potentially suited for applications in chemical sensors or<br />
dye-sensitized solar cells and growth in oxygen-rich conditions led to the formation <strong>of</strong> dense<br />
columnar films (Fig. 2) which may be applied in LEDs.<br />
This study was partially supported by the European Union within European Regional<br />
Development Fund, through grant Innovative Economy (POIG.01.03.01-00-159/08,<br />
"InTechFun").<br />
Fig. 1. Fig. 2.<br />
[1] B. Pecz et al., J. Appl. Phys. 100, 103506 (2006)<br />
[2] M.W. Cho et al., Semicond. Sci. Technol. 20, S13–S21 (2005)<br />
[3] M.A. Borysiewicz et al., Acta Phys. Pol. A 119, 333 (<strong>2011</strong>)<br />
54
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoP17<br />
Comparison <strong>of</strong> the valence band <strong>of</strong> amorphous and crystalline<br />
GeTe and (Ge,Mn)Te layers<br />
W. Kn<strong>of</strong>f 1 , M.A. Pietrzyk 1 , B.A. Orłowski 1 , B. Taliashvili 1 , T. Story 1 , R.L. Johnson 2<br />
1 Institute <strong>of</strong> Physics, PAS, Al. Lotników 32/46, 02-668 Warsaw, Poland<br />
2 Institute <strong>of</strong> Experimental Physics, University <strong>of</strong> Hamburg, Luruper Chaussee 149,<br />
D-22761 Hamburg, Germany<br />
(Ge,Mn)Te belongs to the IV-VI family <strong>of</strong> diluted magnetic semiconductors and attracts<br />
considerable attention due to its ferroelectricity and carrier-induced ferromagnetism governed<br />
by the RKKY indirect exchange mechanism. GeTe is also known as a phase change material<br />
exhibiting at the nanosecond scale a transformation from an amorphous to a polycrystalline<br />
phase, accompanied by insulator to metal transition. In (Ge,Mn)Te layers this structural<br />
transformation results also in paramagnet to ferromagnet transition. The aim <strong>of</strong> this work is to<br />
compare electronic structure <strong>of</strong> amorphous (a-) and crystalline (c-) GeTe and Ge0.9Mn0.1Te<br />
semiconductors using resonant photoemission spectroscopy (RPES) method.<br />
GeTe and (Ge,Mn)Te layers were grown on BaF2 (111) substrates by molecular beam epitaxy<br />
technique using effusion cells with GeTe, Mn, and Te2. For the growth <strong>of</strong> amorphous and<br />
crystalline GeTe and (Ge,Mn)Te layers, the substrate temperature was kept at room<br />
temperature and at T=250 0 C, respectively. Mn content in the layers was determined by energy<br />
dispersive X-ray fluorescence analysis. No evidence <strong>of</strong> crystalline phases was found in<br />
a-GeTe and a-(Ge,Mn)Te layers by X-ray diffraction (XRD) measurements. GeTe and<br />
(Ge,Mn)Te layers deposited at higher temperature (T=250 0 C) grow epitaxially and are<br />
monocrystalline with [111] growth direction. For GeTe and (Ge,Mn)Te layers the<br />
photoemission spectra were measured in the photon energy range 45-60 eV. The three-peak<br />
structure related to 4p and 5p Te orbitals (about 2.6 eV), Ge 4s orbitals (7.6 eV), and Te 5s<br />
orbitals (12 eV) was experimentally observed in the valence band <strong>of</strong> both c-GeTe and a-GeTe<br />
[1-3]. The spectra obtained for c-(Ge,Mn)Te and a-(Ge,Mn)Te in the energy range<br />
corresponding to the transition from Mn 3p to Mn 3d states reveal the same structure <strong>of</strong> peaks<br />
and Mn 3d orbitals contribution at 4 eV.<br />
Intensity (arb.u.)<br />
a)<br />
c-GeTe<br />
a-GeTe<br />
hυ = 50 eV<br />
0 5 10 15<br />
Binding Energy (eV)<br />
0 5 10 15 20 25<br />
Binding Energy (eV)<br />
Fig.1 The valence band photoemission spectra <strong>of</strong> amorphous and crystalline GeTe (figure a) and (Ge,Mn)Te<br />
(figure b). The spectrum <strong>of</strong> (Ge,Mn)Te was measured at the energy corresponding to the Mn 3p-3d threshold.<br />
[1] N.J. Shevchik, J. Tejada, W.W. Langer, M. Cardona, Phys. Rev. Lett., 30 659 (1973).<br />
[2] B.J. Kowalski, M.A. Pietrzyk, W. Kn<strong>of</strong>f, et al., Physics Procedia 3 1357 (2010).<br />
[3] M.A. Pietrzyk, B.J. Kowalski, B.A. Orłowski, et al., Acta Phys. Pol. A 112, 275 (2007).<br />
c-(Ge,Mn)Te<br />
a-(Ge,Mn)Te<br />
hν=51 eV<br />
This work was partially supported by the EC Network SemiSpinNet (PITN-GA-2008-215368).<br />
55<br />
Intensity (arb.u.)<br />
b)
MoP18 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Local Structure Study <strong>of</strong> GaAs:Te Using Te K-edge X-ray Absorption Fine<br />
Structure.<br />
A. Pietnoczka 1 , R. Bacewicz 1 , T. Słupi ski 2 , S. H. Wei 3 , M. Jie 3 , J. Antonowicz 1 ,<br />
T. Drobiazg 1<br />
1 Faculty <strong>of</strong> Physics, Warsaw University <strong>of</strong> Technology, ul. Koszykowa 75, 00-662 Warsaw,<br />
Poland<br />
2 Faculty <strong>of</strong> Physics, University <strong>of</strong> Warsaw, Ho a 69, Warsaw 00-681, Poland<br />
3 NREL, 1617 Cole Blvd., Golden, CO 80401, USA<br />
The annealing <strong>of</strong> heavily doped GaAs:Te can significantly change the value <strong>of</strong> the free<br />
electron concentration in a reversible manner. These changes <strong>of</strong> electrical properties are<br />
accompanied by the structural changes <strong>of</strong> GaAs:Te solid solution.[1] In this contribution we<br />
present an attempt to determine local changes around Te atoms for different states <strong>of</strong> the<br />
GaAs:Te crystals caused by the annealing corresponding to different electron concentrations.<br />
Extended X-ray Absorption Fine Structure (EXAFS) and X-ray Absorption Near Edge<br />
Structure (XANES) techniques have been employed to investigate the local structure around<br />
tellurium atom and its valence. Tellurium K-edge absorption has been measured in a<br />
fluorescence method at the X1 beamline in the HASYLAB. The samples were kept at the<br />
temperature <strong>of</strong> 80 K in order to minimize the thermal disorder.<br />
Various models <strong>of</strong> Te defect complex have been used to fit the data for low<br />
concentration state: VGa- TeAs complexes [2], TeAs+ TeAs pairs [1], as well as several DX-like<br />
configurations <strong>of</strong> tellurium. DX configurations have been calculated using density functional<br />
theory (DFT). The EXAFS and XANES spectra for different Te location have been simulated<br />
and compared with the experimental data. Multi-parameter fitting <strong>of</strong> the EXAFS data<br />
provided information on the local structure around Te (the bond lengths, the coordination<br />
numbers and Debye-Waller factor). The charge state <strong>of</strong> Te has been determined from the<br />
XANES data. The EXAFS and XANES were useful for ruling out the possibility <strong>of</strong> existence<br />
GaTe and Ga2Te3 precipitations.<br />
The XAFS results have been compared with the studies <strong>of</strong> structural disorder using the<br />
X-ray diffuse scattering.<br />
References:<br />
[1] T.Słupi ski, E.Zieli ska-Rohozi ska, Thin Solid Films 367, 227 (2000).<br />
[2] D.T.J. Hurle, J. Appl. Phys. 85, 6957 (1999).<br />
56
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoP19<br />
! " ! # $ % & # # ' (<br />
! ! ) * + # , ! )- . / 0 + , (<br />
! "# # $ # %<br />
& # % '( "# # ) *<br />
+ , # -. "# # $ # %<br />
$ $ #% / 0 ! "# #<br />
$ # %<br />
1# $ "# # ) * + , # -. "# #<br />
$ # %<br />
! " # $ % & '<br />
" " ( # )<br />
$ $ "<br />
$ &*#+ ,'# ) $ (<br />
$ " $<br />
# $ $<br />
$ #<br />
)<br />
- $ # ! . //0 " #<br />
) " " 1 2 $ " $<br />
3# 4 ( $ " $ // $ ( (<br />
$# 5 6 $ ( " 78 ,9 $<br />
: ! $ " :; $ ( $ 2
MoP20 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Investigation <strong>of</strong> thermal properties <strong>of</strong> AlGaAs/GaAs Quantum<br />
Cascade Lasers by thermoreflectance spectroscopy<br />
Małgorzata Iwi ska 1,2 , Dorota Pier ci ska 1 , Kamil Pier ci ski 1 ,<br />
Anna Szerling 1 , Piotr Karbownik 1 , Maciej Bugajski 1,2<br />
1 Institute <strong>of</strong> Electron Technology, al. Lotników 32/46, 02-668 Warsaw, Poland<br />
2 Faculty <strong>of</strong> Physics, WUT, ul. Koszykowa 75, 00-662 Warsaw, Poland<br />
Abstract<br />
Quantum Cascade Lasers (QCLs) are semiconductor unipolar devices in<br />
which intersubband transitions <strong>of</strong> carriers lead to lasing. The QCL design’s<br />
characteristic feature is the active regions cascading scheme. The wavelength <strong>of</strong><br />
emitted radiation in QCLs depends mainly on the width and the depth <strong>of</strong> quantum<br />
wells and to the less extend on the material. QCLs emit wavelengths within the<br />
infrared region <strong>of</strong> the spectrum and which covers most <strong>of</strong> molecules’ absorption<br />
bands. This makes them ideal sources for trace gases detection, air pollution<br />
monitoring, human’s exhaled breath monitoring or biomedical imaging. Most <strong>of</strong><br />
QCL applications require devices operating at room temperature (RT), <strong>of</strong>ten in<br />
CW mode, which is not an easy task to achieve.<br />
For full exploitation <strong>of</strong> QCLs application potential understanding <strong>of</strong><br />
thermal management in the devices is necessary. For CW RT operation the active<br />
region temperature has to be kept under control. Its value depends on such<br />
parameters as the pulse frequency, the pulse width, device mounting technology<br />
and the heat sink temperature. The excessive temperature <strong>of</strong> the active region<br />
causes escape <strong>of</strong> the electrons from the upper laser level to continuum states and<br />
thermal backfilling <strong>of</strong> the lower laser level, both processes deteriorating device<br />
performance by lowering the laser gain.<br />
The AlGaAs/GaAs QCLs investigated in this work are designed and<br />
fabricated at the Institute <strong>of</strong> Electron Technology in Warsaw [1]. The aim <strong>of</strong> the<br />
study was to determine the main factors influencing temperature rise in QCLs and<br />
to optimise their thermal properties. The measurements included temperature<br />
dependence <strong>of</strong> light current (I-V) characteristics and measurements <strong>of</strong> temperature<br />
distributions within devices. The latter were obtained by spatially resolved<br />
thermoreflectance measurements [2,3], which is the technique allowing for high<br />
resolution temperature mapping on working devices.<br />
1. K. Kosiel, M. Bugajski, A. Szerling, J. Kubacka-Traczyk, P. Karbownik, E. Pruszy ska-<br />
Karbownik, J. Muszalski, A. Łaszcz, P. Romanowski, M. Wasiak, W. Nakwaski, I.<br />
Makarowa, P. Perlin , Photonics Letters <strong>of</strong> Poland, vol.1, 16-18, (2009)<br />
2. M. Bugajski, T. Piwoñski, D. Wawer, T. Ochalski, E. Deichsel, P. Unger, B. Corbett,<br />
Materials Science in Semiconductor Processing, vol.9, 188-197 (2006)<br />
D. Wawer, T. J. Ochalski, T. Piwo ski, A. Wójcik-Jedli ska, M. Bugajski, H. Page, phys.<br />
stat. sol. (a) 202, 1227–1232 (2005)<br />
58
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoP21<br />
Luminescence properties <strong>of</strong> Sm 3+ in different phases <strong>of</strong> TiO2<br />
P. Łach 1 , M. G. Brik 2 , I. Sildos 2 , A. Kamińska 1 , and A. Suchocki 1,3<br />
1<br />
- Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences, al. Lotników 32/46, 02-668, Warsaw,<br />
Poland<br />
2<br />
- Institute <strong>of</strong> Physics, University <strong>of</strong> Tartu, Riia 142, Tartu 51014, Estonia<br />
3<br />
- Institute <strong>of</strong> Physics, Kazimierz Wielki University, Weyssenh<strong>of</strong>fa 11, Bydgoszcz 85-072,<br />
Poland<br />
Titanium dioxide (TiO2) belongs to the wide-gap (3.0-3.2 eV) semiconductors and it<br />
has a very good performance as a photocatalyst and an oxygen sensor. Recently, it has been<br />
considered as a host suitable for doping with rare (RE) ions. In particular, samarium-doped<br />
titania reveals interesting phenomena like intense host-emission, high sensitivity <strong>of</strong> the<br />
emission on the ambient gas and enhanced photocatalytic activity [1].<br />
We have carried out studies <strong>of</strong> the photoluminescence <strong>of</strong> Sm 3+ -doped TiO2 within the<br />
temperature range 10 – 300 K in order to show the energy levels <strong>of</strong> the trivalent impurity ion<br />
in two different phases <strong>of</strong> titania. The anatase TiO2 crystallizes in I41/amd space group, in a<br />
tetragonal structure, the lattice parameters are equal to 3.7850 and 9.5140 Ǻ. The rutile TiO2<br />
crystallizes in the P42/mnm space group and lattice parameters are equal to 4.593 and 2.959<br />
Ǻ. In order to obtain crystalline titania powder in both <strong>of</strong> these two phases our samples were<br />
annealed: up to 700°C and at 1100 °C for anatase and rutile phases, respectively. Thanks to<br />
that we have obtained a maximum <strong>of</strong> Sm emission intensity [2], which depends on the<br />
ambient around the samples. The best luminescence signals were observed during the<br />
measurements in the atmospheric oxygen. We have used the over band-gap excitation in both<br />
cases. The PL intensity <strong>of</strong> rutile titania was much lower than in the corresponding anatase<br />
TiO2 at room temperature. In the two cases we have also observed the PL spectra at low<br />
temperatures. Unfortunately in argon atmosphere our samples have not given any signals.<br />
An example <strong>of</strong> PL spectrum <strong>of</strong> Sm 3+ in TiO2 at room temperature under the band-toband<br />
excitation <strong>of</strong> the anatase structure is presented below.<br />
PL intenity [au]<br />
0,007<br />
0,006<br />
0,005<br />
0,004<br />
0,003<br />
0,002<br />
0,001<br />
0,000<br />
6 H7/2<br />
6 H5/2<br />
6 H9/2<br />
6 H11/2<br />
500 550 600 650 700 750<br />
Wavelength [nm]<br />
Sm 3+ in anatase TiO 2<br />
λ exc = 325 nm<br />
T=300K<br />
In order to check the influence <strong>of</strong> high pressure on the photoluminescence properties<br />
<strong>of</strong> both phases titanium oxide doped with Sm we have used the intra-center excitation (488<br />
nm) in a diamond anvil-cell at low temperature. Pressure coefficients <strong>of</strong> the various<br />
luminescence lines were established. Pressure dependence <strong>of</strong> the crystals field parameters f<br />
Sm 3+ ions were calculated on this basis.<br />
Acknowledgements: This work was partly supported by the European Union within European Regional<br />
Development Fund, through the Innovative Economy grant POIG.01.01.02-00-108/2009/3<br />
References:<br />
[1] M.G. Brik et al., Physica B 405 (2010) 2450-2456<br />
[2] V. Kiisk et al., J. Phys. D: Appl. Phys. 42 (2009) 125107<br />
59
MoP22 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Investigation <strong>of</strong> electro-optical properties <strong>of</strong> 2D macroporous silicon<br />
structures with surface nanocrystals<br />
Lyudmila A. Karachevtseva a , Stepan Ya. Kuchmii b , Oleg A. Lytvynenko a ,<br />
Fedor F. Sizov a , Olena J. Stronska a , Alexandr L. Stroyuk b<br />
a V. Lashkaryov Institute <strong>of</strong> Semiconductor Physics <strong>of</strong> NASU, Kyiv, Ukraine<br />
b L.V. Pisarzhevskii Institute <strong>of</strong> Physical Chemistry <strong>of</strong> NASU, Kyiv, Ukraine<br />
One <strong>of</strong> the perspective materials for the development <strong>of</strong> 2D photonic structures is<br />
macroporous silicon that can be obtained by the method <strong>of</strong> photoanodic etching. In view <strong>of</strong><br />
the potential barrier on a macropore surface or heterojunction on «macropore-nanocoating»<br />
boundary, one should take into account processes on the local surface centres at energies<br />
below that <strong>of</strong> the indirect interband transition.<br />
Optical absorption in 2D photonic macroporous silicon structures was investigated<br />
taking into account the linear electro-optical effect. The spectral dependence <strong>of</strong> optical<br />
absorption in the near-IR area (impurity absorption) has oscillating structure and varies under<br />
the “3/2” law at long wavelengths. This correlates with frequency dependence <strong>of</strong> the<br />
imaginary part <strong>of</strong> permittivity for optical transitions between impurity levels and the allowed<br />
bands <strong>of</strong> a crystal in an electric field (the impurity Franz-Keldysh effect) [1]. The<br />
experimental absorption spectra <strong>of</strong> macroporous silicon agree well with the corresponding<br />
spectral dependences <strong>of</strong> the electro-optical energy and the imaginary part <strong>of</strong> permittivity in<br />
the weak electric field approximation, thus confirming realization <strong>of</strong> the impurity Franz-<br />
Keldysh effect. The electric field <strong>of</strong> the reflected electromagnetic wave at the grazing angle <strong>of</strong><br />
light incidence onto macropore surface changes effectively a local surface electric field.<br />
The near-IR light absorption oscillations for 2D macroporous silicon structures with<br />
microporous silicon and as well as CdTe, ZnO surface nanocrystals were investigated taking<br />
into account electro-optical effect within the strong electric field approximation. Wellseparated<br />
oscillations with one order amplitude at spectral ranges <strong>of</strong> the surface bonds were<br />
observed. An analysis <strong>of</strong> the experimental absorption spectra is carried out within the model<br />
<strong>of</strong> the resonant electron scattering on impurity states in the strong electric field with<br />
difference between two resonant energies equaled to Wannier-Stark ladder [2]. The model <strong>of</strong><br />
the resonant electron scattering on impurity states in an electric field <strong>of</strong> heterojunction<br />
«silicon-nanocoating» on macropores surface and the realization <strong>of</strong> Wannier-Stark effect on<br />
the randomly distributed surface bonds were considered. The Wannier-Stark ladders are not<br />
destroyed by impurities due to the longer scattering lifetime relatively the oscillation period<br />
<strong>of</strong> an electron in an external electric field at all investigated spectral region for macroporous<br />
silicon structures with CdTe and ZnO surface nanocrystals.<br />
[1] L.A. Karachevtseva, V.I. Ivanov, O.O. Lytvynenko, K.A. Parshin, O.J. Stronska, Appl.<br />
Surf. Sci. 255(5), 3328 (2008).<br />
[2] L. Karachevtseva, S. Kuchmii, O. Lytvynenko, F. Sizov, O. Stronska, A. Stroyuk, Appl.<br />
Surf. Sci. 257, 3331 (<strong>2011</strong>).<br />
60
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoP23<br />
Investigation <strong>of</strong> multi-phonon excitations in<br />
ZnO textured crystalline films by Raman spectroscopy<br />
George V. Lashkarev 1 , Anatoliy M. Yaremko 2 , Vitaliy A. Karpyna 1<br />
1 I.N.Frantsevich Institute for Problems <strong>of</strong> Material Science, National Academy <strong>of</strong> Sciences <strong>of</strong><br />
Ukraine, 3 Krzizhanovsky st., Kyiv, Ukraine<br />
2 V.E.Lashkarev Institute for Semiconductor Physics, National Academy <strong>of</strong> Sciences <strong>of</strong><br />
Ukraine, 45 Prospect Nauky, Kyiv, Ukraine<br />
ZnO is a direct bandgap semiconductor (Eg = 3.37eV at RT) which has got much<br />
attention due to its ability to generate ultraviolet (UV) radiation. A large binding energy <strong>of</strong><br />
excitons (~60 meV) favors to excitonic transitions at room and higher temperatures. Besides<br />
ZnO has a strong enough constant <strong>of</strong> electron- (exciton) phonon interaction (EPI) what<br />
promtes 8-9 phonon replicas observed in Raman spectrum <strong>of</strong> this crystal. Investigation <strong>of</strong><br />
photoluminescence (PL) and Raman scattering (RS) spectra are essential for understanding<br />
the mechanism <strong>of</strong> light generation and absoorption. In the series <strong>of</strong> papers the phonon replicas<br />
in ZnO were revealed in PL and Raman investigations but without theoretical consideration.<br />
ZnO films were deposited by radio frequency reactive magnetron sputtering on sapphire<br />
(0001) substrate. To grow textured films <strong>of</strong> high structural perfectness we used layer-by-layer<br />
growth mode described in [1]. This growth mode was realized by several stage <strong>of</strong> deposition<br />
at which next ZnO layer grow on the previously grown one, in result, ZnO films <strong>of</strong> improved<br />
quality is formed. The resonant Raman scattering is a powerful method for the investigation<br />
<strong>of</strong> elementary excitations in crystals. Raman measurements were carried out in backscattering<br />
geometry (z(x,x)z� polarization at which z direction oriented parallel to the c-axis) at room<br />
temperature using Horiba Jobin Yvon T64000 system, equipped with an Olympus confocal<br />
optical microscope. The experiment was performed in the wavenumber range from 200 to<br />
5000 cm -1 with a spectral resolution < 2 cm -1 .<br />
Multi-phonon structure <strong>of</strong> RS spectra can be<br />
Intensity, arb.units<br />
50<br />
40<br />
30<br />
20<br />
10<br />
A LO<br />
1<br />
ZnO:Cu/sapphire<br />
2A LO<br />
1<br />
3A LO<br />
1 4ALO<br />
5A<br />
1<br />
LO<br />
1<br />
7A LO<br />
1<br />
6A LO<br />
1<br />
8A LO<br />
1<br />
0<br />
0<br />
0 1000 2000 3000 4000 5000<br />
Raman shift, cm -1<br />
50000<br />
40000<br />
30000<br />
20000<br />
10000<br />
Intensity, arb. units<br />
Fig.1. Comparison <strong>of</strong><br />
experimental RS spectra<br />
(curve 1) <strong>of</strong> ZnO:Cu film with<br />
calculated one (curve 2).<br />
1<br />
2<br />
explained in the frames <strong>of</strong> theoretical approach developed<br />
in [2]. To consider the RS we have to study the response<br />
<strong>of</strong> the system on electromagnetic field in the second order<br />
<strong>of</strong> perturbation theory. Fig.1 shows experimental RS<br />
spectrum and theoretical result. It is seen that positions <strong>of</strong><br />
maxima <strong>of</strong> experimental spectrum are well enough<br />
described by theoretical dependences. In particular, we<br />
can see that number <strong>of</strong> maxima is eight, which is<br />
practically the same as for good quality bulk crystal [3].<br />
The numerical calculations show also that character <strong>of</strong><br />
spectrum, i.e. the number <strong>of</strong> maxima and distribution <strong>of</strong><br />
intensity different line is very sensitive to constant <strong>of</strong> EPI.<br />
The theoretical consideration allows determining<br />
following main parameters, described LO phonons in RS:<br />
energy �LO=72 meV, constant <strong>of</strong> EPI �� 0,1 = 0.2 eV, damping constant � = 10 meV.<br />
[1] A.I. Ievtushenko, V.A. Karpyna, V.I. Lazorenko, et.al., Thin Solid Films 518 4529 (2010)<br />
[2] Yaremko A.M., V.M. Dzhagan, V.O. Yukhymchuk, et.al., J. Mol. Struct., 976 205 (2010)<br />
[3] J. F. Scott, Phys. Rev. B, 2 1209 (1970)<br />
The authors are grateful to V. Strelchuk for Raman measurements <strong>of</strong> ZnO films<br />
61
MoP24 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
&<br />
!<br />
" # $ % & ' ( ) * + " ,<br />
0 -<br />
!" !" ! # $ % !<br />
! $ ' ( ! ) * +(<br />
" # $ , % !<br />
&<br />
'<br />
( )<br />
*(+<br />
,<br />
(- (. / 0 (<br />
&1 - %<br />
& *(((+<br />
23<br />
4- ( &1 - '- 5<br />
- (4) 6 ,<br />
!<br />
' ,<br />
( 7 - 3 )<br />
!<br />
,<br />
' 8<br />
8) %<br />
& *3+<br />
& "<br />
9 % %<br />
*3+ : *4+<br />
%<br />
(<br />
- . / -<br />
, 2 '-( + + , ! ! +<br />
33 . , + 4 /( 1 5 4-' -1 $ + 6 0 1"<br />
( 7 # 8 30 1 ! " ( ! # ( 4<br />
! ( ,0<br />
-<br />
*(+ ; ; , ) # .(( "/ ! 1 -
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoP25<br />
�<br />
�����������������������������������������������������������������������<br />
�������������������������������������<br />
�<br />
�� � ��������� � �������������� � ������������� � ������������� � �������������� � �����������������������<br />
����������� � �������������� � ������������� � ����������� ��<br />
�<br />
�� �������������������������������������������������������������������������������<br />
�� ��������� ����������������������������������������������������<br />
� �����������������������������������������<br />
���� ������������������������������� ����������������������������������������������<br />
���������������������������������������������������������������������������������������<br />
���������� ���������� ��������� ��� ����� ������������ ����� ���� ��� ���� ������������ �����������<br />
����������������������������������������������������������������������������������������������<br />
����������������������������������������������������������������������������������������������<br />
��������������������������������������������������������������������������������������������������<br />
������������������������������������������������������������������������������������������������<br />
�������������������������������������������������������������������������������������������������<br />
�����������������������������������������������������������������������������������������������<br />
����������������<br />
�������������������������������������������������������������������������������<br />
����������������������� �������������������������������������������������������������������������������<br />
�����������������������������������������������������������������������������������������������<br />
���������������� ���������� �������� ��� �������� ������������� ������ ��� �� ��������������� ����� ����<br />
����� ���� ��� ����� ���� �������������������� ����� ���������� ���� ���� ������� ����� ���� �����<br />
�������� ��� ���� ������ ������� ����� ���� ������ ���� �������� ����� ���� ����������� ��� ���� ���<br />
���������� ��� ���� ����� ������ ����� ������ ����� ����� �������� ���� ��� ���� ������ ����� ��� ����<br />
�������������������������������������� ��������� ������������������ ������������������������������<br />
�����������<br />
���������������������������������������������������������������������������<br />
������� ������������<br />
���������������<br />
������<br />
������<br />
����<br />
��������<br />
������� с��<br />
���<br />
������<br />
��������<br />
����������<br />
����<br />
������<br />
��������<br />
���<br />
������������<br />
������������ �� ��<br />
� �� �<br />
����� ������ ����� ������ ����� ��� ����<br />
������������������ ������ ����� ������� ����� ��� ����<br />
��� ���� ������ ���������� ��������������� ���� ��� ������������� ���������� ��� ���� �������<br />
����������� ��� �� ������������ ��� ���� ���������� ��� ���������� ���������� ��������������� ��� ���<br />
���������������������������������������������������������������������������������������������<br />
����������������������������������������������������������������������������������������������<br />
��� ���� ���� ������ ��� ������������ �������� �������������������� ��� ��� ���� ��� ���� ����� ���� ������<br />
�������������� ������ ������� ����� ������� ��� ������ ��� ����� ��� ��������� ����� ���������� ���� ����<br />
������������������������������������������������������������������������������<br />
��������������������������������������������������<br />
���������������������������������������������������������������������������������������������<br />
����������������������������������������������<br />
��������������������������������������������������<br />
�����������������������������������������������������������������������������������<br />
�<br />
63
MoP26 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
�������������������������������������������������������<br />
�������������������������������������������<br />
��������������� � �������������������� � ���������������������� ��<br />
�<br />
�<br />
��������������������������������������������������������������������������������������������<br />
�����������������������������������������������������������������������<br />
�<br />
�������������������������������������������������������������������������������������������<br />
�������������������������������<br />
�<br />
�������������������������������������������������������������������������������������<br />
����� ����� ������ ���� ������������ ����� ��� ���������� �������������� �������� ���� ��� ������ �����<br />
������������� ���� �������� ������ ���� ������������� ��������� ������������ ��� ������� ��������� ��� ����<br />
�������������� ��� ����� ���� ������ ����� ����� ������ ��� �������� ��� ������ �������������� ��� �������<br />
����������������������������������������<br />
� ���� ������� ��� ����� ����� ������ ����� ���������� ������ �������� ��� ������������ �����<br />
������������������������������������������������ ������������������� ������������������������<br />
����� ������� ����� ��� ���������� ��������� ���� ���� ������� ���������� ���������������� ����������� ���<br />
���������������������������<br />
����� ������ �������� ���� �������� ��� ��� �������������� ����� ���� ���������� ��� ���� �����<br />
��������� ��� ���� �������� ����������� ��� ����� ����� ������ ��������� ��� ���� ��������� �����<br />
������������������������������������������������������������������������������������������<br />
���� ����� ������� ���� ��������� ������� ���� ����������� ��� �������� ������������� �������� �����<br />
������ ���� �������� �������� ��� ������������� �������� ��� ����� ������ ���� �������������� ��� ������<br />
�������� ����������� ��� ������������ ������ ����������� ������ ������ ����������� ������������ ����� ����<br />
������������������������������������<br />
���� ���������� ������ ��������� ��� ��� �� ��� ����� ������ ����� ���������� ����� ������<br />
����������� ��� �� ���������� �������� ������� �������� �� �������� ����� ��� ��������������<br />
������������ ��� ������������� �������� ���� ��������� �������� ����� �� ��������� �������� �������<br />
��������������������������������<br />
���� ������������� �������� ��� ����� �� ����� � � ����� ������ ����� ��������� ��� ������ ��� ��<br />
������������� ������������������� ���� ������������� ����� ����� ���������������� ���� ������ ���<br />
�����������������������������������������������<br />
���� ����� �� ����� ����� ������ ����� ������������ ��� ��� ���������� ����� ���������������<br />
��������������������������������������������������������������������������������������������<br />
�����������������������������������������������������������<br />
�<br />
�������������������������������������������������������<br />
����������������������������������������������������������������������������������������������<br />
�����������������������������������������������<br />
�������������������������������������������������������������������������������<br />
������������������������������������������������������������<br />
���� ��� ������������������ ��� ���������������� ��� ��� ������� ��� ������������ �������� ��������<br />
��������������������������<br />
�<br />
�<br />
�<br />
64
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoP27<br />
Properties <strong>of</strong> high-k oxides grown by Atomic Layer Deposition method for<br />
transparent electronics<br />
S. Gierałtowska 1 , Ł. Wachnicki 1 , B.S. Witkowski 1 , T.A. Krajewski 1 , M. Godlewski 1,2 ,<br />
E. Guziewicz 1<br />
1 Polish Academy <strong>of</strong> Sciences, Institute <strong>of</strong> Physics, al. Lotników 32/46, Warszawa 02-668,<br />
Poland<br />
2 Cardinal Stefan Wyszynski University, College <strong>of</strong> Science, Department <strong>of</strong> Mathematics and<br />
Natural Sciences, Warszawa, Poland<br />
Transparent electronics is nowadays an emerging technology for the next generation <strong>of</strong><br />
electronic structures, including IC (integrated circuit), recent devices with 45nm node and<br />
smaller cross-bar memories and transistors for e-paper applications. The essential materials<br />
used in semiconductor manufacturing processes are high-k oxides gate dielectrics. The<br />
introduction <strong>of</strong> high-k for gate dielectrics is one <strong>of</strong> several strategies allowing further<br />
miniaturization <strong>of</strong> microelectronic components, colloquially referred to as extending the<br />
Moore's Law.<br />
Our research was focused on the optimization <strong>of</strong> technological parameters for composite<br />
layers <strong>of</strong> high-k oxides that include hafnium oxide (HfO2) and aluminum oxide (Al2O3). We<br />
obtain high quality thin films using Atomic Layer Deposition at low temperature (60°C –<br />
150°C). Growth parameters are elaborated for the maximum smoothness and amorphous<br />
microstructure required for gating. To fabricate thin film transistors (TFTs) and active<br />
elements <strong>of</strong> cross-bar memories, we deposit ZnO (zinc oxide) as an active channel and high-k<br />
oxides as gate dielectric at low temperature, both grown by the ALD technique. I-V<br />
characteristics <strong>of</strong> our TFT structure will be shown. The combination <strong>of</strong> low temperature and<br />
transparency processing makes the ZnO-TFT with high-k dielectric very promising for the<br />
next generation <strong>of</strong> invisible and flexible electronics.<br />
The research was partially supported by the European Union within European Regional<br />
Development Fund, through grant Innovative Economy (POIG.01.01.02-00-008/08).<br />
65
MoP28 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
High Spin-Low Spin Transitions in Cu0.2Co0.76Cr1.83Se4 Semiconductor<br />
E. Maciążek 1 , T. Groń 2 , A.W. Pacyna 3 , T. Mydlarz 4 and J. Krok-Kowalski 2<br />
1 University <strong>of</strong> Silesia, Institute <strong>of</strong> Chemistry, ul. Szkolna 9, 40-006 Katowice, Poland<br />
2 University <strong>of</strong> Silesia, Institute <strong>of</strong> Physics, ul. Uniwersytecka 4, 40-007 Katowice, Poland<br />
3 The Henryk Niewodniczański Institute <strong>of</strong> Nuclear Physics, Polish Academy <strong>of</strong> Sciences,<br />
ul. Radzikowskiego 152, 31-342 Kraków, Poland<br />
4 International Laboratory <strong>of</strong> High Magnetic Fields and Low Temperatures,<br />
ul. Gajowicka 95, 53-529 Wrocław, Poland<br />
Electrical and magnetic studies carried out on single-crystalline CuxCoyCrzSe4 spinels<br />
showed ferromagnetic ordering and p-type metallic conductivity for y = 0.06, 0.1 and 0.11 as<br />
well as a ferrimagnetic behaviour and n-type electrical semiconductivity for y = 0.23 [1].<br />
Later, structural and electrical investigations carried out on polycrystalline Cu1-xCoxCr2Se4<br />
spinels in the compositional range 0.0 ≤ x ≤ 1.0 revealed a transformation from cubic to<br />
monoclinic structure above x = 0.4 as well as metallic-type conductivity with high values <strong>of</strong><br />
activation energy for x ≤ 0.5 and polaron semiconducting with low activation energy for x ³<br />
0.8 [2]. Recently, X-ray diffraction studies on the single phase Cu1-xCoxCr2Se4 samples (x = 0,<br />
0.2, 0.8, 1) proved the existence <strong>of</strong> the cubic spinel-type structure for x ≤ 0.2 and the<br />
monoclinic Cr3S4-type one for x ³ 0.8 [3]. Magnetization and magnetic susceptibility <strong>of</strong> the<br />
CuxCoyCrzSe4 polycrystals measured in the zero-field-cooled mode showed a transition from<br />
ferromagnetic order via ferrimagnetic one to antiferromagnetic-like behaviour with increasing<br />
Co content. This transition was accompanied with a lowering symmetry from cubic to<br />
monoclinic and for the latter the spin crossover phenomenon occurred [4].<br />
Magnetization, dc and ac magnetic susceptibility <strong>of</strong> Cu0.2Co0.76Cr1.83Se4 were measured<br />
in the zero-field-cooled mode using a vibrating sample magnetometer with a step motor at 4.2<br />
K and in applied external magnetic fields up to 150 kOe, a Faraday type Cahn RG automatic<br />
electrobalance in the temperature range 4.2-370 K and at 800 Oe, and a Lake Shore 7225 ac<br />
'<br />
"<br />
susceptometer, respectively. The in-phase c 1(<br />
T)<br />
and out-<strong>of</strong>-phase c1( T)<br />
components <strong>of</strong> the ac<br />
fundamental susceptibility were recorded in the temperature range 4.2-170 K simultaneously<br />
as a function <strong>of</strong> temperature in an oscillating field Hac = 1 Oe with frequency <strong>of</strong> 120 Hz. The<br />
signals <strong>of</strong> the second (c2) and third (c3) harmonics associated with nonlinear susceptibilities<br />
were detected as a function <strong>of</strong> temperature using an oscillating field <strong>of</strong> 5 Oe with frequency <strong>of</strong><br />
120 Hz without the applied external magnetic field.<br />
'<br />
Two spectacular peaks were observed on the c1( T)<br />
curve at 128 K and 147 K which<br />
corresponded to the strong fall <strong>of</strong> the dc magnetic susceptibility. At the same temperatures<br />
these peaks were observed on the curves <strong>of</strong> the second and third harmonics <strong>of</strong> magnetic<br />
susceptibility. Taking into account also a low value <strong>of</strong> the magnetization <strong>of</strong> 0.22 µB/f.u. at 4.2<br />
K and at 140 kOe as well as a hysteresis between 15 kOe and 60 kOe one can conclude that in<br />
the Cu0.2Co0.76Cr1.83Se4 semiconductor the high spin-low spin transitions may be populated<br />
and cross over in thermal equilibrium.<br />
[1] T. Groń, E. Maciążek, J. Heimann, J. Kusz, I. Okońska-Kozłowska, K. Bärner, and Ch.<br />
Kleeberg, Physica B 254, 84 (1998).<br />
[2] E. Maciążek, A. Molak, and T. Goryczka, J. Alloys Compd. 441, 222 (2007).<br />
[3] V. Svitlyk, and Y. Mozharivskyj, Inorg. Chem. 48, 5296 (2009).<br />
[4] E. Maciążek, T. Groń, A.W. Pacyna, T. Mydlarz, B. Zawisza, and J. Krok-Kowalski, Acta<br />
Phys. Pol. A 119, (<strong>2011</strong>).<br />
66
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoP29<br />
Photoluminescence Study <strong>of</strong> ZnSe Scintillating Crystals Doped with<br />
Isovalent Tellurium and Oxygen<br />
D. Shevchenko 1 , J. Mickevi ius 1 , G. Tamulaitis 1<br />
N. Starzhinskiy 2 , K. Katrunov 2 , V. Ryzhikov 2<br />
1 Institute <strong>of</strong> Applied Research and Semiconductor Physics Department,<br />
Vilnius University, Sauletekio Ave. 9-III, LT-10222 Vilnius, Lithuania<br />
2 Institute for Scintillation Materials, 60 Lenin Ave., 61001 Kharkov, Ukraine<br />
Recently, zinc selenide has attracted considerable attention as a promising scintillating<br />
material for ionizing radiation detection. Compared to conventional CsI(Tl) scintillators,<br />
ZnSe –based scintillators have these advantages: considerably lower hygroscopicity, lower<br />
afterglow level, high light yield, and higher radiation stability (by 3 to 4 orders <strong>of</strong> magnitude).<br />
Thus, scintillators based on ZnSe semiconducting compounds are an attractive choice for<br />
applications in medical and industrial tomography, X-ray introscopy and many others.<br />
Operating photoemission band in the ZnSe scintillators results from optical transition<br />
involving deep-defect states. Deep-level-related radiative recombination efficiency can be<br />
improved by doping with isovalent impurities and subsequent thermal annealing in Zn vapor.<br />
Introduction <strong>of</strong> the isovalent impurities promote formation <strong>of</strong> thermally stable complex<br />
defects, which act as radiative recombination centers. Subsequent annealing in Zn vapor<br />
enhances the concentration <strong>of</strong> the complex defects. Therefore it is important to understand<br />
radiative and non-radiative recombination mechanisms and their relation with the crystal<br />
growth conditions.<br />
The study <strong>of</strong> deep-level-related photoluminescence was carried out in the ZnSe crystals<br />
doped with isovalent tellurium and oxygen. Doping <strong>of</strong> ZnSe by Te was accomplished by<br />
mixing tellurium into the melt for crystal growth. Two methods <strong>of</strong> doping with oxygen were<br />
used: ZnSe(O) sample was doped with O by annealing finelly ground row material in oxygenrich<br />
atmosphere, while ZnSe(O, Al) sample was doped with oxygen by mixing Al2O3 into the<br />
melt for crystal growth. Undoped ZnSe samples were also studied for reference. After growth,<br />
all the samples under study were annealed in Zn vapor in identical conditions.<br />
Photoluminescence (PL) spectra were measured under steady-state conditions using CW<br />
He-Cd laser as an excitation source. The wavelength dependence <strong>of</strong> the absolute quantum<br />
yield was measured using an integrating Ulbricht sphere. Monochromated light <strong>of</strong> a halogen<br />
lamp was used for PL excitation.<br />
It was determined that photoluminescence quantum yield depends on excitation photon<br />
energy. The quantum yield rapidly decreases when the excitation photon energy is increased<br />
above the ZnSe band gap. The highest quantum yield (~ 22 %) is observed in the sample<br />
doped with tellurium and annealed in Zn vapor. The quantum yield <strong>of</strong> this sample was by a<br />
factor <strong>of</strong> 2 higher than that in other crystals. The observed dependence <strong>of</strong> the quantum yield<br />
on excitation wavelength is explained by nonradiative carrier recombination on crystal surface<br />
and direct excitation <strong>of</strong> donor-acceptor pair luminescence in ZnSe crystal volume.<br />
The study <strong>of</strong> PL spectra showed that the deep–level–related photoemission efficiency<br />
can be also enhanced by doping with oxygen. PL intensity <strong>of</strong> ZnSe(O, Al) scintillating crystal<br />
is by a factor <strong>of</strong> 16 higher than that <strong>of</strong> ZnSe(O) scintillator. The photoluminescence spectra<br />
consist <strong>of</strong> two strongly overlapped bands due to recombination via M- and SA-type complex<br />
defects. Annealing results in an increase <strong>of</strong> PL intensity by factor <strong>of</strong> ~3 and a red-shift <strong>of</strong> the<br />
PL band, compared with those in unannealed crystals.<br />
67
MoP30 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Native Deep-Level Defects in MBE-Grown p-Type CdTe<br />
Karolina Olender, Tadeusz Wosiński, Andrzej Mąkosa, Piotr Dłużewski,<br />
Valery Kolkovsky and Grzegorz Karczewski<br />
Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences, 02-668 Warsaw, Poland<br />
We present results <strong>of</strong> investigation <strong>of</strong> deep-level defects in p-type CdTe layers grown by<br />
the molecular-beam epitaxy (MBE) technique. The CdTe layers <strong>of</strong> 5 µm thickness were doped<br />
with nitrogen and grown on (001)-oriented p+-type, Zn-doped GaAs substrate. Prior to<br />
deposition <strong>of</strong> the investigated CdTe:N film, the GaAs substrate was covered with thin, 3monolayer-thick,<br />
undoped ZnTe layer to reduce the strong lattice mismatch between CdTe and<br />
GaAs and to stabilize the growth in the [001] direction.<br />
Deep-level transient spectroscopy (DLTS) was used to identify a set <strong>of</strong> deep electronic<br />
states in the band gap <strong>of</strong> p-CdTe layer employing Schottky barriers, obtained by sputtering<br />
aluminum. Room-temperature free carrier concentration in the CdTe layers was<br />
2.4 × 10 16 cm -3 , as found from capacitance–voltage measurements recorded at a frequency <strong>of</strong><br />
1 MHz.<br />
DLTS measurements, carried out in the temperature range from 90 to 400 K, showed<br />
five hole traps, at low concentrations <strong>of</strong> the order <strong>of</strong> 10 13 cm -3 , called H1 to H5. Explicit<br />
saturation <strong>of</strong> the dependence <strong>of</strong> DLTS-signal amplitude on the filling-pulse time was observed<br />
for the H2 and H3 traps. They were characterized by the exponential kinetics <strong>of</strong> carrier<br />
capture, which means that the observed DLTS peaks originate from isolated point defects or<br />
impurities. The H2 trap, with the activation energy for hole emission <strong>of</strong> 0.46 eV, most likely is<br />
the same trap, which we observed as a minority-carrier trap in n-type CdTe:I layer [1]. We<br />
attributed this trap to the Cd vacancy, VCd. The H3 trap, with the activation energy <strong>of</strong> 0.75 eV,<br />
has been tentatively ascribed to the complex formed <strong>of</strong> VCd and the Te antisite defect, TeCd.<br />
In the case <strong>of</strong> H1 and H4 traps the dependence <strong>of</strong> DLTS-signal amplitude on the filling-<br />
pulse time, observed over the range <strong>of</strong> four orders <strong>of</strong> magnitude <strong>of</strong> the time, was logarithmic.<br />
It suggests that the observed traps are related to the hole states <strong>of</strong> dislocations. Thorough<br />
analysis <strong>of</strong> the dependence <strong>of</strong> DLTS-peak shape on the filling time allowed us to determine<br />
the type <strong>of</strong> electronic states associated with these dislocations. The H1 trap, with the<br />
activation energy <strong>of</strong> 0.22 eV, is characteristic <strong>of</strong> localized states. On the other hand, the H4<br />
trap, with the activation energy <strong>of</strong> 0.49 eV, was assigned to the bandlike states <strong>of</strong> dislocations.<br />
Probably, an electron emission from the same dislocation states was observed in our DLTS<br />
experiments for n-type CdTe:I epitaxial layers [1]. We attributed both the H1 and H4 traps to<br />
threading dislocations, generated at the mismatched interface with the substrate and<br />
propagated through the CdTe layer.<br />
Finally, the H5 trap, displaying the logarithmic capture kinetics, has been associated<br />
with acceptor states at the CdTe surface. We observed a strong influence <strong>of</strong> electric field on<br />
the DLTS-peak position <strong>of</strong> this trap and its apparent activation energy. Measurements <strong>of</strong> the<br />
electric-field dependences <strong>of</strong> hole emission rates, performed at various temperatures, allowed<br />
us to determine the mechanism responsible for the field-induced enhancement <strong>of</strong> hole<br />
ionization rate from the traps as the phonon-assisted tunnelling.<br />
This work has been partially supported by the Polish Ministry <strong>of</strong> Science and Higher<br />
Education under Grant No. N N515 0719 37.<br />
[1] K. Olender, T. Wosinski, A. Makosa, S. Kret, V. Kolkovsky and G. Karczewski, Semicond.<br />
Sci. Technol. 26, 045008 (<strong>2011</strong>).<br />
68
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoP31<br />
Residual Paramagnetism at High-Temperature Semiconductors<br />
CuEu2W2O10 and Cu3Eu2W4O18<br />
P. Urbanowicz 1 , E. Tomaszewicz 2 , T. Groń 1 , H. Duda 1 and Z. Kukuła 1<br />
1 University <strong>of</strong> Silesia, Institute <strong>of</strong> Physics, ul. Uniwersytecka 4, 40-007 Katowice, Poland<br />
2 West Pomeranian University <strong>of</strong> Technology, Department <strong>of</strong> Inorganic and Analytical<br />
Chemistry, Al. Piastów 42, 71-065 Szczecin, Poland<br />
CuEu2W2O10 and Cu3Eu2W4O18 were synthesized by the solid-state reaction method<br />
using CuWO4 and Eu2WO6 as the starting materials. The CuEu2W2O10 and Cu3Eu2W4O18<br />
compounds crystallize in the monoclinic and triclinic system, respectively [1]. Copper<br />
tungstate, CuWO4, belongs to the triclinic distorted wolframite type structure [2–4] in which<br />
every metal ion is surrounded by six oxygen ions. It has a potential technological significance<br />
(scintillation detectors, optical fibres). Thin film <strong>of</strong> CuWO4 is expected to be a promising<br />
positive electrode material for high-performance rechargeable lithium batteries [5,6]. On the<br />
other hand, doped and undoped rare-earth metal molybdates and tungstates (e.g., Eu2WO6) are<br />
host candidates for luminescent applications. They are used for a fabrication <strong>of</strong> white lightemitting<br />
diodes showing high stability, energy-saving and safety [7,8]. In the case <strong>of</strong> Eu2WO6<br />
narrower multiplet widths comparable to the thermal energy cause a weak temperature<br />
dependence <strong>of</strong> the magnetic susceptibility without the Curie-Weiss region as well as a low ntype<br />
conduction [9].<br />
The magnetic susceptibility <strong>of</strong> the powder compounds under study was measured in the<br />
temperature range 2-300 K in the applied external magnetic fields H = 5 kOe using a SQIUD<br />
Magnetometer <strong>of</strong> the Quantum Design MPMS – XL – 5 type. The electrical resistivity has been<br />
measured by a four-probe dc method using the apparatus with Keithley K181 digital<br />
multimeters. The thermoelectric power was measured by a differential method using the<br />
temperature difference <strong>of</strong> about 2 K. For electrical measurements the powder samples were<br />
compacted to disc form (10mm in diameter and 1–2 mm thick) using a pressure <strong>of</strong> 1.5GPa<br />
and they were next sintered through 2 h at 873 K.<br />
The electrical and magnetic measurements <strong>of</strong> CuEu2W2O10 and Cu3Eu2W4O18 showed a<br />
residual paramagnetism, i.e. a low value <strong>of</strong> the magnetic susceptibility and its weak<br />
temperature dependence as well as a semiconducting behaviour at high temperatures. We<br />
have also found that the effective magnetic moment increases gradually from 0.5 µB at 2 K to<br />
5 µB at 300 K, suggesting a transition from the low spin (LS) state <strong>of</strong> Eu 3+ ions to their LS and<br />
high spin (HS) mixed states, as the effective number <strong>of</strong> Bohr magnetons (peff = 9.80 for S = 3,<br />
HS) has not been reached. It means that only 50 % conversion <strong>of</strong> the molecules from LS to<br />
HS occurred. This work is partly founded from science Grant No. N N209 336937.<br />
[1] E. Tomaszewicz, J. Typek, and S.M. Kaczmarek, J. Therm. Anal. Calorim. 98, 409(2009).<br />
[2] L. Kihlborg, and L. Gebert, Acta Crystallogr. B 26, 1020 (1970).<br />
[3] S. Klein, and H. Weitzel, J. Appl. Crystallogr. 8, 54 (1975) (in German).<br />
[4] P.F. Sch<strong>of</strong>ield, K.S. Knight, S.A.T. Redfern, and G. Cressey, Acta Crystallogr. B 53, 102<br />
(1997).<br />
[5] L.G. Van Uitert, and S.T. Preziosi, J. Appl. Phys. 33, 2908 (1962).<br />
[6] R.H. Gillette, Rev. Sci. Instrum. 21, 294 (1950).<br />
[7] S. Neeraj, N. Kijima, and A.K. Cheetham, Chem. Phys. Lett. 387, 2 (2004).<br />
[8] T. Kim, and S. Kang, J. Lumin. 122–123, 964 (2007).<br />
[9] P. Urbanowicz, E. Tomaszewicz, T. Groń, H. Duda, A.W. Pacyna, and T. Mydlarz, Physica<br />
B 404, 2213 (2009).<br />
69
MoP32 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Ferromagnetic nanocomposite Co-Al2O3 as a spintronic material with<br />
engineered magnetic properties<br />
M.V. Radchenko 1 , G.V. Lashkarev 1 , M.E. Bugaiova 1 , V.I. Sichkovskyi 1 ,<br />
V.I. Lazorenko 1 , L.A. Krushynskaya 2 ,Y.A. Stelmakh 2 , W. Kn<strong>of</strong>f 3 , T. Story 3<br />
1 I.M. Frantsevych Institute for Problems <strong>of</strong> Material Science, National Academy <strong>of</strong> Sciences<br />
<strong>of</strong> Ukraine, 3 Krzhizhanovskogo str., Kyiv, Ukraine<br />
2 E.O.Paton Electric Welding Institute, Academy <strong>of</strong> Sciences <strong>of</strong> Ukraine, 68 Antonowich str.,<br />
Kyiv, Ukraine<br />
3 Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences, Al. Lotników 32/46, Warsaw, Poland<br />
Ferromagnetic cobalt nanoparticles (NP) embedded in alumina matrix constitute a new class<br />
<strong>of</strong> two-phase spintronic materials - ferromagnetic nanocomposites (FMNC). FMNC attract<br />
attention as artificially developed materials permitting efficient engineering <strong>of</strong> their magnetic<br />
and magnetotransport properties These materials are closely related to inhomogeneous diluted<br />
magnetic semiconductors containing ferromagnetic clusters. In this work, we study magnetic<br />
and electrical characteristics <strong>of</strong> Cox(Al2O3)100-x (x
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoP33<br />
����������������������������������������������������������������������������<br />
�����������������<br />
�<br />
����������� � �������������� � ������������� � ������������� ��<br />
�<br />
� ����������������������������������������������������������������������������������<br />
� ������������������������������������������������������������������������������������������<br />
�� ������������������������������������������������������������������� �<br />
�<br />
�<br />
����������������������������������������������������������������������������������������<br />
�����������������������������������������������������������������������������������������������<br />
��������������������������������������������������� �� �� � �� �� ����������������������������������<br />
��������������������������������������������������������������������������������������������<br />
����� ���� ���� ��� ����������� ���� ��� ������ ��� ��� ���� ��� ����� ����� ��� ������ ��� ���� ��������<br />
��������������������������������������������������������������������������������������������������<br />
����������������������������������������������������������<br />
������������������� ��������������������� ������������������ ������� ��������������������<br />
�����������������������������������������������������������������������������������������������<br />
�������� ���� ������������ �������� ����� ��������� ��� ������ ��� ���� ����������� ������� ������<br />
������� �������� ������� ���� ����� ���������� ���������� ��� ���� ��������� �������������� ����<br />
�������������������������������������������� � ��������������� � ���������������� ��������������<br />
�������������������������������������������������������������������� ���� ������������������<br />
������� ��� ����������������������������������������� ���� ���� � �����������������������������<br />
���� ���������� ��� ���������� ����������� ���� ������ ���������� �������� ��� ������������ ����<br />
���������� ��� ���� ����� ��������� ���� ��������� � ����� ������������ ���� ���������� ��� ���� �������<br />
�������� ���������� ��� ������ ���� �������� ��� ���� �������� ��������� �������� ������������� ���<br />
������������ ���� ��� ����� ������������ ������ ���� ���������� ����� ������� ��� ���� ���������<br />
�����������������������������������������������������������������������������������������������<br />
������������������������������������������������������������������������������������������������<br />
������������������������������������������������������������������������������������������������<br />
�� ������������ ��� ���� ���������� ��� ������������ ���������� ���������� ����� ���������� ����<br />
���������������������������������������������������������������������������������������������<br />
�������������������<br />
�<br />
�<br />
�������������������������������������������������������������������������������������<br />
���������������<br />
����������������������������������������������������������������������������������<br />
���������������������������<br />
������������������������������������������������������������������������������������<br />
������������������������������<br />
�����������������������������������������������������<br />
��������������������������������������������������������������������������������������������<br />
���������������<br />
�������������������������������������������������������������������������������������������<br />
��������<br />
�<br />
71
MoP34 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Van der Waals surface <strong>of</strong> InSe as a standard nanorelief in the metrology <strong>of</strong><br />
nanoobjects<br />
Dmitriev A. I.<br />
. I. M. Frantsevich Institute for Problems <strong>of</strong> Material Science, National Academy <strong>of</strong><br />
Sciences <strong>of</strong> Ukraine, Krzhizhanivskogo st. 3, 03680 Kyiv, Ukraine, FAX: +38424-2131.<br />
E-mail: dmitr. kiev@gmail.com<br />
Development <strong>of</strong> new high technologies and new materials leading to the fundamental<br />
improvment <strong>of</strong> the technical level <strong>of</strong> production largely dependents on the level <strong>of</strong> metrological<br />
support. Intensive development <strong>of</strong> nanotechnology requires high-precision measurements on<br />
atomic and molecular levels by creating the metrological system for measurements, at the first<br />
turn, the length in the nanometer scale. Such measurements require standard samples <strong>of</strong><br />
nanorelief surface.<br />
At present, the nanorelief surface <strong>of</strong> highly oriented pyrolytic graphite (HOPG) is used for<br />
this purpase. Simultaneously with the scanning <strong>of</strong> an unknown surface the scan <strong>of</strong> the well-<br />
known surface <strong>of</strong> graphite (the period <strong>of</strong> the hexagonal crystal lattice: a = 2,464 ± 0,002 Ǻ) is<br />
carried out. S<strong>of</strong>tware recognition <strong>of</strong> atoms image <strong>of</strong> the graphite allows to generate a control<br />
signal for correcting the microscope manipulator. When cleaving HOPG many stages with<br />
dimensions less than 2x2 mm are formed. They are used as a model <strong>of</strong> nanorelief They appear<br />
at fracture <strong>of</strong> graphite planes, oriented not toward cleavage. Graphite planes have structural<br />
defects associated with dislocations. Different types <strong>of</strong> dislocations manifest themselves in the<br />
topography <strong>of</strong> the surface in many ways. Over time, the accumulation <strong>of</strong> the adsorbate on the<br />
surface, results in poor quality images.<br />
Semiconductor InSe belongs to a group <strong>of</strong> layered compounds A3B6.Each layer <strong>of</strong> this<br />
crystal consists <strong>of</strong> atomic groups Se-In-In-Se with strong covalent bonds in the layer. Se atoms<br />
<strong>of</strong> neighboring layers are linked by weak Van der Waals (VdW) forces and form VdW surface.<br />
High chemical stability and the absence <strong>of</strong> dangling bonds (defects) prevents the appearance <strong>of</strong><br />
the adsorbat. Low surface roughness <strong>of</strong> cleaved surface at the areas up to 5x5 mm allows to<br />
use the VdW surface <strong>of</strong> InSe single crystals as the standard nanorelief. This is evidenced by<br />
the researches <strong>of</strong> morphology for VdW surface by scanning tunneling microscopy [1] in which<br />
images with atomic resolution were obtained. A single crystal InSe, grown by vertical<br />
Bridgman-Stockbarger method. The crystal has -g-polytype rhombohedral structure . with<br />
periods a = 4.003 Å, and c = 24.9553 Å (in hexagonal axes).<br />
Thus InSe is appropriate standard nanorelief for probe microscopy.<br />
1. Physics <strong>of</strong> the Solid State,53, 3, 622 (<strong>2011</strong>).<br />
72
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoP35<br />
Crystal field analysis <strong>of</strong> the Yb 3+ energy level scheme in III-V<br />
semiconductors<br />
A. Kami�ska 1 , C.-G. Ma 2 , M.G. Brik 2 , A. Kozanecki 1 , M. Bo�kowski 3 , E. Alves 4 ,<br />
A. Suchocki 1,5<br />
1 Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences, 02-668 Warsaw, Poland<br />
2 Institute <strong>of</strong> Physics, University <strong>of</strong> Tartu, Riia 142, Tartu 51014, Estonia<br />
3 Institute <strong>of</strong> High Pressures Physics ‘Unipress’, Polish Academy <strong>of</strong> Sciences, 01-142 Warsaw,<br />
Poland<br />
4 Instituto Tecnologico e Nuclear, Estrada Nacional 10, 2686-953 Sacavém, Portugal<br />
5 Institute <strong>of</strong> Physics, University <strong>of</strong> Bydgoszcz, Weyssenh<strong>of</strong>fa 11, 85-072 Bydgoszcz, Poland<br />
Rare earths (RE) doped semiconductors are nowadays attracting a lot <strong>of</strong> attention due<br />
to their unique optical and electrical properties and high potential <strong>of</strong> these materials in new<br />
optoelectronic device applications, for instance in electrically-pumped light-emitting diodes<br />
or lasers [1]. Among the RE doped III–V semiconductors InP:Yb is one <strong>of</strong> the most<br />
intensively studied material. Recently an important effort has been devoted also to study the<br />
optical properties <strong>of</strong> ytterbium doped GaN [2, 3]. These materials reveal strong Yb-related<br />
luminescence at about 1 �m. Yb 3+ ions (4f 13 electron configuration) have a simple energy<br />
level scheme, which consists <strong>of</strong> two states only: 2 F7/2 and 2 F5/2 separated by about<br />
10 000 cm -1 . Such a scheme excludes excited state absorption and all related energy losses.<br />
From this point <strong>of</strong> view, Yb 3+ ion is now the most promising for use as a non-Nd lasing center<br />
in the infrared spectral region.<br />
In the present work we report on combination <strong>of</strong> the high-pressure measurements with<br />
use <strong>of</strong> diamond anvil cell (DAC) <strong>of</strong> the luminescence spectra <strong>of</strong> InP:Yb 3+ and GaN:Yb 3+<br />
crystals with ab initio calculations <strong>of</strong> the electronic and optical properties <strong>of</strong> these systems.<br />
The CASTEP module [4] <strong>of</strong> the Materials Studio was used to calculate the electronic and<br />
optical properties at ambient and elevated pressure. In addition, crystal field analysis <strong>of</strong><br />
splitting <strong>of</strong> the 2 F7/2 and 2 F5/2 states has been performed, with an aim <strong>of</strong> assigning all features<br />
<strong>of</strong> the experimental luminescence spectra.<br />
A thorough analysis <strong>of</strong> the pressure behavior <strong>of</strong> the Yb 3+ luminescence lines in InP<br />
reveals interesting effects, namely: after 6 GPa blue shift <strong>of</strong> the luminescence tends to<br />
saturation, which was explained by pressure-induced shift <strong>of</strong> the InP valence band<br />
approaching the energies <strong>of</strong> the Yb 4f-4f transitions. This overlap behaves exactly as the<br />
luminescence lines with pressure. An analysis <strong>of</strong> the pressure dependence <strong>of</strong> the Yb 3+<br />
luminescence in GaN and fitting the trigonal crystal field Hamiltonian to the experimental<br />
data allowed to determine the ambient pressure values <strong>of</strong> crystal field parameters, spin orbit<br />
parameter and their pressure coefficients.<br />
Acknowledgements: This work was partly supported by the European Union within<br />
European Regional Development Fund, through the Innovative Economy grant<br />
(POIG.01.01.02-00-108/2009/2) and the project No. N N202 203838 <strong>of</strong> Polish Ministry <strong>of</strong><br />
Science and Higher Education.<br />
References:<br />
[1] J. H. Park et al., J. Appl. Phys. 98, 056108 (2005).<br />
[2] W.M. Jadwisienczak et al., Opt. Mater. 23, 175 (2003),<br />
[3] T. Koubaa et al., J. Alloys Compd. 496, 56 (2010).<br />
[4] M. D. Segall et al., J. Phys.: Condens. Matter 14, 2717 (2002).<br />
73
MoP36<br />
_______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Studies <strong>of</strong> Electric Transport Properties <strong>of</strong> Single Layer Devices<br />
Based on the Polyazomethine Thin Films<br />
J. Weszka, M. Domański, J. Jurusik, B. Hajduk, M. Chwastek,<br />
and H. Bednarski<br />
Centre <strong>of</strong> Polymer and Carbon Materials, Polish Academy <strong>of</strong> Sciences, 34 M.<br />
Curie-Sk̷lodowska Str., 41-819 Zabrze, Poland<br />
Organic semiconductors (OS) have been known since a many years for their great potential<br />
applications in opto-electronic devices. Among the conjugated polymers, poly(pphenylenevinylene)<br />
(PPV) and its derivatives (like MEH-PPV) belong to the family<br />
<strong>of</strong> more extensively studied OSs and are known as revealing good properties for optoelectronic<br />
applications. Poly (p-phenyleneazomethine) (PPI), much less known and studied<br />
polymer than PPV, being its iso-elctronic counterpart and having nearly the same<br />
electronic structure, though containing nitrogen atoms in the backbone, has appeared to<br />
be very promising for optoelectronic applications, too [1,2].<br />
Single-layer organic devices have been produced based on chemical vapour deposited,<br />
in a homemade technological set-up [3]. Polyazomethine thin films were grown via polycondensation<br />
process <strong>of</strong> aromatic diamine and dialdehyde and sandwiched between metal<br />
electrodes with different work functions. The choice <strong>of</strong> the electrodes has appeared to<br />
influence greatly the device properties. For these reasons various pairs <strong>of</strong> electrode materials<br />
were used from ITO, Al and Au in order to check practically conditions <strong>of</strong> Schottky<br />
barrier formation and their applicability for photovoltaic devices. Metallic electrodes were<br />
thermally evaporated under vacuum <strong>of</strong> 10 −6 Tr, otherwise glass substrates covered with<br />
ITO layer were used.<br />
To verify quality <strong>of</strong> the as-prepared devices current-voltage characteristics have been<br />
taken as well as measurements <strong>of</strong> the current dependence on time were performed. Results<br />
have been analysed in terms <strong>of</strong> the unified device model for single layer organic light<br />
emitting diodes or photovoltaic cells with charge injection, transport and space charge<br />
effects [4]. The model describes both injection limited and space charge limited current<br />
flow and the transition between them. Determined by us relatively high value <strong>of</strong> the<br />
Schottky barrier height (higher then 0.5 eV) suggests that the current flow in the asprepared<br />
devices is injection charge limited, which is further supported by the net injected<br />
charge being relatively small. Therefore, for small bias voltage in the injection limited<br />
regime the thermionic emission is the dominant injection mechanism.<br />
Relatively low currents density, in comparison to those reported for PPV, recorded<br />
for our studied PPI based devices is attributed to the presence <strong>of</strong> surface states on the<br />
side <strong>of</strong> high work function electrode rather than to steric differences in the backbones <strong>of</strong><br />
the two polymers. This is consistent with the results we obtained on MEH-PPV devices<br />
prepared for comparison reasons.<br />
[1] A. Iwan and D. Sek, Prog. Polym. Sci. 33 (2008) 289-345.<br />
[2] J. Weszka, H. Bednarski and M.Domanski, J. Chem. Phys. 131 (2009) 024901.<br />
[3] J. Weszka, M. Domanski, B. Jarzabek, J. Jurusik, J. Cisowski, A. Burian, Thin<br />
Solid Films 516 (2008) 3098.<br />
[4] P.S. Davids, I.H. Campbell, and D.L. Smith, J. Appl. Phys. 82 (1997) 6319.<br />
74
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoP37<br />
Optical properties <strong>of</strong> black silicon with precipitated silver and gold<br />
nanoparticles<br />
R. Jarimavi i t -Žvalionien 1 , I. Prosy evas 1 , Ž. Kaminskien 1 , S. Lapinskas 2<br />
1 Institute <strong>of</strong> Materials Science, Kaunas University <strong>of</strong> Technology, Savanoriu pr. 271, Kaunas, Lithuania<br />
2 Institute <strong>of</strong> Applied Research, Vilnius University, Sauletekio str. 10, Kaunas, Lithuania<br />
Black silicon (BS) is a new generation material which may be useful for many<br />
applications, such as lasers, gas sensors, solar cells and etc. Black silicon (BS) is a type <strong>of</strong> porous<br />
silicon with low light reflectance and high light absorbance values. BS layers are especially<br />
useful and important in solar energy conversion. Forming BS layers on silicon increase the<br />
effective area and light absorbance as well as solar conversion efficiency. Another new way to<br />
increase solar energy conversion is use surface plasmons, i.e., collective surface oscillations <strong>of</strong><br />
conducting electrons in metal nanostructures that tend to trap optical waves near their surface.<br />
Plasmonics is a rapidly emerging subfield <strong>of</strong> nanotechnologies, which gives a possibility to<br />
control and guide light interaction with matter more effectively. Noble metals like silver and<br />
gold are ideal for this purpose as they do not have many interband transitions and do not absorb<br />
much light as a result. By combining black silicon with precipitated plasmonic nanoparticles in<br />
active layer has the potential to increase light interaction with matter more effectively and can<br />
revolutionary change the market <strong>of</strong> solar cells.<br />
Etching <strong>of</strong> silicon and formation <strong>of</strong> porous surfaces can be performed by many different<br />
methods. But the most simple in use is chemical metal assisted etching. In this work the porous<br />
silicon plates were prepared by wet chemical metal assisted method in a 1-minute single step<br />
liquid etch based upon catalysis by silver nanoparticles formed in a solution containing HF, V2O2<br />
and AgNO3. Processing time was reduced to 10 seconds by heating the samples at 80 o C. For<br />
synthesis <strong>of</strong> plasmonic nanoparticles silver nitrate (AgNO3) and chloroauric acid (HAuCl4) were<br />
used.<br />
Scanning electron microscope (SEM) analysis was done in order to evaluate the form and<br />
size <strong>of</strong> formed pores before precipitation <strong>of</strong> plasmonic nanoparticles and morphology <strong>of</strong> surface<br />
<strong>of</strong> porous silicon after precipitation. Optical properties <strong>of</strong> black silicon samples were investigated<br />
with UV-VIS and FTIR spectrometers and reflectance spectra were recorded during porous<br />
silicon formation at different times.<br />
SEM analysis <strong>of</strong> PS exhibited oval-shaped pores with Ø 150 – 200 nm and Ø 50 – 100<br />
nm in samples with longer and shorter processing times respectively. SEM analysis <strong>of</strong> BS with<br />
precipitated nanoparticles has revealed non-uniform distribution <strong>of</strong> gold and silver nanoparticles<br />
(Ø 70 - 150 nm) on BS surface and inside <strong>of</strong> pores. Reflectance analysis <strong>of</strong> BS has revealed that<br />
reflectance decreases with increase <strong>of</strong> processing time, herewith with pores depth, and increases<br />
after precipitation <strong>of</strong> nanoparticles in both cases. This phenomenon is still under investigation,<br />
but in general we can conclude, that formation <strong>of</strong> BS was successful and precipitation <strong>of</strong><br />
nanoparticles on its surface gives a possibility to control reflectivity <strong>of</strong> BS surfaces and increase<br />
photo efficiency through enhanced plasmonic properties <strong>of</strong> gold and silver nanoparticles.<br />
75
MoP38 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
���������������������������������������������������������������������������<br />
����������������������������������������������������������������<br />
�<br />
�����������������������������������������������������������������<br />
���������������������<br />
�<br />
����������������������������������������������������������������������������������������<br />
��������<br />
�<br />
������ ���������� ��� ���� ����� ��� �������������� ��� ������� ��� ��������� ���� ��������<br />
����������������������������������������������������������������������������������������<br />
�����������������������������������������������������������������������������������������������<br />
�����������������������������������������������������������������������������������������������<br />
������������ ���� ������������� ���� ������� ��� ������������ ������ ��� �������� �������������� ���<br />
�������������������������������������������������������������������������������������������<br />
��� ������������� ��������������� ������ ��� ���������� ������ ��� ��������� ������������� ������<br />
�������������������������������������������������������������������������������������������<br />
������ ���� ������� ��� ��������� ������� ������������ ���� �������� ����������� ��� ���� ������ ���<br />
����������������������������������������������������������������������������������������������<br />
�����������������������������������������������������<br />
������������������������� ��������������������������� �������������������������������������<br />
���� ������������� ��������� ����� ����������� ��� ���� ������ ��� ���� ������ ��� ����������� ����<br />
�����������������������������������������������������������������������������������������������<br />
���� ��� ����� ������������ ��� ���� ������������ ��������� ��� ����������� ���� ������� ��� ����<br />
�������������� � �������������������������������������������������������������������������������<br />
�����������������������������������������������������������������������������������������������<br />
��������������<br />
�������������������� �� �<br />
�������� �� �� �����<br />
����������������������������� �� ��� �� �� �����<br />
���������������������������������� �� ��� �� �� �����<br />
�������������������������������� �� ��� �� �� �����<br />
���������������������������������� �� ��� �� �� �����<br />
���������������������������������� ��� �� �� �� �����<br />
���������������������������������� �� ��� �� �� �����<br />
���������������������������������� �� ��� �� �� �����<br />
�������������������������������������������������������������������������������� � �������<br />
������������������������������������������������������������������������������������������������<br />
�������� ������������ ��� ���� ������������� ���������� ���� ������ ��� �������� ����� ���� �� �� � ����<br />
�������������������������������������������������������������� � ����������������<br />
��������������������������������������������������������������������������������������<br />
�������� �������������������������������������������������������������������������������������<br />
��������������������������������������������������<br />
��� ���� ��� ����� ����� ������ �� ����� ���� �������� ����� ���� ���� ������������� ���������<br />
���������������������������� �� ����������������������������������������������������������������������<br />
��������������������������������������������������������������������������������������������������<br />
���������������������������������������<br />
��� �������� ��� ���������� ������ ��� ���������� ����� ��� ����������� ���� �� ������� ��� ����<br />
����������� ����� ���� ������ ��������� ���� ���������� ��� �� �� ������ �������������� ���� �� �� ���� ���<br />
�������������������������������������������������������������<br />
76
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoP39<br />
Self-Compensation Mechanism in Semi-Insulating CdMnTe:Sn Crystals<br />
Intended for X/γ-Ray Detectors<br />
N.S. Yurtsenyuk, L.A. Kosyachenko, V.M. Sklyarchuk,<br />
O.L. Maslyanchuk, E.V. Grushko<br />
Chernivtsi National University, Optoelectronics Department, 58012 Chernivtsi, Ukraine<br />
Cd1-xMnxTe is a typical diluted magnetic semiconductor with the zinc-blende structure<br />
within the 0 < x < 0.77 range. Owing to its unique magnetic and magneto-optic properties,<br />
Cd1-xMnxTe crystals can be used in many device applications, such as Faraday rotators,<br />
optical isolators, solar cells, magnetic field sensors and substrates for the epitaxial growth <strong>of</strong><br />
Hg1-xCdxTe and Hg1-xZnxTe [1]. More recently, Cd1-xMnxTe <strong>of</strong>fers several potential<br />
advantages over Cd1-xZnxTe as a material for room-temperature X/γ-ray detectors [2,3] that<br />
are widely used in science, medicine, industry, environment monitoring and other fields.<br />
Cd1-xMnxTe has a wide range <strong>of</strong> tunability <strong>of</strong> the band gap due to the large compositional<br />
influence <strong>of</strong> manganese. The band gap range 1.7 eV to 2.2 eV, required for substantiating an<br />
“ideal” radiation detector performance, can be realized with a relatively low (< 50%) amount<br />
<strong>of</strong> Mn compared with that <strong>of</strong> Zn in Cd1-xZnxTe (up to 70%) [4]. In fact, the energy band gap<br />
<strong>of</strong> Cd1-xMnxTe increases about 13 meV per atomic percent <strong>of</strong> Mn compared with 6.7 meV <strong>of</strong><br />
Zn in Cd1-xZnxTe. Another important advantage is that the segregation coefficient <strong>of</strong> Mn in<br />
CdTe is approximately 1 compared with 1.35 for Zn, that is, Cd1-xMnxTe crystals can have a<br />
more homogenous distribution <strong>of</strong> Mn throughout the ingot. These notable characteristics <strong>of</strong><br />
Cd1-xMnxTe increase the possibility <strong>of</strong> growing uniform large-volume stoichiometric crystals<br />
that can potentially be fabricated into different high-quality devices.<br />
In this work we study the conductivity mechanism <strong>of</strong> Cd1-xMnxTe single crystals doped<br />
by Sn. It is known that the introduction <strong>of</strong> Sn in the CdTe, as well as in other II-VI<br />
semiconductors, accompanied by the formation <strong>of</strong> complexes and self-compensation <strong>of</strong><br />
conductivity and leads to a semi-insulating state <strong>of</strong> the semiconductor, which is very<br />
important from the standpoint <strong>of</strong> its use in X/γ-ray detectors. We set a goal to find the<br />
ionization energy <strong>of</strong> the level responsible on electrical conductivity Ei <strong>of</strong> the material and the<br />
degree <strong>of</strong> its compensation ξ = Na/Nd. The band gap and its temperature dependence<br />
(necessary for calculations) were found from the optical transmission curves in the range <strong>of</strong><br />
the absorption edge <strong>of</strong> semiconductor. Based on the experimental dependences <strong>of</strong> the<br />
electrical characteristics <strong>of</strong> the material and statistics <strong>of</strong> electrons and holes, we have<br />
succeeded in determining Ei and ξ. Such a degree <strong>of</strong> compensation reveals to be close to<br />
100%. This is a direct confirmation <strong>of</strong> self-compensation, which provides semi-insulating<br />
state <strong>of</strong> the semiconductor. It seems that the obtained results lead to the conclusions which are<br />
important for technology.<br />
[1] J.K. Furdyna, J. Appl. Phys. 64(4), R29 (1988).<br />
[2] A. Burger, K. Chattopadhyay, H. Chen, J.-O. Ndap, X. Ma, S. Trivedi, S.-W. Kutcher, R. Chen,<br />
and R.-D. Rosemeier. J. Cryst. Growth 198/199, 872 (1999).<br />
[3] A. Mycielski, A. Burger,M. Sowinska, M. Groza, A. Szadkowski, P. Wojnar, B.Witkowska,<br />
W.Kaliszek, and P. Siffert, Phys. Stat. Sol. (c) 2, 1578 (2005).<br />
[4] J.E. Toney, T.E. Schlesinger, and R.B. James, Nucl. Instr. & Meth., A428, 14 (1999).<br />
77
MoP40 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Influence <strong>of</strong> Cu, Ga and Au Dopants and Technology Conditions on the<br />
Magnetic Interactions in HgCr2Se4 Single Crystals<br />
J. Krok-Kowalski 1 , G. Władarz 1 , T. Groń 1 , H. Duda 1 , A.W. Pacyna 2 , K. Nikiforov 3 and<br />
P. Rduch 1<br />
1 University <strong>of</strong> Silesia, Institute <strong>of</strong> Physics, ul. Uniwersytecka 4, 40-007, Katowice, Poland<br />
2 The Henryk Niewodniczański Institute <strong>of</strong> Nuclear Physics, Polish Academy <strong>of</strong> Sciences,<br />
ul. Radzikowskiego 152, 31-342 Kraków, Poland<br />
3 Physico-Mathematical Faculty, K. Tsiolkovski State Pedagogical University,<br />
St. Razin str. 26, Kaluga 248023, Russia<br />
It is well known, that in the ferromagnetic spinel compound HgCr2Se4 (where TC and<br />
CW are <strong>of</strong> order 100 K and 150 K respectively) the divalent Hg 2+ ions occupy tetrahedral<br />
positions only, whereas trivalent Cr 3+ ions are located only in octahedral positions.<br />
Ferromagnetic coupling <strong>of</strong> the magnetic moments in this compound is realized via<br />
superexchange magnetic interactions [1]. It is also well known, that the full substitution in the<br />
tetrahedral positions <strong>of</strong> the divalent Hg 2+ ions by the monovalent, e.g. Cu + ions, leads to both<br />
the mixed valence <strong>of</strong> the chromium ions (it means, Cr 3+ and Cr 4+ in the ratio 1:1) and to the<br />
appearance and domination <strong>of</strong> the another mechanism <strong>of</strong> the magnetic coupling giving very<br />
strong ferromagnetic coupling, namely double exchange interaction. In this case both TC and<br />
CW become <strong>of</strong> order <strong>of</strong> 400 K [2,3]. However, the substitution <strong>of</strong> the Hg 2+ ions by the<br />
trivalent ions, e.g. Ga 3+ , leads to the collinear antiferromagnetism being the result <strong>of</strong> the<br />
superexchange interaction only, with the concentration <strong>of</strong> Ga 3+ ions up to 50%. The one aim<br />
<strong>of</strong> this work is to investigate how influences the substitution <strong>of</strong> Hg 2+ ions by Cu + , Au 2+ and<br />
Ga 3+ ions on the magnetic ordering in the sample, when the concentration <strong>of</strong> the substituting<br />
ions is lower than 10 %. The another one concerns <strong>of</strong> the influence <strong>of</strong> technology conditions<br />
on magnetic ordering in the sample.<br />
Using the chemical transport method the single crystals <strong>of</strong> HgCr2Se4 doped with copper<br />
(7%), gallium (8%) and gold (2%) were obtained. Additionally, the two non doped single<br />
crystals <strong>of</strong> HgCr2Se4 were obtained, in different synthesis conditions. One <strong>of</strong> them was<br />
synthesized with the deficiency <strong>of</strong> Hg (this procedure the produces vacancies), another one<br />
with Hg excess (this procedure brings to elimination <strong>of</strong> vacancies). For all crystals under<br />
study both the dc and ac magnetic susceptibility was measured in the temperature range <strong>of</strong><br />
4.2 – 250 K. The dc measurements were carried out with the use <strong>of</strong> the Cahn magnetic<br />
balance and SQUID susceptometer, whereas the ac measurements were carried out with the<br />
use <strong>of</strong> the Lake Shore 7225 magnetometer.<br />
As it follows from the obtained results, substitution monovalent Cu + ions and divalent<br />
Au 2+ ions in place <strong>of</strong> Hg 2+ ions causes the increase <strong>of</strong> the Curie temperature in comparison<br />
with the Curie temperature <strong>of</strong> undoped HgCr2Se4. Substitution <strong>of</strong> Hg 2+ ions by trivalent Ga 3+<br />
ions causes the lowering <strong>of</strong> TC. On the other hand, ZFC and FC measurements <strong>of</strong> the<br />
magnetic susceptibility shows that in the samples containing Cu + and Ga 3+ some trace<br />
frustration <strong>of</strong> magnetic interactions in spite <strong>of</strong> their ferromagnetic ordering occurs. Similar<br />
effect have been observed for single crystal synthesized with the deficiency <strong>of</strong> Hg. The latter<br />
effect is caused by the occurrence <strong>of</strong> vacancies generated in the sample by the technological<br />
process.<br />
[1] P.K. Baltzer, P.J. Wojtowicz, M. Robbins, and E. Lopatin, Phys. Rev. 151, 367 (1966).<br />
[2] F.K. Lotgering, Proc. Int. Conf. on Magnetism, Nottingham England, (1964) 533.<br />
[3] J. Krok, J. Spałek, J. Juszczyk, and J. Warczewski, Phys. Rev. B 28, 6499 (1983).<br />
78
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoP41<br />
The Photodetectors with Vertical Integration Based on ZnO Films<br />
A.I. Ievtushenko 1 , G.V. Lashkarev 1 , V.I. Lazorenko 1 , Zs.J. Horvath 2,3 , M.G. Dusheyko 4<br />
1 Frantsevich Institute for Problems <strong>of</strong> Materials Science, NASU, Kyiv, Ukraine<br />
2 Research Institute for Technical Physics and Materials Science, HAS, Budapest, Hungary<br />
3 Institute <strong>of</strong> Microelectronics and Technology, Budapest, Hungary<br />
4 National Technical University <strong>of</strong> Ukraine “KPI”, Kyiv, Ukraine<br />
ZnO is a wide-band (Eg 3.3 eV) direct-gap material which attracts a great attention to<br />
the development <strong>of</strong> the photodetectors for near-ultraviolet range. It is an electronic analog <strong>of</strong><br />
GaN, but ZnO have economic and ecological benefits in comparison with GaN. There are<br />
many papers devoted to the formation detectors <strong>of</strong> a planar configuration [1]. One <strong>of</strong> the<br />
promising directions <strong>of</strong> microelectronics is the development and study <strong>of</strong> devices with a<br />
vertical structure. The reason is that in this vertical configuration <strong>of</strong> electrodes the<br />
interelectrode distance can be easily reduced without using expensive lithography processes.<br />
Therefore, the purpose <strong>of</strong> this report is devoted to the study <strong>of</strong> technological aspects for the<br />
deposition <strong>of</strong> ZnO films as well as the investigation <strong>of</strong> the photoelectrical properties <strong>of</strong><br />
structures with vertical configuration <strong>of</strong> the electrodes.<br />
Early we reported the positive role <strong>of</strong> nitrogen in ZnO for UV detectors [2]. Therefore,<br />
the undoped and nitrogen doped ZnO films with different concentrations <strong>of</strong> the nitrogen were<br />
deposited by magnetron sputtering on n-Si and ITO/glass substrates. Structural and optical<br />
properties <strong>of</strong> the films were examined by XRD, EDX and optical measurements. Aluminum<br />
as ohmic contact to Si and Ni as Schottky contact to ZnO were deposited by electron beam<br />
evaporation with thickness control. Ni/ZnO/n-Si/Al and Ni/ZnO/ITO layered structures <strong>of</strong><br />
photodetectors were obtained. Current-voltage curves <strong>of</strong> photodiodes were studied in dark and<br />
under visible and UV irradiations using Keithley-236 Source Measure Unit. Features <strong>of</strong> the<br />
current-voltage characteristics and leakage current mechanisms are discussed. It was found an<br />
increase <strong>of</strong> photosensitivity with increasing nitrogen concentration in nitrogen doped ZnO<br />
films for all types <strong>of</strong> vertical structures. The maximum enhancement <strong>of</strong> reverse current under<br />
UV irradiation was two orders <strong>of</strong> magnitude. The influence <strong>of</strong> deposition technology <strong>of</strong> ZnO<br />
films on the photoelectrical properties <strong>of</strong> detectors as well as the effect <strong>of</strong> nitrogen<br />
incorporation will be presented. Prospects for the application <strong>of</strong> ZnO photodetectors with<br />
vertical integration and requirements for their design will be discussed.<br />
[1] K. Liu, M. Sakurai, M. Aono Sensors, 10, pp.8604-8634 (2010).<br />
[2] A.I. Ievtushenko, G.V. Lashkarev, V.I. Lazorenko, et al. Phys. Stat. Sol. ( ), 207, 7, pp.<br />
1746–1750 (2010).<br />
79
MoP42 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Features <strong>of</strong> the Properties for Nitrogen Doping and Al-N Codoping <strong>of</strong> ZnO<br />
Films<br />
A.I. Ievtushenko 1 , G.V. Lashkarev 1 , V.I. Lazorenko 1 , O.Y. Khyzhun 1 , L.O. Klochkov 1 ,<br />
O.I. Bykov 1 , V.M. Tkach 2 , V.A. Baturin 3 , A.Y. Karpenko 3<br />
1 Frantsevich Institute for Problems <strong>of</strong> Materials Science, NASU, Kyiv, Ukraine<br />
2 Bakul Institute for Superhard Materials, Kyiv, Ukraine<br />
3 Institute <strong>of</strong> Applied Physics, Sumy, Ukraine<br />
Now zinc oxide is almost the most investigated material in the world because <strong>of</strong> its<br />
physical, ecological and economic benefits which make it promising one for photonic,<br />
optoelectronic and acoustooptic devices, etc. However the properties <strong>of</strong> ZnO films strongly<br />
depend on the concentration <strong>of</strong> various defects appearing at the stages <strong>of</strong> film growth and<br />
further processes in as-grown ZnO films.<br />
The nitrogen in ZnO is an acceptor impurity and widely used in attempts to obtain<br />
stable p-type conductivity in ZnO. However, based on our previous studies, we think that<br />
nitrogen doping leads to the passivation <strong>of</strong> oxygen vacancies <strong>of</strong> ZnO and therefore increase<br />
the photosensitivity, response speed and stability <strong>of</strong> photodetectors on their basis. Little<br />
known fact is the effect <strong>of</strong> increasing the concentration <strong>of</strong> nitrogen in ZnO films on their<br />
physical properties by using the increase <strong>of</strong> nitrogen pressure in the chamber at magnetron<br />
deposition. It is promising to investigate the way <strong>of</strong> increasing <strong>of</strong> nitrogen solubility in ZnO<br />
by using Al-N codoping.<br />
ZnO:N films with different nitrogen concentration were deposited on Si, SiNx and<br />
quartz substrates by magnetron sputtering using Zn, Zn:0.7%Al and Zn:1.4%Al targets. The<br />
investigations <strong>of</strong> XRD, AFM, PL, X-ray photoelectronic and EDX spectroscopy were carried<br />
out. In this report the influence <strong>of</strong> different types <strong>of</strong> nitrogen doping on the transformation <strong>of</strong><br />
the structure, optical properties and morphology <strong>of</strong> ZnO:N films will be presented.<br />
80
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoP43<br />
Influence <strong>of</strong> Oxygen Pressure on the Properties <strong>of</strong> AZO Films<br />
A.I. Ievtushenko 1 , G.V. Lashkarev 1 , V.I. Lazorenko 1 , L.O. Klochkov 1 , O.I. Bykov 1 ,<br />
O.M. Kutsay 2 , S.P. Starik 2 , V.A. Baturin 3 , A.Y. Karpenko 3<br />
1 Frantsevich Institute for Problems <strong>of</strong> Materials Science, NASU, Kyiv, Ukraine<br />
2 Bakul Institute for Superhard Materials, Kyiv, Ukraine<br />
3 Institute <strong>of</strong> Applied Physics, Sumy, Ukraine<br />
New oxide materials are very attractive for replacement <strong>of</strong> existing traditional ones in<br />
various fields <strong>of</strong> electronics and optoelectronics. Particularly the problems <strong>of</strong> creation the<br />
transparent conductive electrodes based on them attract more attention.<br />
The major drawback <strong>of</strong> widely-used ITO is a limited world reserve <strong>of</strong> indium.<br />
Opposite to ITO, zinc oxide has ecological and economical benefits and its components are<br />
widely distributed in Earth. Therefore, doped with aluminum (AZO), gallium (GZO) or<br />
indium (IZO) ZnO films are the best suited candidates to replace ITO. The development <strong>of</strong><br />
technology for deposition <strong>of</strong> transparent conductive electrodes based on AZO films will have<br />
the commercial future.<br />
Therefore the high quality aluminum doped ZnO films were deposited on glass<br />
substrates by layer by layer deposition method at a magnetron sputtering <strong>of</strong> Zn:1.4%Al target.<br />
The report presents the results <strong>of</strong> oxygen pressure influence on structural, morphological,<br />
electrical and optical properties <strong>of</strong> AZO films. It is demonstrated that optimal oxygen pressure<br />
<strong>of</strong> 0.03 Pa in the deposition chamber leads to the following AZO films characteristics: the<br />
resistivity is about 10 -3 Ohm cm, transparency - 95% and roughness - 6 nm. Our results on<br />
investigation <strong>of</strong> AZO films magnetron deposition technology will be discussed as well as the<br />
prospects <strong>of</strong> their applications.<br />
81
MoP44 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Thermoelectric Power <strong>of</strong> CuCrxVySe4 p-Type Spinel Semiconductors<br />
H. Duda 1 , E. Malicka 2 , T. Groń 1 , A. Gągor 3 , R. Sitko 2 , J. Krok-Kowalski 1 and P. Rduch 1<br />
1 University <strong>of</strong> Silesia, Institute <strong>of</strong> Physics, ul. Uniwersytecka 4, 40-007 Katowice, Poland<br />
2 University <strong>of</strong> Silesia, Institute <strong>of</strong> Chemistry, ul. Szkolna 9, 40-006 Katowice, Poland<br />
3 Institute <strong>of</strong> Low Temperature and Structure Research, Polish Academy <strong>of</strong> Sciences,<br />
ul. Okólna 2, 50-950 Wrocław, Poland<br />
Detailed studies <strong>of</strong> the thermoelectric power, electrical conductivity, structural and<br />
magnetic properties carried out on polycrystalline CuCrxVySe4 spinels with x = 1.79, 1.64 and<br />
1.49 and y = 0.08, 0.22 and 0.45, respectively, are presented. Powder samples <strong>of</strong> the<br />
CuCrxVySe4 spinel series were prepared by annealing stoichiometric mixtures <strong>of</strong> high purity<br />
( 99.99 %) elements: Cu, Cr, V and Se. A precise atomic content determination <strong>of</strong> each<br />
sample with the aid <strong>of</strong> the energy-dispersive X-ray spectrometry (EDXRF) showed that the<br />
polycrystals <strong>of</strong> the CuCrxVySe4 spinel system revealed the cation deficiencies in the<br />
octahedral sites because x + y<br />
y<br />
2. A Rietveld refinement <strong>of</strong> the spinels under study was done<br />
using an X'Pert PRO X-ray diffractometer and a Jana-2000 program package. The electrical<br />
conductivity has been measured with the four-probe dc method using the apparatus with<br />
Keithley K181 digital multimeters. The thermoelectric power was measured with a<br />
differential method using the temperature gradient T <strong>of</strong> about 2 K. The electrical and thermal<br />
contacts between the sample and the copper rods were maintained with a silver lacquer<br />
mixture (Degussa Leitsilber 200). Magnetization and magnetic susceptibility were performed<br />
using a Faraday type Cahn RG automatic electrobalance at magnetic field up to 7 kOe. The<br />
electrical and magnetic measurements were done in the temperature ranges 77-432 K and 77-<br />
550 K, respectively.<br />
All the compounds under study crystallize in regular system <strong>of</strong> a normal spinel type<br />
MgAl2O4 structure with the space group symmetry Fd3m. With increasing V content a lattice<br />
parameter slightly increases from 1033.66(4) pm for y = 0.08 to 1033.71 (5) for y = 0.45<br />
while the positional anion parameter decreases from 0.2574(3) for y = 0.08 to 0.2568(3) for y<br />
= 0.45. The CuCrxVySe4 spinels are ferromagnets for that a Curie temperature TC and a<br />
paramagnetic Curie-Weiss temperature decrease as the V content increases, i.e. from TC =<br />
406 K and = 383 K for y = 0.08, via TC = 351 K and = 380 K for y = 0.22 via to TC = 282<br />
K and = 277 K for y = 0.45. The electrical conductivity measurements revealed the change<br />
<strong>of</strong> the hole conductivity character from the semiconductive into the metallic one close to TC.<br />
The thermoelectric power analysis was done with the aid <strong>of</strong> the modified Matoba, Anzai and<br />
Fujimori semi-empirical formula [1] including magnetic contribution [2]. The results <strong>of</strong><br />
thermopower analysis <strong>of</strong> the spinels under study show that the intensity <strong>of</strong> the magnon drag<br />
component is smaller in comparison with the single crystal sample <strong>of</strong> comparable V-content<br />
[2]. The magnon excitations, usually driven by the double exchange mechanism, are observed<br />
only at low temperatures. At higher temperatures the contribution to total thermopower<br />
originates mainly from the phonon component. At high temperatures, the diffusion component<br />
dominates and, according to the Mott formula, it is proportional to temperature. The non-zero<br />
value <strong>of</strong> the so-called impurity component <strong>of</strong> the thermopower described by the T 1/2 law<br />
suggests the presence <strong>of</strong> cation deficiencies in the octahedral sites.<br />
This work is partly founded from science Grant No. N N204 145938.<br />
[1] M. Matoba, S. Anzai, and A. Fujimori, J. Phys. Soc. Jpn. 63, 1429 (1994).<br />
[2] E. Malicka, T. Groń, D. Skrzypek, A.W. Pacyna, D. Badurski, A. Waśkowska, S. Mazur,<br />
and R. Sitko, Philos. Mag. 90, 1525 (2010).<br />
82
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoP45<br />
Critical Behaviour <strong>of</strong> the 3D-Heisenberg Ferromagnetic<br />
Semiconductors CdxCeyCr2Se4<br />
H. Duda 1 , P. Rduch 1 , E. Malicka 2 , T. Groń 1 and A. Gągor 3<br />
1 University <strong>of</strong> Silesia, Institute <strong>of</strong> Physics, ul. Uniwersytecka 4, 40-007 Katowice, Poland<br />
2 University <strong>of</strong> Silesia, Institute <strong>of</strong> Chemistry, ul. Szkolna 9, 40-006 Katowice, Poland<br />
3 Institute <strong>of</strong> Low Temperature and Structure Research, Polish Academy <strong>of</strong> Sciences,<br />
ul. Okólna 2, 50-950 Wrocław, Poland<br />
The pure CdCr2Se4 end member <strong>of</strong> the CdxCeyCr2Se4 spinel system combines the p-type<br />
semiconducting and ferromagnetic properties with a Curie temperature TC = 142 K and a<br />
Curie-Weiss temperature qCW = 190 K [1]. Magnetization <strong>of</strong> CdCr2Se4 reaches the full<br />
saturation <strong>of</strong> 5.98 µB per molecule [2] and the ferromagnetic properties are the result <strong>of</strong><br />
dominance <strong>of</strong> large and positive the nearest-neighbour Cr-Cr interactions [3]. Various<br />
theoretical models <strong>of</strong> the critical phenomena, the mean-field, the three-dimensional (3D)<br />
Heisenberg, the 3D-Ising and the tricritical mean-field model used to explain the critical<br />
properties showed that the CdCr2Se4 was in best accordance with the 3D-Heisenberg model<br />
for which the critical exponents: β = 0.36, γ = 1.38 and δ = 4.8 [4,5].<br />
The CdxCeyCr2Se4 spinel single crystals under study have a normal cation distribution,<br />
the Cd and Ce ions being located at the tetrahedral sites, while the Cr ions at the octahedral<br />
ones. Electrical and magnetic measurements revealed the semiconducting properties and a<br />
ferromagnetic order with TC = 132 K which insensibly decreases as the Ce content increases<br />
as well as a strong reduction <strong>of</strong> magnetic moment from 5.8 µB for y = 0.03 to 1.93 µB for y =<br />
0.13. The critical exponents <strong>of</strong> CdxCeyCr2Se4 single crystals were studied by measuring<br />
isothermal dc-magnetization around the paramagnetic-ferromagnetic (PM-FM) phase<br />
transition. Isothermal magnetization was measure with the aid <strong>of</strong> a Quantum Design System<br />
(MPMS XL) in the magnetic field up to 20 kOe and in the temperature range 125-140 K.<br />
The critical exponents (TC, β, γ and δ) were determined by analyzing the magnetization<br />
data in terms <strong>of</strong> various research techniques including modified Arrott plot [6], Kouvel-Fisher<br />
method [7], and critical isotherm analysis. Here β and γ are the critical exponents for the<br />
temperature dependence <strong>of</strong> the spontaneous magnetization just below TC and inverse initial<br />
susceptibility just above TC, respectively. In addition, the critical exponent δ is determined<br />
separately from the isothermal magnetization at TC. The critical exponent values are found to<br />
be consistent with the Widom scaling relation δ = 1 + γ/β, implying the critical exponents are<br />
reliable. The critical exponents characterizing the PM-FM transition are: β = 0.367 ±0.013, γ<br />
= 1.389 ±0.025 and δ = 4.763 ±0.043 at TC = 131 K for the sample Cd0.96Ce0.03Cr2Se4 and β =<br />
0.350 ±0.003, γ = 1.202 ±0.026 and δ = 4.487 ±0.095 at TC = 134 K for the sample<br />
Cd0.84Ce0.13Cr2Se4. These values <strong>of</strong> critical exponents concerning CdxCeyCr2Se4 system are<br />
close to the theoretical ones predicted by the 3D-Heisenberg model for CdCr2Se4.<br />
This work is partly founded from science Grant No. N N204 145938.<br />
[1] P.K. Baltzer, H.W. Lehmann, and M. Robbins, Phys. Rev. Lett. 15, 493 (1965).<br />
[2] R.C. LeCraw, H. von Philipsborn, and M.D. Sturge, J. Appl. Phys. 38, 965 (1967).<br />
[3] P.K. Baltzer, M. Robbins, and P.J. Wojtowicz, J. Appl. Phys. 38, 953 (1967).<br />
[4] L. Zhang, J. Fan, L. Li, R. Li, L. Ling, Z. Qu, W. Tong, S. Tan, and Y. Zhang, A Letters<br />
Journal Exploring the Frontiers <strong>of</strong> Physics (EPL) 91, 57001 (2010).<br />
[5] W. Zarek, Acta. Phys. Pol. A 52, 657 (1977).<br />
[6] A. Arrott, and J.E. Noakes, Phys. Rev. Lett. 19, 786 (1967).<br />
[7] J.S. Kouvel and M.E. Fisher, Phys. Rev. Lett. 136, A1626 (1964).<br />
83
MoP46 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Optical properties <strong>of</strong> ZnO/ZnMgO single quantum wells grown by<br />
molecular beam epitaxy<br />
J.M. Sajkowski, M.A. Pietrzyk, D. Dobosz, M. Stachowicz, A. Droba, E.Przezdziecka,<br />
A.Wierzbicka, A. Kozanecki,<br />
Institute <strong>of</strong> Physics Polish Academy <strong>of</strong> Sciences, Al. Lotników 32/46 02-668, Warsaw, Poland<br />
In this work the high quality ZnMgO layers and ZnO/ZnMgO quantum wells (QWs) showing<br />
no phase separation were grown on (0001) sapphire substrates. ZnMgO layers and<br />
ZnO/ZnMgO heterostructures were grown at temperatures 445 - 460°C. No specific buffer<br />
layers were deposited on sapphire. Prior to growth the substrates were annealed in the growth<br />
chamber in oxygen (p=1,4*10 -5 Torr) at 600° C, and then a ~100 nm ZnMgO (Mg contents 10<br />
- 20%) barrier layer was grown. Then the growth was interrupted for couple <strong>of</strong> minutes and<br />
after this a ZnO wells with different thicknesses were grown. Finally, ZnMgO capping layers<br />
<strong>of</strong> the same composition as the ZnMgO barrier were deposited. After growth the structures<br />
were characterized using X-ray diffraction for the analysis <strong>of</strong> their crystalline quality and<br />
AFM was used for surface morphology studies. AFM images revealed very good flatness <strong>of</strong><br />
the surface – rms value was 1-2 nm.<br />
We present results <strong>of</strong> optical spectroscopy for a series <strong>of</strong> ZnO/ZnMgO single quantum wells<br />
<strong>of</strong> different widths using photoluminescence. Temperature dependent PL spectra were<br />
measured within a temperature range <strong>of</strong> 4-300K. We observed a blue shift <strong>of</strong> excitonic<br />
emission in comparison with bulk ZnO values which demonstrates quantum confinement in<br />
the ZnO wells with thicknesses smaller than 4 nm. We also demonstrate that excitonic<br />
emission from QWs is dominated by excitons bound to donors. The characteristic energy <strong>of</strong><br />
PL decay <strong>of</strong> the dominant excitonic lines agrees well with the values <strong>of</strong> localization energies<br />
<strong>of</strong> excitons to donor centers. For wider ZnO wells the quantum confined Stark effect shifts the<br />
PL spectra below the band gap <strong>of</strong> ZnO. This is a direct consequence <strong>of</strong> the very large built-in<br />
electric field developed in ZnO/ZnMgO QWs grown along the polar c-direction.<br />
Acknowledgement: Work supported in part within European Regional Development Fund,<br />
through grant Innovative Economy (POIG.01.01.02-00-008/08).<br />
84
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoP47<br />
Electronic structure <strong>of</strong> CdTe/PbEuTe/CdTe<br />
B.A. Orlowski 1 , S. P. Dziawa 1 , A. Reszka 1 , K. Gas 1 , S. Mickievicius 2 , S. Thiess 3 ,<br />
W. Drube 3 .<br />
! " # $<br />
$ % &<br />
' ( $) $ $ $ $ ) ( $ ' * + , - $ ! $ $<br />
,- * . $ "/ ,! ' ( $) % $<br />
Top part <strong>of</strong> the semiconductor nanostructure CdTe/Pb0.95Eu 0.05Te/CdTe with Pb0.95Eu 0.05Te<br />
buried under thin layer <strong>of</strong> CdTe (the layer transparent for part <strong>of</strong> photoemitted electrons) was<br />
studied with application <strong>of</strong> High Energy X-ray Photoemission Spectroscopy. The Energy<br />
Distribution Curves (EDC) corresponding to the valence band and core levels electrons <strong>of</strong> the<br />
buried layer and top cover CdTe layer were measured with application <strong>of</strong> synchrotron<br />
radiation <strong>of</strong> high energy h = 3510eV to obtain big escape depth <strong>of</strong> electrons photoemitted<br />
from buried layer. The measured peaks corresponding to the buried layer atoms were<br />
observed in the valence band region and in the high binding energy region for core levels<br />
Pb4f, Pb3d and Eu 3d. For top cover layer the contribution <strong>of</strong> valence band, Cd4d and<br />
Cd3d core levels were obtained. Measured Te4d, Te 3d and Te4d spectra possesses<br />
contribution as well from buried as from the top cover layer.<br />
The nanostructure was grown by MBE deposition method in the Institute <strong>of</strong> Physics, Polish<br />
Academy <strong>of</strong> Sciences in Warsaw [1]. The CdTe buffer layer was evaporated on GaAs wafer<br />
and Pb0.95Eu0.05Te layer <strong>of</strong> 6nm thick, evaporated on it. The layer <strong>of</strong> Pb0.95Eu 0.05Te was<br />
covered by CdTe layer <strong>of</strong> thickness 22 nm. The structure was exposed to the air and<br />
photoemission spectra were performed ex situ using the Tunable High Energy X-ray<br />
Photoemission Spectrometer at HASYLAB, Hamburg [2]. After sputtering <strong>of</strong> CdTe top layer<br />
the photoemission signal <strong>of</strong> buried layer elements remarkably increases (see Fig.1, Pb5d).<br />
Main parameters <strong>of</strong> the electronic structure <strong>of</strong> CdTe/Pb0.95Eu 0.05Te/CdTe were determined.<br />
Intensity [a. u.]<br />
1 . 4<br />
1 . 2<br />
1 . 0<br />
0 . 8<br />
0 . 6<br />
0 . 4<br />
0 . 2<br />
0 . 0<br />
C d T e / P b E u T e / C d T e<br />
H I G H E N E R G Y X P S<br />
h ν = 3 5 1 0 e V<br />
S p u t t e r i n g<br />
2 n d<br />
1 s t<br />
n o<br />
v b<br />
C d 4 d<br />
P b 5 d<br />
T e 4 d<br />
- 1 0 0 1 0 2 0 3 0 4 0 5 0<br />
B i n d i n g E n e r g y [ e V ]<br />
Fig.2. Set <strong>of</strong> EDC’s <strong>of</strong> the valence band, Cd4d, Pb5d and Te4d electrons region and measured after following<br />
steps <strong>of</strong> cleaning <strong>of</strong> the sample by sputtering CdTe 22nm top layer.<br />
The authors acknowledge support by MSHE <strong>of</strong> Poland research Projects DESY/68/2007 and by the European<br />
Community FP6 Programme "Integrating Activity on Synchrotron and Free Electron Laser Science" at DESY.<br />
1. M. Szot, L. Kowalczyk, E. Smajek, V. Domukhovski, J. Domagała, E. Łusakowska, B. Taliashvili,<br />
P. Dziawa, W. Kn<strong>of</strong>f, W. Wiater, T. Wojtowicz, T. Story, Acta Phys. Pol. 114, 1397 (2009).<br />
2. Th. Eickh<strong>of</strong>f, V. Medicherla, W. Drube, Jour.Electr. Spectr.Rel. Phen. 137-140, 85-88 (2004)<br />
orbro@ifpan.edu.pl<br />
85<br />
1 7<br />
0 4<br />
0 2
MoP48 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
!" # $<br />
%& ' ()* + * , -& & - , & - . * , ! / 0 ! /<br />
! ! " ! " "<br />
!"# !<br />
$ % & '#( % )* +'#(,)<br />
- ./<br />
0 1. 0 ) 2 * ( *<br />
3 4 )5<br />
- 6 % & '#( % )* +'#(,) 3 )<br />
% & + 3,*3'#( +3 7 / / / /) / /2 / /. / /1 / / 2<br />
/ , 8<br />
9 +9$:,<br />
% & '#( % )* +'#(,) !"#<br />
- $<br />
))2 +: , 2;; +< , &<br />
- % & '#(<br />
-<br />
= =) =( > 6 ? -<br />
= + , % & '#( $ - % )* +'#(,)<br />
$ !"#<br />
;.. ! % & '#(<br />
% )* +'#(,) 3 $<br />
% & '#( % )* +'#(,) 3 )<br />
% & + 3,*3'#( !<br />
!"# % )* +'#(,)<br />
- $<br />
- "<br />
4 5 " * $ @ & < " @ A B C #$ % 123<br />
222 2D( + //1,<br />
4 5 @ E & : " * @ $ & 45 D(; D22 + / ,<br />
4)5 A F & #$ % 164 1)1 12( + / /,<br />
86
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoP49<br />
Multi-interface layered P-doped silicon structures for third generation<br />
photovoltaics<br />
Mikael Hosatte 1 , Marek Basta 1 , Bartłomiej S. Witkowski 2 , Zbigniew T. Kuznicki<br />
1 and Marek Godlewski 2<br />
1 Photonic Systems Laboratory, Pole Api, Blvd. Sebastien Brandt, 67414 Illkirch-<br />
Graffenstaden, France<br />
2 Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences, Al. Lotnikow 32/46, 02-668 Warszawa,<br />
Poland<br />
Nanoscale Si-layered systems represent an attractive way to enlarge optical and electrical<br />
functions in Si optoelectronic, photonic and PV technology. Physical interactions transform<br />
the initial Si material to a new Si-based metamaterial. The device architecture also plays a<br />
role in specific nonlinear features. The seemingly paradoxical behavior requires a better<br />
insight into understanding the mechanisms determining the macroscopic performance. We<br />
report here some specific optical and electrical properties resulting from the complexity <strong>of</strong><br />
different test structures. Optoelectronic properties <strong>of</strong> nanostructured materials can differ<br />
greatly comparing to those <strong>of</strong> bulk, or even thin film. The complexity <strong>of</strong> low-dimensional<br />
interactions renders impossible to directly use the description <strong>of</strong> the phenomena developed for<br />
previous device generations [1]. This is particularly true for Multi-interface Novel Devices<br />
(MINDs) which results mainly from the superposition <strong>of</strong> a PN junction [2], a nanoscale<br />
system [3] and a multi-interface architecture. In MINDs, whose collection efficiencies have<br />
been improved since the first generation, the buried substructure is presented as a<br />
metamaterial layer because it exhibits physical properties different than bulk silicon. In<br />
particular, it allows low energy-carrier multiplication.<br />
a) b)<br />
Figure 1. Architecture <strong>of</strong> MIND test structure (a) and SEM measurement showing buried nanoscale<br />
substructure as well as metamaterial layer (b)<br />
[1]P. Havu, V. Havu, M.J. Puska, M.H. Hakala, A. Foster, and R.M. Nieminen, “Finiteelement<br />
implementation for electron transport innanostructures”,J.Chem. Phys.124, 054707<br />
(2006)<br />
[2]C.T Sah, R.N. Noyce, W. Shockley, Proc. IRE 45, 1228 (1957). (2001).<br />
[3]Z. T. Kuznicki, Multi-interface novel devices, model with a continuous substructure, “3 rd<br />
Generation Photovoltaics for High Efficiency through Full Spectrum Utilization”, ed. A.<br />
Luque& A. Marti (Institute <strong>of</strong> Physics Publishing, Bristol, UK, 2003) pp. 165-195.<br />
87
MoP50 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
! ! " # $ " !<br />
" % " &<br />
! " # $<br />
! " ! ! # " ! # ! # !<br />
! ! $ ! % !<br />
" & !# !#"<br />
' ( ) ! # ! ! *+#',- %<br />
. ! ! ! #<br />
! # ! *+#',-% /# /0) 1 -!<br />
' 1-' + 2 ' +2' !<br />
$ ! % +2' " ! ! ! %<br />
3 & 0'1 " ! 45 ! %<br />
3 /0) " ! 267' %8 8 %4 ! ! 4 5<br />
49 " ! !! & ! % + ! & !<br />
! $<br />
! %% 3 !! ! 267'<br />
& # 5 !! & ! ! % 3 !<br />
!! !!<br />
88 8: % +! ! ',- ! "<br />
% . # ! !! 88<br />
" % 6 ! !<br />
! % 6 *; "<br />
" !!<br />
% 3 !<br />
49 % + 8 ! 5 (<br />
! ! !<br />
% 3 !! ! & %<br />
+ ! < 6 - 0 ! ) " ! 2<br />
. " " - * .=% 4% 4% 8# # >? > %<br />
88
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoP51<br />
ZnO-based Nanotubes Obtained by the Oxidation <strong>of</strong> ZnTe<br />
and ZnTe/Zn Nanowires<br />
K. Gas 1* , E. Dynowska 1 , E. Janik 1 , A. Kami ska 2 , S. Kret 1 , J.F. Morhange 3 ,<br />
I. Pasternak 4 , M. Wiater 1 , W. Zaleszczyk 1 , R. Hołyst 2 , E. Kami ska 4 , T. Wojtowicz 1 ,<br />
W. Szuszkiewicz 1<br />
1 Institute <strong>of</strong> Physics PAS, Al. Lotników 32/46, Warsaw, Poland<br />
2 Institute <strong>of</strong> Physical Chemistry, PAS, ul. Kasprzaka 44/52, Warsaw, Poland<br />
3 Institut des Nanosciences de Paris, UMR 7588, UMPC, 4 Place Jussieu, Paris, France<br />
4 Institute <strong>of</strong> Electron Technology, Al. Lotników 32/46, Warsaw, Poland<br />
One-dimensional (1D) semiconductor nanostructures like, e.g., nanowires (NWs) or<br />
nanobelts recently attracted a great deal <strong>of</strong> attention due to their potential application in<br />
electronic and optoelectronic nanodevices. Among many materials ZnO is particularly<br />
intensively studied due to its wide, direct energy gap (3.37 eV at 295 K) and large exciton<br />
binding energy <strong>of</strong> 60 meV. The non-intentionally doped ZnO usually exhibits n-type<br />
conductivity and it is difficult to convert it to p-type. The lack <strong>of</strong> p-type ZnO is the major<br />
obstacle in possible device applications <strong>of</strong> this material. Contrary to ZnO, ZnTe can be<br />
relatively easy highly p-type doped. Moreover, as it was already shown thermal oxidation <strong>of</strong><br />
nitrogen-doped ZnTe allows to obtain p-type ZnO [1], therefore one possible way to produce<br />
p-type, ZnO-based NW structure would be through the oxidation <strong>of</strong> p-type ZnTe NWs. In this<br />
paper we report a successful fabrication <strong>of</strong> the ZnO-containing nanotubes obtained by thermal<br />
oxidation. For this purpose MBE-grown ZnTe and ZnTe/Zn core/shell NWs were oxidized by<br />
annealing in flowing O2 gas using both a rapid thermal annealing apparatus and a furnace.<br />
Some samples were annealed in flowing Ar gas for a comparison.<br />
Scanning and transmission electron microscopy (SEM and TEM, respectively)<br />
demonstrated tubular character and some changes in the NWs’ morphology after the oxidation<br />
process. The TEM analyses also revealed that the nanotubes are built <strong>of</strong> small ZnO and TeO2<br />
crystallites with random crystallographic orientations. The average inner diameter <strong>of</strong> the<br />
nanotubes is about 40 nm whereas their outer diameter can reach up to 100 nm (measured in<br />
the central part <strong>of</strong> the NW). We determined the chemical composition <strong>of</strong> the samples using<br />
energy dispersive X-ray spectroscopy. Complementary information on the structure properties<br />
<strong>of</strong> the NWs was obtained by X-ray diffraction, the optical properties <strong>of</strong> the NWs were also<br />
investigated by Raman scattering. The influence <strong>of</strong> the technological conditions on the<br />
evolution <strong>of</strong> the nanostructures is shown and discussed. The results <strong>of</strong> our studies suggest that<br />
the most probable mechanism <strong>of</strong> formation <strong>of</strong> such tubular nanostructures consists <strong>of</strong> two<br />
major steps. Firstly, the ZnTe is partially decomposed by the reaction with oxygen both at the<br />
surface <strong>of</strong> the NW and along its axis at the center <strong>of</strong> the NWs. Due to very fast oxygen<br />
diffusion along the [111] direction a creation <strong>of</strong> a cavity starts in the central part <strong>of</strong> the NW.<br />
Next, the Zn from the surface produce the ZnO shell and the remaining Te aggregate into<br />
small nanocrystals, which decorate the hollow inner part <strong>of</strong> the ZnO NW [2] or (after an<br />
oxidation) form TeO2.<br />
The studies were partially supported by the European Union within European<br />
Regional Development Fund, through grant Innovative Economy (POIG.01.01.02-00-008/08).<br />
* Corresponding author, e-mail address: kgas@ifpan.edu.pl<br />
[1] E. Prze dziecka et al., Semicond. Sci. Technol. 22, 10 (2007).<br />
[2] P. Lu, D.J. Smith, phys. stat. sol. (a) 107, 681 (1988).<br />
89
MoP52 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Composition-dependentthermalresistance<strong>of</strong>multilayeredstructurestakingintoaccountphononreflection,scatteringandtunneling.<br />
DariuszŻak 1,2 ,WłodzimierzNakwaski 2<br />
1 Institute<strong>of</strong>ElectronTechnology,<br />
al.Lotników32/46,02-668Warsaw,Poland.<br />
2 PhotonicsGroup,Institute<strong>of</strong>Physics,TechnicalUniversity<strong>of</strong>Lodz,<br />
ul.Wolczanska219,90-924Lodz,Poland.<br />
Abstract<br />
Developmentinthesynthesisandprocessing<strong>of</strong>nanoscaledevicescreatesthe<br />
demandtopredicttheirthermalproperties.Nowadays,superlatticestructures<br />
arebasecomponents<strong>of</strong>lightemittingandlightdetectingdevicesinmiddle<strong>of</strong><br />
infra-redspectrumrange.Thermalpropertiestakeimportantplaceindesigning<br />
process.Heatspreadinginmultilayersstructuresismuchdifferentthanthatin<br />
bulkmaterials.Mainpart<strong>of</strong>heattransportinthosestructures,isgovernedby<br />
thephonontransport.Thepresence<strong>of</strong>multipleinterfacesinmultilayerstructuresimpairssignificantlythephonontransport<strong>of</strong>heat,whichresultsinthe<br />
reducedthermalconductivities<strong>of</strong>thelayersandamountstoincreasedthermal<br />
resistivity<strong>of</strong>thedevices.Thebehaviour<strong>of</strong>thephonon/heattransportinthe<br />
presence<strong>of</strong>interfacesisachallengeinanalyticaldescription.Inthisarticlea<br />
semi-analyticalexpressionbasedontheacousticmismatchmodelandthediffuse<br />
mismatchmodelisproposed,thatpredictsthethermalconductivity<strong>of</strong>superlatticestructuresatroomtemperaturesaccurately.Themodelwasusedtopredictthethermalconductivities<strong>of</strong>Si/Ge,Si/Si0.3Ge0.7.Si/Gesuperlatticestructuresconsist<strong>of</strong>60,80,100,200,300periodsandhavedifferentlayerssizeratio<br />
foreachsample.AllSi/Si0.3Ge0.7superlatticestructureswithperiodwidths<br />
300˚A,150˚A,75˚A,45˚Aare3µmthinkandhave2:1layerratio.Theobtained<br />
calculations<strong>of</strong>thermalconductivityinrange<strong>of</strong>temperaturefrom200Kto300K<br />
areingoodagreementwiththeexperimentaldata.Thebehaviour<strong>of</strong>thermal<br />
conductivityinthepresents<strong>of</strong>changingnumbers<strong>of</strong>interfacespredictedbythis<br />
modelisconfirmedbymeasurements.<br />
1<br />
90
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoP53<br />
Pb1-xGexTe surface with Sm 2+ and Sm 3 + doping<br />
A. Reszka 1 , B.A. Orlowski 1 , M.A. Pietrzyk 1 , A. Szczerbakow 1 , S. Mickievicius 2 ,<br />
S. Balakauskas 2 , R.L. Johnson 3<br />
1 Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences, Al. Lotników 32/46, 02-668 Warsaw,<br />
Poland<br />
2 Semiconductor Physics Institute, A. Gostauto 11, 2600 Vilnius, Lithuania<br />
3 Institut für Experimentalphysik, Universität Hamburg, Luruper Chaussee 149, 22761<br />
Hamburg, Germany<br />
In the present work we report the experimental results <strong>of</strong> resonant photoemission study<br />
<strong>of</strong> single crystal Pb1-xGexTe surface doped with Sm. Doping <strong>of</strong> IV-VI semiconductors with<br />
rare earth atoms turns these materials into diluted magnetic semiconductors, exhibiting<br />
a number <strong>of</strong> specific properties [1].<br />
The main goals <strong>of</strong> the presented studies were:<br />
– Determination <strong>of</strong> Sm 4f electrons contribution to the valence band <strong>of</strong> Sm/Pb1-xGexTe.<br />
– Verification <strong>of</strong> Sm ions charge states (Sm 2+ and Sm 3+ ) to determine their structure<br />
and binding energies.<br />
– Investigation <strong>of</strong> the changes <strong>of</strong> Sm 2+ /Sm 3+ ratio during the process <strong>of</strong> Sm incorporation into<br />
the host lattice.<br />
The Pb1-xGexTe (x=0.03) samples were grown by the Bridgman method in the Institute<br />
<strong>of</strong> Physics Polish Academy <strong>of</strong> Sciences in Warsaw. The photoemission experiments were<br />
performed at the FLIPPER II beam line in HASYLAB in Hamburg, Germany.<br />
Typical feature <strong>of</strong> rare-earth photoemission spectroscopy (PES) is a characteristic<br />
behaviour <strong>of</strong> its intensity related to strongly localized 4f-shell electrons. Therefore, the PES<br />
investigations <strong>of</strong> Sm/Pb1-xGexTe were carried out for the varying photon energy through the<br />
region corresponding to Sm 4d 4f Fano resonance (h = 130-160 eV) [2]. In order to get<br />
a clean surface for experiment the sample was sputtered by Ar + ions to remove possible oxide<br />
contaminations and annealed at the temperature 300 ºC under UHV conditions. Samarium<br />
was deposited on the top <strong>of</strong> Pb1-xGexTe sequentially to obtain 1, 3 and 7 ML capping layers.<br />
Afterwards sample was annealed at 250 ºC for 0.5, 1.5 and 6.5 hours to incorporate Sm atoms<br />
into Pb1-xGexTe crystal matrix. After each deposition and annealing processes the set <strong>of</strong><br />
energy distribution curves (EDC) spectra was measured. Deposited samarium ions occur as<br />
Sm 2+ and Sm 3+ at the surface. Annealing led to the decrease <strong>of</strong> Sm 2+ /Sm 3+ ratio indicating<br />
change in the interaction <strong>of</strong> Sm ions with their neighbourhood.<br />
The authors acknowledge support by MSHE <strong>of</strong> Poland research Projects DESY/68/2007 and by the<br />
European Community via the Research Infrastructure Action under the FP6 Structuring the European<br />
Research Area" Programme (through the Integrated Infrastructure Initiative "Integrating Activity on<br />
Synchrotron and Free Electron Laser Science") at DESY. Partially supported by Innovative Economy<br />
(POIG.01.01.02-00-108/09) and (POIG.01.01.02-00-008/08)<br />
[1] T. Story. Semimagnetic Semiconductors Based on Lead Chalcogenides.<br />
Lead Chalcogenides: Physics and Applications. Taylor & Francis, New York 2002.<br />
[2] U. Fano, Phys. Rev. B 23 (1961) 1866.<br />
91
MoP54<br />
_______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Studies <strong>of</strong> Optical Properties <strong>of</strong> Protonated Poliazomethine<br />
Thin Films<br />
H. Bednarski 1 , J. Weszka 1 , M. Domański 1 and V. Cozan 2<br />
1 Centre <strong>of</strong> Polymer and Carbon Materials, Polish Academy <strong>of</strong> Sciences, 34 M.<br />
Curie-Sk̷lodowska Str., 41-819 Zabrze, Poland<br />
2 Petru Poni Institute <strong>of</strong> Macromolecular Chemistry, Aleea Gr. Ghica Voda 41 A ,<br />
700487 Iasi, Romania<br />
The poly(p-phenyleneazomethine) (PPI) belongs to alternately conjugated polymers<br />
revealing semiconducting properties, thus belonging to the family <strong>of</strong> organic semiconductors.<br />
PPI is isoelectronic counterpart <strong>of</strong> poly(p-phenylene-vinilene) (PPV), which is well<br />
known for its wide applications in organic optoelectronics. Having nearly the same π electron<br />
system as PPV [1], the PPI differ structurally in the presence <strong>of</strong> nitrogen atoms. This<br />
structural feature opens an additional way <strong>of</strong> chemical doping, by nitrogen atoms lone<br />
pair protonation. Doping <strong>of</strong> conjugated polymers is a well established way <strong>of</strong> influencing<br />
on important electronic, opto-electronic and mechanical properties <strong>of</strong> semiconducting<br />
polymers.<br />
Doping <strong>of</strong> poliazomethines is realized mainly based on acid chemistry. Such a technique<br />
makes the process complex and difficult to control. Apart from intentional protonation<br />
always exist possibility <strong>of</strong> occurrence <strong>of</strong> unwanted side reactions like hydrolysis, decomposition<br />
etc. [2]. Moreover, the routine experimental methods such as IR, UV-vis, provide<br />
only averaged quantities, thus microscopic details are not fully resolved. Generally, the<br />
microscopic mechanism <strong>of</strong> protonation <strong>of</strong> polyazomethines is not well understood and<br />
many reported results are unclear.<br />
The purpose <strong>of</strong> this work is tw<strong>of</strong>old, namely experimental studies <strong>of</strong> optical properties<br />
<strong>of</strong> protonated poliazomethine thin films, as well as theoretical studies <strong>of</strong> protonation<br />
mechanism <strong>of</strong> this important group <strong>of</strong> materials. Performed theoretical calculations are<br />
aimed to rationalize observed experimental finding such as a red shift <strong>of</strong> the maximum <strong>of</strong><br />
the absorption band due to protonation.<br />
[1] J. Weszka, H. Bednarski and M.Domanski, J. Chem. Phys. 131 (2009) 024901.<br />
[2] A. Iwan and D. Sek, Prog. Polym. Sci. 33 (2008) 289-345.<br />
92
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoP55<br />
The reasons <strong>of</strong> destruction <strong>of</strong> nanoporous GaAs matrix<br />
fabricated by electrochemical etching<br />
A. V. Atrashchenko 1 , V. P. Ulin 1 , V. P. Evtikhiev 1<br />
1 I<strong>of</strong>fe Physical-Technical Institute <strong>of</strong> the Russian Academy <strong>of</strong> Sciences,<br />
Politekhnicheskaya 26, 194021, St Petersburg, Russia<br />
The goal <strong>of</strong> the present study is to find out the reasons <strong>of</strong> the destruction <strong>of</strong> porous<br />
matrices based on n-type GaAs oriented in (100) created by using anodic electrochemical<br />
etchinganddefineparameters<strong>of</strong>theporouslayers(poresdiameter, wallsthickness, depth,<br />
period <strong>of</strong> branching) for further application <strong>of</strong> such matrices in nanocomposite materials<br />
(matrixes with filled with metals or alloys (Ga, In, Ga-Au, In-Au)) [1,2].<br />
Aperspectivetechnique<strong>of</strong>creatingsuchmatricesisthemethod<strong>of</strong>anodicelectrochemical<br />
etching which allows one to obtain nanoporous matrices on large areas. Experiments<br />
in [3,4] show that a creation <strong>of</strong> nanoporous matrices by electrochemical etching method<br />
meets with a problem <strong>of</strong> destruction <strong>of</strong> porous layer with the increasing <strong>of</strong> the etching<br />
depth.<br />
Based on previously obtained data [4] and carried out experiments in this paper there<br />
were suggested three hypotheses explaining the destruction <strong>of</strong> the porous layer with the<br />
increasing <strong>of</strong> depth. 1) During the etching occurs a catastrophic heating which leads to a<br />
change <strong>of</strong> the electrical parameters <strong>of</strong> the reaction. The temperature measurements made<br />
while executing every experiment has refuted this hypothesis. 2) There is a different speed<br />
<strong>of</strong> movement <strong>of</strong> the electrochemical reaction (relative to the front <strong>of</strong> etching) in different<br />
halides. To check this hypothesis we changed the electrolyte composition by leaving only<br />
one halide. This led to predictable changes in the pores parameters [5], but didn’t remove<br />
the problem <strong>of</strong> porous layer destruction. Occasionally we found that with the increasing<br />
<strong>of</strong> the pore size the undamaged layer depth increases. 3) Mechanical destruction <strong>of</strong> the<br />
porous layer is due to high pressure <strong>of</strong> products <strong>of</strong> the etching. In etching form the<br />
products whose volume is larger than etched volume. The products go outside through<br />
nanochannels. The increase <strong>of</strong> the nanochannel length leads to drastic amplification <strong>of</strong><br />
resistance to the products outlet. At some moment a pressure <strong>of</strong> the etching products on<br />
a pore walls exceeds the value <strong>of</strong> the tensile strength so that the pressure at the front <strong>of</strong><br />
pore formation is higher than one in the middle <strong>of</strong> the porous layer. This explains why<br />
at stopping <strong>of</strong> the etching, when the destruction starts, we can see the destruction only<br />
at the front <strong>of</strong> etching.<br />
We have defined the reason <strong>of</strong> the porous matrices destruction while increasing <strong>of</strong> the<br />
porous layer depth by anodic electrochemical etching which is a mechanical destruction<br />
<strong>of</strong> the pores walls due to amplification <strong>of</strong> the etching products pressure on the walls.<br />
We have defined the limits <strong>of</strong> the parameters variation for subsequent usage them as a<br />
medium for creation <strong>of</strong> nanocomposite metal-semiconductor A3B5 materials: the pores<br />
diameter is from 20 nm to 70 nm (up to 200 nm in fluoride electrolytes with an abrupt<br />
decrease <strong>of</strong> the structural order), the walls thickness lies within 10-20 nm, the depth <strong>of</strong><br />
branching ranges from 150 nm to 1200 nm. We have outlined the methods allowing one<br />
to increase the porous layers depth by varying pulse current while etching.<br />
[1] J. B.Pendry, Phys. Rev. Lett. 85, 3966 (2000);<br />
[2] M. G. Silveirinha, C. A. Fernandes, Phys. Rev. B 78, 033108 (2008);<br />
[3] M. Christophersen et al., Phys. Stat. Sol. (a) 197, 197, (2003);<br />
[4] V. P. Ulin, S. G. Konnikov,Semiconductors 41, 832 (2007);<br />
93
MoP56 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Effect <strong>of</strong> Substitution <strong>of</strong> Ti for Cd in CdCr2Se4 p-Type Semiconductor<br />
E. Malicka 1 , T. Groń 2 , A.W. Pacyna 3 , H. Duda 2 and J. Krok-Kowalski 2<br />
1 University <strong>of</strong> Silesia, Institute <strong>of</strong> Chemistry, ul. Szkolna 9, 40-006 Katowice, Poland<br />
2 University <strong>of</strong> Silesia, Institute <strong>of</strong> Physics, ul. Uniwersytecka 4, 40-007 Katowice, Poland<br />
3 The Henryk Niewodniczański Institute <strong>of</strong> Nuclear Physics, Polish Academy <strong>of</strong> Sciences,<br />
ul. Radzikowskiego 152, 31-342 Kraków, Poland<br />
The CdCr2Se4 spinel combines the p-type semiconducting and ferromagnetic properties<br />
with Curie temperature TC = 142 K and Curie-Weiss temperature qCW = 190 K [1,2].<br />
Magnetization <strong>of</strong> CdCr2Se4 reaches the full saturation <strong>of</strong> 5.98 µB per molecule [3]. The<br />
ferromagnetic properties <strong>of</strong> CdCr2Se4 are a result <strong>of</strong> dominating interactions between the<br />
nearest-neighbour chromium ions and <strong>of</strong> weaker superexchange couplings between the more<br />
distant chromium ones [4]. The Cr 2p XPS spectra <strong>of</strong> CdCr2Se4 showed the spin-orbit<br />
splitting between the final Cr 2p3/2 and Cr 2p1/2 states <strong>of</strong> 9.5 eV. The Cr 2p3/2 states are split<br />
into two peaks at 574.2 and 575.2 eV. The peak separation with the binding energy difference<br />
ΔE about 1 eV is typical <strong>of</strong> the 3d 3 elements with localized magnetic moment <strong>of</strong> 3 µB [5].<br />
CdCr2Se4 crystallizes in the cubic structure (Fd3m). The X-ray refinements showed that the<br />
(Cd) ions have a preference to be located in the tetrahedral sites and the [Cr] ions prefer to be<br />
located in the octahedral sites <strong>of</strong> the spinel structure [6].<br />
Chemical composition and structure refinement gave the sample composition and cation<br />
distribution according to the spinel formula: (Cd0.88Ti0.08)[Cr2]Se4. Magnetization, ac and dc<br />
magnetic susceptibility <strong>of</strong> (Cd0.88Ti0.08)[Cr2]Se4 were measured in the zero-field-cooled mode<br />
using a Lake Shore 7225 dc magnetometer/ac susceptometer at 4.3 K and in applied external<br />
magnetic fields up to 60 kOe, and a Faraday type Cahn RG automatic electrobalance in the<br />
temperature range 4.3-300 K and at 1 kOe and 2 kOe, respectively.<br />
The dc magnetic measurements <strong>of</strong> (Cd0.88Ti0.08)[Cr2]Se4 showed ferromagnetic order<br />
with Curie temperature TC = 129 K and Curie-Weiss temperature qCW = 138 K. Saturation<br />
magnetization <strong>of</strong> 4.73 µB/f.u. at 4.3 K and at 60 kOe seems to be strongly reduced in<br />
comparison with the CdCr2Se4 matrix. Such considerable lowering <strong>of</strong> magnetic moment<br />
induced by titanium substitution could be explained with only a transition <strong>of</strong> the certain<br />
number <strong>of</strong> Cr ions from the high-spin states to the low-spin states. Additionally, the ac<br />
magnetic measurements revealed a spectacular peak at 450 Oe and 1 kOe in the real part <strong>of</strong><br />
the first harmonic susceptibility curve near TC, similar to the Hopkinson peak. Zero-value <strong>of</strong><br />
its imaginary part below TC indicates a lack <strong>of</strong> the energy loss connected with the spin<br />
rearrangement processes. This is consistent with zero values <strong>of</strong> second and third harmonic ac<br />
susceptibility in the temperature range <strong>of</strong> 4.5-160 K, suggesting the lack <strong>of</strong> short-range<br />
magnetic interactions.<br />
This work is partly founded from science Grant No. N N204 145938.<br />
[1] H.W. Lehmann, Phys. Rev. 163, 488 (1967).<br />
[2] P.K. Baltzer, H.W. Lehmann, and M. Robbins, Phys. Rev. Lett. 15, 493 (1965).<br />
[3] R.C. LeCraw, H. von Philipsborn, and M.D. Sturge, J. Appl. Phys. 38, 965 (1967).<br />
[4] P.K. Baltzer, M. Robbins, and P.J. Wojtowicz, J. Appl. Phys. 38, 953 (1967).<br />
[5] V. Tsurkan, St. Plogmann, M. Demeter, D. Hartmann, and M. Neumann, Eur. Phys. J. B<br />
15, 401 (2000).<br />
[6] T. Groń, A. Krajewski, J. Kusz, E. Malicka, I. Okońska-Kozłowska, A. Waśkowska, Phys.<br />
Rev. B 71, 035208 (2005).<br />
94
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoP57<br />
Thermoelectric power in epitaxial Bi2Se3/Si(111) layers<br />
K. Dybko , M. Szot, T. Story and G. Karczewski,<br />
Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences,<br />
Al. Lotników 32/46, 02-668 Warszawa, Poland<br />
S. Schreyeck, C. Schumacher, K. Brunner and L.W. Molenkamp<br />
! "<br />
Bi2Se3 belongs to the class <strong>of</strong> three-dimensional topological insulators characterized<br />
by a bulk insulating state and conducting two-dimensional surface states with linear Diraclike<br />
dispersion. As grown Bi2Se3 samples are usually n-type due to inevitably present charged<br />
Se vacancies. Consequently, the conductivity <strong>of</strong> the system consists <strong>of</strong> parallel bulk and<br />
surface states conductivities. The observation <strong>of</strong> the surface states electrical properties<br />
requires minimalization <strong>of</strong> the bulk states contribution. It has been demonstrated in high field<br />
magnetotransport experiment [1] that beyond magnetic field corresponding to quantum limit<br />
for 3D carriers the clear Shubnikov de Haas oscillations <strong>of</strong> 2D character appear. This high<br />
field signature <strong>of</strong> surface states unambiguously proved the coexistence <strong>of</strong> both conducting<br />
channels in bulk samples.<br />
Here we study how the surface states affect thermoelectric properties <strong>of</strong> the whole<br />
electronic system in Bi2Se3. The samples were grown by molecular beam epitaxy on<br />
reconstructed (7x7)–(111) surface <strong>of</strong> Si semi-insulating substrate. Typically, samples are 100<br />
nm thick and have electron concentration <strong>of</strong> the order <strong>of</strong> 10 19 cm -3 . The Seebeck effect was<br />
measured over the temperature range <strong>of</strong> 10-300 K in ab-plane <strong>of</strong> the samples. The magnitude<br />
<strong>of</strong> the thermoelectric power decreases smoothly with lowering temperature until around 80 K,<br />
where it develops into a broad peak with maximum at 45-50 K. The absolute height <strong>of</strong> this<br />
peak is within 10 percent equal to the room temperature value (-60 V). The observed<br />
anomaly could be attributed to the theoretically predicted enhancement <strong>of</strong> the thermopower<br />
for surface states <strong>of</strong> topological insulators [2,3]. Here we measure the sum <strong>of</strong> individual<br />
contributions to the thermopower originating from bulk and surface states weighted by their<br />
electrical conductivities. Therefore, we also consider an alternative explanation that the<br />
effect is due to phonon drag, which may lead to the enhancement <strong>of</strong> the low temperature<br />
thermopower in semiconductors.<br />
[1] J.G. Analytis et al. , Nature Phys. 6, 960 (2010).<br />
[2] R. Takahashi and S. Murakami, Phys. Rev. B 81, 161302 (2010).<br />
[3] P. Ghaemi, R.S.K. Mong and J.E. Moore, Phys. Rev. Lett. 105, 166603 (2010).<br />
95
MoP58<br />
_______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Diamagnetism <strong>of</strong> the pre-paired electronic systems:<br />
the flow equation study<br />
M. Zapalska and T. Domański<br />
Institute <strong>of</strong> Physics, M. Curie Sk̷lodowska University, 20-031 Lublin, Poland<br />
Magnetic ordering <strong>of</strong> the electronic systems can originate from a variety <strong>of</strong> mechanisms.<br />
It can be explained for instance by the energetic reasons (Pauli paramagnetism), assigned<br />
to the orbital effects (diamagnetism) or to the mutual electron-electron interactions which<br />
through the exchange coupling induce either the ferromagnetic or antiferromagnetic order.<br />
Among these phenomena the particular version <strong>of</strong> ordering (perfect diamagnetism)<br />
appears whenever the Fermi liquid becomes unstable towards the electron pair formation<br />
and the system becomes superconducting. It is well known that superconductivity goes<br />
hand in hand together with the perfect diamagnetism (Meissner effect). In his seminal<br />
paper P.W. Anderson explained this in terms <strong>of</strong> the Anderson-Higgs mechanism when the<br />
phasal Goldstone mode is swallen producing the massive term A 2 .<br />
Recent studies <strong>of</strong> the Princeton group [L. Li et al, Phys. Rev. B 81, 054510 (2010) by<br />
means <strong>of</strong> the high-presission torque magnetometry reveiled that in cuprate materials the<br />
remarkable diamagnetic response survies to temperatures much higher than critical Tc. It<br />
is expected that apparantly the preformed electron pairs (which above Tc are not capable<br />
to establish the long-range coherence) are responsible for such residual diamagnetism. We<br />
shall analyze this problem using the nonperturbative flow equation technique designed<br />
within the numerical renormalization group scheme.<br />
96
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors MoP59<br />
Quantum Monte Carlo vs. Density Functional Methods for the prediction <strong>of</strong><br />
relative energies <strong>of</strong> small Si-C clusters<br />
Nevill Gonzalez Szwacki and Jacek A. Majewski<br />
Faculty <strong>of</strong> Physics, University <strong>of</strong> Warsaw, ul Hoża 69, 00-681 Warszawa, Poland<br />
In many systems, prediction <strong>of</strong> the structural properties requires correct description <strong>of</strong><br />
the electron correlation, clearly exceeding the accuracy <strong>of</strong> the available exchange correlation<br />
functionals in the density functional theory (DFT). This is also the case in small systems with<br />
very non-homogenous distribution <strong>of</strong> electronic charge, like for example Si–C clusters, which<br />
play an important role are in technological processes, such as chemical vapor deposition used<br />
for fabrication <strong>of</strong> SiC thin films. The simulation <strong>of</strong> the growth processes is out <strong>of</strong> scope <strong>of</strong> abinito<br />
methods and has to be done using effective approaches after ‘coarse graining’ procedure.<br />
The quality <strong>of</strong> the effective potentials is crucial for reliability <strong>of</strong> the predictions. Therefore,<br />
the effective potentials are usually fitted to the results <strong>of</strong> the first-principles calculations for<br />
building blocks <strong>of</strong> a material.<br />
Numerous experimental and theoretical studies have been performed to investigate<br />
equilibrium geometries, vibrational frequencies, and relative energies <strong>of</strong> small SinCm cluster<br />
isomers [1,2]. Most <strong>of</strong> the calculations employed standard approximations to the DFT.<br />
In order to examine the accuracy <strong>of</strong> various DFT exchange-correlation (XC)<br />
functionals in predicting the relative energies <strong>of</strong> cluster isomers and to provide new<br />
benchmark data, we have performed diffusion quantum Monte Carlo (DMC) calculations for<br />
small SinCm clusters. We compare our DMC results with DFT ones obtained using the local<br />
density approximation (LDA), Perdew-Burke-Ernzerh<strong>of</strong> generalized gradient approximation<br />
(GGA), and the popular B3LYP hybrid Hartree-Fock/DFT functional. On the basis <strong>of</strong> our<br />
DMC results for Si-C clusters, we conclude that LDA and GGA XC functionals are not<br />
reliable for predicting the relative stability <strong>of</strong> various isomers <strong>of</strong> covalent SiC clusters,<br />
B3LYP does a little bit better, and generally the errors <strong>of</strong> various methods are completely<br />
unsystematic. This is illustrated in Fig. 1, where we plotted the energy (relative to the<br />
average) <strong>of</strong> four SinCm isomers. In particular, it is easily seen that the LDA approximation<br />
underestimates the relative energy <strong>of</strong> the planar (1) and (2) clusters, whereas the GGA<br />
approximation is unable to capture correctly the relative energy <strong>of</strong> the 3D clusters (2) and (3).<br />
The present studies clearly demonstrate that<br />
the determination <strong>of</strong> the effective potentials<br />
on the basis <strong>of</strong> DFT calculations with<br />
approximated exchange-correlation<br />
functionals can be misleading and<br />
computational schemes going beyond this<br />
approximation are very desirable.<br />
This paper has been supported by the project<br />
“SiCMAT” (Innovative Economy,<br />
POIG.01.03.01-14-155/09-00)<br />
[1] M. Bertolus, F. Finocchi, and P. Millie, J. Chem. Phys. 120, 4333 (2004).<br />
[2] C. R. Hsing et al., Phys. Rev. B 79, 245401 (2009).<br />
97
MoP60 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Analysis <strong>of</strong> a Spatial Hole Burning Effect in a Square and<br />
Triangular Lattice Photonic Crystal Laser<br />
Marcin Koba 1,2 , and Pawe̷l Szczepański 1,2<br />
1 Warsaw University <strong>of</strong> Technology, Koszykowa 75, 00-662 Warsaw, Poland<br />
2 National Institute <strong>of</strong> Telecommunications, Szachowa 1, 04-894 Warsaw, Poland<br />
In this paper we present an approximate analysis <strong>of</strong> the nonlinear operation <strong>of</strong> the<br />
two-dimensional (2D) photonic crystal (PC) lasers taking into account nonlinear effects<br />
(gain saturation, spatial hole burning).<br />
Our analysis is conducted for the laser with an active medium that is confined in<br />
the square region with circular holes arranged in square or triangular Bravais’ lattice.<br />
The analysis is based on two-dimensional coupled wave equations [1]-[4] for TE and TM<br />
modes, modified by introduction <strong>of</strong> nonlinear gain and energy theorem developed earlier<br />
forvarioustypes<strong>of</strong>laser[5], includingDFBstructures[6]-[8]. Similarly, aswehavedonein<br />
e.g. [6]-[8] the field distributions appearing in the energy theorem (including nonlinearity)<br />
were approximated by those existing in the linear structure (i.e. at threshold).<br />
Following our derivation presented in [4],[9],[10] we introduce an approximate equations<br />
describing the small signal gain coefficient α0 as a function <strong>of</strong> system parameters,<br />
where we have introduced power saturation and spatial hole burning effects. We use these<br />
equations to calculate laser characteristics illustrating hole burning effect impact on cw<br />
operation <strong>of</strong> square and triangular lattice PC laser structure above the threshold. With<br />
thehelp<strong>of</strong>thepresentedtechniqueitispossibletocalculatetheoptimalcouplingstrength<br />
providing maximal power efficiency <strong>of</strong> the given 2D photonic laser structure.<br />
[1] K. Sakai, E. Miyai, and S. Noda, Opt. Express 15, 3981 (2007).<br />
[2] K. Sakai, E. Miyai, and S. Noda, IEEE J. Quantum Electron. 46, 788 (2010).<br />
[3] K. Sakai, J. Yue, and S. Noda, Opt. Express 16, 6033 (2008).<br />
[4] M. Koba, P. Szczepanski, and T. Kossek, IEEE J. Quantum Electron. 47, (13) <strong>2011</strong>.<br />
[5] L. Wosińska, P. Szczepański, and W. Woliński, Opto-Electron. Rev. 1, 26 (1992).<br />
[6] R. Paszkiewicz, A. Tyszka-Zawadzka, and P. Szczepański, Opt. Commun. 270, 314<br />
(2007).<br />
[7] P. Szczepański, D. Sikorski, and W. Woliński, IEEE J. Quantum Electron. 25, 871<br />
(1989).<br />
[8] P. Szczepanski, Appl. Opt. 24, 3574 (1985).<br />
[9] M. Koba, and P. Szczepanski, IEEE J. Quantum Electron. 46, 1003 (2010).<br />
[10] M. Koba, T. Osuch, and P. Szczepanski, J. Mod. Opt. submited for publication.<br />
98
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors TuI1<br />
Optical control <strong>of</strong> one and two spins in interacting quantum dots<br />
Alex Greilich, Samuel G. Carter, Danny Kim, Allan S. Bracker and Daniel Gammon<br />
Naval Research Laboratory, Washington DC 20375, USA<br />
The interaction between two quantum bits enables entanglement, the two-particle<br />
correlations that are at the heart <strong>of</strong> quantum information science. In semiconductor quantum<br />
dots much work has been focused on demonstrating single spin qubit control using optical<br />
techniques. However, optical control <strong>of</strong> two spin qubits remains a major challenge for scaling<br />
to a full-fledged quantum information platform. Here we combine advances in verticallystacked<br />
quantum dots with ultrafast laser techniques to achieve optical control <strong>of</strong> the<br />
entangled state <strong>of</strong> two spins.<br />
We demonstrate ultrafast optical control <strong>of</strong> two interacting electron or hole spins in<br />
two separate QDs using optical initialization, single-qubit gates with short pulses, and twoqubit<br />
gates with longer pulses or through precession in the exchange field. Local<br />
entanglement between the two spins is inferred from the coherent evolution <strong>of</strong> superposition<br />
states as measured in Ramsey fringes.<br />
The two-qubit gate speeds achieved here are over an order <strong>of</strong> magnitude faster than in<br />
other systems. These results demonstrate the viability and advantages <strong>of</strong> optically controlled<br />
quantum dot spins for multi-qubit systems.<br />
[1] Danny Kim, Samuel G. Carter, Alex Greilich, Allan S. Bracker and Daniel Gammon,<br />
Nature Physics 7, 223 (<strong>2011</strong>).��<br />
99
TuI2 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Graphene Transport Properties�<br />
Shaffique Adam<br />
Center for Center for Nanoscale Science and Technology, National Institute <strong>of</strong> Standards and<br />
Technology<br />
Arguably, one <strong>of</strong> the most intriguing properties <strong>of</strong> graphene transport is the nonvanishing<br />
``minimum conductivity” at the Dirac point. The carrier density in these single<br />
monatomic sheets <strong>of</strong> carbon can be continuously tuned from electron-like carriers for large<br />
positive gate bias to hole-like carriers for negative bias. The physics close to zero carrier<br />
density (also called the intrinsic or Dirac region), is now understood to be dominated by<br />
fluctuations in local disorder potential, breaking the landscape into puddles <strong>of</strong> electrons and<br />
holes [1]. I will discuss the competing effects <strong>of</strong> disorder, electron-electron interactions, and<br />
quantum interference on graphene’s transport properties and demonstrate that, remarkably, a<br />
semi-classical effective medium theory captures the physics in the experimentally relevant<br />
regime [2]. Figure 1 shows a comparison <strong>of</strong> our theoretical predictions [3] with subsequent<br />
experimental data from Columbia, Manchester, Maryland and Exeter universities showing<br />
good agreement. To further test the range <strong>of</strong> validity <strong>of</strong> our model, I compare our results to a<br />
fully quantum-mechanical numerical calculation [4] <strong>of</strong> the conductivity and find that while<br />
the two theories are incompatible at weak<br />
disorder, they are compatible for strong<br />
disorder. Armed with this success, I will<br />
discuss how future graphene experiments<br />
could shed light on some long-standing<br />
open questions in condensed matter<br />
physics.<br />
Figure 1: Comparison <strong>of</strong> the self-consistent<br />
theory [3] with experimental data from<br />
various groups around the world. (See Ref.<br />
[2] for details). Solid blue line shows the<br />
Dirac point conductivity for graphene in<br />
the presence <strong>of</strong> charged impurities <strong>of</strong><br />
concentration nimp. The dashed line shows the quantum mechanical ballistic result 4e 2 /�h.<br />
Early results claimed that the conductivity was universal with a value <strong>of</strong> 4e 2 /h (dotted line).<br />
References:<br />
[1] S. Das Sarma, S. Adam, E. H. Hwang, E. Rossi, “Electronic transport in two dimensional<br />
graphene”, arXiv:1003.4731, Rev. Mod. Phys. (in press).<br />
[2] S. Adam, “Graphene carrier transport theory”, invited book chapter for Graphene<br />
Nanoelectronics: metrology, synthesis, properties and applications (Springer book series on<br />
nanoscience and technology).<br />
[3] S. Adam, E. H. Hwang, V. Galitski, and S. Das Sarma “A self-consistent theory for<br />
graphene transport” Proc. Nat. Acad. Sci. USA 104, 18392 (2007).<br />
[4] S. Adam, P. W. Brouwer, and S. Das Sarma “Crossover from quantum to Boltzmann<br />
transport in graphene” Phys. Rev. B Rapid Communications 79, 201404 (2009).<br />
100
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors TuI3<br />
Theoretical search for non-Abelian statistics in fractional quantum Hall states<br />
Arkadiusz Wójs<br />
Institute <strong>of</strong> Physics, Wroc�aw University <strong>of</strong> Technology, Wroc�aw, Poland<br />
Convincing demonstration <strong>of</strong> non-Abelian anyon quantum statistics (corresponding to<br />
the multi-dimensional realization <strong>of</strong> the braid group, allowed in two spatial dimensions) is a<br />
major challenge in today’s condensed matter physics. Promising physical systems for this<br />
quest are sought among correlated many-electron states a high magnetic field, responsible for<br />
the fractional quantum Hall effect at such Landau level fillings as �=5/2 or 12/5. In particular,<br />
the 5/2 state is believed to realize a superfluid condensate <strong>of</strong> p-paired “composite fermions”<br />
(electrons binding vortices <strong>of</strong> the many-body wave function) in an effectively compensated<br />
magnetic field. This hypothetical state, described by the “Pfaffian” wave function <strong>of</strong> Moore<br />
and Read, supports elementary charge excitations in form <strong>of</strong> fractional (e/4) quasiparticles<br />
which obey non-Abelian anyon quantum statistics. Anticipation <strong>of</strong> the Pfaffian ground state<br />
relies mostly on numerics, and it is confronted with rather ambiguous experimental evidence,<br />
including recent reports <strong>of</strong> the (unexpected) depolarization <strong>of</strong> spin at �=5/2. Therefore, the<br />
focus <strong>of</strong> this talk will be on available evidence concerning Pfaffian dynamics (more generally,<br />
non-Abelian quantum statistics) in realistic conditions <strong>of</strong> the fractional quantum Hall effect.<br />
We will begin by revisiting the question <strong>of</strong> spin polarization at �=5/2. The combination<br />
<strong>of</strong> exact diagonalization and quantum Monte Carlo in spherical geometry confirms quantum<br />
ferromagnetism in clean systems, as expected for the Pfaffian phase, but also reveals stability<br />
<strong>of</strong> Skyrmions (topological spin excitations) in sufficiently thick systems [1]. As pairing nature<br />
<strong>of</strong> the ferromagnetic (Pfaffian) ground state implies that Skyrmions at �=5/2 carry twice the<br />
charge (e/2) <strong>of</strong> spinless quasiparticles, the (realistic) lateral disorder aids Skyrmion formation,<br />
in turn leading to local depolarization – in agreement with its photoluminescence signatures.<br />
Having established the role <strong>of</strong> spin we will confine further analysis to the spin-polarized<br />
channel and turn to the question <strong>of</strong> adiabatic connection (i.e., essential equivalence) between<br />
the exact Pfaffian state and the realistic Coulomb ground state. The problem is subtler than at<br />
�=1/3 (where the famous Laughlin wave function is firmly established), because <strong>of</strong> a longer<br />
correlation length, a more fragile gap, and a competition between two- and three-body effects<br />
(the Pfaffian state being a unique zero-energy ground state <strong>of</strong> the contact triplet repulsion).<br />
Since the analysis <strong>of</strong> overlaps or pair correlation functions computed in the largest tractable<br />
systems is hardly conclusive, we studied features linked directly to the non-Abelian statistics.<br />
Specifically, in addition to the neutral boson pair-pair collective mode with the characteristic<br />
roton dispersion, the Pfaffian ground state should have a neutral fermion excitation associated<br />
with an unpaired particle (analogous to the Bogoliubov-deGennes excitation <strong>of</strong> a conventional<br />
superconductor). In the relevant weakly paired phase, the neutral fermion dispersion should<br />
have a minimum near the Fermi wave vector. More importantly, the neutral fermion mode is<br />
predicted to become gapless in the presence <strong>of</strong> distant quasiparticles – as a consequence <strong>of</strong><br />
their non-Abelian statistics within the Ising model with degenerate fusion channels (1 and �).<br />
Our calculations confirm [2] these qualitative features, in support <strong>of</strong> exotic statistics at �=5/2.<br />
Other issues will include Pfaffian/anti-Pfaffian duality, effects <strong>of</strong> Landau level mixing<br />
(e.g., on particle-hole symmetry) [3], and the pairing transition <strong>of</strong> composite fermions [4].<br />
[1] A. Wójs, G. Möller, S. H. Simon, and N. R. Cooper, Phys. Rev. Lett. 104, 086801 (2010).<br />
[2] G. Möller, A. Wójs, and N. R. Cooper, http://arxiv.org/abs/1009.4956<br />
[3] A. Wójs, C. T�ke, and J. K. Jain, Phys. Rev. Lett. 105, 096802 (2010).<br />
[4] A. Wójs, C. T�ke, and J. K. Jain, Phys. Rev. Lett. 105, 196801 (2010).<br />
101
TuI4 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Polariton condensation in dynamic acoustic lattices<br />
D. Krizhanovskii 1 , E. Cerda-Méndez 2 , M.Wouters 3 , K. Biermann 3 , K.Guda 1 , R.Bradley 1 ,<br />
D.Sarker 1 , P. V. Santos 3 , R. Hey 3 and M. S. Skolnick 1<br />
1 Department <strong>of</strong> Physics and Astronomy, University <strong>of</strong> Sheffield, Sheffield, United Kingdom<br />
2 Paul-Drude-Institut für Festkörperelektronik, Berlin, Germany<br />
3 ITP, Ecole Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland<br />
Microcavity polariton condensates<br />
arising either from Bose-Einstein condensation<br />
(BEC) or from Optical Parametric Oscillation<br />
(OPO) exhibit collective superfluid-like<br />
behavior [1](OPO), spatial and temporal<br />
coherence (OPO, BEC) and vortices (OPO,<br />
BEC). Here, we study spatial coherence <strong>of</strong><br />
OPO polariton condensates formed by<br />
polariton-polariton scattering from the pump in<br />
external periodic potentials created by Surface<br />
Acoustic Waves (SAW)[2]. The effect <strong>of</strong><br />
strong polariton-phonon coupling results in<br />
modulation <strong>of</strong> the polariton dispersion with<br />
marked formation <strong>of</strong> Brillouin zones <strong>of</strong> period<br />
k=π/λSAW and energy stop band widths up to ~0.5<br />
meV (Fig.1a).<br />
Without SAW the OPO “signal” condensate<br />
is formed at k=0 above threshold with a coherence<br />
length <strong>of</strong> about ~20-25 µm. As the height <strong>of</strong> the<br />
SAW potential is increased we observe dramatic<br />
reduction <strong>of</strong> the spatial coherence length (Ly) along<br />
the SAW direction by a factor <strong>of</strong> 3-4 (Fig.1c), a result <strong>of</strong> suppressed tunnelling between the<br />
adjacent SAW minima. A smaller decrease <strong>of</strong> the coherence length Lx by a factor <strong>of</strong> 2<br />
perpendicular to the SAW direction (Fig.1d) is attributed to the increased phase fluctuations,<br />
predicted theoretically for 1D system <strong>of</strong> the non-equilibrium OPO. The observed effect<br />
corresponds to transition from condensate to insulator state. Nevertheless, this is not a Mott<br />
insulator: the number <strong>of</strong> particles per potential well is still large compare to the case <strong>of</strong><br />
superfluid-to Mott-insulator transition in cold atom system. Interestingly, at intermediate SAW<br />
powers the emission peaks at the edges <strong>of</strong> the 1 st Fig.1 a) Dispersion <strong>of</strong> polaritons subject to SAW<br />
potential. b) Image <strong>of</strong> condensate emission c)-d)<br />
Spatial coherence length <strong>of</strong> OPO signal parallel<br />
(LX) and perpendicular (LY) SAW at different<br />
pump powers.<br />
Brillouin zone (Fig.1b), indicating a phase<br />
difference <strong>of</strong> π between adjacent condensates. A likely reason for this is an additional scattering<br />
mechanism between the main pump beam and its diffraction replica.<br />
By contrast, inter-particle interactions at high densities screen the acoustic potential,<br />
partially reversing its effect on spatial coherence. Figs.1 c)-d) shows that with increasing<br />
excitation power the reduction <strong>of</strong> spatial coherence lengths occurs at higher SAW powers, thus<br />
indicating efficient screening <strong>of</strong> SAW potential at low acoustic powers as the polariton density<br />
is increased.<br />
Our work on polariton condensation in acoustic lattices provides new technology to<br />
investigate fundamental effects in hybrid light-matter system, such as coherent transport <strong>of</strong><br />
condensates, creation <strong>of</strong> entangled states and Josephson tunneling.<br />
1. A. Amo, et al, Nature 457, 291 (2009)<br />
Ly (P =140 mW)<br />
laser<br />
40<br />
Ly (P =200 mW)<br />
laser<br />
Ly (P =240 mW)<br />
laser<br />
30<br />
Ly (P =300 mW)<br />
laser<br />
-5 0 5 10 15<br />
0 5 10 15<br />
2. E. A. Cerda-Méndez, D. N. Krizhanovskii et al, Phys. Rev. Lett. 105, 116402 (2010)<br />
102<br />
Energy (eV)<br />
1.5365<br />
1.5360<br />
1.5355<br />
1.5350<br />
1.5345<br />
1.5340<br />
1.5365<br />
1.5360<br />
1.5355<br />
1.5350<br />
1.5345<br />
1.5340<br />
a)<br />
b)<br />
-2 -1 0 1 2<br />
k-vector ky<br />
c)<br />
d)<br />
Ly (P laser =68 mW)<br />
SAW power (dBm)<br />
50<br />
20<br />
10<br />
0<br />
40<br />
30<br />
20<br />
10<br />
0<br />
Coherence length L y (µm)<br />
Coherence length Lx (µm)
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors TuO1<br />
Resonant Terahertz Absorption by Magnetoplasmons<br />
in Grating-Gate GaN/AlGaN-based Field-Effect Transistors<br />
K. Nogajewski 1 , K. Karpierz 1 , M. Grynberg 1 , W. Knap 2 , R. Gaska 3 ,<br />
J. Yang 3 , M. S. Shur 4 , and J. ̷Lusakowski 1<br />
1 Faculty <strong>of</strong> Physics, University <strong>of</strong> Warsaw, ul. Ho˙za 69, 00-681 Warsaw, Poland<br />
2 CNRS - Université Montpellier2, Pl. E. Bataillon, 34095 Montpellier, France<br />
3 Sensor Electronic Technology, Inc., Columbia, South Carolina 29209, USA<br />
4 Rensselaer Polytechnic Institute, Troy, New York 12180, USA<br />
An interest to fabricate voltage-tunable detectors <strong>of</strong> THz radiation based on plasma<br />
excitations in field-effect transistors (FETs) is marked by an effort to test new types <strong>of</strong><br />
structures and new materials. High electron mobility FETs based on GaN/AlGaN heterostructures<br />
<strong>of</strong>fer a possibility <strong>of</strong> high-voltage/high-current THz applications in agressive<br />
environments. However, to support technical developments, basic physical investigation<br />
must be carried out on technologically advanced devices.<br />
The goal <strong>of</strong> the present work is to describe magnetoplasmon excitations in a twodimensional<br />
electron gas in a GaN/AlGaN heterostructure. Our experiments were carried<br />
out on samples that have been recently shown to exhibit gate-voltage tunable plasmon<br />
resonances at zero magnetic field (B) [1]. The samples were large-area (about<br />
1.6 mm×1.6 mm) FETs with the gate electrode composed <strong>of</strong> two interdigitated comb-like<br />
structures. Five transistors with the gate periodicity between 1.5µm and 4µm were processed<br />
on a high-quality GaN/AlGaN heterostructure. The metal grating served both as<br />
a gate electrode controlling the concetration <strong>of</strong> a two-dimensional electron gas (n) and as<br />
a coupler between magnetoplasmons and incident THz radiation.<br />
Transmission ( a.u. )<br />
ν FIR = 2.52THz<br />
−3.5 −3 −2.5 −2 −1.5 −1 −0.5 0<br />
Gate Voltage ( V )<br />
B = 8T<br />
B = 6T<br />
B = 4T<br />
B = 2T<br />
B = 0T<br />
Fig. 1: Transmission <strong>of</strong> 2.52 THz radiation<br />
through a GaN/AlGaN FET as a function <strong>of</strong><br />
VG and B. Minima correspond to magnetoplasmon<br />
resonances.<br />
Bothmagnetoresistanceand magnetotransmission<br />
measurements were carried<br />
out at 4.2 K and B up to 10 T as a function<br />
<strong>of</strong> the gate polarization (VG). Shubnikov-de<br />
Haas oscillations <strong>of</strong> the magnetoresistance<br />
allowed us to estimate n as<br />
a function <strong>of</strong> the gate polarization (VG)<br />
and then an n-dependent electron quantum<br />
relaxation time τ. We found that τ<br />
dependsinanon-monotonicwayonnand<br />
ranges from 0.03 ps to about 0.1 ps.<br />
An example <strong>of</strong> magnetotransmission<br />
data shown in Fig. 1 gives a clear evidence<strong>of</strong>excitation<strong>of</strong>magnetoplamonresonances<br />
tunable by B and VG. Magnetoplasmons<br />
frequency, ωmp, can only approximately<br />
be described by a classical<br />
formula ωmp = �<br />
ω2 c +ω2 p. Deviations<br />
from this dependence are explained as a result <strong>of</strong> interaction <strong>of</strong> plasmons in gated and<br />
ungated parts <strong>of</strong> the transistor channel.<br />
This work was partially supported by the JU ENIAC MERCURE project No. 220120.<br />
[1] A. V. Muravjov et al., Appl. Phys. Lett. 96, 042105 (2010).<br />
103
TuO2 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Structural and electronic properties <strong>of</strong> functionalized graphene<br />
Karolina Milowska, Magdalena Birowska, and Jacek A. Majewski<br />
Faculty <strong>of</strong> Physics, University <strong>of</strong> Warsaw, ul. Hoża 69, PL-00-681 Warszawa, Poland<br />
We present extensive ab initio studies in the framework <strong>of</strong> the density functional theory<br />
(DFT) <strong>of</strong> graphene layers functionalized with simple molecules, which provide novel<br />
predictions for electronic structure <strong>of</strong> the functionalized graphene mono- and bilayers. The<br />
calculations reveal that the functionalization <strong>of</strong> graphene by simple molecules (e.g., such as<br />
OH, NH, and COOH) causes opening <strong>of</strong> the electronic band gap, just indicating new<br />
perspectives for design <strong>of</strong> field effect transistors (FETs) based on single and bilayer graphene.<br />
Remarkable electronic, mechanical and thermal properties <strong>of</strong> graphene have made it a<br />
promising candidate for a new generation <strong>of</strong> electronic devices [1]. However, the single and<br />
bilayer graphene have zero energy band gaps. Therefore, it hinders direct application <strong>of</strong> these<br />
systems in FETs. On the other hand, the functionalization <strong>of</strong> the graphene could change its<br />
electronic structure. Since quantitatively this effect is rather unknown, we have undertaken<br />
theoretical studies <strong>of</strong> this issue.<br />
In the present paper, we study the effects <strong>of</strong> functionalization <strong>of</strong> graphene with simple<br />
organic molecules (NHn, OH, and COOH) focusing on the stability and band gaps <strong>of</strong> the<br />
structures. We perform DFT calculations for graphene supercells with various numbers <strong>of</strong> the<br />
attached molecules. Employing the numerical package SIESTA, we completely optimize the<br />
geometry <strong>of</strong> the structures getting very good agreement with existing experimental data [2],<br />
and other theoretical calculations [3]. All studied groups bind covalently to the graphene layer<br />
implying sp 2 to sp 3 rehybridization and local deformation <strong>of</strong> the graphene plane. As example<br />
<strong>of</strong> our results, below we depict band<br />
structures <strong>of</strong> graphene layer<br />
functionalized with one single<br />
molecule per 3x3 graphene<br />
superlattice. The molecules are (a) -<br />
OH, (b) - NH, (c) – NH2, and (d) –<br />
COOH. The red dotted lines indicate<br />
band structure <strong>of</strong> pure graphene, the<br />
black ones the electronic structure <strong>of</strong><br />
functionalized graphene. In all cases,<br />
the zero band gap at K-point <strong>of</strong> pure graphene opens, being equal to 0.11, 0.12, 0.25, and 0.24<br />
eV for OH, NH, NH2, and COOH, respectively. Generally, the stability <strong>of</strong> the functionalized<br />
graphene layers decreases with the growing concentration <strong>of</strong> functionalizing molecules.<br />
Interestingly, for higher (1x1) concentrations <strong>of</strong> OH, we observe separation <strong>of</strong> O and H,<br />
which cover now other sides <strong>of</strong> the graphene layer. On the other hand, quite naturally, the<br />
band gap increases with the concentration <strong>of</strong> the attached molecules, e.g., in the case <strong>of</strong> 2x2<br />
cell functionalized with hydroxyl group (graphene’s surface is strongly bent), the band gap<br />
reaches 1.04 eV. All this shows clearly that the band gap <strong>of</strong> graphene layers can be tuned<br />
within fairly large range. Further, it turns out that the functionalized graphene layers exhibit<br />
magnetic moments, just implying some possible spintronic applications.<br />
This paper has been supported by the project “SiCMAT” (Innovative Economy, POIG.01.03.01-14-155/09-00).<br />
1. S. Park, and R. S Ru<strong>of</strong>f, Nature Nanotech., 4, 217 (2009).<br />
2. H. B. Su, R. J. Nielsen, A. C. T. van Duin, and W. A. Goddard, Phys. Rev. B 75 134107 (2007).<br />
3. Jia-An Yan, M. Y. Chou, Phys. Rev. B 82 125403 (2010).<br />
104
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors TuO3<br />
Analysis <strong>of</strong> the magnetic anisotropy in ultrathin GaMnAs<br />
O. Proselkov 1 , W. Stefanowicz 1 , S. Dobkowska 1 , J. Sadowski 1,2 ,<br />
T. Dietl 1,3 , M. Sawicki 1<br />
1 Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences, Warszawa, Poland<br />
2 MAX-Lab, Lund University, Lund, Sweden<br />
3 Institute <strong>of</strong> Theoretical Physics, University <strong>of</strong> Warsaw, Poland<br />
(Ga,Mn)As is the main test-bed material for testing concepts <strong>of</strong> future spintronics devices<br />
and its magnetic anisotropy is assumed to be reasonably well known both on the ground <strong>of</strong><br />
understanding <strong>of</strong> the experimental data and on the level <strong>of</strong> a description its microscopic<br />
origins. However, some very recent studies have brought new insights into the magnetic<br />
anisotropy <strong>of</strong> thin and ultrathin (Ga,Mn)As. Firstly, in very thin (Ga,Mn)As layers, despite an<br />
adequate high Mn concentration to assure their homogenous metallic character, a blocked<br />
superparamagnetic response has been found [1]. Then, it was postulated that the strength <strong>of</strong><br />
the ever-present in-plane uniaxial magnetic anisotropy was related to nanometer-scale ripples<br />
on the (Ga,Mn)As surface [2]. Furthermore, a collapse in the magnitude <strong>of</strong> the anomalous<br />
Hall effect was found for thin layers at low temperatures [3]. Finally, a cycloidal spin<br />
arrangement and the accompanying uniaxial in-plane anisotropy <strong>of</strong> diagonal [110]/[1-10]<br />
directions was suggested to form in very thin layers [4].<br />
As all these findings indicate that the long celebrated macrospin approach [5] to the<br />
phenomenological description <strong>of</strong> the magnetic anisotropy may cease its validity in ultrathin<br />
layers, an examination <strong>of</strong> the conditions <strong>of</strong> its applicability in this technological important<br />
case is timely and important. Simultaneously, means for assessment <strong>of</strong> the abovementioned<br />
concepts are to be defined and tested. For these purposes we investigate in greater detail<br />
temperature dependence <strong>of</strong> the magnetic anisotropy <strong>of</strong> 4 nm Ga0.925Mn0.075As, a layer that<br />
according do [1] exhibits two types <strong>of</strong> magnetic responses: a temperature reversible one<br />
typical for a long-range-coupled magnetization and an irreversible one possessing<br />
characteristics similar to a blocked superparamagnet. We find that we can self consistently<br />
describe our findings on the grounds <strong>of</strong> the macrospin model as in [5] only above a<br />
temperature at which signatures <strong>of</strong> blocked superparamagnetic response disappears (25 K in<br />
this case). Below, the same model yields conflicting results which depend on the magnetic<br />
history <strong>of</strong> the sample. However, if applied at this temperature range, this approach implies<br />
disappearance <strong>of</strong> magnetic anisotropy at temperatures corresponding to the mean blocking<br />
temperature <strong>of</strong> the superparamagnetic part <strong>of</strong> the layer and a change <strong>of</strong> sign <strong>of</strong> the cubic<br />
anisotropy at the lowest temperatures. The last findings point to a strong coupling between<br />
these two major constituencies <strong>of</strong> the magnetic response in such a layers.<br />
The work was supported in part by the European Research Council through the FunDMS<br />
Advanced Grant within the "Ideas" 7th Framework Programme <strong>of</strong> the EC, EC Network<br />
SemiSpinNet (PITN-GA-2008-215368) and Polish MNiSW 2048/B/H03/2008/34 grant.<br />
[1] M. Sawicki, et al., Nature Phys. 6, 22 (2010); S. Dobkowska, et al., this conference.<br />
[2] S. Piano, et al., arXiv:1010.0112.<br />
[3] D. Chiba et al., Phys. Rev. Lett. 104, 106601 (2010).<br />
[4] A. Werpachowska and T. Dietl, Phys. Rev. B 82, 085204 (2010).<br />
[5] K.-Y. Wang, et al., Phys. Rev. Lett., 95, 217204 (2005).<br />
105
TuO4 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Spin-lattice relaxation <strong>of</strong> a single Mn 2+ ion in a CdTe quantum dot<br />
M. Goryca 1,2 , P. Plochocka 2 , P. Wojnar 3 , T. Kazimierczuk 1 , M. Potemski 2<br />
and P. Kossacki 1,2<br />
1 Institute <strong>of</strong> Experimental Physics, Faculty <strong>of</strong> Physics, University <strong>of</strong> Warsaw, Warsaw, Poland<br />
2 Grenoble High Magnetic Field Laboratory, CNRS, Grenoble, France<br />
3 Polish Academy <strong>of</strong> Sciences, Warsaw, Poland<br />
From the point <strong>of</strong> view <strong>of</strong> miniaturization <strong>of</strong> magnetic memory devices a quantum dot<br />
(QD) containing a single magnetic impurity is an interesting system. It has been demonstrated<br />
that it gives unique possibility to read and manipulate the electronic spin state <strong>of</strong> the single<br />
atom [1-3]. Although principles <strong>of</strong> using such system as a memory device are known, further<br />
studies <strong>of</strong> interaction <strong>of</strong> the magnetic ion with its neighborhood are still needed. Particularly a<br />
detailed knowledge about the spin-lattice relaxation is <strong>of</strong> a great importance, as this kind <strong>of</strong><br />
relaxation is a main factor limiting the information storage time.<br />
In this work we present spectroscopic studies <strong>of</strong> the spin-lattice relaxation <strong>of</strong> a single<br />
Mn 2+ ion embedded in a CdTe/ZnTe QD, performed in high magnetic field. The experiment<br />
was done in a micro-photoluminescence setup at temperature <strong>of</strong> about 15 K. The magnetic<br />
field up to 12 T was produced with a superconductive magnet and applied in the Faraday<br />
configuration, parallel to the growth axis <strong>of</strong> the sample. The sample contained a single layer<br />
<strong>of</strong> self-assembled QDs. The PL <strong>of</strong> the QDs was excited using a tunable dye laser in the range<br />
570−610 nm. The spin-conserving excitation transfer between two QDs [4] was used to inject<br />
spin-polarized excitons into the QD containing the Mn 2+ ion and to orient its spin [3].<br />
The spin-lattice relaxation time <strong>of</strong> the Mn 2+ ion was determined in two independent<br />
experiments. In the first one we measured the efficiency <strong>of</strong> the optical orientation <strong>of</strong> the Mn 2+<br />
ion [3] as function <strong>of</strong> excitation power and magnetic field. Thus we were able to examine in<br />
detail the competition between two processes: the optical orientation <strong>of</strong> the Mn spin by spinpolarized<br />
excitons injected into the QD and thermalization <strong>of</strong> the magnetic ion in external<br />
magnetic field caused by spin-lattice relaxation. When these processes exactly compensate<br />
each other, i.e. the spin orientation rate is exactly equal to the relaxation rate, then the Mn 2+ is<br />
completely depolarized. Thus the spin orientation rate can be estimated using the exciton<br />
injection rate to the QD and probability <strong>of</strong> the Mn spin flip per one exciton [3].<br />
In the second, time-resolved experiment we measured directly the Mn spin relaxation<br />
time. We used the same mechanism as previously to orient the magnetic ion. Once it reached<br />
the steady state we turned the excitation <strong>of</strong>f for a certain period. Then we turned the excitation<br />
on again to probe the Mn spin. By varying the length <strong>of</strong> the dark period in the range <strong>of</strong> 20 ns –<br />
500 µs we were able to measure the relaxation time in the wide range <strong>of</strong> magnetic fields.<br />
Both experiments gave the same results within experimental accuracy. The spin-lattice<br />
relaxation time <strong>of</strong> the Mn 2+ ion in the QD for low magnetic field (around 1 T) reaches<br />
hundreds <strong>of</strong> microseconds and is comparable to the value measured for bulk, diluted<br />
(Cd,Mn)Te material [5]. It is, however, decreasing rapidly for high magnetic field and at 12T<br />
reaches value <strong>of</strong> tens <strong>of</strong> nanoseconds, which is over two orders <strong>of</strong> magnitude smaller than the<br />
one observed for bulk material. Possible origins <strong>of</strong> this surprising difference include strain<br />
fields, usually high in structures containing QDs.<br />
[1] L. Besombes et al., Phys. Rev. Lett. 93, 207403 (2004).<br />
[2] C. Le Gall et al, Phys. Rev. Lett. 102, 127402 (2009).<br />
[3] M. Goryca et al, Phys. Rev. Lett. 103, 087401 (2009).<br />
[4] T. Kazimierczuk et al, Phys. Rev. B 79, 153301 (2009).<br />
[5] T. Strutz et al., Phys. Rev. Lett. 68, 3912 (1992).<br />
106
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors TuO5<br />
Magnetophotoluminescence <strong>of</strong> CdTe/ZnTe Quantum Dots:<br />
G-factor and Diamagnetic Shift Variation in a Single Dot<br />
Tomasz Kazimierczuk 1 , Mateusz Goryca 1,2 , Piotr Wojnar 3 ,<br />
Andrzej Golnik 1 , Piotr Kosacki 1,2<br />
1 Institute <strong>of</strong> Experimental Physics, Faculty <strong>of</strong> Physics,<br />
University <strong>of</strong> Warsaw, ul. Hoża 69, PL-00-681 Warsaw, Poland<br />
2 LNCMI-CNRS (UJF, UPS, INSA), BP 166, 38042 Grenoble Cedex 9, France<br />
3 Institute Physics, Polish Academy <strong>of</strong> Science,<br />
Al. Lotników 32/46, PL-02-668 Warsaw, Poland<br />
Typically, self-assembled quantum dots (QDs) are characterized by strong confining<br />
potential, which defines carrier wavefunctions. A flat shape <strong>of</strong> such QDs lifts the degeneracy<br />
between light and heavy holes in favor <strong>of</strong> the latter ones. Due to the selection rules, the<br />
recombination occurs between an electron and a hole <strong>of</strong> opposite spin directions. Thus, in the<br />
magnetic field along growth direction, all transitions with carriers in the ground state exhibit<br />
Zeeman given by a difference between electron and hole Landé factors. Particularly, the same<br />
Zeeman splitting should be observed for X, X + , X - , and XX transitions. Such a predictiction<br />
was tested already in early stages <strong>of</strong> QD research demonstrating equality between Landé<br />
factors for neutral and charged exciton with precision better than 1% [1].<br />
In this work we present a comparison <strong>of</strong> magnetic field influence on various excitonic<br />
transitions in single CdTe/ZnTe QDs. The experiments included PL measurements in<br />
magnetic field up to 28T applied either<br />
perpendicular or parallel to the growth direction.<br />
Recorded data allowed us to extract two basic<br />
parameters: excitonic Landé factor related to linear<br />
Zeeman effect and diamagnetic shift related to<br />
quadratic field dependence. Contrary to the picture<br />
described previously, we discovered a clear<br />
systematic difference between g-factors inferred<br />
from X, X + , and X - transitions in Faraday<br />
configuration (Fig. 1). Landé factor inferred from<br />
the XX transition follows exactly values related to<br />
X transition, as in both cases the spectral effect is<br />
related to splitting <strong>of</strong> the same state (being final or<br />
initial one respectively). The average difference<br />
between X + and X - g-factors yields 21%.<br />
Similar but smaller difference between g-factors inferred from different excitonic<br />
transitions was found in the in-plane field geometry. In such a geometry, qualitatively similar<br />
behavior was reported for pyramidal InGaAs/AlGaAs QDs [2] and interpreted in terms <strong>of</strong><br />
Coulomb correlations. In our case, such an interpretation is supported by analysis <strong>of</strong><br />
diamagnetic coefficients. The lack <strong>of</strong> significant systematic difference in diamagnetic shift<br />
allows us to reject an alternative explanation <strong>of</strong> g-factor variation by increase/decrease <strong>of</strong><br />
barrier penetration by weakly confined hole.<br />
[1] J. J. Finley et al., Phys. Rev. B 66, 153316 (2002).<br />
[2] D. Y. Oberli et al., Phys. Rev. B 80, 165312 (2009).<br />
107<br />
g-factor <strong>of</strong> X + , X - , and XX<br />
3.0<br />
2.5<br />
2.0<br />
1.5<br />
XX<br />
X +<br />
X -<br />
1.0<br />
1.0 1.5 2.0 2.5 3.0<br />
g-factor <strong>of</strong> X<br />
Fig. 1. Correlation between Landé factors<br />
inferred from different excitonic transitions<br />
in 24 different dots.
TuO6 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
ControloverCdTeQuantumDotsEmissionusingZnTe-based<br />
MicropillarCavities<br />
T.Jakubczyk 1 ,W.Pacuski 1 ,A.Golnik 1 ,C.Kruse 2 ,D.Hommel 2 ,<br />
H.Hilmer 3 ,R.Schmidt-Grund 3 and J.A.Gaj 1<br />
1 Institute<strong>of</strong>ExperimentalPhysics,University<strong>of</strong>Warsaw,ul.Hoża69,00-681Warsaw,<br />
Poland<br />
2 Institute<strong>of</strong>SolidStatePhysics,University<strong>of</strong>Bremen,Postfach330440,D-28334,<br />
Bremen,Germany<br />
3 InstituteforExperimentalPhysicsII,UniversitätLeipzig,Linnéstr.5,D-04103,<br />
Leipzig,Germany<br />
Pillarmicrocavitiescontainingquantumdots(QDs)werereportedassystemsenablingcontroloverspatial,energeticandtemporalaspects[1]<strong>of</strong>lightemission.Interms<strong>of</strong><br />
applicationsinquantuminformationprocessing[2],coherentcouplingbetweenlightand<br />
matterobtainedinsuchsystemsis<strong>of</strong>primaryimportance.Inthiscontextanextension<br />
<strong>of</strong>theresearchfromtheGaAs-basedsystemstowiderband-gapII-VIbasedonesispromising.Therobustexcitonicstatesandastrongercarrierconfinementinthesematerials<br />
extendthefunctionalityrange<strong>of</strong>suchQDsystemstohighertemperatures.<br />
Inthiscommunicationwereportamicrophotoluminescencestudy<strong>of</strong>pillarcavities<br />
basedonsuperlattice-baseddistributedBraggreflectors(DBRs)[3],lattice-matchedto<br />
ZnTe.Micropillars<strong>of</strong>diametersrangingfrom0.7to5µmwereetchedbyfocusedion<br />
beam(FIB)out<strong>of</strong>aplanarmicrocavitybasedontheseDBRsandcontainingasingle<br />
plane<strong>of</strong>CdTe/ZnTequantumdots.<br />
Boththeemission<strong>of</strong>asingleQDaswellasemission<strong>of</strong><br />
anensemble<strong>of</strong>themwasusedtoexaminedifferentproperties<strong>of</strong>thestructures.Wepresentresultsprovingtheability<strong>of</strong>ZnTe-basedpillarstoenhancetheQDsemission(see<br />
Figure).Furthermore,wepresenttheirabilitytoguidethis<br />
emissionresultinginanenhancedcollectionrate.Thesefeaturesstronglyindicatethepossibility<strong>of</strong>constructingefficientsinglephotonsourcesbasedonCdTeQDs.<br />
Toinhibittheunwantedemissionintotheleakymodes<br />
anewapproachwasused.AlateralDBRmade<strong>of</strong>yttriastabilizedzirconiaandaluminawassuccessfullydeposited.Wepresentadetailedphotoluminescencestudy<strong>of</strong>theobtainedstructures.<br />
[1]J.M.Gérard,B.Sermage,B.Gayral,B.Legrand,E.Costard,andV.Thierry-Mieg,<br />
Phys.Rev.Lett.81,1110<br />
(1998).<br />
[2]J. P. Reithmaier, G. S ֒ ek, A. Löffler, C. H<strong>of</strong>mann,<br />
S.Kuhn,S.Reitzenstein,L.V.Keldysh,V.D.Kulakovskii,T.L.Reinecke,andA.Forchel,Nature432,197<br />
(2004).<br />
[3]W.Pacuski,C.Kruse,S.Figge,andD.Hommel,Applied<br />
PhysicsLetters94,191108(2009).<br />
108
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors TuP1<br />
Exciton and Mn spin dynamics in a nonresonantly coupled pair<br />
<strong>of</strong> (Cd,Mn)Te quantum dots<br />
T. Smoleński 1 , ̷L. Cywiński 2 , M. Goryca, 1 and P. Kossacki 1<br />
1 Institute <strong>of</strong> Experimental Physics, Faculty <strong>of</strong> Physics, University <strong>of</strong> Warsaw, ul. Ho˙za<br />
69, 00-681 Warsaw, Poland<br />
2 Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences, Al. Lotników 32/46, 02-668 Warsaw,<br />
Poland<br />
It has been shown recently that a spin <strong>of</strong> a single Mn ion residing inside a quantum<br />
dot can be optically oriented by circularly polarized light creating spin-polarized excitons<br />
in the dot [1,2]. In case <strong>of</strong> experiments in which the absoprtion line corresponding to a<br />
specific spin state <strong>of</strong> the Mn was excited resonantly [2], it was argued that the spin relaxation<br />
<strong>of</strong> holes (due to phonon scattering and spin-orbit interaction, leading to transitions<br />
between bright and dark exciton states seen in experiment in Ref. [2]) can explain the<br />
existence <strong>of</strong> the Mn optical orientation [3].<br />
In experiment from Ref. [1] the spin-polarized excitons were created in the Mn free<br />
dot which was nonresonantly coupled to an adjacent dot [4] containing a single Mn spin<br />
[1,5]. The exciton (X) tunnels out <strong>of</strong> the first dot in a few ps [4], but the timescale on<br />
which it relaxes to its ground state in the second dot (thereby lowering its energy by<br />
≈ 0.2 eV) is not precisely know. Depending on whether this relaxation is fast or slow<br />
compared to the dynamics <strong>of</strong> the exciton coupled to the Mn spin, different states <strong>of</strong> the<br />
X+Mn system are expected to be realized after the relaxation. In order to model both the<br />
dynamics <strong>of</strong> the photoluminescence from the second dot, and the Mn spin dynamics (due<br />
to the sp-d exchange coupling with the exciton), we use the Lindblad generator formalism<br />
to describe all the relaxation process: the transition from the first dot to the ground<br />
state <strong>of</strong> the second dot, the radiative recombination, and the carrier spin relaxation. In<br />
this way we model both the coherent and the incoherent the processes leading to the<br />
brightening <strong>of</strong> dark excitons [5], possible population trapping in the dark state, and Mn<br />
optical orientation.<br />
This work was supported by InTechFun (Grant No. POIG.01.03.01-00-159/08), by the<br />
Homing and the Start programmes <strong>of</strong> the Foundation for Polish Science supported by the<br />
EEA Financial Mechanism, and by the Polish Ministry <strong>of</strong> Science and Higher Education<br />
as research grants in years 2010-<strong>2011</strong>.<br />
[1] M. Goryca, T. Kazimierczuk, M. Nawrocki, A. Golnik, J.A. Gaj, P. Kossacki, P. Wojnar,<br />
and G. Karczewski, Phys. Rev. Lett. 103, 087401 (2009).<br />
[2] C. Le Gall, L. Besombes, H. Boukari, R. Kolodka, J. Cibert, and H. Mariette, Phys.<br />
Rev. Lett. 102, 127402 (2009).<br />
[3] ̷L. Cywiński, Phys. Rev. B 82, 075321 (2010).<br />
[4] T. Kazimierczuk, J. Suffczyński, A. Golnik, J.A. Gaj, P. Kossacki, and P. Wojnar,<br />
Phys. Rev. B 79, 153301 (2009).<br />
[5] M. Goryca, P. Plochocka,T. Kazimierczuk, P. Wojnar, G. Karczewski, J.A. Gaj, M. Potemski,<br />
and P. Kossacki Phys. Rev. B 82, 165323 (2010).<br />
109
TuP2 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Three-dimensional anisotropy studies <strong>of</strong> CdTe quantum dots<br />
J. Kobak 1 , W. Pacuski 1 , T. Kazimierczuk 1 , J. Suffczy ski 1 , T. Jakubczyk 1 , A. Golnik 1 ,<br />
P. Kossacki 1 , M. Nawrocki 1 , J.A. Gaj 1 , C. Kruse 2 , D. Hommel 2<br />
1 Institute <strong>of</strong> Experimental Physics, Faculty <strong>of</strong> Physics, University <strong>of</strong> Warsaw,<br />
ul. Ho a 69, PL-00-681 Warszawa, Poland<br />
2 Institute <strong>of</strong> Solid State Physics, University <strong>of</strong> Bremen, PO.Box 330 440, D-28334 Bremen,<br />
Germany<br />
In standard microphotoluminescence study <strong>of</strong> quantum dots (QDs) optical axis is<br />
oriented parallel to the growth axis <strong>of</strong> the sample. Therefore polarization <strong>of</strong> emitted photons is<br />
perpendicular to the growth axis <strong>of</strong> the sample. In this work we present the results <strong>of</strong> microand<br />
macro-photoluminescence study <strong>of</strong> QDs in a configuration with optical axis perpendicular<br />
to the growth axis. Such measurements were possible since the samples were cleaved and<br />
their edge was exposed. In our measurements we explore both polarizations parallel and<br />
perpendicular to the growth axis.<br />
We studied selforganized CdTe QDs in ZnTe matrix grown by molecular beam<br />
epitaxy (MBE). Two techniques were used in order to form dots from thin CdTe layer. In the<br />
first technique the CdTe thin layer was covered by amorphous tellurium at a low temperature.<br />
Subsequently amorphous tellurium was evaporated at growth temperature before overgrowing<br />
with ZnTe. In the second technique, the CdTe thin layer was overgrown by ZnTe with high<br />
Zn content, without changing the temperature. For spectroscopy samples were placed under<br />
the microscope lens which position was controlled by three xyz piezo translation stages.<br />
Whole system was immersed in superfluid helium in a magneto-optical cryostat. Lines<br />
associated to the single QDs we identified with use <strong>of</strong> single photon correlation<br />
measurements.<br />
Two emission bands were observed in the macro-photoluminescence measurements.<br />
Band seen at lower energy was attributed to quantum dots. This band is strongly polarized in<br />
direction perpendicular to the growth axis <strong>of</strong> the sample. Such effect is expected for heavy<br />
hole excitons. The band located at higher energy was identified as coming from the wetting<br />
layer. It shows a small degree <strong>of</strong> linear polarization with a stronger component in direction<br />
parallel to the growth axis.<br />
In micro-photoluminescence measurements we have found that more than 99% <strong>of</strong><br />
QDs show the complete polarization perpendicular to the growth axis. This is in agreement<br />
with macro-photoluminescence results and with prediction done for heavy hole excitons.<br />
Surprisingly, we have found also sharp emission lines exhibiting a weak polarization degree.<br />
Moreover, for them the stronger polarization was parallel to the growth axis. Apart from<br />
polarization properties, such lines exhibit typical properties <strong>of</strong> QDs: excitation power<br />
dependence and temporal evolution such as biexciton – exciton cascade observed in photon<br />
correlation measurements.<br />
We have done magneto-optical measurements to get a deeper understanding <strong>of</strong> the<br />
observed unusual polarization anisotropy. We applied two configurations: with a magnetic<br />
field in a direction parallel or perpendicular to the growth axis. Results <strong>of</strong> the polarization<br />
resolved magnetooptical measurements are discussed in terms <strong>of</strong> multi-axis anisotropy <strong>of</strong><br />
QDs.<br />
110
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors TuP3<br />
Probing electron concentration in epitaxial graphene<br />
using Raman spectroscopy<br />
K.Grodecki 1,2 , W.Strupinski 2 , A.Wysmołek 1 , R.Stępniewski 1 , and J.M.Baranowski 2,1<br />
1 Faculty <strong>of</strong> Physics, University <strong>of</strong> Warsaw, Hoża 69, 00-681 Warsaw, Poland<br />
kacper.grodecki@fuw.edu.pl<br />
2 Institute <strong>of</strong> Electronic Materials Technology, Wólczynska 133, 01-919 Warsaw, Poland<br />
Over the past years, a lot <strong>of</strong> attention has been attracted to graphene. Its unique<br />
properties could make it the successor to silicon in electronic applications. Now one <strong>of</strong> the<br />
essential problems that must be solved in graphene technology is accurate control <strong>of</strong> the<br />
carrier concentration in the structure. Within this communication we would like to let know<br />
that this problem can be effectively solved by the use <strong>of</strong> Raman spectroscopy. The Raman<br />
spectra <strong>of</strong> graphene-like structures consist <strong>of</strong> several bands. The most prominent among them<br />
are so called G and 2D bands. The position <strong>of</strong> the 2D band is predominantly strain sensitive<br />
whereas the position <strong>of</strong> the G band depends both on the intensity and the doping [1, 2].<br />
In this communication we show how frequency measurements <strong>of</strong> G and 2D-band may<br />
be used to evaluate free carrier concentration in graphene layer. In order to validate this<br />
method we have examined a set <strong>of</strong> graphene samples with known electron concentration.<br />
The measured graphene layers were grown on Si face <strong>of</strong> 4H-SiC on-axis substrates by<br />
chemical vapor deposition technique [3]. As determined from the Hall measurements the<br />
electron concentration <strong>of</strong> different samples remained between 6x10 12 and 2.4x10 13 cm -2 .<br />
Micro-Raman measurements were performed at room temperature using single grating<br />
spectrometer equipped with longpass filters and CCD camera. The size <strong>of</strong> exciting light spot<br />
from 532nm Nd:YAG continuous wave laser at the sample surface was approximately 2mm<br />
in diameter.<br />
The micro-Raman measurements performed on the samples with different electron<br />
concentration revealed pronounced shifts <strong>of</strong> the G and 2D band positions. Basing on the<br />
strain dependences <strong>of</strong> these bands from literature data [2] the electron concentration induced<br />
shift was calculated. This result - 1cm -1 /10 12 cm 2 - remains in compliance with the<br />
measurements performed within this concentration range for a gated structure freestanding<br />
graphene measurements [1]. This proves that Raman spectroscopy can be very useful as a fast<br />
contactless method which allows to determinate the local carrier concentration and strain (up<br />
to micrometer scale) in graphene structures.<br />
[1] A. Das, S. Pisana, B.Chakraborty, S. Piscanec, S. K. Saha, U. V. Waghmare, K. S.<br />
Novoselov, H. R. Krishnamurthy, A. K. Gaim, A. C. Ferrari and A. K. Sood, Nature<br />
Nanotechnology 3, 210 (2008)<br />
[2] N. Ferralis, Journal <strong>of</strong> Materials Science 45, 19, (<strong>2011</strong>)<br />
[3] W. Strupinski, K. Grodecki, A. Wysmolek, R. Stepniewski, T. Szkopek, P. E. Gaskell, A.<br />
Gruneis, D. Haberer, R. Bozek, J. Krupka, and J. M. Baranowski, NANO Letters (<strong>2011</strong>) DOI:<br />
10.1021/nl200390e<br />
111
TuP4 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Dynamical control <strong>of</strong> excitonic fine structure with nanomechanical strain<br />
M. Zielinski 1 , G. W. Bryant 2 , N. Malkova 2 , J. Sims 3 , W. Jaskolski 1 , J. Aizupura 4<br />
1 <strong>Instytut</strong> <strong>Fizyki</strong> UMK, ul. Grudziądzka 5, 87-100 Toruń, Poland<br />
2 Atomic Physics Division and Joint Quantum Institute, NIST, 100 Bureau Drive,<br />
Gaithersburg, Maryland 20899-8423, USA<br />
3 Information Technology Laboratory, NIST, 100 Bureau Drive, Gaithersburg, Maryland<br />
20899-8423, USA<br />
4 Centro Mixto de Fisica de Materiales CSIC-UPV/EHU and Donostia International Physics<br />
Center, Paseo Manuel Lardizabal 4, Donostia-San Sebastian 20018, Spain<br />
We show how nanomechanical strain [1,2] can be used to achieve dynamical control<br />
<strong>of</strong> electronic and optical properties <strong>of</strong> semiconductor quantum dots, including exciton fine<br />
structure <strong>of</strong> single quantum dots and charge distributions in double quantum dots with<br />
potential applications in quantum dot based schemes for entangled photon-pair generation and<br />
manipulating interacting qubits for quantum information processing.<br />
We use atomistic tight-binding theory [1-4] to describe the response <strong>of</strong> confined states<br />
in a self-assembled quantum dot embedded in a nanomechanical cantilever. The internal strain<br />
due to the lattice mismatch, the external strain and the internal readjustment to minimize the<br />
applied strain must all be accounted for to model correctly the strain effects [1,2]. We show<br />
that applied strain can be used to manipulate the fine structure splitting <strong>of</strong> mechanoexcitons<br />
by distorting electron and hole charge distributions and rotating hole orientation. Electrons<br />
and hole levels and charge distributions can shift together or in opposite directions depending<br />
on how the strain is applied. This gives control for tailoring band gaps and optical response.<br />
The strain can also be used to transfer electrons and holes between vertically or laterally<br />
coupled dots, giving a mechanism for manipulating transition strengths and interacting qubits<br />
for quantum information processing.<br />
We demonstrate that nanomechanical strain tunes the exchange splittings and rotates<br />
the polarization <strong>of</strong> mechanoexcitons, providing energy and phase control <strong>of</strong> excitons. We<br />
further demonstrate that nanomechanical strain is a tool to shift electron and hole levels,<br />
transfer carriers between dots, manipulate mechanoexciton shape, orientation, fine-structure<br />
splitting and optical transitions rates.<br />
[1] G.W. Bryant, M. Zielinski, N. Malkova, J. Sims, W. Jaskolski and J. Aizpurua, Phys. Rev.<br />
Lett. 105, 067404 (2010).<br />
[2] G.W. Bryant, M. Zielinski, N. Malkova, J. Sims, W. Jaskolski, J. Aizpurua, prepared for<br />
publication in Phys. Rev. B<br />
[3] W. Jaskolski, M. Zielinski, G.W. Bryant, J. Aizpurua, Phys. Rev. B 74, 205309 (2006).<br />
[4] M. Zielinski, M. Korkusinski, and P. Hawrylak, Phys. Rev. B 81, 085301 (2010).<br />
112
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors TuP5<br />
Non-Markovian noise at the Fermi-edge singularity in quantum<br />
dots<br />
Katarzyna Roszak 1 and Tomáˇs Novotn´y 2<br />
1 Institute <strong>of</strong> Physics, Wroc̷law University <strong>of</strong> Technology, 50-370 Wroc̷law, Poland<br />
2 Department <strong>of</strong> Condensed Matter Physics, Faculty <strong>of</strong> Mathematics and Physics,<br />
Charles University, 12116 Prague, Czech Republic<br />
The Fermi-edge singularity is a phenomenon originating from the interaction <strong>of</strong> conduction<br />
electrons with localised perturbations and is characterised by a power-law divergence.<br />
It was first predicted theoretically for X-ray absorption in metals [1] and later<br />
verified experimentally [2]. The theory then developed [1,3] has been used to describe<br />
other situations involving a similar Hamiltonian and leading to the same power-law divergence,<br />
such as resonant tunnelling through localised levels. The proper description <strong>of</strong><br />
the transport set-up is done by an extension <strong>of</strong> the original Fermi-edge singularity model,<br />
where no charge transfer between the continuum and the localized level(s) is considered,<br />
to the interacting resonant level model, which has served recently as an important benchmark<br />
for novel quantum transport techniques [4].<br />
The Fermi-edge singularity in transport through quantum dots occurs in the regime<br />
where the energy <strong>of</strong> the quantum dot level(s) is similar to that <strong>of</strong> one <strong>of</strong> the leads. The<br />
change <strong>of</strong> occupation in the local level(s) during the tunnelling process leads to sudden<br />
changes in the scattering potential and hence, to truncated singular behaviour <strong>of</strong> the<br />
current through the quantum dot at resonance and power-law dependence <strong>of</strong> the current<br />
away from resonance [5,6]. As shown in recent experiments [6], noise in this regime<br />
displays characteristic behaviour (including a super-Poissonian maximum) which cannot<br />
be accounted for by the Markovian theory. Since the Fermi-edge singularity transport<br />
setup involves both many-body correlations and quantum coherence it is reasonable to<br />
expect large non-Markovian corrections in the singularity region. We show that this is<br />
indeed the case and that including the non-Markovian effects allows us to account for all<br />
<strong>of</strong> the qualitative features <strong>of</strong> the noise.<br />
[1] G. D. Mahan, Phys. Rev. 163, 612 (1967).<br />
[2] P. H. Citrin, Phys. Rev. B 8, 5545 (1973).<br />
[3] G. D. Mahan, Many-Particle Physics (Kluwer, New York, 2000).<br />
[4] E. Boulat, H. Saleur, and P.Schmitteckert, Phys. Rev. Lett.101, 140601 (2008).<br />
[5] H. Frahm, C. von Zobeltitz, N. Marie, and R. J. Haug, Phys. Rev. B74,035329 (2006).<br />
[6] N. Marie, F. Hohls, T. Lüdtke, K.Pierz, R. J. Haug, Phys. Rev. B 75, 233304 (2007).<br />
113
TuP6 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Transport and Spin Properties <strong>of</strong> CdTe/CdMgTe Quantum Point Contacts<br />
M. Czapkiewicz, J. Wróbel, V. Kolkovsky, P. Nowicki, M. Aleszkiewicz, M. Wiater and<br />
T. Wojtowicz<br />
<strong>Instytut</strong> <strong>Fizyki</strong> <strong>PAN</strong>, Warszawa, Poland<br />
The spin properties <strong>of</strong> one-dimensional (1D) semiconductor devices are more strongly influenced<br />
by electron-electron interactions as compared to two-dimensional (2D) systems. Up to<br />
now, however, studies <strong>of</strong> spin characteristics <strong>of</strong> 1D transport channels are almost exclusively<br />
devoted to III-V heterostructures and quantum wells [1]. Here we report on fabrication and<br />
low temperature magnetotransport measurements <strong>of</strong> quantum point contacts, patterned from ntype<br />
CdTe/CdMgTe modulation doped quantum well. We expect that the many-body effects are<br />
quite important in this material since the effective mass is larger and the dielectric constant is<br />
smaller as compared to GaAs.<br />
Two-terminal quantum point contacts<br />
have been made <strong>of</strong> high quality 2D electron<br />
gas with concentration n2D= 4.6×10 11 cm −2<br />
and mobility µ = 0.2×10 6 cm 2 /Vs. The<br />
nanojunctions <strong>of</strong> length L≈0.2 µm and lithographic<br />
width Wlith = 0.45±0.01 µm are<br />
patterned by e-beam lithography and deepetching<br />
techniques. The carrier density in the<br />
device is controlled by means <strong>of</strong> V-shaped<br />
side gates which are separated from the constriction<br />
area by narrow etched grooves, see<br />
Fig. 1. The differential conductances G have<br />
been measured in a He-3 cryostat as a function<br />
<strong>of</strong> dc source-drain excitation and inplane<br />
magnetic field by employing a standard<br />
low-frequency lock-in technique.<br />
SG<br />
5<br />
10<br />
15<br />
µm<br />
etched grooves<br />
SG<br />
Figure 1: Atomic force microscopy image<br />
<strong>of</strong> quantum point contact formed by chemical<br />
etching. The width <strong>of</strong> nano-junction is controlled<br />
by side gates (SG).<br />
Data show that G is quantized vs gate voltage, however, conductance steps are reduced<br />
down to≈ 0.6×(2e 2 /h) and superimposed on reproducible conductance fluctuations (UCF’s).<br />
We have also found, that the quantization/UCF patterns change considerably when voltages are<br />
applied asymmetrically to the side gates. Therefore, we argue that transport in studied devices<br />
is strongly influenced by short range potential fluctuations present in the junction area. However,<br />
in spite <strong>of</strong> a disorder, we have been able to observe the Zeeman splitting <strong>of</strong> 1D transport<br />
channels at milikelvin temperatures. By comparing obtained splittings with the nonlinear conductance<br />
data we have been able to determine the effective Landé factor in the constriction. We<br />
have found that |g ∗ | = 1.7±0.1 in remarkable agreement with bulk value reported for CdTe,<br />
independently on 1D level index. We conclude that there is no enhancement <strong>of</strong> electronic g factor.<br />
This is an unexpected result, since such enhancement, attributed to exchange interaction,<br />
was always observed for quantum point contacts and quantum wires made <strong>of</strong> GaAs [2].<br />
We acknowledge the support from the Polish Ministry <strong>of</strong> Science and Higher Education,<br />
project number N202/103936 and partial support by the European Union within European Regional<br />
Development Fund (grant Innovative Economy POIG.01.01.02-00-008/08).<br />
[1] K.-F. Berggren and M. Pepper, Phil. Trans. R. Soc. A 368, 1141 (2010).<br />
[2] K. J. Thomas et al., Phys.Rev. B 68, 4846 (1998).<br />
114
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors TuP7<br />
The Role <strong>of</strong> Strong Coupling in the Superradiance <strong>of</strong> Ensembles<br />
<strong>of</strong> Quantum Dots.<br />
Micha̷l Kozub, Pawe̷l Machnikowski<br />
1 Institute <strong>of</strong> Physics, Wroc̷law University <strong>of</strong> Technology, 50-370 Wroc̷law, Poland<br />
We present a theoretical description <strong>of</strong> spontaneous emision from a dense ensemble<br />
<strong>of</strong> self-assembled quantum dots (QDs) in the limit <strong>of</strong> weak excitation. We show that<br />
the decay rate <strong>of</strong> the spontaneous emission from the system depends on the number <strong>of</strong><br />
collectively emitting dots, thus confirming the experimental conclusion [1] on the appearance<br />
<strong>of</strong> collective (superradiant) emission in the inhomogeneous ensamble <strong>of</strong> interacting<br />
dots. However our simulations indicate that interactions over a common electromagnetic<br />
reservoir are not sufficiently strong to account for the observed increase <strong>of</strong> the decay rate.<br />
We therefore introduce additional terms in the interaction potential to make up for the<br />
discrepancy between the experiment and our simulations.<br />
In the experiment [1], the decay rate <strong>of</strong> the luminescence signal was analysed as a<br />
function <strong>of</strong> the size <strong>of</strong> mesas cut out in a planar ensemble <strong>of</strong> QDs, that is, <strong>of</strong> the number<br />
<strong>of</strong> dots collectively interacting with the electromagnetic field. The observed decay <strong>of</strong> luminescence<br />
after a resonant excitation had an approximately exponential character with<br />
the decay rate increasing as the mesas became larger. This was interpreted as a collective<br />
effect, due to long range coupling between the dots.<br />
In this work we model the ensemble <strong>of</strong> QDs as a system <strong>of</strong> interacting and collectively<br />
emitting two-level atoms. In analogy with the experimental procedure, we study the<br />
emission <strong>of</strong> systems <strong>of</strong> dots with randomly selected transition energies that are randomly<br />
placed (at a fixed density) on squares <strong>of</strong> different sizes. The evolution <strong>of</strong> the system<br />
is simulated using the equations derived in the Power-Zinau-Wooley picture and in the<br />
Markov approximation, which corresponds to the method <strong>of</strong> Ref. [2], generalized to nonidentical<br />
emitters placed in a dielectric medium. We make the additional assumption that<br />
at most one exciton is present in the system in the initial state, coherently delocalized<br />
over all the dots (which corresponds to resonant excitation with a very short pulse). We<br />
calculate the time dependence <strong>of</strong> the average number <strong>of</strong> excitons in the dots for system<br />
parameters corresponding to the experimental situation [1] and show that the population<br />
decay indeed has an approximately exponential character with rates very close to<br />
those observed experimentally. This result could not be achieved for dipole-coupled dots,<br />
without taking into account another type <strong>of</strong> QD interaction. We show that including a<br />
short range tunnel coupling between the dots leeds to an increase <strong>of</strong> the recombination<br />
rate which is indistinguishable from that obtained for an artificially enhanced long-range<br />
dipole coupling. In this way we show that the coupling between the quantum dot emitters<br />
and the electromagnetic vacuum cannot be held fully responisble for the variation <strong>of</strong> the<br />
emission rate as proposed in [1].<br />
[1] M. Scheibner et al., Nature Physics, 3, 106 (2007).<br />
[2] R. H. Lehmberg, Phys. Rev. A, 2, 883 (1970).<br />
115
TuP8 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Photoluminescence Linewidth Analysis<br />
<strong>of</strong> Single CdMnTe Quantum Dots<br />
M. Szymura 1;2 , Ł. Kłopotowski 2 , P. Wojnar 2 , K. Fronc 2 , T. Kazimierczuk 3 ,<br />
G. Karczewski 2 and T. Wojtowicz 2<br />
1 Department <strong>of</strong> Mathematics and Natural Sciences,<br />
Cardinal Stefan Wyszyński University, Warsaw, Poland<br />
2 Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences, Warsaw, Poland<br />
3 Faculty <strong>of</strong> Physics, University <strong>of</strong> Warsaw, Warsaw, Poland<br />
Quantum dots (QDs) doped with transition metal ions are interesting from the point <strong>of</strong> view <strong>of</strong><br />
both basic studies and applications in quantum information processing. Introducing e.g. Mn,<br />
substantially changes magnetooptical properties <strong>of</strong> QDs. In particular, the photoluminescence<br />
(PL) linewidth (γ) for a CdTe QD is about 200 μeV, while for CdMnTe with 20% <strong>of</strong><br />
manganese increases by a factor <strong>of</strong> 100. In this report, we present a comprehensive study <strong>of</strong><br />
the PL linewidth performed on a large number <strong>of</strong> dots differing with respect to size and<br />
manganese concentration.<br />
Two kinds <strong>of</strong> structures with CdMnTe QDs were investigated: those with ZnTe barrier and<br />
with ZnMnTe barrier. All quantum dots were grown by molecular beam epitaxy (MBE) on<br />
GaAs substrate. A 4 μm thick buffer CdTe layer was first deposited. Next a ZnTe (and<br />
ZnMnTe) barrier layer was grown. QDs were developed from 6 monolayers <strong>of</strong> CdMnTe. PL<br />
measurements were performed as a function <strong>of</strong> the magnetic field up to 5T at a temperature <strong>of</strong><br />
2K. We study samples with manganese concentration between 1% to 20%.<br />
The presence <strong>of</strong> manganese ions caused giant Zeeman splitting <strong>of</strong> electronic states <strong>of</strong> QDs.We<br />
-<br />
analyzed two circular polarizations. In practice, the transitions in polarization quickly<br />
disappear as a result <strong>of</strong> efficient spin relaxation between the Zeeman split subbands. We<br />
reproduce the transition energies with a Brillouin function, which allows us to evaluate<br />
manganese concentration and temperature.<br />
Broadening <strong>of</strong> the PL transitions from Mn-containig QDs is due to magnetization fluctuations.<br />
Therefore, γ depends on magnetic field, QD volume and manganese concentration and<br />
temperature [1]. At the absence <strong>of</strong> magnetic field, for a QD with 1.5% manganese we find γ<br />
≈4.5 meV. As the magnetic field is increased, γ decreases by about a factor <strong>of</strong> 2 in 5T. This<br />
decrease manifests the freezing <strong>of</strong> the magnetic fluctuations in increased magnetic field. We<br />
reproduce this narrowing <strong>of</strong> the linewidth with magnetic field with a formula derived from<br />
fluctuation dissipation theorem [1]. We obtain a good agreement between the results <strong>of</strong><br />
calculations and experiment. Moreover, the above analysis allows us to evaluate the QD<br />
volume. We find that QDs emitting in higher energies have smaller volume, as expected.<br />
[1] G. Bacher et al. Phys. Rev. Lett. 89, 127201 (2002).<br />
116
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors TuP9<br />
Statistical Study <strong>of</strong> the Inter-Dot Excitation Transfer in CdTe/ZnTe<br />
Quantum Dots.<br />
Maciej Koperski 1 , Tomasz Kazimierczuk 1 , Mateusz Goryca 1 , Andrzej Golnik 1 ,<br />
Jan A. Gaj 1 , Michał Nawrocki 1 , Piotr Wojnar 2 and Piotr Kossacki 1<br />
1 Institute <strong>of</strong> Experimental Physics, Faculty <strong>of</strong> Physics, University <strong>of</strong> Warsaw,<br />
Hoża 69, 00-681 Warsaw, Poland<br />
2 Polish Academy <strong>of</strong> Sciences, Lotników 32/46, 02-668 Warsaw, Poland<br />
Photoluminescence (PL) experiments are the most basic tool for characterization <strong>of</strong><br />
semiconductor quantum dots (QDs). They provide broad range <strong>of</strong> information on the<br />
excitonic complexes, such as neutral excitons (X), trions (X + , X - ) and biexcitons (XX),<br />
confined in a zero-dimensional system. Measurements <strong>of</strong> PL spectra as a function <strong>of</strong><br />
excitation laser energy is known as photoluminescence excitation technique (PLE). Features<br />
in PLE spectra are usually related to excited exciton states or phonon-replica. Additionally, in<br />
the system <strong>of</strong> self organized CdTe/ZnTe quantum dots, very sharp resonances at high energy<br />
are <strong>of</strong>ten observed. They were attributed to the efficient excitation transfer between<br />
neighboring dots [1]. Diverse experiments have been reported, taking advantage <strong>of</strong> the<br />
excitation transfer, such as polarization conversion [1] and orientation <strong>of</strong> the magnetic ion in<br />
dots with single manganese atom [2]. Despite extensive studies, the detailed mechanism <strong>of</strong> the<br />
inter-dot tunneling remains not entirely clear.<br />
Here we present the results <strong>of</strong> detailed statistical analysis <strong>of</strong> the energy distribution <strong>of</strong><br />
resonances detected in PLE spectra. We studied MBE grown samples containing single layer<br />
<strong>of</strong> self organized CdTe/ZnTe quantum dots. The samples were placed in a microphotoluminescence<br />
setup at low temperature (1.5 - 15 K). The PL was excited using a tunable<br />
dye laser in the range 570−610 nm. During measurements we have selected more than 200<br />
single quantum dots for which exciton complexes (X, X+, X- and XX) were identified, and<br />
for which the sharp resonances related to inter-dot transfer were observed. Each resonance<br />
correspond to the absorption with creation <strong>of</strong> neutral exciton, after which the exciton is<br />
transferred to the second dot and subsequently recombines. The difference between energies<br />
<strong>of</strong> neutral excitons in the two dots varied in our experiments between 80 meV and 300 meV<br />
and was mainly limited by the experimental setup. This distance is larger than typical distance<br />
between s-shell and p-shell groups <strong>of</strong> lines which yields about 50meV in our case. We<br />
analyzed the occurrence probability versus the energy distance between neutral exciton line in<br />
the emitting dot and the exciton energy in the absorbing dot (resonance energy).<br />
This analysis revealed relatively flat distribution <strong>of</strong> the resonances, which shows that<br />
levels participating in the tunneling are not collected in separate groups (p-shell, d-shell etc.).<br />
This might be interpreted as a result <strong>of</strong> smearing discrete distribution into continues one due<br />
to the statistical scatter between different dots or significant broadening <strong>of</strong> the excited states<br />
in each emitting dot. In any case, the observed flat distribution suggest that the density <strong>of</strong><br />
states participating in the tunneling is flat or the tunneling occurrence is not limited by the<br />
presence <strong>of</strong> excited states in the emitting dot.<br />
[1] T. Kazimierczuk, J. Suffczyński, A. Golnik, J. A. Gaj, P. Kossacki and P. Wojnar, Phys.<br />
Rev. B 79, 153301 (2009).<br />
[2] M. Goryca, T. Kazimierczuk, M. Nawrocki, A. Golnik, J. A. Gaj, P. Kossacki, P. Wojnar<br />
and G. Karczewski, Phys. Rev. Lett. 103, 087401 (2009)<br />
117
TuP10 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Phonon influence on the weak measurement <strong>of</strong> double quantum<br />
dot spin states.<br />
̷L. Marcinowski 1 , M. Krzy˙zosiak 1,2 , K. Roszak 1 , P. Machnikowski 1 ,<br />
R. Buczko 3 , and J. Mostowski 3<br />
1 Institute <strong>of</strong> Physics, Wroc̷law University <strong>of</strong> Technology, 50-370 Wroc̷law, Poland<br />
2 Beijing University <strong>of</strong> Technology, 100124 Beijing, China<br />
3 Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences, 02-668 Warsaw, Poland<br />
The occupation <strong>of</strong> a double quantum dot (DQD) can be measured by coupling one <strong>of</strong> the<br />
dots to a quantum point contact (QPC) and monitoring the current flowing through the<br />
QPC [1]. This originates form the interaction between the quantum dot and QPC electrons<br />
which shifts the energy level in the QPC depending on the dot state influencing the<br />
tunneling rate through the point contact. A single tunneling event is a weak measurement<br />
<strong>of</strong> the dot and provides a small amount <strong>of</strong> information about the system state. A series<br />
<strong>of</strong> tunneling events constitutes a continous measurement process in which the descrete<br />
current yields a growing amount <strong>of</strong> information on the system state, but also gradually<br />
destroys quantum coherence and localizes the state, consistently with the von Neumann<br />
projection principle.<br />
The same measurement scheme can be used to distinguish two-electron spin-singlet<br />
and spin-triplet states in DQDs [2]. This is based on Pauli exclusion: when the spin-triplet<br />
state requires the electrons to be localized in different dots (only QD ground states are<br />
taken into account), double dot occupation is allowed for the spin-singlet state. Hence,<br />
monitoring the fluctuations in the QPC current allows one to perform a measurement in<br />
the singlet-triplet basis.<br />
In this work we study the effect <strong>of</strong> phonon-induced transitions which are present in this<br />
solid state system [3] on the QPC current and the measurement-influenced evolution <strong>of</strong> the<br />
DQD state. In the strong bias regime, when the measurement is possible in the absence<br />
<strong>of</strong> phonons [2], phonon-induced relaxation leads to a reduction <strong>of</strong> the double occupation,<br />
and hence, to quenching <strong>of</strong> the current response. In the weak bias case the energy transfer<br />
from the tunneling electron is insufficient to excite the QD to a doubly occupied state<br />
and the phonon reservoir serves as a source <strong>of</strong> noise which makes the excitation possible;<br />
this is reflected in the current trace.<br />
[1] T. M. Stace and S. D. Barrett, Phys. Rev. Lett. 92, 136802 (2004).<br />
[2] S. D. Barrett and T. M. Stace, Phys. Rev. B 73, 075324 (2006).<br />
[3] K. Roszak and P. Machnikowski, Phys. Rev. B 80, 195315 (2009).<br />
118
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors TuP11<br />
Influence<strong>of</strong>CarrierTrappingontheOpticalProperties<strong>of</strong><br />
InAs/InPQuantumDashes<br />
P.Kaczmarkiewicz,A.Musiał,G.Sęk,P.Podemski,J.Misiewicz,<br />
P.Machnikowski<br />
Institute<strong>of</strong>Physics,WrocławUniversity<strong>of</strong>Technology,50-370Wrocław,Poland<br />
Wemodeltheopticalproperties<strong>of</strong>highlyanisotropicInAs/InPquantumdots,also<br />
referredtoasquantumdashes(QDashes)[1]. Thosestructuresarecharacterizedbya<br />
non-uniformshapeandpresence<strong>of</strong>widthfluctuations[2]whichcanprovideadditional<br />
confinementinthesystem. Trapping<strong>of</strong>carriersinthepotentialfluctuationsleadsto<br />
quantitativeandqualitativechangeintheopticalproperties<strong>of</strong>thesystem.Theexciton<br />
groundstateexhibitsQD-like(stronglyconfined[3])propertieswhichcanexplainthe<br />
temperaturedependence<strong>of</strong>thedegree<strong>of</strong>linearpolarization<strong>of</strong>emittedradiation.<br />
WeshowhowthetransitionratesdependonQDashshapeparameters(amplitude<br />
andposition<strong>of</strong>thewidening)andcalculateemissionspectraforsingleQDashesandfor<br />
ensembles<strong>of</strong>QDashes. Wealsostudythepolarizationproperties<strong>of</strong>radiationemitted<br />
bythesystem. WeconfirmthataQDashwidthfluctuationleadstostrongtrapping<br />
<strong>of</strong>carriers,whichresultsinloweringtheanisotropy<strong>of</strong>anexcitongroundstateinspite<br />
<strong>of</strong>strongelongation<strong>of</strong>thewholestructure. Suchacharacter<strong>of</strong>theconfiningpotential<br />
manifestsitselfintheexperimentalresultsbyaloweredvalue<strong>of</strong>thedegree<strong>of</strong>linear<br />
polarizationatlowtemperaturesandlowexcitationconditions.<br />
Theinvestigation<strong>of</strong>theopticalproperties<strong>of</strong>QDashesisimportantfromthepoint<strong>of</strong><br />
view<strong>of</strong>possibleapplications.IthasbeenshownthatQDashesmayhaveseveraladvantagesoverothernanostructures,especiallyinlaserapplicationsfortelecommunication[1].<br />
Understanding<strong>of</strong>thepolarizationpropertiesandthereforecarrierconfinementregimeis<br />
alsoimportantforconstructingpolarization-insensitiveopticalamplifiers[4].<br />
Webaseourmodeling<strong>of</strong>theelectron-holesystemontheeffectivemassmethodwith<br />
light-heavyholebandmixingtakenintoaccount[5].Theholestatesareassumedtoconsist<br />
mostly<strong>of</strong>aheavy-holecomponentwithsmalladmixture<strong>of</strong>light-holestates.Thislightholeadmixturedependsontheanisotropy<strong>of</strong>theholewavefunctionviathemomentum<br />
dependence<strong>of</strong>therelevantelement<strong>of</strong>the k·pHamiltonian. Interferencebetweenthe<br />
radiationemittedbythelight-andheavy-holecomponentsleadstotheappearance<strong>of</strong><br />
apreferredpolarizationaxis,paralleltothestructureelongation. Thepresence<strong>of</strong>an<br />
additionalconfinementduetotheshapefluctuationdecreasestheanisotropy<strong>of</strong>anexciton<br />
groundstateandreducestheDOP.Inourmodelingweconsiderthecase<strong>of</strong>asingle<br />
excitonconfinedinaQDash,whichcorrespondstoexperimentalmeasurementsinthe<br />
weakexcitationregime. Coulombcorrelationsareincludedwithintheconfigurationinteractionscheme.<br />
[1]J.P.Reithmaier,G.Eisenstein,A.Forchel,Proc.IEEE95,1779(2007).<br />
[2]H.Deryetal.,J.Appl.Phys.95,6103(2004).<br />
[3]G.Sęketal.,J.Appl.Phys.105,086104(2009).<br />
[4]P.Podemskietal.,Appl.Phys.Lett.93,171910(2008).<br />
[5]P.Kaczmarkiewiczetal.,ActaPhys.Pol.A(inprint).<br />
119
TuP12 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
CdSe/ZnS Colloidal Quantum Dots with Alloyed Core/Shell Interfaces:<br />
a Photoluminescence Dynamics Study<br />
Konrad Dziatkowski 1,2 , Daniel Ratchford 1 , Thomas Hartsfield 1 , Xiaoqin Li 1 ,<br />
Yan Gao 3 and Zhiyong Tang 3<br />
1 Department <strong>of</strong> Physics, University <strong>of</strong> Texas at Austin, Austin TX 78712, USA<br />
2 Institute <strong>of</strong> Experimental Physics – University <strong>of</strong> Warsaw, 00-681 Warsaw, Poland<br />
3 National Center for Nanoscience and Technology, 100190 Beijing, P. R. China<br />
The colloidal semiconducting nanocrystals – and quantum dots (QDs) in particular –<br />
have been a subject to extensive studies driven by their promising photonic or photovoltaic<br />
applications. There are numerous features <strong>of</strong> colloidal QDs, to mention only long term<br />
photostability, that make them superior to other single emitters, e.g dye molecules.<br />
Nevertheless, their optical properties are not yet optimal from the viewpoint <strong>of</strong> technological<br />
utilization. One <strong>of</strong> the main unresolved issues with respect to colloidal QDs is fluctuations<br />
<strong>of</strong> photoluminescence (PL), or so called “blinking”. Despite intense experimental<br />
and theoretical research effort, the detailed microscopic mechanism <strong>of</strong> PL intermittency<br />
remains obscure, and so far no single theoretical model accounts for all observations [1].<br />
The most comprehensive explanation postulates that when a QD is charged, a non-radiative<br />
Auger ionization suppresses PL and makes the emitter appearing dark [2]. Recently,<br />
the diminishing <strong>of</strong> Auger process and, in consequence, quenching <strong>of</strong> blinking was observed<br />
in selenium-based ternary QDs, even though these QDs possessed extra charge [3].<br />
Subsequent theoretical study claimed this observation to be a general feature <strong>of</strong> the emitters<br />
with smoothed quantum confinement potential, since under such conditions the transition<br />
matrix element for the Auger recombination is greatly reduced [4].<br />
This paper reports a comprehensive study <strong>of</strong> PL dynamics in chemically-synthesized<br />
CdSe/ZnS QDs with alloyed core/shell interfaces. Due to the s<strong>of</strong>tening <strong>of</strong> the structural<br />
interface between the core and the shell such emitters manifest a quantum yield <strong>of</strong> up<br />
to 80 % [5]. The employed technique <strong>of</strong> time-correlated single photon counting enabled<br />
measurements <strong>of</strong> the decay <strong>of</strong> exciton luminescence from both the ensemble and individual<br />
QDs. For decreasing emission wavelength, the ensemble data revealed systematic increase<br />
<strong>of</strong> total decay rates with greater variation, as it is expected that smaller emitters are more<br />
influenced by the surface/interface trap states. In experiments performed on single QDs,<br />
the PL trajectories exhibited familiar two-state blinking pattern, despite alloying between<br />
CdSe and ZnS. We suggest that in core/shell QDs with a large band gap <strong>of</strong>fset, as in case<br />
<strong>of</strong> CdSe/ZnS system, the compositionally graded energy pr<strong>of</strong>ile at the interface may not<br />
be smooth enough to suppress non-radiative Auger recombination and prevent blinking.<br />
[1] P. Frantsuzov, M. Kuno, B. Janko, and R. A. Marcus, Nature Physics 4, 519 (2008).<br />
[2] A. L. Efros and M. Rosen, Physical Review Letters 78, 1110 (1997).<br />
[3] X. Wang, X. Ren, K. Kahen, M. A. Hahn, M. Rajeswaran, S. Maccagnano-Zacher,<br />
J. Silcox, G. E. Cragg, A. L. Efros, and T. D. Krauss, Nature 459, 686 (2009).<br />
[4] G. E. Cragg and A. L. Efros, Nano Letters 10, 313 (2010).<br />
[5] W. K. Bae, K. Char, H. Hur, and S. Lee, Chemistry <strong>of</strong> Materials 20, 531 (2008).<br />
120
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors TuP13<br />
Spectroscopy <strong>of</strong> Indirect Excitons in Vertically Stacked CdTe<br />
Quantum Dot Structures<br />
K. Kukliński, ̷L. K̷lopotowski, K. Fronc, P. Wojnar, T. Wojciechowski,<br />
M. Czapkiewicz, J. Kossut, G. Karczewski, and T. Wojtowicz<br />
Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences, Al. Lotników 32/46,<br />
02–668 Warsaw, Poland<br />
We show that by means <strong>of</strong> an electric field we can tune the energy levels in vertical<br />
single quantum dot (QD) pairs and study transitions related to recombination <strong>of</strong> indirect<br />
excitons.<br />
Embedding QDs in vertical field effect structures enables electrical control <strong>of</strong> the QDs<br />
charge state and allows to study properties related to Coulomb interactions. There are<br />
many reports on studies <strong>of</strong> direct excitons in single CdTe/ZnTe QDs. However, for this<br />
material system, no data is reported on indirect excitons, i.e. excitons related to recombination<br />
<strong>of</strong> an electron and a hole localized in two separate QDs. Obtaining a controlled<br />
coupling between two QDs is interesting from the point <strong>of</strong> view <strong>of</strong> basic research and for<br />
realization <strong>of</strong> a quantum gate, the building block <strong>of</strong> a quantum computer.<br />
In this communication, we present the results <strong>of</strong> spectroscopic studies <strong>of</strong> indirect excitons<br />
in vertically stacked QD structures. The QDs were embedded in the insulating<br />
region <strong>of</strong> a p–i–Schottky diode grown by molecular beam epitaxy on a GaAs substrate.<br />
The layer sequence was the following: a 4-µm thick p–doped ZnTe buffer, a 100 nm <strong>of</strong><br />
undoped ZnTe, two QD layers separated by a 4 or 8 nm ZnTe spacer, a 50 nm thick<br />
ZnTe cap and a 50 nm ZnMgTe blocking barrier. QDs were formed from a 2D CdTe layer<br />
six monolayers thick. Ohmic contacts were established to the p–type ZnTe and a Al/Au<br />
Schottky contacts were deposited on top <strong>of</strong> the sample. Single QD pairs were accessed<br />
through 200 nm diameter shadow mask apertures. The photoluminescence (PL) signal<br />
was excited with a 532 nm laser beam focused to a 2 µm spot with a microscope objective.<br />
PL spectra were measured at 10 K as a function <strong>of</strong> a bias voltage.<br />
At zero bias we observe PL transitions from single pairs <strong>of</strong> QDs. We identify charge<br />
states from uncoupled dots. As the bias is increased, transitions shift due to the quantum<br />
confined Stark effect. Evidence <strong>of</strong> the presence <strong>of</strong> indirect excitons in the measured PL<br />
spectra are transitions strongly shifting in the electric field, as a consequence <strong>of</strong> greater<br />
value <strong>of</strong> dipole moment compared with direct excitons. We observe two types <strong>of</strong> lines:<br />
these for which Stark shift is about 0.5–1 meV/V and these with 4–7 meV/V. In accordance<br />
with the above conclusion, we identify the first transitions as related to direct<br />
excitons, and the latter as indirect. We assume that the dipole moment <strong>of</strong> the indirect excitons<br />
is pind = ed, where d is the spacer layer thickness. Therefore, the Stark shift <strong>of</strong> the<br />
indirect exciton allows us to calibrate the electric field in our structure. We find that this<br />
field is a factor <strong>of</strong> four smaller than expected from a capacitor formula: F = (U−Ubi)/W,<br />
where U and Ubi is the applied and built–in voltage, respectively, and W is the width <strong>of</strong><br />
the intrinsic region. Having calibrated F, we find that the dipole moment for the direct<br />
exciton pdir ∼ 1 nm, thus pind ≈ 4(8)pdir. With decreasing reverse bias we observe both<br />
the red– and blue–shifted indirect exciton transitions. We assume that due to strain–<br />
enhanced nucleation, the QDs in the top layer are larger than the bottom dots. Basing<br />
on the band pr<strong>of</strong>iles <strong>of</strong> our structures, we conclude that the blue–shifted transitions are<br />
related to the recombination <strong>of</strong> the electron and hole localized in the top and the bottom<br />
dot, respectively. In turn, red–shifted indirect excitons are associated with the recombination<br />
<strong>of</strong> the electron and hole in the bottom and the top dot, respectively.<br />
121
TuP14 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Collective spontaneous emission from pairs <strong>of</strong> quantum dots:<br />
the role <strong>of</strong> coupling and system geometry<br />
Wildan Abdussalam, Anna Sitek, Pawe̷l Machnikowski<br />
Institute <strong>of</strong> Physics, Wroc̷law University <strong>of</strong> Technology, 50-370 Wroc̷law, Poland<br />
We study the role <strong>of</strong> various types <strong>of</strong> coupling between semiconductor quantum dots<br />
(QDs) in the collective spontaneous emission <strong>of</strong> double dot systems. Excitons delocalized<br />
in closely spaced QDs recombine in a different way than in a single QD due to collective<br />
interaction <strong>of</strong> the two emitters with the quantum radiation field [1]. While for noninteracting<br />
dots this collective effect appears only for almost identical dots (with the<br />
transition energies within the emission line width), coupling between the dots leads to<br />
accelerated or slowed down emission even for dots with different transitions energies,<br />
which is manifested in the linear and non-linear response from these systems [2].<br />
In this contribution, we study the spontaneous emission from an exciton confined in<br />
a double quantum dot. We focus on the similarities and differences between the cases <strong>of</strong><br />
radiative (long-range, dipole) and tunnel coupling between the excitons in the dots. We<br />
show that for strictly identical dots the oscillating nature <strong>of</strong> the dipole coupling on long<br />
distances leads to non-monotonic dependence <strong>of</strong> the radiative decay rate on the inter-dot<br />
separation, which is not present in the case <strong>of</strong> tunnel coupling. However, for a double<br />
dot system with a realistic, technologically feasible mismatch <strong>of</strong> transition energies, the<br />
collective effects disappear completely well before these oscillations become relevant. In<br />
this case, there is no qualitative difference between the two physically different coupling<br />
types and the system dynamics in the two cases becomes indistinguishable for appropriate<br />
values <strong>of</strong> the coupling parameters.<br />
We believe that these findings may shed some light on the interpretation <strong>of</strong> the experiment<br />
[3] in which enhanced emission was observed in a quantum dot ensemble in which<br />
the dipole coupling energies on the typical inter-dot distances were much smaller than the<br />
average transition energy mismatch between the dots.<br />
[1] A. Sitek, P. Machnikowski, Phys. Rev. B 75, 035328 (2007).<br />
[2] A. Sitek, P. Machnikowski, Phys. Rev. B 80, 115319 (2009); 80, 115301 (2009).<br />
[3] M. Scheibner, T. Schmidt, L. Worschech, A. Forchel, G. Bacher, T. Passow, D. Hommel,<br />
Nature Physics 3, 106 (2007).<br />
122
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors TuP15<br />
Tunneling Transfer Protocol in a Quantum Dots Chain Immune<br />
to Inhomogeneity<br />
Kamil Korzekwa and Pawe̷l Machnikowski<br />
Institute <strong>of</strong> Physics, Wroc̷law University <strong>of</strong> Technology, 50-370 Wroc̷law, Poland<br />
We propose a quantum dot (QD) implementation <strong>of</strong> a quantum state transfer channel.<br />
The proposed channel consists <strong>of</strong> N vertically stacked QDs with the nearest neighbour<br />
tunnel coupling, placed in an axial electric field. The QD chain isdoped with oneelectron.<br />
The terminal QDs are assumed to provide a stronger binding for the electron. In this<br />
way, the states with electron on the terminal dots are energetically separated from other,<br />
but remain indirectly coupled via the QD chain. The inhomogeneity <strong>of</strong> the QD chain<br />
is included by using normal distribution for the values <strong>of</strong> tunnel couplings and electron<br />
energy levels. By applying an external electric field along the chain, the terminal QDs<br />
are brought to resonance, which leads to an efficient occupation transfer.<br />
Our proposal requires a global electric field as the only control means, therefore it<br />
satisfies the requirement <strong>of</strong> simplicity. We study the conditions <strong>of</strong> getting the quantum<br />
state transfer and compare the results with the previously proposed protocol [1]. We show<br />
that in our model the transfer fidelity for a homogenous QD chain is independent <strong>of</strong> the<br />
chains length. We show also that controlling the global electric field is enough to make<br />
the system resistant to inhomogeneity <strong>of</strong> both QD energies and tunnel coupling. Within<br />
our model we use the real QD parameters and show that almost perfect tunneling transfer<br />
is possible.<br />
Sinceourprotocolfulfillsthesimplicityrequirementitisexperimentallyfeasible. Since<br />
thetransferisindependent<strong>of</strong>theQDinhomogeneityitcouldbeusedtotransferexcitation<br />
in a real QD chain.<br />
a)<br />
|P| 2<br />
1<br />
0.75<br />
0.5<br />
0.25<br />
0<br />
0 25 50 75 100<br />
t [ns]<br />
b)<br />
|P| 2<br />
1<br />
0.75<br />
0.5<br />
0.25<br />
0<br />
0 25 50 75 100<br />
Evolution <strong>of</strong> the occupation <strong>of</strong> on the initial QD (solid line), the final QD (dashed line) and the<br />
QDs inside the chain (dotted line) <strong>of</strong> 10 QDs spin chain. Electron energies in the QD’s normally<br />
distributed with standard deviation ∆ = 10meV and tunnel couplings J = 10meV. Terminal<br />
QDs energy E = 50meV a) No external electric field; b) Terminal QDs tuned to resonance by<br />
an external field.<br />
[1] A. Wójcik, T. ̷Luczak, P. Kurzyński, A. Grudka, T. Gdala, and M. Bednarska, Phys.<br />
Rev. A 72, 034303<br />
123<br />
t [ns]
TuP16 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Thermal Quenching <strong>of</strong> Photoluminescence from epitaxial InGaAs/GaAs<br />
Quantum Dots with High Lateral Aspect Ratio<br />
Anna Musiał 1 , Grzegorz S k 1 , Aleksander Mary ski 1 , Paweł Podemski 1 ,<br />
Janusz Andrzejewski 1 and Jan Misiewicz 1<br />
Andreas Löffler 2 , Sven Höfling 2 , Stephan Reitzenstein 2 , Johann Peter Reithmaier 2,*<br />
and Alfred Forchel 2<br />
1 Institute <strong>of</strong> Physics, Wrocław University <strong>of</strong> Technology, Wrocław, Poland<br />
2 Technische Physik, Universität Würzburg, Wilhelm Conrad Röntgen-Center for Complex<br />
Material Systems, Am Hubland, D-97074 Würzburg, Germany<br />
Self-assembled In0.3Ga0.7As quantum dots (QDs) [1] have already been demonstrated as<br />
good candidates for testing QED phenomena due to enhanced exciton oscillator strength [2]<br />
being a result <strong>of</strong> increased structure volume and weaker carrier confinement [3].<br />
This report concerns temperature dependence <strong>of</strong> photoluminescence (PL) in both<br />
ensemble and single QD regime. At 5K, PL spectra consist <strong>of</strong> two bands related to the wetting<br />
layer (WL) and QD ensemble, separated by rather small energy (~ 35 meV) indicated shallow<br />
confining potentials for holes and electrons. At low temperatures WL emission dominates and<br />
there is a rather insignificant carrier transfer to the dots because most <strong>of</strong> the WL states are<br />
localized [3]. At higher temperatures (> 30K) carriers are released becoming extended into the<br />
WL plane and carrier redistribution increases the QD emission with respect to the WL one.<br />
Thermal quenching <strong>of</strong> PL from QD ensemble reveals activation energy <strong>of</strong> about 30 meV<br />
corresponding well with the confinement energy identifying a primary carrier loss mechanism<br />
– release <strong>of</strong> carriers to WL. This has also been confirmed at the single QD level. The<br />
activation energy varies between 15-30 meV reflecting the dot size and hence carrier<br />
confinement distribution. In case <strong>of</strong> single QD the impact <strong>of</strong> WL is stronger, because the<br />
probability that a carrier returns to the same dot and emits is lower than the probability <strong>of</strong><br />
carrier to get back to any <strong>of</strong> the dots contributing to the ensemble PL. In a very few cases a<br />
fingerprint <strong>of</strong> 2 nd significantly smaller activation energy is observed, suggesting that carrier<br />
localization inside those structures is not present or localization energy is very low (below 1<br />
meV - the resolution <strong>of</strong> the method) meaning that at low temperatures excitons are always<br />
extended over the entire QD and experience large coherence volume. This was further<br />
supported by polarization-resolved temperature dependent PL to probe the symmetry <strong>of</strong><br />
confining potential by obtaining the degree <strong>of</strong> linear polarization. It is low at 5K (~6%) and<br />
decreases slightly with temperature due to emission from higher energy states reflecting lower<br />
effective anisotropy [4]. This can be understood as a result <strong>of</strong> very weak confining potential<br />
for electrons in agreement with 8 band kp calculations giving DOP values below 10%.<br />
[1] A. Löffler, J. P. Reithmaier, A. Forchel, A. Sauerwald, D. Peskes, T. Kümmell and G.<br />
Bacher, J. Cryst. Growth 286, 6 (2006).<br />
[2] J. P. Reithmaier, G. S k, A. Löffler, C. H<strong>of</strong>mann, S. Kuhn, S. Reitzenstein, L. Keldysh,<br />
V. Kulakovskii, T. Reinecke and A. Forchel, Nature 432, 197 (2004).<br />
[3] G. S k, A. Musiał, P. Podemski, M. Syperek, J. Misiewicz, A. Löffler, S. Heling, L.<br />
Worschech and A. Forchel, J. Appl. Phys. 107, 96106 (2010).<br />
[4] P. Kaczmarkiewiecz, A. Musiał, G. S k, P. Podemski, P. Machnikowski and J. Misiewicz,<br />
Acta Phys. Pol. A (<strong>2011</strong>), in press.<br />
* Currently at: Institute <strong>of</strong> Nanostructure Technologies and Analytics, Technische Physik, Universität Kassel<br />
124
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors TuP17<br />
Excitonic Magnetoabsorption <strong>of</strong> Cylindrical Quantum Disks<br />
P. Schillak and G. Czajkowski<br />
Department <strong>of</strong> Theoretical Physics, University <strong>of</strong> Technology and Life Sciences<br />
Al. Pr<strong>of</strong>. S. Kaliskiego 7, 85-789 Bydgoszcz, Poland<br />
Potential applications <strong>of</strong> semiconducting nanostructures in novel optoelectronic devices<br />
gain the interest in the optical properties <strong>of</strong> quantum disks, quantum wires and the<br />
other low-dimensional objects. Here we investigate the influence <strong>of</strong> excitonic effects on<br />
the optical properties <strong>of</strong> quantum disks in the external magnetic field. We propose the<br />
theoretical approach which enables to calculate the optical functions for low-dimensional<br />
structureswithcylindricalsymmetryexposedtotheexternalmagneticfielddirectedalong<br />
the symmetry axis. We consider an interacting electron-hole pair inside the semiconducting<br />
nanostructure with hard-wall potential <strong>of</strong> finite height (different for an electron and<br />
a hole) at the boundaries. The different anisotropic effective masses are taken inside and<br />
outside the structure. The novelty <strong>of</strong> our approach is that the six-dimensional eigenvalue<br />
problem is transformed into the equivalent eigenvalue problem given by the system <strong>of</strong> the<br />
coupled two-dimensional second order differential equations. Differential equations are<br />
solved numerically. As an example, we present in Fig. 1 the magnetic field dependent<br />
excitonic energy spectrum for the InP disk with the radius R = 8 nm and the height<br />
d = 4 nm. Additionally, numerical results are compared with experimental data taken<br />
from the paper by Dewitz et al. [1]. Having the energies and wave functions, we apply<br />
the real density matrix approach [2,3]<br />
which gives the optical functions including<br />
magnetoabsorption and the<br />
magnetic field dependent dielectric<br />
tensor for light- and heavy-hole excitons.<br />
In this approach the linear<br />
response is described by the set <strong>of</strong><br />
coupledequationsfortwo-pointcorrelation<br />
function Y(re,rh) and the<br />
Maxwellianfieldequation. TheconstitutiveequationforY(re,rh)hasa<br />
form <strong>of</strong> Schrödinger’s equation with<br />
a source term, which reflects the<br />
light-matter interaction. Numerical<br />
computations were performed<br />
for In0.55Al0.45As/Al0.35Ga0.65 and<br />
CdSe quantum disks. Our numerical<br />
results are in a good agreement<br />
with experimental data.<br />
relative energy [meV]<br />
120<br />
100<br />
80<br />
60<br />
40<br />
20<br />
0<br />
0 20 40 60<br />
magnetic field B [T]<br />
Figure 1: The calculated (full lines) and measured<br />
(circles) relative positions <strong>of</strong> energy levels<br />
as functions <strong>of</strong> the applied magnetic field.<br />
[1] C. v. Dewitz, F. Hatami, M. Millot, J. M. Broto, J. Léotin, and W. T. Masselink,<br />
Appl. Phys. Letters 95, 151105 (2009).<br />
[2] G. Czajkowski, F. Bassani, and L. Silvestri, Rivista del Nuovo Cimento, 26(5-6), pp.<br />
1-150 (2003).<br />
[3] P. Schillak and G. Czajkowski, phys. stat. sol. (c), 5 2495 (2008).<br />
125
TuP18 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Spontaneous emission from double quantum dots: collective<br />
effects and carrier-phonon kinetics<br />
Pawe̷l Karwat, Anna Sitek, Pawe̷l Machnikowski<br />
Institute <strong>of</strong> Physics, Wroc̷law University <strong>of</strong> Technology, 50-370 Wroc̷law, Poland<br />
We study theoretically the evolution <strong>of</strong> luminescence from a double quantum dot in the<br />
presence <strong>of</strong> carrier-phonon interaction. We show that the spontaneous emission from<br />
this system is determined by the interplay <strong>of</strong> collective effects, which are due to the<br />
interaction between the two emitters and their common radiative reservoir, and phononinduced<br />
transitions (typically on a much shorter time scale).<br />
The time-resolved luminescence from double quantum dots is an interesting subject <strong>of</strong><br />
study as it yields information on the carrier kinetics under the joint action <strong>of</strong> two reservoirs:<br />
electromagnetic vacuum to which the exciton ultimately decays in the spontaneous<br />
emission (radiative recombination) process and the lattice excitations which may dephase<br />
the charge states and lead to transitions between the confined states. One <strong>of</strong> the experiments<br />
[1] revealed surprising temperature dependence <strong>of</strong> the luminescence life time which<br />
first grows as the temperature is increased and then decreases. This enhancement <strong>of</strong> the<br />
exciton lifetime with increasing temperature is in striking contrast to the single dot case<br />
where the life time drops down with temperature, which can easily be associated with<br />
various possible thermally activated non-radiative channels.<br />
In this presentation, we model the system kinetics using a Markovian Lindblad generator<br />
for the spontaneous emission process [2] and a non-Markovian time-convolutionless<br />
dissipator that accounts for the phonon-related kinetics [3]. We show that the quasiequilibrium<br />
distribution <strong>of</strong> the occupations <strong>of</strong> the two lowest energy eigenstates sets up<br />
within several picoseconds after the excitation. Since one <strong>of</strong> these states is more active<br />
optically (“brighter”) this leads to temperature dependence <strong>of</strong> the emission rate and to<br />
non-monotonic dependence <strong>of</strong> the luminescence life time similar to that observed in the<br />
experiment.<br />
[1] C. Bardot, M. Schwab, M. Bayer, S. Fafard, Z. Wasilewski, P. Hawrylak, Phys. Rev.<br />
B 72, 035314 (2005).<br />
[2] A. Sitek, P. Machnikowski, Phys. Rev. B 80, 115319 (2009).<br />
[3] P. Machnikowski, K. Roszak, A. Sitek, Acta Phys. Pol. A 116, 818 (2009).<br />
126
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors TuP19<br />
Cathodoluminescence studies <strong>of</strong> the II – VI semiconducting quantum dots<br />
grown by molecular beam epitaxy<br />
P. Łach 1 , A. Reszka 1 , G. Karczewski 1 , P. Wojnar 1 , T. Wojtowicz 1 , A. Kamińska 1 ,<br />
and A. Suchocki 1,2<br />
1<br />
- Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences, al. Lotników 32/46, 02-668, Warsaw,<br />
Poland<br />
2<br />
- Institute <strong>of</strong> Physics, Kazimierz Wielki University, Weyssenh<strong>of</strong>fa 11, Bydgoszcz 85-072,<br />
Poland<br />
Self-assembled quantum dots (QDs) as zero-dimensional structures are very<br />
intensively studied optically and electrically on account <strong>of</strong> their shape and small sizes. Our<br />
QDs consisted <strong>of</strong> II-VI compounds were grown on (100)-oriented GaAs substrate by Stranski-<br />
Krastanov mode by molecular beam epitaxy method (MBE). The obtained objects have<br />
diameter and height <strong>of</strong> the order <strong>of</strong> 10 - 40 nm and 1 - 14 nm, respectively. The QDs were<br />
embedded in a ZnTe matrix with thickness <strong>of</strong> about 1.5 µm and they were composited <strong>of</strong> 6<br />
monolayers. A whole was covered by a thin cap (100 nm). The concentration <strong>of</strong> the QDs was<br />
equal to 10 9 – 10 10 <strong>of</strong> QDs per 1 cm 2 .<br />
We have measured the cathodoluminescence (CL) spectra <strong>of</strong> such structures as a<br />
function <strong>of</strong> the temperature. The luminescence was excited by electron beam with<br />
accelerating voltage <strong>of</strong> the order <strong>of</strong> 5 – 15 kV. This experiment was carried out for CdTe,<br />
CdMnTe and CdSe QDs. We have checked behaviour <strong>of</strong> the CL signals in different places on<br />
these samples. This dependence shows certain non-uniform distribution <strong>of</strong> QDs in the<br />
examined samples. With increase <strong>of</strong> the temperature the spectra became considerably wider<br />
and their intensities decreased. It results from excitons redistribution between QDs (the<br />
carriers were intercepted from a wetting layer to a bigger QD which emits a light in lower<br />
energies). The possible mechanisms <strong>of</strong> the temperature luminescence quenching are nonradiative<br />
exciton recombination due to the Auger effect and disappearance <strong>of</strong> the quantum<br />
confinement at higher temperatures. We discuss the both mechanisms in this paper.<br />
The examples <strong>of</strong> the results <strong>of</strong> the photoluminescence (PL) and cathodoluminescence (CL)<br />
measurements (on the left) and the temperature dependences for CdTe QDs (on the right) are<br />
presented below for comparison.<br />
CL intensity [au]<br />
PL intensity [au]<br />
40000<br />
30000<br />
20000<br />
10000<br />
0<br />
20K<br />
2,00 2,05 2,10 2,15 2,20 2,25 2,30<br />
50000<br />
40000<br />
30000<br />
20000<br />
10000<br />
0<br />
Energy [eV]<br />
70K<br />
60K<br />
50K<br />
40K<br />
30K<br />
1,95 2,00 2,05 2,10 2,15 2,20 2,25 2,30 2,35<br />
Energy [eV]<br />
T=60K<br />
T=57K<br />
T=53K<br />
T=48K<br />
T=43K<br />
T=34K<br />
T=28K<br />
T=20K<br />
T=11K<br />
CL intensity [au]<br />
14000<br />
12000<br />
10000<br />
8000<br />
6000<br />
4000<br />
2000<br />
10 20 30 40 50 60 70<br />
Temperature [K]<br />
10 20 30 40 50 60<br />
In this case we have measured the PL and CL spectra to about 100K. The CdMnTe and CdSe<br />
QDs have given the CL signals even over 200K.<br />
Acknowledgement: This work was partially supported by the grant <strong>of</strong> the Polish Ministry <strong>of</strong><br />
Science and Higher Education for years 2007-<strong>2011</strong>.<br />
127<br />
PL intensity [au]<br />
14000<br />
12000<br />
10000<br />
8000<br />
6000<br />
4000<br />
2000<br />
Temperature [K]
TuP20 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Raman spectroscopy <strong>of</strong> CdTe/ZnTe quantum dots<br />
E. Zielony 1 , E. Popko 1 , Z. Gumienny 1 , P. Kamyczek 1 , A. Henrykowski 1 , J. Jacak 1 ,<br />
G.Karczewski 2<br />
1 Institute <strong>of</strong> Physics, Wroclaw University <strong>of</strong> Technology, Wybrzeze Wyspianskiego 27,<br />
50-370 Wroclaw, Poland<br />
2 Institute <strong>of</strong> Physics, Institute <strong>of</strong> Physics Polish Academy <strong>of</strong> Science, al. Lotnikow 32/46,<br />
02-668 Warsaw, Poland<br />
Semiconducting low-dimensional structures <strong>of</strong> CdTe quantum dots embedded in ZnTe matrix<br />
have been investigated by micro-Raman spectroscopy. A reference ZnTe sample (without<br />
dots) was also studied for comparison. Both samples were grown by molecular beam epitaxy<br />
technique on the p-type GaAs substrate. The QD sample consists <strong>of</strong> 3µm thick p + -type<br />
ZnTe:N buffer deposited on the p-type GaAs substrate, 1µm undoped ZnTe, a layer <strong>of</strong> CdTe<br />
quantum dots and 0.3µm <strong>of</strong> undoped ZnTe cap. The CdTe self assembled quantum dots were<br />
grown in the atomic layer epitaxy (ALE) growth mode by deposition <strong>of</strong> six monolayers <strong>of</strong><br />
CdTe. The process <strong>of</strong> QD formation was induced by covering the CdTe layers with an<br />
amorphous tellurium layer and its subsequent thermal desorption [1]. The presence <strong>of</strong><br />
quantum dot structure was confirmed by photoluminescence measurements. The Raman<br />
measurements have been performed at room temperature with the help <strong>of</strong> the T64000 Jobin<br />
Ivon spectrometer configured in the triple subtractive mode <strong>of</strong> operation. The samples were<br />
excited by an Ar 2+ laser working at a wavelength <strong>of</strong> 514.5 nm. The Raman spectra have been<br />
recorded for different acquisition parameters <strong>of</strong> the measurement. For the reference and QD<br />
sample localized longitudinal (LO) phonons <strong>of</strong> 210cm -1 wavenumber associated with the<br />
ZnTe layer [2] are observed. In the case <strong>of</strong> QD sample another broad band corresponding to<br />
the LO CdTe phonon related to the QD-layer appears at a wavenumber <strong>of</strong> 160 cm -1 [2]. Such<br />
behavior does not exhibit the Raman spectra for the reference sample. For both samples<br />
additionally tellurium - related peaks at wavenumbers around 120 cm -1 and 140 cm -1 are<br />
detected. It has been noticed that the intensity <strong>of</strong> CdTe and ZnTe LO phonon peaks decreases<br />
with increasing time <strong>of</strong> a laser beam exposition while the signal associated with Te - like<br />
Raman peaks increases. The latter is caused by the laser damage in the ZnTe layer followed<br />
by the formation <strong>of</strong> Te aggregates on the ZnTe surface. The Raman measurements confirm<br />
the presence <strong>of</strong> CdTe layer <strong>of</strong> quantum dots in the investigated material.<br />
[1] P. Wojnar, J. Suffczynski, K. Kowalik, A. Golnik, M. Aleszkiewicz, G. Karczewski, and<br />
J. Kossut, Nanotechnology 19, 235403 (2008).<br />
[2] V. S. Vinogradov, et al., Physics <strong>of</strong> the Solid State, Vol. 50, No. 1, 2008.<br />
128
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors TuP21<br />
Quantum interference in charge transport via the quantum dots<br />
coupled between the metallic and superconducting leads<br />
J. Barański and T. Domański<br />
Institute <strong>of</strong> Physics, M. Curie Sk̷lodowska University, 20-031 Lublin, Poland<br />
We analyze the effects <strong>of</strong> quantum interference observable in the Andreev current<br />
induced through the double quantum dots coupled between an isotropic superconductor<br />
and metallic leads. Superconducting properties are transfered on such nanostructures<br />
due to the proximity effect and they have qualitative influence on the nonequillibrium<br />
phenomena, especially in a regime <strong>of</strong> the subgap bias voltage |V | ≤ ∆/|e|. We will<br />
discuss condictions necesarry for observing the Fano resonances and shall try to account<br />
for their interplay with the strong correlation effects.<br />
129
TuP22 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Bound Magnetic Polaron Molecule in Diluted Magnetic<br />
Semiconductors within the Heitler-London Approximation<br />
Henryk Bednarski 1 and Jozef Spa̷lek 2<br />
1 Centre <strong>of</strong> Polymer and Carbon Materials, Polish Academy <strong>of</strong> Sciences, 34 M.<br />
Curie-Sk̷lodowska Str., 41-819 Zabrze, Poland<br />
2 Marian Smoluchowski Institute <strong>of</strong> Physics, Jagiellonian University, Reymonta 4,<br />
30-059 Kraków, Poland<br />
Recently developed by us a complete microscopic theory <strong>of</strong> the bound magnetic polaron<br />
molecule (BMPM) in diluted magnetic semiconductors [1] is reformulated in a reduced<br />
spin-state space. Namely, we solve the BMPM problem by taking into account the<br />
Heitler-London form <strong>of</strong> the two-electron wave function in the four dimensional spin-state<br />
space (comprising one singlet and three triplet states) and include the thermodynamic<br />
fluctuations <strong>of</strong> the magnetization due to the localized 3d spins in the Gaussian form.<br />
In this manner, the role <strong>of</strong> covalency effects can be clearly identified and its influence<br />
analysed in detail. Moreover, within this formulation the quantum aspect <strong>of</strong> the BMPM<br />
problem is also accounted for, as it results from quantum-mechanical indistinguishability<br />
<strong>of</strong> the two impurity electrons and is contained in the antisymmetric nature <strong>of</strong> their<br />
two-electron wave function. Therefore, the formulation <strong>of</strong> the BMPM theory provides us<br />
with the opportunity <strong>of</strong> studying the physical origin <strong>of</strong> the competition between ferroand<br />
antiferro-magnetic interactions <strong>of</strong> the impurity pair <strong>of</strong> electrons in diluted magnetic<br />
semiconductors.<br />
[1] H. Bednarski, J. Spa̷lek, arXiv:0912.0662v4, submitted to Phys. Rev. B.<br />
130
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors TuP23<br />
Intersubband polaritons in strongly pumped microcavities<br />
M. Zału ny and A. Kozłowski<br />
Institute <strong>of</strong> Physics, M. Curie-Skłodowska University, Pl. M. Curie-Skłodowskiej 1, 20-031<br />
Lublin, Poland<br />
It is well known that resonant microstructures have an important role in the<br />
enhancement <strong>of</strong> nonlinear optical processes in multiple quantum well (MQWs) structures. So<br />
far most research has concentrated on the nonlinear effects connected with excitonic coupling<br />
to the microcavity (MC) mode [1]. In our recent paper [2] we have discussed theoretically the<br />
nonlinear intersubband response <strong>of</strong> the MQWs embedded in MCs. The simplest case, when<br />
strong probe wave acts alone, has been considered. It has been shown, that in the strongcoupling-regime<br />
the saturation effect leads to the evolution <strong>of</strong> the absorption spectra from a<br />
pair <strong>of</strong> simple harmonic oscillators to highly anharmonic ones.<br />
The experimental and theoretical results reported in [1] indicate that in MCs the pumpprobe<br />
nonlinear excitonic effects [leading to the formation <strong>of</strong> the so-called Mollow absorption<br />
spectrum (MAS) [3]] are much more easily observed due to the enhancement <strong>of</strong> the optical<br />
field by the MC. Stimulated by this fact in this paper we discuss theoretically the problem <strong>of</strong><br />
the formation <strong>of</strong> the intersubband MAS in the MQW-MC systems.<br />
Like in our previous paper, we employ a semiclassical approach based on the transfer<br />
matrix formalism (TMF) and the so-called sheet model [2,4] (The sheet, modeling the optical<br />
response <strong>of</strong> the QW, is characterized by the 2D conductivity.) For simplicity we neglect the<br />
influence <strong>of</strong> the electron-electron interaction on the nonlinear intersubband response. It has a<br />
good justification when the surface electron concentration in QW is relatively small (�10¹¹<br />
cm²) and the wavelength <strong>of</strong> incident radiation is not too large ( �10 m) [5]. To achieve a<br />
sufficiently strong coupling between intersubband excitation and incident radiation the<br />
experimentally studied systems usually contain a large number <strong>of</strong> QWs. Practically, the QWs<br />
occupy almost the whole space between mirrors. Unfortunately, in such type <strong>of</strong> systems,<br />
contrary to those studied in [1], the strength <strong>of</strong> the optical field acting on each QW is<br />
different. The numerical simulations reported in this paper are performed taking into account<br />
also the above mentioned effect.<br />
The calculations <strong>of</strong> the angle resolved (probe) absorption spectra are decomposed into<br />
two sequential stages. First, the pump wave propagation in the system is described neglecting<br />
the energy transfer between the (strong) pump and (weak) probe waves. The spatial pump<br />
field distribution in the MC-MQW system is calculated taking into account the saturation<br />
effect. The case when the response to pump beam has bistable character [2] is also considered.<br />
At second stage, the linear problem <strong>of</strong> the probe beam propagation in the system is solved<br />
taking into account fact the that the Mollow 2D (intersubband) conductivity <strong>of</strong> each QW [3,6]<br />
depends on the pump field intensity, at the position <strong>of</strong> the QW, determined in the first stage.<br />
For better understanding <strong>of</strong> the role <strong>of</strong> the spatial variation <strong>of</strong> the pump field in the MC we<br />
also present a simplified approach based on a Fabry-Perot model and the “mean field”<br />
approximation [2].<br />
[1] B. Beveaurd, et al. CR Acad. Sci. IV-Phys., 2, 1439 (2001) and references therein.<br />
[2] M. Zału ny and C. Nalewajko, J. Appl. Phys. 107, 123106 (2010).<br />
[3] B. R. Mollow, Phys. Rev. 188,1969 (1969); Phys. Rev. A 5, 227 (1972).<br />
[4] M. Zału ny and C. Nalewajko, J. Appl. Phys. 99, 026104 (2006.<br />
[5] R. A. Kaindl, et al., Phys. Rev. B 62, 161308(R) (2001).<br />
[6] P. Meystre and M. Sargent III, Elements <strong>of</strong> quantum optics (Springer, Berlin, 1990).<br />
131
TuP24 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Magneto-Optical Studies <strong>of</strong> Narrow Band-Gap Heterostructures with Type<br />
II Quantum Dots InSb in an InAs Matrix<br />
Mikhail S. Mukhin 1 , Yakov V. Terent’ev 1 , Leonid E. Golub 1 , Mikhail O.Nestoklon 1 ,<br />
Boris Ya. Meltser 1 , Alexey N. Semenov 1 , Victor A. Solov’ev 1 , Alla A. Sitnikova 1 , Alexey A.<br />
Toropov 1 and Sergey V. Ivanov 1<br />
1 I<strong>of</strong>fe Physical-Technical Institute <strong>of</strong> Russian Academy <strong>of</strong> Sciences, 26 Polytekhnicheskaya,<br />
St Petersburg 194021, Russian Federation<br />
Spin is the only electron internal degree <strong>of</strong> freedom, and utilizing it in the new<br />
generation <strong>of</strong> semiconductor devices is the main goal <strong>of</strong> semiconductor spintronics. Today<br />
spintronics focuses mainly on diluted magnetic semiconductors (DMS) where ferromagnetism<br />
and giant Zeeman splitting can be obtained due to exchange interaction between free carriers<br />
and Mn ions. Most <strong>of</strong> the work has been focused on II-VI DMS such as CdMnTe or ZnMnSe<br />
and some others. Enhanced magnetic properties <strong>of</strong> these materials exist only at cryogenic<br />
temperatures due to paramagnetic behavior <strong>of</strong> the magnetic ions. A lot <strong>of</strong> recent investigations<br />
have been focused on doping III-V semiconductors into a DMS state. Although III-V DMS<br />
demonstrate higher Curie temperature, Mn is much less soluble here than in II-VI<br />
semiconductors [1]. Another issue is that in III-V compounds magnetic doping harmfully<br />
affects emission properties and, moreover, changes conductivity to p-type. We have proposed<br />
a new approach to the problem based on using narrow band-gap III-V compounds possessing<br />
the largest intrinsic electronic g-factor, which are InSb and InAs [2].<br />
Present research is focused on spin-related phenomena in low-dimensional InSb/InAs<br />
heterostructures and continues our preliminary studies [3]. Magneto-optical investigations <strong>of</strong><br />
type-II InSb QDs in an InAs matrix and their theoretical consideration are reported. In(Sb,As)<br />
heterostructures incorporating monolayer-scale InSb insertions in the InAs matrix, possessing<br />
high recombination efficiency, were fabricated by MBE. Circularly polarized<br />
photoluminescence has been measured in the<br />
polarization, %<br />
100<br />
80<br />
60<br />
40<br />
20<br />
0<br />
-20<br />
T=2K<br />
B=4T<br />
0,1 1 10<br />
Excitation Intensity, W/cm 2<br />
Faraday geometry, allowing direct probing <strong>of</strong> the<br />
electron spin state. Experiments were carried out in<br />
the temperature range from 2 to 140 K and<br />
excitation density 0.1 – 20 W/cm 2 . It was found that<br />
the polarization degree varies form 100% -minus<br />
up to 15% -plus, depending on the excitation<br />
intensity and temperature. The physical model <strong>of</strong> the<br />
phenomenon is developed, based on the selection<br />
rules for optical transitions involving heavy holes,<br />
dependence <strong>of</strong> the oscillator strength on the excitation density, inherent for indirect in space<br />
optical transitions that only are allowed here, and an analysis <strong>of</strong> equilibrium population <strong>of</strong><br />
energy levels. The detailed calculation <strong>of</strong> the energy spectrum <strong>of</strong> charge carries, performed<br />
within a tight-binding approximation, is also presented. The results <strong>of</strong> the theoretical<br />
treatment are in good agreement with experimental data. The simulation <strong>of</strong> the experimental<br />
data revealed that the oscillator strength <strong>of</strong> the optical transitions <strong>of</strong> electrons with the spin<br />
oriented either along or against the magnetic field vector differs by approximately 1,8 times.<br />
[1] T.T. Chen, C.H. Chen, W.S. Su et al., J. Appl. Phys. 93, 9655 (2002).<br />
[2] Ya.V. Terent’ev, A.A. Toropov, B.Y. Meltser et al., Semiconductors 44, 194 (2010).<br />
[3]Ya.V. Terent’ev, O.G. Lyublinskaya, A.A. Toropov et al., Semiconductors 43, 635 (2009).<br />
132
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors TuP25<br />
Phonon-assisted tunneling between hole states in double<br />
quantum dots<br />
Krzyszt<strong>of</strong> Gawarecki, Pawe̷l Machnikowski<br />
Institute <strong>of</strong> Physics, Wroc̷law University <strong>of</strong> Technology, 50-370 Wroc̷law, Poland<br />
In this work, we present a theoretical analysis <strong>of</strong> hole states in double quantum dots<br />
(DQDs). The system under consideration, consist two, coupled, vertically stacked dots<br />
with one trapped hole. We study hole states in a realistic model which takes into account<br />
the system geometry and strain in the DQD. In present work, we also investigate<br />
phonon-related processes in such a system. We derive phonon-assisted relaxation between<br />
two lowest hole states and compare those results with the previous one for electrons [1,2].<br />
The relaxation rates are calculated as a function <strong>of</strong> external axial electric field. Carrierphonon<br />
couplings via both deformation potential and piezoelectric interactions can be<br />
predominant for different parameters. As shown previously [3] the ground state in such<br />
molecule can be antibonding [3], which corresponds to recent experiments[4]. In consequence,<br />
for some distance between dots (relevant to ground state bonding–antibonding<br />
transition) relaxation rate can be very small.<br />
DQDs, can be used for designing quantum-coherent devices, including spin-based<br />
quantum bits. Information can be encoded on carrier states in such structures. However,<br />
phonon-related processes are inevitable in a cristal environment and may limit the<br />
feasibility <strong>of</strong> implementing quantum control in these systems. In consequence, it is essential<br />
to understand not only the spectral properties <strong>of</strong> various kinds <strong>of</strong> semiconductor<br />
nanostructures but also the relevant phonon-induced processes.<br />
Our model [1] is based on a k·p approximation for a single particle in a strained selfassembled<br />
structure. The local band structure in a such a system is obtained from the<br />
8-band Hamiltonian with strain-induced terms (Bir-Pikus Hamiltonian).We solve the 4x4<br />
part (light- and heavy holes with two possible spin orientation) <strong>of</strong> this hamiltonian. Interation<br />
with the conduction band, was included via quasi-degenerate perturbation theory.<br />
Next, we calculate the the valence band edges and the effective mass for heavy and light<br />
holes as a function <strong>of</strong> the spatial position. The hole wave functions are calculated within<br />
a multi-component envelope function approximation combined with a Ritz variational<br />
method. Finally, the Fermi golden rule is used to obtain the relaxation rates between the<br />
lowest energy eigenstates.<br />
[1] K. Gawarecki, M. Pochwa̷la, A. Grodecka-Grad and P. Machnikowski,<br />
Phys. Rev. B 81, 245312 (2010).<br />
[2] K. Gawarecki, P. Machnikowski, Acta Phys. Pol. A (in press), arxiv:1007.1577 (2010).<br />
[3] J.I. Climente et al., Phys. Rev. B 78, 115323 (2008).<br />
[4] M.F. Doty et al., Phys. Rev. Lett. 102, 047401 (2009).<br />
133
TuP26 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Intermediate band formation for electrons and holes in an<br />
inhomogeneous chain <strong>of</strong> quantum dots<br />
Igor Bragar, Krzyszt<strong>of</strong> Gawarecki, Pawe̷l Machnikowski<br />
Institute <strong>of</strong> Physics, Wroc̷law University <strong>of</strong> Technology, 50-370 Wroc̷law, Poland<br />
We study the electron and hole states <strong>of</strong> a chain <strong>of</strong> non-identical, vertically stacked quantum<br />
dots (QDs). It has been proposed [1] that inserting such a structure in the intrinsic<br />
region <strong>of</strong> a p-i-n junction solar cell could increase the photocurrent due to sequential<br />
absorption <strong>of</strong> low energy (below-bandgap) photons. While this idea has been gaining<br />
growing experimental support in the recent years [2], it still seems unclear whether the<br />
underlying physics corresponds to the intermediate band concept [3] as the band formation<br />
in a quantum dot chain may be strongly suppressed by the energy inhomogeneity.<br />
In this presentation we use a simple model <strong>of</strong> a tunnel-coupled chain <strong>of</strong> dots containing<br />
one electron or one hole. By using the energy and coupling parameters obtained from<br />
realistic k.p calculations [4], we are able to relate the spectral properties <strong>of</strong> the chain as<br />
well as the localization properties <strong>of</strong> the carrier wave functions to the actual geometry <strong>of</strong><br />
the system (dot sizes and inter-dot separations). We study how the pseudo-band formed<br />
<strong>of</strong> the ground states confined in the QDs disintegrates upon increasing the inhomogeneity<br />
<strong>of</strong> the electron and hole energies. We discuss the impact <strong>of</strong> localization on the interband<br />
absorption from the pseudo-band to extended (bulk) states, which is a prerequisite for the<br />
functionality <strong>of</strong> the photovoltaic device. We formulate the conditions for the strength <strong>of</strong><br />
the coupling (hence the dot separation) as compared to the degree <strong>of</strong> the inhomogeneity<br />
that must be satisfied in order for the system to support delocalized states.<br />
[1] V. Aroutiounian, S. Petrosyan, A. Khachatryan, K. Touryan, J. Appl. Phys. 89, 2268<br />
(2001).<br />
[2] Y. Okada, T. Morioka, K. Yoshida, R. Oshima, Y. Shoji, T. Inoue, T. Kita, J. Appl.<br />
Phys. 109, 024301 (<strong>2011</strong>).<br />
[3] A. Luque and A. Martí, Phys. Rev. Lett. 78, 5014 (1997).<br />
[4] K. Gawarecki, M. Pochwa̷la, A. Grodecka-Grad, P. Machnikowski, Phys. Rev. B 81,<br />
245312 (2010).<br />
134
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors TuP27<br />
Modelling <strong>of</strong> 1D-2D structural transitions in small Yukawa<br />
clusters<br />
Olga Rancova, Egidijus Anisimovas<br />
Department <strong>of</strong> Theoretical Physics, Vilnius University, Saul˙etekio al. 9, LT-10222<br />
Vilnius, Lithuania<br />
We study structural transitions that take place in the simplest finite-size systems and<br />
are analogous to phase transitions in the macroscopic limit [1]. The model system consists<br />
<strong>of</strong> a small number <strong>of</strong> identical interacting charged particles in an anisotropic trap, where<br />
the particles form stable ordered structures known as Wigner crystals [2]. These are<br />
observed in diverse systems as electrons on liquid He, ions in traps, 2D semiconductor<br />
quantum dots, dusty plasmas, macroscopic model systems [3]. Recent developments <strong>of</strong><br />
experimental techniques allowed direct investigation <strong>of</strong> structural and dynamic properties<br />
<strong>of</strong> systems consisting <strong>of</strong> just a few or several tens <strong>of</strong> particles under different external<br />
conditions that can be modelled numerically.<br />
In present research we model 1D-2D dimensional structural transition caused by decreasing<br />
anisotropy <strong>of</strong> the confining potential that is one <strong>of</strong> the simplest cases <strong>of</strong> continuous<br />
phase transitions. Such dimensional transitions called ”zigzag transitions” are studied<br />
experimentally and modelled numerically [4, 5].<br />
The model system consists <strong>of</strong> N identical charged particles interacting through pairwiseYukawapotentialVin<br />
= r −1 exp(−κr)withscreeningparameterκandconfinedby2D<br />
asymmetric parabolic potential Vc = x 2 +α 2 y 2 . The potential interaction energy strongly<br />
dominates over the kinetic energy. Thus, we are concerned with the minimization <strong>of</strong> the<br />
total potential energy. α > 1 squeezes the confining potential inducing structural transitions<strong>of</strong>thesystem.<br />
ThenumericaltechniquecombiningtheMetropolisprocedureandthe<br />
Newtonoptimizationmethodisusedtoobtaintheenergeticallyfavourableconfigurations.<br />
The calculations are performed for different κ changing the value <strong>of</strong> α.<br />
In present research we investigate the simplest systems up to N = 9 undergoing<br />
1D-2D structural transition. Changing the potential parameter α gradually we explore<br />
dependence <strong>of</strong> the order parameter <strong>of</strong> the system 〈y〉, defined as average system width<br />
in the anisotropy dimension, on the potential anisotropy parameter looking for evidences<br />
<strong>of</strong> the phase transition. When α reaches a critical value αc the system order parameter<br />
drops to zero and we obtain a power-law behaviour <strong>of</strong> the order parameter 〈y〉 ∝ (αc−α) γ<br />
in the vicinity <strong>of</strong> this critical point (γ is a critical exponent). We find that in this area<br />
γ ≈ 0.5 in all cases investigated. Whereas [5] suggests γ = 0.387 in N = 5,κ = 3 case.<br />
We obtain that the value <strong>of</strong> the critical exponent changes when the potential anisotropy<br />
parameter value departs from the critical point.<br />
Acknowledgments O. Rancova is funded by Lithuanian Science Council (grant No.<br />
SF-PD-2009-08-17-0054).<br />
[1] O. Rancova, E. Anisimovas, and T. Varanavičius, Phys. Rev. E 83, 036409 (<strong>2011</strong>).<br />
[2] E. Wigner, Phys. Rev. 46, 1002 (1934); C. C. Grimes and G. Adams, Phys. Rev.<br />
Lett. 42, 795 (1979); E. Y.Andrei, G. Deville, D. C. Glattli, F. I. B. Williams, E.Paris,<br />
and B. Etienne, Phys. Rev. Lett. 60, 2765 (1988).<br />
[3] A. V. Filinov et.al., Phys. Rev. Lett. 86, 3851 (2001); M. Saint Jean et.al., Europhys.<br />
Lett. 55, 45 (2001); D. Block et.al., Phys. Plasmas 15, 040701 (2008).<br />
[4] A. Melzer, Phys. Rev. E 73, 056404 (2006).<br />
[5] T. E. Sheridan and K. D. Wells, Phys. Rev. E 81, 016404 (2010).<br />
135
TuP28 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
EfficientCoulombmixingbetweensingle-and<br />
biexcitonstatesinInAsnanocrystals<br />
P.Kowalski,P.Machnikowski<br />
Institute<strong>of</strong>Physics,WrocławUniversity<strong>of</strong>Technology,50-370Wrocław,Poland<br />
Multipleexcitongeneration(impactionization)innanocrystalsisaprocess,inwhich<br />
absorption<strong>of</strong>asinglephotonleadstogeneration<strong>of</strong>multipleelectron-holepairs. This<br />
effecthasbeenobservedexperimentally[1,2]andmayincreasetheefficiency<strong>of</strong>solarcells.<br />
Whileearlierpapersreportedquantumefficienciesreaching700%[3],laterreportssuggestthatthosefigureswereoverestimatedandtheactualyielddoesnotexceed170%[4].<br />
Thiscontroversymightberesolvediftheprocessisbetterunderstoodtheoretically.<br />
Inourpreviouswork[5]wehaveshownthatcoherenttwo-pairgenerationwithasingle<br />
absorbedphotonispossibleduetoCoulombindirectmixing<strong>of</strong>single-exciton(onepair)<br />
andbiexciton(two-pair)configurations.However,forlowenergystatesthisprocessisin<br />
generalveryinefficientsincethetypicalmagnitude<strong>of</strong>theCoulombmatrixelement(tens<br />
<strong>of</strong>meV)ismuchsmallerthantheaverageseparationbetweentheenergylevels(hundreds<br />
<strong>of</strong>meV).<br />
Forhighenergystatesthespectrum<strong>of</strong>two-pairconfigurationsismuchmoredense.In<br />
thatrangetypicalseparation<strong>of</strong>statescoupledtoasinglepairisontheorder<strong>of</strong>100meV<br />
(forthenanocrystalradius<strong>of</strong>7nm).Thestrength<strong>of</strong>thecouplingstillreaches100meV.<br />
Hence,weexpectstrongmixingbetweenthe1-pairand2-pairconfigurationswhichcan<br />
leadtoamoreefficientbiexcitongeneration.<br />
[1]R.D.Schaller,V.I.Klimov,Phys.Rev.Lett.92,186601(2004).<br />
[2]R.D.Schaller,J.M.Pietryga,V.I.Klimov,NanoLett.7,3469(2007).<br />
[3]R.D.Schaller,M.Sykora,J.M.Pietryga,V.I.Klimov,NanoLett.6,424(2006).<br />
[4]M.TuanhTrinhet.al.,NanoLett.8,1713(2008).<br />
[5]P.Kowalski,P.Machnikowski,ActaPhys.Pol.114,1187(2008).<br />
136
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors TuP29<br />
Strongly disordered wetting layer in the InAs/GaAs self-assembled<br />
quantum dots system<br />
K. Gołasa 1 , M. Molas 1 , J. Borysiuk 1 , Z. R. Wasilewski 2 and A. Babiński 1<br />
1 Faculty <strong>of</strong> Physics, University <strong>of</strong> Warsaw, Hoża 69, 00-681 Warszawa, Poland<br />
2 Institute for Microstructural Sciences, National Research Council, Ottawa, Canada<br />
A wetting layer (WL) is an inevitable result <strong>of</strong> the formation <strong>of</strong> quantum dots (QDs) in<br />
the Stranski–Krastanow growth mode. It is usually assumed that the WL is a thin, highly<br />
strained quantum well, on which three-dimensional islands grow. However, in real systems<br />
the morphology <strong>of</strong> the WL can be more complex. In particular substantial indium fluctuations<br />
in the WL, which give rise to confinement <strong>of</strong> carriers have recently been identified [1].<br />
In this communication we address the question <strong>of</strong> growth conditions necessary to form<br />
such strongly disordered WL. We investigate two samples A and B grown with different<br />
parameters using molecular beam epitaxy. While growing both samples, a substrate rotation<br />
was stopped during the InAs layer deposition in order to obtain a gradient <strong>of</strong> InAs coverage<br />
across the wafer. The amount <strong>of</strong> deposited InAs was chosen in such a way as to have a<br />
continuous variation <strong>of</strong> the dot density, starting with just InAs WL to fairly high density <strong>of</strong><br />
InAs QDs on each wafer. In the reference sample A, grown in usual conditions, such the<br />
procedure resulted in an expected evolution <strong>of</strong> the low-temperature photoluminescence across<br />
the wafer. Low InAs coverage resulted in the WL-related emission centred at 1.44 eV, which<br />
disappeared from the spectrum with increasing intensity <strong>of</strong> the QDs emission due to<br />
increasing density <strong>of</strong> QDs grown. In the<br />
sample B the scenario was different. The<br />
lowest InAs coverage resulted in QDs<br />
emission but no emission from the WL (see<br />
lowest spectrum in Fig. 1). At the expected<br />
energy range <strong>of</strong> the WL-emission several<br />
sharp lines due to recombination <strong>of</strong> confined<br />
single excitons and biexcitons were<br />
observed. At higher InAs coverage a weak<br />
emission related to a thin WL (denoted with<br />
an arrow in Fig. 1) was observed, which<br />
disappeared at even higher InAs coverage<br />
leaving just the QDs emission in the<br />
1,25 1,30 1,35 1,40 1,45<br />
spectrum. The observed behaviour suggests that the WL does not form a continuous twodimensional<br />
layer before the onset <strong>of</strong> the QDs formation in the sample B. Instead, twodimensional<br />
platelets form, which can confine electrons and holes. With higher InAs coverage<br />
the platelets merge giving rise to usually observed WL. Our conclusions are supported with<br />
results <strong>of</strong> transmission electron microscopic study <strong>of</strong> the samples morphology.<br />
We present details <strong>of</strong> the growth procedure and discus their effect on the morphology<br />
<strong>of</strong> the investigated samples. We also discuss possible applications <strong>of</strong> self-assembled QDs<br />
grown with no continuous WL.<br />
[1] A. Babiński, J. Borysiuk, S. Kret, M. Czyż, A. Golnik, S. Raymond and Z. R. Wasilewski,<br />
Appl. Phys. Lett. 92, 171104 (2008).<br />
137<br />
PL intensity [arb.u.]<br />
Energy [eV]<br />
WL<br />
Fig.1. The μ-PL spectra from three positions <strong>of</strong><br />
the investigated sample.<br />
InAs nominal coverage
TuP30 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Optical transformation <strong>of</strong> zero-dimensional confinement in the<br />
CdTe/CdMgTe multiple quantum wells<br />
M. Molas 1,* , K. Gołasa 1 , J. Łusakowski 1 , T.Wojtowicz 2 , and A. Babi ski 1<br />
1 Faculty <strong>of</strong> Physics, University <strong>of</strong> Warsaw, Ho a 69, 00-681 Warszawa, Poland<br />
2 Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences, Al. Lotników 32/46, 02-668 Warszawa, Poland<br />
Zero-dimensional confinement in semiconductor structures, which is crucial for many optoelectronic<br />
applications, may result from several factors influencing potential energy <strong>of</strong> carriers. Most intensely<br />
investigated are quantum dots (QDs) formed in Stranski-Krastanov growth mode [1,2] or due to composition<br />
fluctuations in thin quantum wells (QWs) [3]. There are, however, systems in which the confinement results<br />
from a more complicated potential energy pattern [4,5]. In this communication we report on QDs which in our<br />
opinion fall to the latter category as they can be transformed by optical illumination.<br />
An active part <strong>of</strong> the investigated structure consisted <strong>of</strong> 20 CdTe QWs, each <strong>of</strong> a 16 nm width,<br />
separated by a 48 nm-wide Cd0.8Mg0.2Te barriers. Both the QWs and the barriers were uniformly doped with 2 ×<br />
10 16 cm -3 iodine atoms acting as donors. Broad optical emission band related to the QWs was observed around<br />
1.6 eV. Moreover, a few discrete emission lines were observed at a lower energy. A more detailed analysis <strong>of</strong><br />
these lines, based on excitation power dependence <strong>of</strong> their intensity and polarization-sensitive measurements <strong>of</strong><br />
their optical anisotropy, allowed for their attribution to 0D-confined neutral excitons and biexcitons.<br />
A characteristic feature <strong>of</strong> the observed single-dot spectra was their instability against a high excitation powerdensity.<br />
Series <strong>of</strong> spectra in excited with a relatively low (I = 0.1 W) excitation power-density separated by 2<br />
min exposure to a high power light (I = 350 W) are shown in Fig. 1. It can be appreciated that the spectra<br />
change as a result <strong>of</strong> a high intensity illumination (see Fig.1 a - c) and a new lineshape, stable against subsequent<br />
high-density illumination was obtained (see Fig. 1 c - d).<br />
An analysis <strong>of</strong> the observed metastable behaviour <strong>of</strong> the spectra led us to conclusion that the<br />
confinement <strong>of</strong> carriers, responsible for the spectra, results partially from intrinsic electric fields due to ionized<br />
Luminescence (a.u.)<br />
d)<br />
c)<br />
b)<br />
a)<br />
1.535 1.540 1.545 1.550<br />
Energy (eV)<br />
1.555 1.560 1.565<br />
Figure 1. The -PL spectra from one position <strong>of</strong> the<br />
investigated sample<br />
donors and partially due to local potential<br />
fluctuations inherent to the QW growth process.<br />
Existence <strong>of</strong> such mixed QDs (MQDs) have<br />
previously been proposed on the basis <strong>of</strong> farinfrared<br />
spectroscopic measurements <strong>of</strong> the<br />
investigated structure [4, 5]. A large (from 20<br />
eV to nearly 120 eV) dispersion <strong>of</strong> the optical<br />
anisotropy <strong>of</strong> excitons confined in the MQDs<br />
suggests a substantial asymmetry <strong>of</strong> the<br />
respective lateral potential. The observed<br />
transform-ation <strong>of</strong> MQDs is proposed to be<br />
related to ionization <strong>of</strong> donors which changes<br />
the potential fluctuations pattern. A longer<br />
exposure to a high intensity light leads to<br />
formation <strong>of</strong> a different potential fluctuations<br />
pattern which is reflected by a different<br />
luminescence lineshape. Discussion <strong>of</strong> obtained<br />
results in terms <strong>of</strong> potential fluctuations in doped QW is presented.<br />
Partial financial support by Polish Ministry <strong>of</strong> Higher Education contract N N515071937<br />
is acknowledged.<br />
[1] M. Bayer, et al, Phys. Rev. B 65, 195315 (2002).<br />
[2] G. Karczewski, et al, Appl. Phys. Lett. 74, 3011 (1999).<br />
[3] L. Besombes, et al, phys. stat. sol. (a) 178, 197 (2000).<br />
[4] K. Karpierz, et al, Acta Phys. Polonica A 112, 237 (2007).<br />
[5] K. Nogajewski, et al, Acta Phys. Pol. A 114, 1259 (2008).<br />
* corresponding author : maciej.molas@fuw.edu.pl<br />
138
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors TuP31<br />
OpticalProperties<strong>of</strong>InGaAsQuantumDotson(100)GaAs<br />
SubstratesFormedbyDropletEpitaxy<br />
MichałKozub 1 ,AnnaMusiał 1 ,GrzegorzSęk 1 ,JanMisiewicz 1 ,<br />
VerenaZürbig 2 andJohannPeterReithmaier 2<br />
1 Institute<strong>of</strong>Physics,WrocławUniversity<strong>of</strong>Technology,50-370Wrocław,Poland<br />
2 Institute<strong>of</strong>NanostructureTechnologiesandAnalytics,TechnischePhysik,Universität<br />
Kassel,Heinrich-Plett-Str.40,34132Kassel,Germany<br />
Thepresence<strong>of</strong>awettinglayer(WL)inalasingsemiconductormicrostructureintroducesanaditionalandunwantedchannelforcarrierlossandnonradiativerelaxation,<br />
significantlyreducingtheefficiency<strong>of</strong>thedevice. However,inacommonIII-Vsystem<br />
grownbyself-assemblytheexistence<strong>of</strong>thewettinglayerisanintrinsicproperty<strong>of</strong>the<br />
growthmodeandisunlikelytobeavoided.Because<strong>of</strong>theWLsdeterioratinginfluence,<br />
techniques<strong>of</strong>growth<strong>of</strong>lowdimensionalstructuresaimingtoreducetheWLorevento<br />
allowthegrowth<strong>of</strong>nanostructureswithouttheWLatallarebeingsought.<br />
Weattemptasystematicstudy<strong>of</strong>theinfluence<strong>of</strong>temperatureandflowrateonthe<br />
formation<strong>of</strong>awettinglayerinanInGaAs/GaAsquantumdot(QD)systemgrownby<br />
the’dropletepitaxy’method[1,2],andfurtherhowthegrowthconditionchangesaffect<br />
theopticalproperties<strong>of</strong>theentirestructure.SeveralQDsamplesgrownattemperatures<br />
between 410and 500 o Candgrowthrates<strong>of</strong>80-110nm/hhavebeeninvestigatedby<br />
photoluminescence(PL)andcontactlesselectroreflectance(CER).Incase<strong>of</strong>thelatter,<br />
duetoenhancedsensitivity<strong>of</strong>themodulationtechniqueitwaspossiblet<strong>of</strong>ollownot<br />
onlytheQDs,butalsotheWLsopticaltransitionsalongthevariatinggrowthcondition<br />
parameters.Ourfindingssuggestthatwithdecreasingthesubstratetemperature,surface<br />
diffusioncanbeefficientlyreducedthusenablingustohindertheassembly<strong>of</strong>thewetting<br />
layer. TheWL-relatedopticaltransitionsarewellresolvedathighertemperatures(i.e.<br />
500 o C),furtherfollowedbyacorrespondingCERsignalintensitydropandbroadening,<br />
t<strong>of</strong>inallydisappearfortemperaturesaslowas 410 o C.Weassumethatthedecrement<strong>of</strong><br />
growthtemperature(and/orgrowthrate)renderstheWLdiscontinuousandcausesitto<br />
coverasmallfraction<strong>of</strong>thesurfaceonlyforthelowesttemperatures(growthrates).<br />
Asboththechangesinthegrowthconditionsanddisappearance<strong>of</strong>theWLareexpectedtoaffecttheopticalproperties<strong>of</strong>quantumdots,theQDopticaltransitionshave<br />
alsobeeninvestigatedvsthetechnologicalparameters. First<strong>of</strong>all,thecombination<strong>of</strong><br />
PLandCERallowedustodeterminethetransitionenergiesrelatedtoQDs.Then,we<br />
derivetherespectiveactivationenergiesfromPLthermalquenchingmeasurementsina<br />
temperaturerangefrom5to300Kinordertodetectareduction<strong>of</strong>thermally-induced<br />
losseswithdiminishingWL.Eventually,thechangesinthesurfacedensity<strong>of</strong>thedots<br />
havebeenstudiedbymeans<strong>of</strong>PLwiththespatialresolution<strong>of</strong>about1micrometer,<br />
whichallowedthedisclosure<strong>of</strong>thesubstructure<strong>of</strong>emissionpeaksfromtheensemble<br />
relatedtosinglequantumdotemissionlinesoreventhesplit<strong>of</strong>spectraintosharplines<br />
relatedtosingledots.<br />
[1]N.Koguchi,S.Takahashi,T.Chikyow,J.CrystalGrowth111,688(1991).<br />
[2]V.Zrbig,A.Gushterov,L.Lingys,M.Benyoucef,J.P.Reithmaier,Int.MBEConf.,<br />
Berlin,Germany(August,2010).<br />
139
TuP32 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Magnetooptical study <strong>of</strong> excitons confined in potential fluctuations in<br />
the CdTe/CdMgTe quantum well<br />
M. Molas 1,* , K. Gołasa 1 , T. Kazimierczuk 1 , J.Łusakowski 1 , T.Wojtowicz 2 ,<br />
and A. Babi ski 1<br />
1 Faculty <strong>of</strong> Physics, University <strong>of</strong> Warsaw, ul. Ho a 69, 00-681 Warszawa, Poland<br />
2 Institute <strong>of</strong> Physics, PAS, Al. Lotników 32/46, 02-668 Warszawa, Poland<br />
The low dimensional systems based on quantum dots (QDs) are intensely investigated<br />
for their applications in efficient light sources and detectors tuned in a wide range <strong>of</strong><br />
wavelengths from near infrared to ultraviolet. A three-dimensional confinement <strong>of</strong> excitons<br />
can be achieved in self-assembled QDs [1], on fluctuations at interfaces <strong>of</strong> thin quantum<br />
wells (QWs) or in disordered thin QWs [2, 3]. However, it was recently shown that also in<br />
wider single or multiple doped QWs local potential fluctuations can also lead to the QD-like<br />
confinement [4, 5].<br />
We report on magneto-optical properties <strong>of</strong> sharp excitonic photoluminescence (PL)<br />
lines related potential fluctuations, which are observed from a sample with a single QW <strong>of</strong> a<br />
modulated thickness. An active part <strong>of</strong> the sample consisted <strong>of</strong> a 30.1 nm wide CdTe QW,<br />
sandwiched between two Cd0.8Mg0.2Te barriers. A 5 nm wide iodine doping layer was added<br />
in the Cd0.8Mg0.2Te barrier. Results <strong>of</strong> low-temperature micro-PL measurements, performed<br />
in magnetic field are presented.<br />
Optical emission from the QW was observed in energy range between 1.81 eV to<br />
1.83 eV, which reflected the change in the QW thickness. At lower energy, at several<br />
positions on the sample a few discrete emission lines were observed. The effect <strong>of</strong> sample<br />
temperature, magnetic field, and detection polarisation was studied. Neutral biexcitons and<br />
excitons confined to a single fluctuation-related QD were identified. We found that the<br />
optical anisotropy <strong>of</strong> observed excitonic features ranged from about 10 eV to nearly<br />
160 eV, which suggests a broad distribution <strong>of</strong> lateral potential asymmetry. Energy<br />
dispersion <strong>of</strong> neutral exciton states amounts to 22 meV, which suggests a substantial<br />
distribution <strong>of</strong> the confining potential. The values <strong>of</strong> effective excitonic Lande factor <strong>of</strong><br />
excitons ranged from |g*| = 1.8 to 2.<br />
We propose to explain our observations in a model <strong>of</strong> potential fluctuations in the QW<br />
resulting from a strongly disordered layer <strong>of</strong> iodine-doping. In regions <strong>of</strong> the highest iodine<br />
surface density the effect <strong>of</strong> electrostatic potential on the QW is the strongest. Confined<br />
electronic and hole states in the QW in such regions are strongly affected by the Quantum<br />
Confined Stark Effect (QCSE), which leads to a local decrease <strong>of</strong> the QW band-gap. In areas<br />
with a lower iodine density the effect is weaker and the QW bandgap is only weakly affected.<br />
Fluctuations <strong>of</strong> the effective QW bandgap due to the QCSE resulting from fluctuations <strong>of</strong><br />
remote ionized iodine donor density are responsible in our opinion for observed QD-like<br />
confinement <strong>of</strong> electrons and holes in the investigated structure.<br />
[1] For review, see D. Bimberg et al, Quantum Dot Heterostructures (Wiley, New York, 1999).<br />
[2] L. Besombes, et al, Phys. Stat. Sol. (a) 178, 197 (2000).<br />
[3] D. Gammon, et al, Phys. Rev. Lett. 76, 3005 (1996).<br />
[4] K. Karpierz, et al, Acta Phys. Pol. A 112, 237 (2007).<br />
[5] K. Nogajewski, et al, Acta Phys. Pol. A 114, 1259 (2008).<br />
* corresponding author : maciej.molas@fuw.edu.pl<br />
140
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors TuP33<br />
Mixed Correlation Phases In Elongated Quantum Dots<br />
A. Ballester 1 , J. M. Escartín 2 , J. L. Movilla 1 , M. Pi 2 , and J. Planelles 1<br />
1 Departament de Química Física I Analítica, Universitat Jaume I, Box 224, E-12080,<br />
Castelló, Spain<br />
2 Departament ECM, Facultat de Física, IN2UB, Universitat de Barcelona, E-08028,<br />
Barcelona, Spain<br />
Nanorods (NRs) are an elongated variant <strong>of</strong> quantum dots (QDs) with a highly<br />
anisotropic confining potential. These nanostructures constitute the bridge between zerodimensional<br />
QDs and one-dimensional quantum wires (QWs). Nanodumbbells (NDs) are<br />
another type <strong>of</strong> elongated structures. These structures are NRs sandwiched between two<br />
spherical caps, typically <strong>of</strong> a different material. The theoretical study <strong>of</strong> colloidal<br />
semiconductor NRs and NDs is <strong>of</strong> particular interest because <strong>of</strong> the ability to synthesize these<br />
kinds <strong>of</strong> nanostructures with precise size and shape control [1,2].<br />
The unusual long length <strong>of</strong> the NRs leads to characteristic pr<strong>of</strong>iles <strong>of</strong> the electron<br />
density distribution in the NR growth direction, which depend on the ratio among the NR<br />
length and the number <strong>of</strong> electrons populating the conduction band <strong>of</strong> the nanostructure. In<br />
the case <strong>of</strong> diluted N-electron NRs, the particles are distributed in an ordered way forming a<br />
Wigner crystal [3]. The Coulomb energy predominates so that correlations dominate the<br />
electronic structure. In the opposite limit, the particles interact weakly and the kinetic energy<br />
dominates the Coulomb repulsion. Then, the so-called Fermi-liquid phase appears [4]. The<br />
transition to the Fermi liquid goes through an intermediate phase (called charge-density wave<br />
within the local spin-density-functional theory) which can be considered as a partially melted<br />
Wigner molecule [5]. In this contribution, we present a comprehensive study on the electron<br />
density distribution in both, NRs under applied electric fields and nanodumbbells. We show<br />
that two <strong>of</strong> the above-mentioned phases may coexist simultaneously in the same structure –<br />
but in different regions – giving rise to new phases that we refer to as mixed correlation<br />
phases [6].<br />
[1] X. Peng, L. Manna, W. Yang, J. Wickham, E. Scher, A. Kadanich, and A. P. Alivisatos,<br />
Nature (London) 404, 59 (2000); S. Kan, T. Mokari, E. Rothenberg, and U. Banin, Nature<br />
Mater. 2, 155 (2003).<br />
[2] J. E. Halpert, V. J. Porter, J. P. Zimmer, and M. G. Bawendi, J. Am. Chem. Soc. 128, 12590<br />
(2006).<br />
[3] J. Planelles, M. Royo, A. Ballester, and M. Pi, Phys. Rev. B 80, 045324 (2009).<br />
[4] V. Filinov, M. Bonitz, and Yu. E. Lozovik, Phys. Rev. Lett. 86, 3851 (2001).<br />
[5] M. Koskinen, M. Manninen, and S. M. Reimann, Phys. Rev. Lett. 79, 1389 (1997).<br />
[6] A. Ballester, J. M. Escartín, J. L. Movilla, M. Pi, and J. Planelles, Phys. Rev. B 82, 115405<br />
(2010).<br />
141
TuP34 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Simulation and Realization <strong>of</strong> Photonic Crystals in Light<br />
Emitting Devices<br />
Jakob Ebeling, Timo Aschenbrenner, Stephan Figge, and Detlef Hommel<br />
Institute <strong>of</strong> Solid State Physics, Univerity <strong>of</strong> Bremen, Germany<br />
Indium gallium nitride (InGaN) based light emitting diodes (LEDs) have been very<br />
successful in the blue and green spectral range. However, the light extraction efficiency<br />
<strong>of</strong> conventional devices suffers from the total internal reflection at the semiconductor-air<br />
interface due to the relatively high optical refractive index (GaN: n≈2.5) limiting the numerical<br />
extraction efficiency <strong>of</strong> conventional, planar LEDs based on GaN to about 4% [1].<br />
To overcome this issue, different concepts have been theoretically studied and partially<br />
realized. Among these are encapsulation with a lower index material, surface roughening,<br />
thin-film LEDs and structuring the surface with photonic crystal (PC) structures,<br />
which are periodic variations <strong>of</strong> the refractive index at the length scale <strong>of</strong> the emission<br />
wavelength. The most common concept <strong>of</strong> a one-dimensional PC is a distributed Bragg<br />
reflector (DBR).<br />
Additionally to LEDs, PCs can be used to improve the properties <strong>of</strong> laser diodes.<br />
Commonly known are vertical-cavity surface-emitting laser diodes with DBR mirrors,<br />
but also distributed feedback (DFB) and DBR-based edge emitting laser diodes have<br />
been realized [2][3].<br />
We present numerical simulations <strong>of</strong> different types <strong>of</strong> PCs and their realization on<br />
various structures. A one-dimensional photonic crystal grating was fabricated by focused<br />
ion beam (FIB) milling at the resonator ends <strong>of</strong> an edge-emitting GaN-based laser device<br />
forming a lateral GaN-air-DBR to enhance the reflectivity <strong>of</strong> the resonator mirrors.<br />
Optoelectronic measurements were made to compare devices with and without additional<br />
gratings.<br />
Moreover, green emitting InGaN-based quantum dot LED structures were grown on<br />
sapphire substrates with metal-organic vapour-phase epitaxy. Different two-dimensional<br />
PCs were structured by FIB milling as well as e-beam lithography with subsequent dry<br />
etching. The structure surfaces are characterized by scanning electron microscopy and<br />
atomic force microscopy and a comparison between the two technological methods is<br />
given. Optical measurements obtained by micro-photoluminescence are used to quantify<br />
the light extraction enhancement achieved by PC structuring. The results <strong>of</strong> the different<br />
structures are compared with each other and with previous simulations.<br />
[1] C. Wiesmann, K. Bergenek, N. Linder, and U.T. Schwarz, Laser & Photonics Review<br />
3, 3 (2009).<br />
[2] D. H<strong>of</strong>stetter, R.L. Thornton, L.T. Romano, D.P. Bour, M. Kneissl, and R.M. Donaldson,<br />
Appl.Phys.Lett. 73, 2158 (1998).<br />
[3] J. Cho, S. Cho, B.J. Kim, S. Chae, C. Sone, O.H. Nam, J.W. Lee, Y. Park, and T.I.<br />
Kim, Appl.Phys.Lett. 76, 12 (2000).<br />
142
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors TuP35<br />
Electro-optical characterization <strong>of</strong> Ti/Au-ZnTe Schottky diodes with CdTe<br />
quantum dots<br />
E. Zielony 1 , E. Popko 1 , Z. Gumienny 1 , G. Karczewski 2<br />
1 Institute <strong>of</strong> Physics, Wroclaw University <strong>of</strong> Technology, Wybrzeze Wyspianskiego 27,<br />
50-370 Wroclaw, Poland<br />
2 Institute <strong>of</strong> Physics, Institute <strong>of</strong> Physics Polish Academy <strong>of</strong> Science, al. Lotnikow 32/46,<br />
02-668 Warsaw, Poland<br />
In this study the deep level transient spectroscopy (DLTS) and micro-Raman spectroscopy<br />
techniques have been applied to investigate ZnTe (p-type) – Ti/Au Schottky diodes containing<br />
a layer <strong>of</strong> CdTe self-assembled quantum dots (SAQDs). The reference sample was the ZnTe –<br />
Ti/Au diode without dots. Both samples were grown by molecular beam epitaxy technique.<br />
The dots were formed in the atomic layer epitaxy (ALE) growth mode by deposition <strong>of</strong> three<br />
monolayers <strong>of</strong> CdTe. Micro-Raman measurements confirmed the presence <strong>of</strong> CdTe layer <strong>of</strong><br />
quantum dots in the investigated material. A broad band corresponding to the longitudinal<br />
(LO) CdTe phonon related to the QD-layer has been detected at a wavenumber <strong>of</strong> 160 cm -1<br />
[1] in the case <strong>of</strong> the QD sample. Another peak observed for the wavenumber <strong>of</strong> 210 cm -1 has<br />
been assigned to localized LO phonon associated with the ZnTe layer in both samples [1].<br />
Moreover two tellurium - related bands at wavenumbers around 120 cm -1 and 140 cm -1 have<br />
been identified [1]. DLTS measurements for the sample with QDs reveal the presence <strong>of</strong> two<br />
hole-related signals with thermal activation energies equal to EH1=0.2eV and EH2=0.4eV. For<br />
the reference ZnTe-Ti/Au diode only the H2 signal is observed. It may be concluded that the<br />
H1=0.2 eV level can be assigned to the hole emission from defects accompanying quantum<br />
dots formation. The 0.4eV trap is attributed to the ZnTe bulk material [2].<br />
[1] V. S. Vinogradov, et al., Physics <strong>of</strong> the Solid State, Vol. 50, No. 1, 2008.<br />
[2] E. Placzek-Popko, et al., Physica B 404 5173-5176 (2009).<br />
143
TuP36 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Magneto-photoluminescence studies <strong>of</strong> many body scattering processes<br />
in two-dimensional hole gas<br />
J. Jadczak1 1 , L. Bryja 1 , A. Wojs 1 , G. Bartsch 2 , D.R. Yakovlev 2 , M. Bayer 2<br />
and M. Potemski 3<br />
1 Institute <strong>of</strong> Physics, Wroclaw University <strong>of</strong> Technology, Wroclaw, Poland<br />
2 Experimentelle Physik 2, Technische Universität Dortmund, D-44227 Dortmund, Germany<br />
3 Grenoble High Magnetic Field Laboratory, CNRS, Grenoble, France<br />
Fig. 1. PL vs magnetic field for<br />
the symmetric 15 nm QW.<br />
The 2D hole gas due to the much higher effective mass<br />
create a unique possibility to study a variety <strong>of</strong> many<br />
body scattering processes, also not observed in n-type<br />
systems. In this work we report on detailed lowtemperature<br />
(T=1.8÷4.2K), high-field (B≤28T), polarization-resolved<br />
magneto-photoluminescence (MPL) studies<br />
<strong>of</strong> 2DHG in GaAs quantum wells. In the symmetric 15<br />
nm wide well with a hole concentration p=1.5x10 11 /cm -2<br />
we detected in MPL spectra shake-up hole replicas <strong>of</strong><br />
positively charged exciton (trion) on acceptor bound [1]<br />
and free states (Fig. 1). The former one for the first time.<br />
In asymmetric samples with different width<br />
(w=18÷30nm) and concentrations (p=1.4÷2.3x10 11 /cm -2 ) we observed an opposite process<br />
with hole cyclotron replicas going to higher energies with the magnetic field. The most intriguing<br />
effect, never reported before, was the observation <strong>of</strong> a coupling between the free and<br />
acceptor bound positively charged excitons (Fig. 2). The two<br />
states: free and bound are brought into resonance by an exchange<br />
<strong>of</strong> a quantum <strong>of</strong> a hole cyclotron energy in a scattering<br />
process with surrounding 2D holes. For the 22 nm wide<br />
sample the resonance is observed in magnetic field around<br />
B=13T. Further to our previous studies [3] we observed, that<br />
the essential coupling occur between free trion - X + and a<br />
pair <strong>of</strong> hole cyclotron replicas <strong>of</strong> the acceptor bound trion -<br />
CR-AX + . The coupling can be observed in photoluminescence<br />
spectra due to a nearly energy degeneracy <strong>of</strong> all final<br />
states involve in a radiative recombination. Aided with numerical<br />
calculations [3] we have explained the existence <strong>of</strong><br />
two hole replicas as the transitions <strong>of</strong> bright and dark doublet<br />
states <strong>of</strong> AX + . The dark state is enabled in an optical<br />
transition by transfer <strong>of</strong> an access momentum to one <strong>of</strong> 2D<br />
Fig. 2. PL vs magnetic field<br />
for asymmetric 22 nm QW<br />
surrounding holes. In three coupling states model we have calculated energy and intensity <strong>of</strong><br />
all observed PL lines and we have obtained a very good agreement with experimental data.<br />
The coupling is observed in spite <strong>of</strong> very small coupling constants (determined from experiment)<br />
only because two replicas are involved.<br />
[1] L. Bryja et al., Phys. Rev. B 75, 035308 (2007).<br />
[2] J. Jadczak et al. Acta Physica Polonica (in press)<br />
[3] A. Wojs, Phys. Rev. B 76, 085344 (2007); Phys. Rev. Lett. 104, 086801 (2010).<br />
144
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors TuP37<br />
Norm. PhotoVoltaic Response (arb. Units)<br />
1.5<br />
1.0<br />
T = 10K<br />
Spin related effect in Si-MOSFETs THz Photoresponse<br />
Hadley Videlier 1 , Nina Dyakonova 1 , Frederic Teppe 1 , Cristophe Consejo 1 ,<br />
Wojciech Knap 1 , Jerzy Lusakowski 2 , Daniel Tomaszewski 3 , Jacek Marczewski 3 , Piotr<br />
Grabiec 3<br />
1 Laboratoire Charles Coulomb UMR 5221 CNRS- Université Montpellier 2, 34095<br />
Montpellier, France<br />
2 Institute <strong>of</strong> Experimental Physics, Warsaw University, Warsaw, Poland<br />
3 Institute <strong>of</strong> Electron Technology, Al. Lotnikow 32/46, 02-668 Warsaw, Poland<br />
Photoresponse to sub-terahertz and Terahertz (THz) radiations in Silicon Metal Oxide<br />
Semiconductor Field Effect Transistors (Si-MOSFETs) has been studied previously [1-3]. The<br />
obtained values <strong>of</strong> responsivity and Noise Equivalent Power have shown the potential <strong>of</strong> Si-<br />
MOSFETs as sensitive detectors at room temperature [1]. The photovoltaic response<br />
dependences on the gate length and the gate bias were in good agreement with the Dyakonov-<br />
Shur plasma wave detection theory [4]. The detection signal results from the rectification <strong>of</strong><br />
high frequency currents induced by the incident radiation in the transistor channel. The<br />
rectification takes place due to a nonlinear response <strong>of</strong> the gated two dimensional electron gas<br />
and a source-drain asymmetry. The main source <strong>of</strong> nonlinearity is the superposition <strong>of</strong> two<br />
radiation-induced effects: i) the modulation <strong>of</strong> the carrier density in the channel by variations<br />
<strong>of</strong> the gate potential and ii) current oscillations due to variations <strong>of</strong> the drain potential.<br />
Recently, monolithically integrated THz focal-plane arrays including antennas and amplifiers<br />
on a single silicon die has been designed to work at room temperature [5].<br />
In this work we have studied the effect <strong>of</strong> high magnetic field on the Si-MOSFETs<br />
photovoltaic response subjected to sub-terahertz radiation (185-300 GHz ) at low temperature.<br />
Frequency (GHz)<br />
400<br />
300<br />
200<br />
100<br />
0<br />
0 3 5 8 10 13 15<br />
B (Tesla)<br />
!" # $ %$" &$ ' (<br />
% ) * + ,$ -<br />
( . %$<br />
185 GHz<br />
In Figure 1, photovoltaic signal is presented at 10 K<br />
as a function <strong>of</strong> applied magnetic field for three<br />
different incident frequencies. A very well<br />
0.5<br />
pronounced structure shifts to higher fields by<br />
0 2 4 6 8 10<br />
Magnetic Field (T)<br />
12 14 16<br />
increasing incident frequency. The positions <strong>of</strong> peaks<br />
is plotted in the inset <strong>of</strong> Fig.1 where the straight line<br />
follows equation f = gµ BB<br />
/ h and g = 2 . The calculation is in good qualitative agreement<br />
with experiments indicating a possible link between the observed structures in the<br />
photovoltaic response and a spin related phenomenon. This effect has been studied as a<br />
function <strong>of</strong> different parameters as temperature and applied source-drain current. However, a<br />
full quantitative interpretation <strong>of</strong> our results requires more complete experimental and<br />
theoretical developments.<br />
[1] R. Tauk, F. Teppe, S; Boubanga, et al. Appl. Phys. Lett. 89, 253511 (2006)<br />
[2] W. Knap, F. Teppe, Y. Meziani, et al. Applied Physics Letters 85 (4) 675-677 (2004)<br />
[3] W. Stillman, M.S. Shur, D. Veksler, S. Rumyantsev and F. Guarin, El. Lett., V43 (2007)<br />
[4] M. Dyakonov and M. Shur. El. Dev., IEEE Transactions on, 43(3):380–387 (1996)<br />
[5] E. Ojefors et al., IEEE Journal <strong>of</strong>, 44(7):1968–1976 (2009)<br />
145
TuP38 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
THz emission related to hot plasmons and plasma wave instability in<br />
field effect transistors<br />
Nina Dyakonova 1,2 , Abdelouahad El Fatimy 3 , Yahya Meziani 4 , Dominique<br />
Coquillat 1,2 , Wojciech Knap 1,2 , Frederic Teppe 1,2 , Petre Buzatu 1,2 , Luca Varani 5 , Hugues<br />
Marinchio 5 , Jeremy.Torres 5 , Pilippe Nouvel 5<br />
1 Université Montpellier 2, Laboratoire Charles Coulomb UMR 5221, Montpellier, France<br />
2 CNRS, Laboratoire Charles Coulomb UMR 5221, Montpellier, France<br />
3 Cardiff School <strong>of</strong> Physics and Astronomy, Cardiff University, Cardiff, United Kingdom<br />
4 Departamento de Física Aplicada, Universidad de Salamanca, Salamanca, Spain<br />
5 IES, UMR CNRS 5214, Université Montpellier 2, Montpellier, France<br />
The plasma waves in two-dimensional channel can be generated thermally by hot electrons<br />
(hot plasmons) [1] or can be due to plasma wave instability [2]. The THz emission due to<br />
radiative decay <strong>of</strong> non resonant hot plasmons is characterized by broad non tunable spectrum<br />
and a smooth onset. The mechanism plasma wave instability in a field effect transistor was<br />
proposed in Ref [3]. It has been shown that the electron flow in the channel should be<br />
unstable because <strong>of</strong> plasma wave amplification as a result <strong>of</strong> reflection from the device<br />
boundaries. One <strong>of</strong> characteristic features <strong>of</strong> the emission due to plasma wave instability is a<br />
threshold like onset. In gated part <strong>of</strong> the channel the fundamental frequency <strong>of</strong> plasma waves<br />
can be tuned by the gate bias. The theory developed recently [4] has shown that in realistic<br />
transistor geometry also oblique modes <strong>of</strong> plasma waves exist. Moreover, it has been shown a<br />
new type <strong>of</strong> instability related to plasma waves propagating along the gate boundary<br />
perpendicularly to the current direction. This instability should lead to a broad non tunable<br />
spectrum.<br />
In this work we present studies <strong>of</strong> InGaAs/InAlAs and GaN/AlGaN based field effect<br />
transistors .We show that origin <strong>of</strong> THz emission changes depending on the transistor<br />
configuration. The experimental results are supported by numerical simulations.<br />
We observed several types <strong>of</strong> THz emission. First, the emission onset as a function <strong>of</strong> drainsource<br />
voltage was <strong>of</strong> two kinds: (i) a smooth increasing <strong>of</strong> emission integrated signal and (ii)<br />
a threshold like appearance. For (i) case we observed non tunable broad emission spectra. The<br />
same results we observed for the most <strong>of</strong> the (ii) cases. However, for a transistor <strong>of</strong> special<br />
design we observed a voltage tunable THz emission. The gate <strong>of</strong> the transistor was covered by<br />
the field plate deposited between gate and drain terminals. We suppose that the plasma wave<br />
excitation under the gate is favored by the presence <strong>of</strong> the field plate through providing the<br />
needed boundary conditions<br />
We discuss our experimental results supposing that the emission origin depends on the<br />
potential distribution in the channel. In particular, simulation results show a step like behavior<br />
<strong>of</strong> the potential pr<strong>of</strong>ile near the border <strong>of</strong> the gate in the case <strong>of</strong> the standard transistor and a<br />
smooth potential pr<strong>of</strong>ile in the case <strong>of</strong> the transistors with a field plate.<br />
[1] Höpfel R. A and Gornik E. Surface Science, 142, 412-422, (1984)<br />
[2] El Fatimy A, Dyakonova N., Meziani Y., Otsuji T., Knap W., Vandenbrouk S.,. Madjour<br />
K, Théron D., Gaquiere C., Poisson M. A., Delage S., Prystawko, P. and Skierbiszewski C. , J.<br />
Appl. Phys., 107, 024504, (2010)<br />
[3] Dyakonov M. and Shur M. Phys. Rev. Lett., 71, 2465-2468. (1993)<br />
[4] Dyakonov M., Semiconductors, 42, 984, (2008)<br />
146
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors TuP39<br />
A Comb-Like Shape <strong>of</strong> the Cyclotron Resonance Line<br />
in GaAs/AlGaAs Heterostructure<br />
M. Bia̷lek 1 , M. Czapkiewicz 2 , K. Fronc 2 , J. Wróbel 2 , V. Umansky 3 ,<br />
M. Grynberg 1 , and J. ̷Lusakowski 1<br />
1 Faculty <strong>of</strong> Physics, University <strong>of</strong> Warsaw, ul. Ho˙za 69, 00-681 Warsaw, Poland<br />
2 Institute <strong>of</strong> Physics, PAS, al. Lotników 32/46, 02-668 Warsaw, Poland<br />
3 Weizmann Institute <strong>of</strong> Science, Rehovot 76100, Israel<br />
The present work originates from a larger program devoted to investigation <strong>of</strong> plasmon<br />
excitations in a two-dimensional electron gas (2DEG) perturbed by a periodic potential.<br />
ThesamplesusedinthepresentstudywereprocessedonahighmobilityGaAs/AlGaAs<br />
heterostructure with the electron concentration <strong>of</strong> about 3×10 11 cm −2 . Each sample was<br />
supplied with two ohmic contacts and on some smples a metallic gate was deposited. The<br />
samples were placed in the magnetic field (up to 12 T) at 4.2 K. The source <strong>of</strong> radiation<br />
was a molecular laser with the lines between 70 µm and 202 µm. The signal measured<br />
was a photovoltage induced between the ohmic contacts.<br />
An overall shape <strong>of</strong> the signal as a function <strong>of</strong> the<br />
magnetic field is very complicated. We observe indications<br />
<strong>of</strong> an interference <strong>of</strong> edge magnetoplasmons,<br />
Shubnikov-de Haas oscillations and a cyclotron resonance<br />
(CR) transition. In the present communication<br />
we concentrate on the CR only. A typical shape <strong>of</strong><br />
the signal in the vicinity <strong>of</strong> a CR is shown in Fig. 1<br />
and has a comb-like shape. To understand the origin<br />
<strong>of</strong> this multi-line structure, we have processed samples<br />
with different gate geometries: a flat gate without any<br />
periodicity, a gate <strong>of</strong> a thickness periodically changed,<br />
acomb-likemulti-fingergate, andameander-likegate.<br />
We also changed the period <strong>of</strong> the gate modulation.<br />
We found that 1) the comb-like shape <strong>of</strong> the CR line<br />
is present in all samples, with or without the gate; 2)<br />
photovoltage [V]<br />
3e-04<br />
3e-04<br />
2e-04<br />
2e-04<br />
1e-04<br />
5e-05<br />
0e+00<br />
-5e-05<br />
1 1.5 2 2.5 3 3.5 4 4.5 5 5.5<br />
B [T]<br />
Fig. 1: Photovoltaic signal measured<br />
on a sample with a meanderlike<br />
gate illumianted with a 165 µm<br />
radiation.<br />
the distance between the components <strong>of</strong> the CR line shown in Fig. 1 does not depend on<br />
gate geometry and does not depend on the magnetic field. We concluded that the observed<br />
splitting is related to the ohmic contacts or sample edges. To verify this hypothesis<br />
we carried out a transmission experiment which showed a Lorentzian-like CR line thus<br />
confirming that “bulk” electrons are not resposible for the observerd comb-like structure.<br />
We discuss two mechanisms <strong>of</strong> the comb-like shape <strong>of</strong> the CR line; both <strong>of</strong> them are<br />
based on a theoretical work [1] which indicates that the border between a 2DEG and a<br />
metal (an ohmic contact in our case) is a region <strong>of</strong> an extremly strong and nonuniform<br />
electric field generated by the incident electromagnetic wave. In one mechanism, we<br />
expect splitting <strong>of</strong> the CR line due to the gradient <strong>of</strong> the electric field, as it was observed<br />
in investigation <strong>of</strong> gases [2]. In the other, we consider an interaction <strong>of</strong> the CR transition<br />
with the acoustic waves generated within the contact area by a piezoelectric effect [3].<br />
This work was partially supported by a Foundation for Polish Science grant POMOST<br />
within EU Innovative Economy National Cohesion Strategy Program.<br />
[1] S. A. Mikhailov, Phys. Rev. B 83, 155303 (<strong>2011</strong>).<br />
[2] A. V. Rokhlenko, Zh. Eksp. Teor. Fiz. 69, 169 (1975).<br />
[3] N. A. Zimbovskaya, J. L. Birman, J. Phys.:Condens. Matter 12, 3337-3348 (2000).<br />
147
TuP40 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Impurity-Related Noise in Si �-doped Single-Barrier GaAs/AlAs/GaAs<br />
Resonant Tunneling Devices<br />
Jacek Przybytek, Marta Gryglas-Borysiewicz ��������������<br />
University <strong>of</strong> Warsaw, Faculty <strong>of</strong> Physics, �����������-681 Warsaw, Poland<br />
We have measured the current noise in single-barrier GaAs/AlAs/GaAs heterostructures<br />
with Si �-doping in the center <strong>of</strong> the 10nm-thick barrier for vertical mesas <strong>of</strong> diameter ranging<br />
from 50�m-300�m. Currents flowing through the structure ranged from a few pA to 0.1mA.<br />
The current noise measurements have been performed by means <strong>of</strong> crosscorrelation technique<br />
[1] with two home-made transimpedance amplifiers. In this technique two amplified signals<br />
from the sample are provided by means <strong>of</strong> two independent signal-carrying and amplifying<br />
channels to the analog-digital converter. The power spectral density <strong>of</strong> the sample-related<br />
correlated signal has been numerically calculated as a crosscorrelation spectra from two<br />
channels. The biasing <strong>of</strong> the sample has been achieved by applying bias voltage between<br />
virtual grounds <strong>of</strong> operational amplifiers used.<br />
Changing the bias voltage applied to the tunneling structure we change also the<br />
mechanism <strong>of</strong> the electron transport through the barrier [2]. For biases U0.3V the generation-recombination-like noise superimposes on the white shot noise.<br />
However, the Fano factor still remains below 1. In the paper we will show the impact <strong>of</strong> the<br />
resonant tunneling through the Si impurities on the low-frequency current noise. We will<br />
present the current noise spectral density measured at temperature T=4.2K for three samples<br />
with the same structure but different doping level in the Si-�-layer.<br />
The integrated low-frequency noise vs bias voltage for various samples with different �doping<br />
level reveals two types <strong>of</strong> noise generated in the heterostructure:<br />
1. for low voltages 0.3V
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors TuP41<br />
Current Driven Terahertz Detection <strong>of</strong> Quantum Cascade Laser<br />
Emission by Plasma Waves in Nano-Transistors<br />
F. Teppe 1 , C. Consejo 1 , J. Torres 2 , B. Chenaud 1 , P. Solignac 1 , Z.R. Wasilewski 3 ,<br />
M. Zholudev 1,4 , N. Dyakonova 1 , D. Coquillat 1 , A. El Fatimy 5 , W. Knap 1<br />
1 L2C, UMR N°5221 au CNRS, Université Montpellier II, 34095 Montpellier, France<br />
2 IES, UMR N°5214 au CNRS, Université Montpellier II, 34095 Montpellier, France<br />
3 Institute for Microstructural Sciences, National Research Council, Ottawa, Canada<br />
4 Institute for Physics <strong>of</strong> Microstructures, RAS, Nizhny Novgorod, Russia<br />
5<br />
1. Introduction: The plasma waves in gated 2D electron gas have a linear dispersion law. The<br />
transistor channel is acting as a resonator for the plasma waves, which can reach frequencies<br />
in the Terahertz (THz) range for a sufficiently short gate length Field Effect Transistors<br />
(FETs) [1,2]. It was shown that the nonlinear properties <strong>of</strong> plasma oscillations can be utilized<br />
for terahertz detectors, mixers and emitters. We have demonstrated in various materials that<br />
the detection <strong>of</strong> THz radiations by plasma waves might be strongly modified by the<br />
application <strong>of</strong> a dc drain current [3,4]. We found that driving a transistor into the saturation<br />
region enhances the non-resonant detection and can lead to the resonant detection [5,6,7] even<br />
if the condition ω 0 τ >> 1 is not satisfied ( ω 0 is the fundamental plasma wave frequency and<br />
τ is the momentum relaxation time), since the effective decay time for plasma oscillations is<br />
1/ τ = 1/ τ − 2 v / L , where v is the electron drift velocity.<br />
eff<br />
2. Experimental results: In this work, we have demonstrated the detection <strong>of</strong> 3.1 THz<br />
radiation <strong>of</strong> a Quantum Cascade Laser (QCL) with a Field Effect Transistor (FET). The<br />
measured photoresponse is plotted in Fig. 1 as a function <strong>of</strong> the gate voltage for three values<br />
<strong>of</strong> the applied drain voltage. The signal exhibits a wide peak centred at about -0.2 V and its<br />
amplitude is increasing with the applied drain voltage. The position <strong>of</strong> this peak corresponds<br />
to the fifth harmonics <strong>of</strong> the resonant plasma mode in the transistor’s channel. The detection<br />
signal is attributed to the plasma wave resonance in the transistor’s channel. The applied DC<br />
current increases the detection amplitude as well as the quality factor <strong>of</strong> the resonant line. The<br />
THz system formed by the QCL source and the FET plasma wave detector has been validated<br />
up to liquid nitrogen temperature.<br />
Photoresponse (arb. units)<br />
2,0<br />
1,5<br />
1,0<br />
0,5<br />
0,0<br />
V d = 0,8 V<br />
V d = 0,7 V<br />
V d = 0,6 V<br />
-0,5 -0,4 -0,3 -0,2 -0,1 0,0<br />
Gate Voltage (V)<br />
[1] M. Dyakonov and M. S. Shur, Phys. Rev. Lett. 71, 2465 (1993)<br />
[2] M. Dyakonov and M. S. Shur, IEEE Trans. Electron Devices 43, 380 (1996)<br />
[3] D. Veksler, F. Teppe, AP. Dmitriev, et al. Phys. Rev. B. 73 (12): 125328 (2006)<br />
[4] F. Teppe, D. Veksler, V. Yu. Kachorovski, et al. Appl. Phys. Lett. 87, 022102 (2005)<br />
[5] F. Teppe, W. Knap, D. Veksler, et al., Appl. Phys. Lett. 87, 052107 (2005)<br />
[6] F. Teppe, M. Orlov, A. E. Fatimy, et al., Applied Physics Letters 89, 222109 (2006)<br />
[7] S. Boubanga-Tombet, F. Teppe, D. Coquillatet al., Appl. Phys. Lett. 92, 212101 (2008)<br />
149<br />
!"
TuP42 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Influence <strong>of</strong> SiC substrate orientation on epitaxial graphene quality studied by Raman<br />
spectroscopy<br />
G. K pisty 1 , K.Grodecki 1,2 , W.Strupi ski 2 , A.Wysmołek 1 , R.St pniewski 1 ,<br />
and J.M.Baranowski 2,1<br />
1 Faculty <strong>of</strong> Physics, University <strong>of</strong> Warsaw, Ho a 69, 00-681 Warsaw, Poland<br />
gk276975@okwf.fuw.edu.pl<br />
2 Institute <strong>of</strong> Electronic Materials Technology, Wólczynska 133, 01-919 Warsaw, Poland<br />
High temperature annealing, which induces sublimation <strong>of</strong> Si atoms from SiC<br />
substrates at high temperatures is one <strong>of</strong> the commonly used methods <strong>of</strong> graphene layers<br />
fabrication. It was shown that sublimation process can be successfully performed in standard<br />
CVD reactors used for SiC epitaxial growth. The final result <strong>of</strong> the process depends on the<br />
annealing temperature and time as well as any other parameters like the pressure <strong>of</strong> the noble<br />
gas (usualy argon) in the CVD reactor. In contrast to the fact that the influence <strong>of</strong> the argon<br />
pressure in the reactor on the growth rate <strong>of</strong> the graphene structures is well established [1], the<br />
role <strong>of</strong> the substrate orientation on the effectiveness <strong>of</strong> the process <strong>of</strong> SiC sublimation is still<br />
not well known.<br />
In the case <strong>of</strong> epitaxy use <strong>of</strong> slightly misoriented substrates favors two dimensional<br />
growth modes, which is crucial in the case <strong>of</strong> flat and homogeneous epilayer growth. Thus the<br />
use <strong>of</strong> on-axis silicon carbide samples can’t be the optimal choice for this method. It could be<br />
argued that misorientation <strong>of</strong> the substrate could have positive influence on out-diffusion <strong>of</strong><br />
silicon atoms during SiC decomposition.<br />
In this communication the influence <strong>of</strong> SiC substrate orientation on epitaxial graphene<br />
is studied by Raman spectroscopy, which provides interesting information about strain state<br />
and homogeneity <strong>of</strong> the graphene structures. Here we focus our attention on the most<br />
prominent bands, so called the G and the 2D bands.<br />
The investigated samples were fabricated in the hot-wall CVD reactor Epigress VP508 at the<br />
temperature <strong>of</strong> 1600 0 C. Two different argon pressures <strong>of</strong> 50 mbar and 100 mbar in the reactor<br />
were maintained during the growth process on the 4H-SiC substrates with different<br />
misorientation <strong>of</strong> 0, 4, and 8 degrees.<br />
Micro-Raman scattering experiments were performed in back scattering geometry,<br />
using the 532nm line <strong>of</strong> a continuous wave Nd-YAG laser. The laser spot size on the sample<br />
was about 2µm in diameter. In order to collect information about strain and concentration<br />
homogeneity micro-Raman maps consisting <strong>of</strong> 530 points on a 2.3x2.2mm 2 sample area were<br />
performed.<br />
It was found that the average position <strong>of</strong> the 2D band, and thus biaxial strain present in<br />
graphene structures is not very sensitive to argon pressure on substrates with different<br />
misorientation. On the other hand, the 2D band average frequency increases with the<br />
misorientation angle. This is direct evidence that the average strain built in the graphene<br />
structures increases significantly with the misorientation angle. Similar behavior was found in<br />
the case <strong>of</strong> the 2D band broadening, for which the width is <strong>of</strong> about 30 cm -1 higher for 8 o<br />
misoriented substrate, as compared to the on-axis samples. The observed broadening depends<br />
also on the pressure <strong>of</strong> argon during the epitaxial process.<br />
The obtained results strongly suggest that the quality <strong>of</strong> graphene layers seems to<br />
decrease with misorientation <strong>of</strong> substrate. This provides additional information about<br />
mechanisms important in the growth <strong>of</strong> epitaxial layers by SiC sublimation.<br />
[1] A. Drabi ska, K. Grodecki, W. Strupi ski, R. Bo ek, K. P. Korona, A. Wysmołek,<br />
R. St pniewski, and J. M. Baranowski, Phys. Rev. B 81, 245410 (2010)<br />
150
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors TuP43<br />
Fermi edge and band-to-band recombination dynamics in n-doped<br />
GaAs/AlGaAs quantum well<br />
M. Syperek, B. Bujko, J.Jadczak, M. Kubisa, L. Bryja, and J. Misiewicz<br />
Institutute <strong>of</strong> Physics, Wroclaw University <strong>of</strong> Technology,<br />
Wybrzeże Wyspiańskiego 27, 50-370 Wrocław, Poland<br />
We performed comprehensive studies on emission properties and carrier dynamics in a<br />
high mobility (μ=1.8÷2.5x10 5 Vs/cm -2 ) n-doped (n=2.1÷3.4x10 11 /cm -2 ) 15 nm-wide<br />
GaAs/AlGaAs quantum well by means <strong>of</strong> time-integrated (PL) and time-resolved<br />
photoluminescence (TRPL) techniques. The low-temperature (T=4.5 K) PL spectrum reveals<br />
a broad emission band centered at the energy <strong>of</strong> 1.531 eV, as is shown in Fig.1. It corresponds<br />
to a photogenerated minority carriers recombination at zero momentum (k=0). Moreover, due<br />
to the higher density <strong>of</strong> a two-dimensional electron gas we are able to observe an additional<br />
photoluminescence feature at the high energy side <strong>of</strong> the PL peak emission. The so-called<br />
Fermi Edge Singularity (FES) is related to the phase-space filling <strong>of</strong> the first confined<br />
electronic subband [1]. Thus, weakly-k-conserving optical transitions between Fermi-sea<br />
electrons (at k=kf ) and localized holes in the valence subband can contribute to the PL<br />
spectrum as well. In the TRPL experiments in the low, non-resonant pumping regime, we<br />
observed the fast PL rise time <strong>of</strong> ~25 ps and the decay time <strong>of</strong> 130 ps. Both time constants are<br />
taken from the fit to the TRPL trace positioned at the maximum <strong>of</strong> the PL spectrum,<br />
corresponding to k=0 transitions. The rise time in this case is related to a hot-carrier cooling<br />
process whereas the decay time is tentatively attributed to a free-carrier recombination time.<br />
Surprisingly, the observed PL decay time taken at the energy corresponding to the FES is<br />
found to be comparable or even shorter than that related to the band-to-band recombination.<br />
We attribute this experimental finding to a small localization area <strong>of</strong> valence band holes<br />
which cause a considerable spread <strong>of</strong> the hole wavefunction in k–space, allowing for the<br />
recombination <strong>of</strong> kf>0 electrons from the edge <strong>of</strong> the Fermi sea. The photoluminescence<br />
dynamics can be also influenced by the non-radiative processes.<br />
Acknowledgement. This work has been done within the NLTK infrastructure (NLTK project<br />
POIG.02.02.00-00-003/08-00)<br />
[1] M. S. Skolnik et al, Phys. Rev. Lett. 58, 2130 (1987).<br />
151<br />
Fig.1 (left panel) TRPL<br />
intensity map at T=4.5 K. (right<br />
panel) The PL emission band<br />
with indicated band-to-band<br />
(BB) and Fermi Edge<br />
Singularity (FES) transitions<br />
with respective TRPL traces
TuP44 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
! " # $ % $ # ! $<br />
& ! ' ( ) " $ * ( & $ & ( + $ ,<br />
# $ ! " $ ,<br />
! " # $ % & ' ( ) **<br />
!" #<br />
! $ % &<br />
' ( ) *<br />
* '<br />
+ , -<br />
. # # " #<br />
/ 0 1 2<br />
# 3 µ ≈ 44 & 56 ≈ 78 4 4 & "<br />
µ ≈ 244 & 56 ≈ +8 4 & " )9<br />
# # &4: #<br />
; # * #<br />
#<br />
; # % - 0 % ! $ ' ! < = * 1><br />
? - / $ - &4 4"<br />
& . $ @ / - ! / 1 . A / ( $ B / $0/ / = ) ! /<br />
1 ! C D ' ( < E / F - / F * < / ? ./01<br />
23 2, 4 ,22562 &4 "<br />
2 * ! < 0 < F F 0 A $ / . $ /<br />
F * < / 1 = = - G2G &44H"<br />
152
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors TuP45<br />
Interaction <strong>of</strong> epitaxial graphene with SiC substrates studied by Raman<br />
spectroscopy<br />
K.Grodecki 1,2 , J.A.Blaszczyk 3 , A.Dominiak 3 , W.Strupinski 2 , A.Wysmolek 1 ,<br />
R.Stepniewski 1 , A.Drabinska 1 , M.Sochacki 4 and J.M.Baranowski 1,2<br />
1 Faculty <strong>of</strong> Physics, University <strong>of</strong> Warsaw, Poland<br />
2 Institute <strong>of</strong> Electronic Materials Technology, Warsaw, Poland<br />
3 Institute <strong>of</strong> Heat Engineering, Warsaw University <strong>of</strong> Technology, Poland<br />
4 Institute <strong>of</strong> Microelectronics and Optoelectronics, Warsaw University <strong>of</strong> Technology, Poland<br />
Epitaxial graphene on silicon carbide is emerging as an attractive process alternative to<br />
the graphite crystals exfoliation. Such way <strong>of</strong> growth is suitable for fabrication <strong>of</strong> large area<br />
graphene sheets, and is more compatible with current Si processing techniques. Commonly<br />
epigraphene is obtained by sublimating Si from SiC surface at high temperature (S-EG).<br />
Recently, it was shown that high-quality epitaxial graphene can be obtained by Chemical<br />
Vapor Deposition technique (CVD) [1].<br />
In this communication we use Raman spectroscopy to compare epitaxial graphene<br />
grown by sublimation and CVD techniques on 4H-SiC (0001).<br />
Raman spectra <strong>of</strong> graphene-like structures consists <strong>of</strong> several bands. Here we focus on<br />
the 2D band which is strain sensitive. It is induced by the interaction with the substrate.<br />
In order to acquire more detailed information on the interaction mechanisms between<br />
graphene layers and the substrate additional analysis were performed. We prepared the<br />
micro-Raman spatial maps at room-temperature and investigated the temperature dependence<br />
<strong>of</strong> the 2D band frequency in the temperatures range 300-600K.<br />
It was found that in the case <strong>of</strong> graphene samples obtained by the sublimation method<br />
(S-EG layer) the mean position <strong>of</strong> the 2D line is about 60 cm -1 higher that observed on<br />
unstrained exfoliated layers. [2] Observed shift difference <strong>of</strong> the 2D line indicates that the S-<br />
EG layer is under strong compressive strain. On the other hand, for CVD-EG structure mean<br />
position <strong>of</strong> the 2D line was found approximately 30 cm -1 higher than for freestanding<br />
graphene. This observation strongly suggests that the compressive strain in the CVD-EG<br />
structure is about two times smaller than in the S-EG one.<br />
The observed difference in strain between S-EG and CVD-EG graphene is compliant<br />
with the data obtained from the temperature dependence <strong>of</strong> the 2D band position.<br />
The thermal experiments <strong>of</strong> S-EG samples revealed thermal shift <strong>of</strong> the 2D band <strong>of</strong><br />
0.17 cm -1 /K. This result is in good agreement with the data reported by other groups [3]. On<br />
the other hand the thermal shift <strong>of</strong> 2D band for graphene samples grown by the CVD method<br />
remains between 0.095 and 0.034 cm -1 /K. The obtained lowest value corresponds to the<br />
exfoliated graphene transferred to the silicon wafers [3,4].Thus our results suggest that the Si<br />
sublimation growth mode always results in layers strongly pinned to the SiC(0001) substrate,<br />
on the other hand, the CVD growth produces layers which are only partially pinned to the<br />
surface.<br />
The interaction mechanisms responsible for different behavior <strong>of</strong> sublimated and CVD<br />
grown graphene layers are discussed.<br />
[1] W.Strupinski et al. Nano Leters (<strong>2011</strong>)<br />
[2] J. Röhrl,et al. Appl. Phys. Lett. 92, 201918 (2008)<br />
[3] N. Ferralis, et al. Phys. Rev. B 83 (<strong>2011</strong>)<br />
[4] I. Calizo, A. Balandin, W. Bao, F. Miao, and C. Lau, Nano Lett. 7, 2645 (2007)<br />
153
TuP46 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Antenna-Equipped Field Effect Transistors on CdTe/CdMgTe<br />
Quantum Wells as Terahertz Detectors<br />
K. Nogajewski 1 , H. Boukari 2 , P. Kopyt 3 , W. Gwarek 3 , T. Wojtowicz 4 ,<br />
H. Mariette 2 , M. Grynberg 1 , and J. ̷Lusakowski 1<br />
1 Faculty <strong>of</strong> Physics, University <strong>of</strong> Warsaw, ul. Ho˙za 69, 00-681 Warsaw, Poland<br />
2 Institut Néel-CNRS, 25 Av. des Martyrs, BP 166, 38042 Grenoble, France<br />
3 Institute <strong>of</strong> Radioelectronics, Warsaw University <strong>of</strong> Technology, ul. Nowowiejska 15/19,<br />
00-665, Warsaw, Poland<br />
4 Institute <strong>of</strong> Physics, PAS, Al. Lotników 32/46, 00-641 Warsaw, Poland<br />
Averyhighquality<strong>of</strong>CdTe/CdMgTequantumwellsgrownat theInstitute<strong>of</strong>Physics,<br />
Polish Academy <strong>of</strong> Sciences, Warsaw, allow to consider them as promising semiconductor<br />
structures for terahertz applications. One <strong>of</strong> the most important factors for such applications,<br />
the electron mobility, can reach the values <strong>of</strong> the order <strong>of</strong> 10 5 cm 2 /Vs at liquid<br />
helium temperatures and thus becomes comparable to that <strong>of</strong> high quality GaAs/AlGaAs<br />
heterostructures.<br />
The idea <strong>of</strong> the present project was to use such quantum wells to process field-effect<br />
transistors for detection <strong>of</strong> THz radiation. A new factor <strong>of</strong> the present work, absent in<br />
previous studies carried out by our group, is incorporation <strong>of</strong> precisely designed antennas<br />
into the transistor structure. The role <strong>of</strong> an antenna is tw<strong>of</strong>old. First, it increases a<br />
coupling <strong>of</strong> the radiation with plasma oscillations in the transistor channel thus increasing<br />
itssensitivity. Second,itpreciselydefinesadistribution<strong>of</strong>theelectricfield<strong>of</strong>theincoming<br />
wave around the channel which allows to control electromagnetic forces exciting a twodimensional<br />
plasma in the quantum well.<br />
Special antennas were designed by a numerical solution <strong>of</strong> the Maxwell equations<br />
with a finite difference time domain method and their dimensions were optimized for the<br />
frequency <strong>of</strong> 2.52 THz (corresponding to a photon wavelength <strong>of</strong> 118 µm - a strong line<br />
<strong>of</strong> our molecular laser). Three basic geometries were considered which differed by the<br />
topology <strong>of</strong> the antenna shape.<br />
The next step was a design and fabrication <strong>of</strong> masks for the lithographic process. In<br />
preparation <strong>of</strong> masks we used the three basic geometries and their modifications to verify<br />
sensitivity <strong>of</strong> the design to perturbation <strong>of</strong> its parameters (i.e., dimensions <strong>of</strong> different<br />
parts <strong>of</strong> antennas and their mutual orientation). The masks were subsequently used in a<br />
standard UV (365nm) and deep UV (240nm) lithography performed on a few wafers <strong>of</strong><br />
CdTe/CdMgTe quantum wells grown by the molecular beam epitaxy. Mesas were defined<br />
by a precisely calibrated dry plasma etching. Metallization <strong>of</strong> source, drain and gate<br />
electrodes was done with aluminum. In order to diminish the gate leakage currents a film<br />
<strong>of</strong> silicon dioxide was introduced bewteen the cap layer and the layer <strong>of</strong> metal forming a<br />
gate electrode. Ohmic contacts were prepared by etching tranches over a quantum well,<br />
filling it with aluminum and then exertion <strong>of</strong> a force with a hot tip.<br />
At the moment <strong>of</strong> the abstract submission, no THz test have yet been performed on<br />
fabricated transistors to compare their performance with numerical calculations. Such<br />
experiments are planned to be carried out in the nearest future. Both already existing<br />
numerical calculations and technological achievements will hopefully be supplied with<br />
experimental results during the Conference.<br />
This work was partially supported by a Polish Ministry <strong>of</strong> Science and Higher Education<br />
project N N515 071937.<br />
154
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors TuP47<br />
Effect <strong>of</strong> homogeneous electric field on exciton dispersion in wide quantum<br />
well<br />
A.D. Chegodaev 1 , D.K. Loginov 1<br />
1 Departament <strong>of</strong> Physics <strong>of</strong> Saint-Petersburg State University, Ulyanovskaya ul. 1,<br />
Petrodvorts, St.- Petersburg, Russia<br />
Exciton in an external homogeneous electric field was studied earlier either in bulk<br />
materials [1-3] or in narrow quantum wells, in which it is two-dimensional one [4-6]. These<br />
conditions did not allow to investigate the influence <strong>of</strong> a weak electric field on the motion <strong>of</strong><br />
the exciton as a whole, because in both cases it is impossible to observe states with a large<br />
momentum. Such states can be observed in optical spectra <strong>of</strong> nanostructures with wide<br />
quantum wells, which widths are at least ten times greater than the exciton Bohr radius.<br />
Correspondingly, the effect <strong>of</strong> weak electric fields, which magnitude F does not exceed the<br />
critical value for exciton ionization, can be studied.<br />
In our study, we theoretically investigate the influence <strong>of</strong> an external homogeneous electric<br />
field on the dispersion <strong>of</strong> energy <strong>of</strong> exciton motion. The electric field is oriented<br />
perpendicular to the wave vector <strong>of</strong> exciton, K<br />
field on the spectra <strong>of</strong> light reflection from a heterostructure GaAs/AlGaAs containing a wide<br />
quantum well.<br />
We have found that the transverse external electric field induces mixing <strong>of</strong> 1s-ground and 2pexcited<br />
states <strong>of</strong> the exciton, which can be described by the perturbation term in exciton<br />
Hamiltonian:<br />
)<br />
2 2<br />
2 h Kˆ<br />
V = ( e / c ) Fr^<br />
,<br />
m m<br />
where e<br />
m and h<br />
m are the effective masses <strong>of</strong> electron and hole; ^<br />
r r<br />
F ^ . We also model the influence <strong>of</strong> such a<br />
e<br />
h<br />
r is the distance between an<br />
K ^ r r<br />
F r r<br />
|| ); Kˆ electron and a hole in the direction <strong>of</strong> the electric field vector ( ^ , ^<br />
operator <strong>of</strong> the momentum <strong>of</strong> translation motion <strong>of</strong> the exciton. The mixing provides<br />
modification <strong>of</strong> the dependence <strong>of</strong> exciton energy on wave vector Kˆ for the 1s-ground and<br />
2p-excited states <strong>of</strong> exciton. In rough approximation, it can be described as a change <strong>of</strong><br />
exciton translational mass due to the electric field. Preliminary calculations have shown that<br />
the effect is sufficient enough to be detected in optical experiments.<br />
References:<br />
h is the<br />
[1] V. Keldysh, JETP 33, 994 (1957)<br />
[2] W. Franz, Einfluss eines electrischen Felden auf eine optische Abrorptionskante, Z.<br />
Naturforschung 13, 484 (1958)<br />
[3] C. B. Duke, M.E. Alferieff, Phys. Rev. 145, 583 (1966)<br />
[4] D.A. Miller, D.S. Chelma, T.C. Damen, A.C. Gossard, W. Weigmann, T.H. Wood, C.A.<br />
Burrus, Phys. Rev. B 32, 1043 (1985)<br />
[5] R.T. Collins, K. v. Klitzing, K. Ploog, Phys. Rev.B, 33, 4378 (1986)<br />
[6] R. T. Collins, L. Vina, W. I. Wang, L.L. Chang, L. Esaki, K. v. Klitzing, K. Ploog, Phys.<br />
Rev.B 36, 1531 (1987)<br />
155
TuP48 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Theoretical Analysis <strong>of</strong> Optical Losses in CdS/CdTe Solar Cells<br />
V.Ya. Roshko, L.A. Kosyachenko, E.V. Grushko<br />
Chernivtsi National University, Optoelectronics Department, 58012 Chernivtsi, Ukraine<br />
Since the early 2000's the mass production <strong>of</strong> solar modules based on thin-film<br />
CdS/CdTe has been implemented. In 2009-2010, annual capacity production <strong>of</strong> such devices <strong>of</strong><br />
only one company, First Solar, exceeded 1 GW and in <strong>2011</strong> it is planned to exceed 2 GW [1].<br />
It is predicted the rapid development <strong>of</strong> this solar energy sector, mainly because the cost <strong>of</strong><br />
CdTe modules is much lower compared to those on Si wafers. The challenge facing<br />
researchers and technologists is increasing the efficiency <strong>of</strong> CdS/CdTe modules. After all, the<br />
efficiency <strong>of</strong> these modules in the production level is only about 10-11 %, while the<br />
theoretical limit amounts to 28-30%. The main causes <strong>of</strong> reduced efficiency <strong>of</strong> CdS/CdTe<br />
solar cell are optical, electrical and recombination losses.<br />
In the present work, the study focuses on optical losses due to absorption and reflection<br />
at the interfaces. Calculations have been carried out being based on the optical constants <strong>of</strong><br />
materials used, the refraction index and extinction coefficient. Calculations <strong>of</strong> losses in<br />
glass/TCO/CdS/CdTe solar cells have been carried out taking into account reflections at the<br />
interfaces and absorption in the TCO (ITO and SnO2:F) and CdS layers. According to the<br />
Fresnel equations, when the light is at near-normal incidence, the reflection coefficient<br />
(reflectivity) from the interface between two contacting materials is determined by their<br />
refraction indexes n1 and n2. In the case <strong>of</strong> electrically conductive material, refraction index<br />
contains an imaginary part and can be written as n ∗ = n – iκ, where n is the refraction index,<br />
and κ is the extinction coefficient. The reflection coefficient from the interface is defined as<br />
the square <strong>of</strong> the modulus ( ∗ 1 n – ∗ 2 n )/( ∗ 1 n + ∗ n 2 ) [2]. We have calculated the short-circuit current<br />
density Jsc for AM1.5 total solar radiation using Tables ISO 9845-1:1992 [3].<br />
It has been shown that optical losses caused by CTO layer with thickness <strong>of</strong> 200 nm are<br />
about 20% and 10% for ITO and SnO2: F, respectively. Thickening <strong>of</strong> the ITO layer to 500-<br />
700 nm leads to unacceptably high optical losses (~ 50%), while using SnO2:F the losses are<br />
reduced to a few percentage points only. In both cases, the presence <strong>of</strong> CdS layer, even when<br />
its thickness is 50 nm leads to further increase in the optical losses by approximately 10% and<br />
its thickening to 100 nm increases the losses by another 7-8%. The theoretical analysis allows<br />
us to formulate some important practical conclusions. In the case <strong>of</strong> the SnO2:F transparent<br />
electrode, the presence <strong>of</strong> even very thin CdS layer limits the density <strong>of</strong> short circuit current<br />
Jsc at ∼ 23 mA/cm 2 , and the achievement <strong>of</strong> this value in ITO/CdS/CdTe solar cell is possible<br />
only with an ultra-thin ITO layer. A short-circuit current density above ∼ 25 mA/cm 2 is impossible<br />
if to use CdS as the window layer due too narrow width <strong>of</strong> the CdS band gap (2.42 eV).<br />
However, it becomes possible (Jsc ≈ 26 mA/cm 2 ) for use <strong>of</strong> modified CdS films with wider<br />
band gap (in CTO/ZTO/CdS/CdTe devices) when interdiffusion between the CdS and<br />
Zn2SnO4 films "consumes" CdS film during device fabrication or using a wider band gap<br />
Cd1-xZnxS ternary alloy to replace CdS as the window layer. The results <strong>of</strong> calculations show<br />
that integrating a ZTO buffer layer into a CdTe cell with a view to modifying the CTO layer<br />
(rather than CdS) results in a very insignificant increase in Jsc, since the optical losses in the<br />
CTO are mainly due to reflection rather than absorption.<br />
[1] http://www.firstsolar.com/en/modules.php.<br />
[2] T.S. Moss, G.J. Burrel, D. Ellis. Semiconductor Optoelectronics. Butterworth Publishers, 1973.<br />
[3] Reference solar spectral irradiance at the ground at different receiving conditions.<br />
Standard <strong>of</strong> International Organization for Standardization ISO 9845-1:1992<br />
156
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors TuP49<br />
Anisotropy <strong>of</strong> B=0 spin splitting <strong>of</strong> holes in symmetric GaAs/Ga(1-x)AlxAs<br />
quantum wells<br />
M. Kubisa, K. Ryczko and J. Misiewicz<br />
Institute <strong>of</strong> Physics, Wrocław University <strong>of</strong> Technology, Wybrze e Wyspia skiego 27,<br />
50-370 Wrocław, Poland<br />
The bulk inversion asymmetry (BIA) <strong>of</strong> GaAs crystal is known to remove the spin<br />
degeneracy <strong>of</strong> hole states even in the absence <strong>of</strong> external fields, resulting in a wave-vectordependent<br />
spin splitting. The BIA splitting strongly influences hole physics in low dimensions<br />
and was recently observed in metal-nonmetal transitions [1], magnetic focusing experiments<br />
[2] and the spin-Hall effect [3]. We study theoretically the BIA splitting <strong>of</strong> heavy hole<br />
subbands in symmetric GaAs quantum wells with various widths and orientations.<br />
Calculations are performed using the standard envelope function method. The hole states are<br />
described in the framework <strong>of</strong> Luttinger model with added BIA terms derived using the theory<br />
<strong>of</strong> invariants [4]. We show that the spin splitting, proportional to k close to the subband edge,<br />
becomes at higher values <strong>of</strong> k nonlinear and anisotropic. Along some directions <strong>of</strong> the<br />
Brillouin zone the splitting decreases with k and the spin-split subbands anticross. We also<br />
examine the magnitude <strong>of</strong> BIA splitting as a function <strong>of</strong> well size for different growth<br />
directions.<br />
[1] S. J. Papadakis, E. P. De Poortere, H. C. Manoharan, M. Shayegan, and R. Winkler,<br />
Science 283, 2056 (1999).<br />
[2] L. P. Rokhinson, V. Larkina, Y. B. Lyanda-Geller, L. N. Pfeiffer, and K. W. West, Phys.<br />
Rev. Lett. 93, 146601 (2004).<br />
[3] J. Wunderlich, B. Kaestner, J. Sinova, and T. Jungwirth, Phys. Rev. Lett. 94, 047204<br />
(2005).<br />
[4] R. Winkler, Spin-Orbit Coupling Effects in Two-Dimensional Electron and Hole Systems,<br />
Springer Tracts in Modern Physics Vol. 191 (Springer-Verlag, Berlin, 2003).<br />
157
TuP50 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Terahertz Properties <strong>of</strong> Gold Layers on GaAs<br />
J. Szczytko 1 , P. Arcade 2 , E. Papis 3 , A. Bara ska 3 ,<br />
B. Pi tka 1 , and J. Łusakowski 1<br />
1 Faculty <strong>of</strong> Physics, University <strong>of</strong> Warsaw, ul. Ho a 69, 00-681 Warsaw, Poland<br />
2 CNRS-Université Montpellier2, Pl. E. Bataillon, 34950 Montpellier, France<br />
3 Institute <strong>of</strong> Electron Technology, Al. Lotników 32/46, 02-668 Warsaw, Poland<br />
In recent years there is a much interest in using field-effect transistors (FETs) for<br />
detection and emission <strong>of</strong> THz radiation. One <strong>of</strong> the most important group <strong>of</strong> FETs used in<br />
such investigation are transistors processes on GaAs/AlGaAs heterostructures. There are two<br />
reasons <strong>of</strong> this choice. First, a relatively high electron mobility increases a detector response<br />
<strong>of</strong> a GaAs/AlGaAs FET at room temperature. Second, an extreme quality <strong>of</strong> such<br />
heterostructures makes it possible to observe an influence <strong>of</strong> basic quantum phenomena, like<br />
the cyclotron resonance transition or Shubnikov - deHaas oscillations on detection at low<br />
temperatures and high magnetic fields.<br />
To understand THz properties <strong>of</strong> a transistor one should know the properties <strong>of</strong> its<br />
constituent parts. In particular, a question arises about properties <strong>of</strong> thin Au layers deposited<br />
on a semiconductor surface. Such layers are <strong>of</strong>ten used as a gate electrode; this is, in fact, the<br />
case <strong>of</strong> most <strong>of</strong> FETs investigated by us. Recently, one <strong>of</strong> our main directions <strong>of</strong> investigation<br />
concerns FETs processed on a high electron mobility GaAs/AlGaAs heterostructures which in<br />
a natural way leads to the need to understand THz properties <strong>of</strong> Au layers on a GaAs surface.<br />
Investigation <strong>of</strong> THz properties <strong>of</strong> such layers deposited on di_erent semiconductors is<br />
not a new subject. However, one can expect that these properties are strongly dependent on a<br />
particular fabrication process, deposition technique and other technological parameters. For<br />
this reason, it is important to investigate gold layers which are prepared by the method used<br />
subsequently in fabrication <strong>of</strong> investigated FETs.<br />
A series <strong>of</strong> Au layers was prepared on a (100) surface <strong>of</strong> an epi-ready semi-insulating<br />
GaAs by dc magnetron sputtering using Leybold L400cp system. The deposition process was<br />
performed in Ar+ plasma at working pressure 3 10 -3 mbar, the power <strong>of</strong> 300 W and a gas flow<br />
<strong>of</strong> Ar + = 100 sccm. The thickness <strong>of</strong> the Au layer changed between 50 nm and 1.5 m. Since<br />
the layers were too thick to allow for a transmission measurements, only reflection <strong>of</strong> light in<br />
the range from 1 THz to 3 THz was investigated at room temperature by a Fourier<br />
spectroscopy. It was found that the reflection coeffcient depends on the THz light wavelength<br />
and also on the Au layer thickness. To understand the dependence on the layer thickness, we<br />
analyzed the layers surface with a scanning tunneling microscope and a pr<strong>of</strong>ilometer. We<br />
found that the surface morphology strongly depends on the layer thickness. Thus we explain<br />
the dependence <strong>of</strong> THz reflectivity <strong>of</strong> gold layers by their surface properties. This work opens<br />
the way to further investigation <strong>of</strong> THz properties <strong>of</strong> metallic gates, including studying an<br />
influence <strong>of</strong> the electrical polarization <strong>of</strong> the gate and properties <strong>of</strong> gates deposited on<br />
heterostructures with an embedded two-dimensional electron gas.<br />
This work was partially supported by a Foundation for Polish Science grant POMOST<br />
within the EU Innovative Economy National Cohesion Program and a Polish Ministry <strong>of</strong><br />
Science and Higher Education project N N515 608139.<br />
158
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors TuP51<br />
Towards Optically Tunable Terahertz Plasmonic Detectors<br />
P. Sznajder, B. Pi tka, J. Szczytko, and J. Łusakowski<br />
Faculty <strong>of</strong> Physics, University <strong>of</strong> Warsaw, ul. Ho»a 69, 00-681 Warsaw, Poland<br />
Nonlinear plasma oscillations in field-effect transistors (FETs) allow to use them as<br />
emitters and detectors <strong>of</strong> THz radiation. There are, however, problems which await for an<br />
optimal solution. First, the coupling <strong>of</strong> radiation with the plasma is weak because the<br />
wavelength <strong>of</strong> a THz photon (~100 m) is much larger than the channel dimensions (typically<br />
<strong>of</strong> a few m). Second, THz performance <strong>of</strong> FETs is only weakly tuned by the electrical<br />
polarization <strong>of</strong> the gate. Third, a response <strong>of</strong> FETs is spectrally very broad.<br />
One direction <strong>of</strong> studies aims at a fabrication <strong>of</strong> voltage-tunable and resonant devices<br />
based on plasmons generation in a periodically modulated 2D plasma. It is known that<br />
deposition <strong>of</strong> a periodically structured gate enables for an optical excitation <strong>of</strong> plasmons in the<br />
transistor channel [1]. In recent years several groups in the world come back to this idea<br />
trying to fabricate either THz detectors [2] or emitters [3] in a form <strong>of</strong> a FET with a<br />
periodically structured gate. This method suffers from the fact that the wavevectors <strong>of</strong> excited<br />
plasmons are defined by the periodicity <strong>of</strong> a lithographically deposited gate which essentially<br />
limits tunability <strong>of</strong> the device.<br />
In the present work we propose an alternative approach [4] which assumes a modulation<br />
<strong>of</strong> the plasma with an interference pattern generated by light. This method allows to change in<br />
a continuous way the period <strong>of</strong> the interference fringes by a simple mechanical adjustment <strong>of</strong><br />
the system and set preferable conditions for plasmons excitation. Final details <strong>of</strong> the<br />
experimental set up are currently under the construction, but in the meantime we proceed with<br />
analytical considerations <strong>of</strong> the system parameters.<br />
We have performed a theoretical analysis <strong>of</strong> the interference image versus the angles <strong>of</strong><br />
incidence <strong>of</strong> the two interfering beams and their polarizations. In the model, two plain waves<br />
are superposed on the sample surface, one beam passing a linear polarizer, the other one - a<br />
linear polarizer and a =2 plate. The resulting image is not trivial, as it cannot be considered<br />
as a simple superposition <strong>of</strong> two interference images <strong>of</strong> crossed linear polarizations. One has<br />
to note that the light polarization in subsequent fringes is changing from one to another (e.g.,<br />
between two different elliptical polarizations with the opposite chirality). The period <strong>of</strong> this<br />
oscillations is two times longer than that <strong>of</strong> the oscillations <strong>of</strong> the intensity. We show how the<br />
contrast <strong>of</strong> the interference pattern changes versus the angles <strong>of</strong> beams polarization and angels<br />
<strong>of</strong> incidence.<br />
Experimental verification <strong>of</strong> the theoretical analysis will be carried out on a set up with<br />
a Mach-Zender type interferometer and a CCD camera. A polarization system laced in front <strong>of</strong><br />
the CCD allows us to probe different polarization components <strong>of</strong> the pattern. A comparison <strong>of</strong><br />
the experimentally observed pattern with theoretical analysis ill allow us to optimize<br />
parameters <strong>of</strong> the system which is crucial for a proper design <strong>of</strong> low-temperature set-up.<br />
This work was partially supported by a Foundation for Polish Science grant POMOST<br />
within EU Innovative Economy National Cohesion Strategy Program.<br />
[1] S. J. Allen et al., Phys. Rev. Lett. 38, 980 (1977).<br />
[2] A. V. Muravjov et al., Appl. Phys. Lett. 96, 042105 (2010).<br />
[3] T. Otsuji et al., J. Phys.: Condens. Matter 20, 384206 (2008).<br />
[4] D. Weiss et al., Europhys. Lett. 8, 179 (1989).<br />
159
TuP52 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Mobility <strong>of</strong> Holes in Nanometer Ge-on-Si p-type<br />
Metal-Oxide-Semiconductor Field-Effect Transistors at Low<br />
Temperatures<br />
I. Grigelionis 1 , K. Fobelets 2 , B. Vincent 3 , J. Mitard 3 , B. De Jaeger 3 ,<br />
E. Simoen 3 , T. Y. H<strong>of</strong>fman3, D. Jaworski 1 , and J. Łusakowski 1<br />
1 Faculty <strong>of</strong> Physics, University <strong>of</strong> Warsaw, ul. Ho a 69, 00-681 Warsaw, Poland<br />
2 Imperial College London, Exhibition Road, SW7 2BT London, United Kingdom<br />
3 IMEC, Kapeldreef 75, 3001 Leuven, Belgium<br />
I In spite <strong>of</strong> the fact that the modern electronics is based on silicon metal-oxidesemiconductor<br />
field-effect transistors (MOSFETs) new materials are tested in a quest for even<br />
faster and more powerful solutions. One <strong>of</strong> approaches concerns incorporation <strong>of</strong> germanium<br />
into a Si-MOSFET structure. For instance, Ge has been mixed with Si in order to induce<br />
strain in a Si channel which in many cases increases the carrier mobility.<br />
A very recent idea is to use Ge as a MOSFET channel since this material benefits from<br />
both higher hole and electron mobility. In such technology, a thick layer <strong>of</strong> Ge is grown on a<br />
Si substrate and covered with a few monolayers (ML) <strong>of</strong> Si which enables preparation <strong>of</strong> an<br />
oxide under the transistor gate and then the gate stack. These novel emerging devices require<br />
a comprehensive characterization; a part <strong>of</strong> such studies has been provided by the present<br />
work.<br />
We investigated transistors processed on a wafer with 4 ML <strong>of</strong> Si grown on Ge. The<br />
channel length (L) varied between 0.125 m and 10 m (10 different values <strong>of</strong> L) and its<br />
width was either 1.8 m or 9.8 m. About 200 transistors were tested at room temperature<br />
with a needle system. The threshold voltage (VTH) determined from transfer characteristics<br />
was found to be approximately constant for L less than about 0.5 m and equal to about -70<br />
mV; VTH increased to about 260 mV for L = 5 m and 10 m.<br />
Three transistors with L = 0.16 m, 0.19 m and 1.03 m were selected for<br />
magnetoresistance (MR) measurements at high magnetic fields (up to 10 T) and low<br />
temperatures (4.2 K). The hole mobility determined by a direct current MR (dc MR) was<br />
found to be about 300 cm 2 /V s and showed an increase with increasing hole concentration in<br />
the channel. To take into account the access resistance, we carried out also MR measurements<br />
with a simultaneous modulation <strong>of</strong> the gate polarization (ac MR). We developed a new<br />
method to determine the carrier mobility from ac MR data which was based on a solution <strong>of</strong><br />
an appropriate differential equation and a one-parameter fitting. This method replaced a<br />
numerically unstable four parameter fitting procedure used before. The mobility determined<br />
from the ac MR data was about two times higher than that determined from dc R which<br />
allows also to estimate an influence <strong>of</strong> the access resistance on the mobility measurements.<br />
This work was partially supported by a Foundation for Polish Science grant POMOST<br />
within EU Innovative Economy National Cohesion Strategy Program.<br />
160
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors TuP53<br />
Vertical InAs nanowires as biological probes<br />
Pawel Utko 1 , Morten H. Madsen 1 , Trine Berthing 2 , Sara Bonde 2 , Claus B.<br />
Sørensen 1 , Karen L. Martinez 2 , and Jesper Nyg˚ard 1<br />
1 Niels Bohr Institute & Nano-Science Center, University <strong>of</strong> Copenhagen,<br />
DK-2100 Copenhagen, Denmark<br />
2 Department <strong>of</strong> Neuroscience and Pharmacology & Nano-Science Center, University <strong>of</strong><br />
Copenhagen, DK-2100 Copenhagen, Denmark<br />
High-aspect ratio nanostructures, like semiconductor nanowires (NWs) and carbon<br />
nan<strong>of</strong>ibers, can cross the membrane <strong>of</strong> a living cell without causing significant damage.<br />
Vertically-aligned arrays <strong>of</strong> such nanostructures might thus become a useful tool for intracellular<br />
delivery <strong>of</strong> biomolecules or label-free electrical measurements <strong>of</strong> intra- and<br />
extracellular events.<br />
Here, we focus on nanowires <strong>of</strong> InAs, a III-V compound semiconductor with favorable<br />
optical and electrical properties. We use patterned arrays <strong>of</strong> nanowires to fabricate vertical<br />
nanoelectrode devices. Initial results [1] indicate their low-invasiveness and biocompatibility<br />
when interfaced with mammalian cells. We perform electrochemical characterization<br />
<strong>of</strong> the InAs nanoelectrodes, followed by electrophysiological tests.<br />
[1] T. Berthing, S. Bonde, C. B. Sørensen, P. Utko, J. Nyg˚ard, K. L. Martinez, Small 7,<br />
640 (<strong>2011</strong>).<br />
161
TuP54 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Towardsbetterlight-confinementinpillarcavities<br />
M.Ściesiek 1 ,T.Jakubczyk 1,2 ,W.Pacuski 1 ,A.Golnik 1 ,P.Kossacki 1 ,<br />
C.Kruse 2 andD.Hommel 2<br />
1 Institute<strong>of</strong>ExperimentalPhysics,Faculty<strong>of</strong>Physics,University<strong>of</strong>Warsaw,Hoża69,<br />
00-681Warszawa,Poland<br />
2 Institute<strong>of</strong>SolidStatePhysics,University<strong>of</strong>Bremen,Postfach330440,D-28334<br />
Bremen,Germany<br />
Semiconductorquantumdots(QDs)embeddedinphotonicstructurescalledmicropillarshavebeenstudiedforserevalrecentyears.Thisisbecausetheyexhibitinterestingproperties,suchashighphotonextractionefficiencyorPurcelleffect[1].Micropillarconsist<strong>of</strong><br />
microcavityembeddedbetweenasystem<strong>of</strong>DistributedBraggReflectors(DBRs)onthe<br />
topandonthebottom[2,3].Thereisalsoplanarconfinementwhichistheeffect<strong>of</strong>etching<br />
thepillarsout<strong>of</strong>thesample.WhereashorizontallythemicrocavityissurroundedbyDBRs<br />
thereisstillsomeleaking<strong>of</strong>photonsverticallysincethereonlyexistsabarrierontheborder<strong>of</strong>highrefractiveindexmicropillars’materialandlowrefractiveindex<strong>of</strong>theairorva-<br />
cuum.<br />
DBRsfabricatedforourmicropillarshaveawidestopbandwithintherange<strong>of</strong>QDsphotoluminescence.Neverthelessinmicropillarsthere<br />
existsaverythinbandpass-thecavitymode.<br />
ByembeddingtheQDsintothecavityonecan<br />
expecttheenhancement<strong>of</strong>microluminescence<br />
forthisresonantenergy.Thankstothelowvolume<strong>of</strong>microcavityandhighqualityfactorQonecansignificantlyenhancelight-matercoupling.InthispaperwepresentstepstowardsobtainingmicropillarswithlatheralDBRs,which<br />
wouldleadtobetterconfinement<strong>of</strong>lightinside<br />
themicrocavity.First,10µmpillarswereetchedbyionetching.Thennextstepwasdone<br />
withFocusedIonBeam(FIB)anddiameterwas<br />
reducedto1–5µm.Attheendweobtained<br />
pillarswhichcanbeeasilyreachablefromthe<br />
side,whichallowplacingthelatheralDBRson<br />
them.Thespectra<strong>of</strong>microphotoluminescence<br />
Fig.1.Photoluminescencespectrafor<br />
micropillarswithvariousdiameters.<br />
confirmsthattwostepetching(ionetching+FIB)resultsinhigh-qualitymicropillars<br />
(Fig.1.).<br />
[1]A.Dousse,J.Suffczyński,A.Beveratos,O.Krebs,A.Lemaître,I.Sagnes,J.Bloch,<br />
P.Vosin,P.Senellart,Nature466,217(2010).<br />
[2]W.Pacuski,C.Kruse,S.Figge,andD.Hommel,Appl.Phys.Lett.94,191108(2009).<br />
[3]T.JakubczykT.KazimierczukA.GolnikP.Bienias,W.Pacuski,C.KruseD.Hommel<br />
Ł.KłopotowskiT.WojtowiczJ.A.Gaj,ActaPhysicaPolonicaA116,888(2009).<br />
162
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors TuP55<br />
Investigation <strong>of</strong> ion-implanted SiC properties by means <strong>of</strong> a light<br />
absorption technique<br />
M. Rawski, J. uk, A. Dro dziel, K. Pyszniak, M. Turek<br />
1 Institute <strong>of</strong> Physics, Maria Curie-Skłodowska University, Pl. M.Curie-Skłodowskiej 1,<br />
20-031 Lublin, Poland<br />
Silicon carbide is considered to be a very suitable material for manufacturing high<br />
temperature, high frequency as well as high power electronic devices [1]. The most interesting<br />
electronic and material properties appropriate for further applications have been found in the<br />
epitaxial layers <strong>of</strong> 4H-SiC and 6H-SiC.<br />
When optical and electronic properties are investigated the absorption technique becomes<br />
very useful. Light-wave dependent measurements <strong>of</strong> the absorption coefficient can provide<br />
information about the energy band gap and transitions between the valence band, conduction<br />
band and dopant energy levels. This, with the knowledge <strong>of</strong> the properties <strong>of</strong> unimplanted<br />
material gives deeper insight into the electronic energy structure <strong>of</strong> the examined implanted<br />
material. However, absorption measurements require <strong>of</strong> a specimen light transparency and, in<br />
the case <strong>of</strong> semiconductors, can only be applied to samples polished on both sides.<br />
In this work, the light absorption spectra <strong>of</strong> the implanted silicon carbide are presented. The<br />
information about the energy band gap and post-implantation disorder obtained by absorption<br />
edge function-fitting and deconvolution is discussed. Moreover, apart from the interband<br />
absorption, the transitions between the dopant states and conduction band continuum can be<br />
observed. The origin <strong>of</strong> these transitions is believed to be related to the Fano resonance [2].<br />
[1] K. Rottner et. al., Materials Science and Engineering B61–62, 330 (1999)<br />
[2] P.J. Wellmann, R. Weingartner, Materials Science and Engineering B102, 262 (2003)<br />
163
TuP56 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
SiC Schottky barrier diode studied by admittance spectroscopy<br />
P. Kamyczek 1 , E. Placzek-Popko 1 , Łukasz Gelczuk 2 , Maria D browska-Szata 2<br />
1 Institute <strong>of</strong> Physics, Wroclaw University <strong>of</strong> Technology, Wybrzeze Wyspianskiego 27,<br />
50-370 Wroclaw, Poland<br />
2 Faculty <strong>of</strong> Microsystem Electronics and Photonics, Wrocław University <strong>of</strong> Technology,<br />
Janiszewskiego 11/17, 50-372 Wrocław, Poland<br />
Silicon carbide (SiC) is a semiconductor material with indirect bandgap. The most<br />
popular polytypes <strong>of</strong> SiC are 4H-SiC and 6H-SiC. Both have a wurtzite type crystal structure.<br />
The energy gap for SiC (Eg=3.0 ÷ 3.2 eV) is close to the energy gap <strong>of</strong> GaN (Eg= 3.4 eV)<br />
and therefore it is widely used as an alternative to GaN. Silicon carbide has many<br />
applications in micro- and optoelectronics. It is used in light emitting diodes, JFET and BJT<br />
transistors. SiC has low resistance and can work at high temperature.<br />
In this paper we report on the electrical properties <strong>of</strong> levels related to defects present<br />
in commercially available SiC Schottky barrier rectifiers (Cree Inc.). The motivation <strong>of</strong> this<br />
work were the results reported at [1]. It was found that some <strong>of</strong> the diodes exhibited<br />
significant reverse leakage current and proposed that such behavior may be due to the<br />
presence <strong>of</strong> defects leading to defect-assisted tunneling through the Schottky barriers [2]. To<br />
characterize deep impurities the capacitance-voltage-frequency (C-V-f) and admittance<br />
spectroscopy (AS) method were used. AS measurements were performed at temperatures<br />
ranging from 300K down to 90K and for frequencies between 2kHz to 1MHz. C-V-f<br />
measurements yield apparent free carrier concentration in the order <strong>of</strong> 2.2 x10 15 cm -3<br />
irrespective to the applied frequency. A single relaxation maximum is visible on the<br />
conductance/frequency versus frequency plot within the 200K-260K temperature range. The<br />
maximum is not sensitive to external bias. Thus it may be assumed that it is related to some<br />
point defect. Activation energy determined from related Arrhenius plot is equal to<br />
0.338±0.004eV and capture cross section 6.22x10 -15 cm 2 . Obtained results confirm DLTS<br />
studies run on the same samples [2]. DLTS revealed a single dominant deep-level defect <strong>of</strong><br />
signature close to the one observed by us. So far there were no reports on a deep level <strong>of</strong><br />
similar signature in SiC. This level is presumably responsible for afore mentioned<br />
discrepancies observed in I-V characteristics in the studied diodes.<br />
[1] Z. Synowiec, Mater. Electron., 32 (2004), 5.<br />
[2] L.Gelczuk et al., Materials Science–Poland - in press<br />
164
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors TuP57<br />
! " # $ !<br />
!"-# " $<br />
% $ & ' ( - ) * + ) ,!"<br />
-- ." * + / +<br />
- ! " #<br />
$ % # " " & $ % '() "<br />
* + " ! " " $ %<br />
# $ % +<br />
" "<br />
+ # $<br />
% " !<br />
! $ +<br />
+ " "<br />
+ " , -$<br />
% +<br />
. / . +<br />
" $ %<br />
+ " - +<br />
0 ! $ %<br />
" +<br />
$<br />
). 1 + *<br />
+ - &$ &$23<br />
&$44 &$56 # " $ $<br />
1 " * / " 0<br />
$ % " " - –<br />
# / + "<br />
+ / +- $ %<br />
" 1<br />
$ "<br />
$ +<br />
( %) *$ % 1<br />
+ *<br />
+ - #<br />
$<br />
+ / *<br />
# /<br />
–<br />
+ $<br />
% % " * + + 7 $ 7 78 8<br />
5&4&43 '7 $<br />
, - $ 1 / 9$ / 9$ / :$ ; / 1 ( 1 & ''/ &84 < && $<br />
165<br />
-
TuP58 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Sub����10 nm Epitaxial Graphene Nanoribbon FETs<br />
K. Tahy 1 , W. S. Hwang 1 , J.L. Tedesco 2 , R.L. Myers����Ward 2 , P.M. Campbell 2 ,<br />
C.R. Eddy 2 Jr., D.K. Gaskill 2 , H. Xing 1 , A. Seabaugh 1 , and D. Jena 1<br />
1 Dept. <strong>of</strong> Electrical Engineering, University <strong>of</strong> Notre Dame, Notre Dame, IN 46556,<br />
USA<br />
2 U.S. Naval Research Laboratory, Washington, DC 20375, USA<br />
This document provides instructions for preparing an abstract for “Jaszowiec” and is<br />
written in the format according to the guidelines given below.<br />
Graphene is being investigated as a promising candidate for electronic devices. For digital a<br />
substantial bandgap is necessary. It is possible to open a bandgap in graphene by quantum<br />
confinement <strong>of</strong> the carriers in patterned graphene nanoribbons (GNRs); GNRs with width W<br />
nm have a bandgap Eg~1.3/W eV. This implies that sub-10 nm wide ribbons can enable roomtemperature<br />
operation <strong>of</strong> GNRs as traditional semiconductors, but with ultimate vertical<br />
scaling, and still take advantage <strong>of</strong> high current drives. To date, GNRs have been fabricated<br />
from exfoliated graphene and operated by back gates, or nanometer scale ribbons were<br />
produced by ‘explosive’ methods that are not controlled and reproducible. These methods are<br />
not suitable for large-area device fabrication. In this work, we report lithographically<br />
patterned GNRs on epitaxial graphene on SiC substrates. Specifically, we show the first topgated<br />
GNR transistors on epi-graphene substrates that exhibit the opening <strong>of</strong> a substantial<br />
energy bandgap (exceeding ~0.2 eV at a ribbon width <strong>of</strong> 10 nm), respectable carrier mobility<br />
(700-800 cm 2 /Vs), high current modulation (10:1 at 300 K), and high current carrying<br />
capacity (0.3 A/mm at VDS = 1 V) at the same time. Both single GNR and GNR array devices<br />
are reported.<br />
Fig: (Left): Array <strong>of</strong> devices on large-area epitaxial graphene. (Center): Measured device<br />
characteristics <strong>of</strong> ~10 nm wide single graphene nanoribbon (GNR) field- effect transistor.<br />
(Right): Scaling to large areas - a GNR FET made <strong>of</strong> a parallel array <strong>of</strong> 30X GNRs which are<br />
~13 nm wide each. High modulation shows that the opening <strong>of</strong> a bandgap is robust, and can<br />
be scaled to multiple graphene nanoribbons.<br />
166
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors WeI1<br />
Spintronics with antiferromagnetic materials<br />
Tomas Jungwirth 1,2<br />
1 Institute <strong>of</strong> Physics ASCR, v.v.i., Cukrovarnická 10, 162 53 Praha 6, Czech Republic<br />
2 School <strong>of</strong> Physics and Astronomy, University <strong>of</strong> Nottingham, Nottingham NG7 2RD, United<br />
Kingdom<br />
To date spintronics research and applications <strong>of</strong> magnetically ordered systems have<br />
focused on ferromagnets (FMs). There are, however, fundamental physical limitations for FM<br />
materials which may make them impractical to realize the full potential <strong>of</strong> spintronics. Metal<br />
FMs <strong>of</strong>fer high temperature operation but the large magnetic stray fields make them<br />
unfavorable for high-density integration and metals are unsuitable for transistor and<br />
information processing applications. FM semiconductors on the other hand do not allow for<br />
high-temperature operation. We present a concept in which these limitations are circumvented<br />
in spintronics based on antiferromagnets. The concept is based on relativistic magnetic and<br />
magneto-transport anisotropy effects in nanodevices whose common characteristics is that<br />
they are an even function <strong>of</strong> the microscopic magnetic moment vector, i.e., can be equally<br />
strong in AFMs as in FMs. As a demonstration we present our experimental observation <strong>of</strong><br />
>100% tunneling anisotropic magnetoresistance in a device with an IrMn AFM tunnel<br />
electrode [1]. As candidate high-temperature AFM semiconductors for this spintronics<br />
concept we introduce relativistic ab initio calculations and growth and characterization <strong>of</strong><br />
bulk and epitaxial I-Mn-V compounds [2-4].<br />
[1] B. G. Park, J.Wunderlich, X. Marti, V. Holy, Y. Kurosaki, M. Yamada, H. Yamamoto, A.<br />
Nishide, J. Hayakawa, H. Takahashi, A. B. Shick, T. Jungwirth, Nature Mat. 10, 347 (<strong>2011</strong>).<br />
[2] T. Jungwirth, V. Novák, X. Marti, M. Cukr, F. Máca, A. B. Shick, J. Mašek, P. Horodyská,<br />
P. N�mec, V. Hol�, et al., Phys. Rev. B 83, 035321 (<strong>2011</strong>).<br />
[3] R. J. Cava, Physics 4, 7 (<strong>2011</strong>).<br />
[4] F. Maca, J. Masek, O. Stelmakhovych, X. Marti, K. Uhlirova, P. Beran, H. Reichlova, P.<br />
Wadley, V. Novak, and T. Jungwirth (<strong>2011</strong>), arXiv:1102.5373.<br />
167
WeI2 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Transport in topological insulators<br />
Ewelina M. Hankiewicz<br />
Institute <strong>of</strong> Theoretical Physics and Astrophysics,<br />
Würzburg University, Germany<br />
An emergent topic in physics is the discovery <strong>of</strong> new phases <strong>of</strong> matter. The quantum<br />
Hall state has been the first state <strong>of</strong> matter that is not classified by symmetry breaking but<br />
forms a topologically distinct phase, which is unaffected by small perturbations. In the last<br />
few years, another class <strong>of</strong> materials where topology plays a crucial role has emerged in<br />
physics, the so- called topological insulators [1]. Topological insulators (TIs) have a bulk<br />
energy gap that separates the highest occupied band from the lowest unoccupied band like in<br />
ordinary insulators. However, the edge (for 2D TIs) or the surface (for 3D TIs) <strong>of</strong> a<br />
topological insulator exhibits gapless electronic states that are protected by time reversal<br />
symmetry.<br />
In this talk I will focus on transport properties <strong>of</strong> topological insulators in two different<br />
regimes: (1) when the Fermi energy probes the helical edge states (counter-propagating<br />
gapless spin edge states) or gapless surface states and (2) when the Fermi energy is deep in<br />
the conduction band where the material behaves as a metal with Dirac-like band dispersion<br />
[2]. In both <strong>of</strong> these regimes, the Dirac-like Hamiltonian (Dirac fermions) governs the physics<br />
<strong>of</strong> the system.<br />
In the first regime, I will discuss the magnetotransport properties <strong>of</strong> the helical edge<br />
channels and surface states. I will show that in two-dimensional topological insulators in the<br />
quasiballistic regime, the spin edge channels do not mix in a magnetic field transverse to the<br />
quantum well, but they persist in strong quantizing fields when the Fermi level lies in the gap<br />
[3]. With the shift <strong>of</strong> the Fermi level into the Landau-quantized conduction band, I will<br />
analyze a transition between topological insulator and quantum Hall regimes. In threedimensional<br />
topological insulators, the magnetic field dependence <strong>of</strong> galvanic responses <strong>of</strong><br />
the system shows anomalies due to broken time-reversal symmetry <strong>of</strong> the surface quantum<br />
Hall state. Here, I will talk about exciting novel features in magnetotransport as linear bulk dc<br />
magnetoresistivity and quadratic field dependence <strong>of</strong> the Hall angle [4].<br />
In the second regime <strong>of</strong> doped topological insulators, I will talk about original crossover<br />
between symplectic and unitary classes in weak localization corrections as a function <strong>of</strong> the<br />
carrier concentration for a zero magnetic field [5].<br />
The relevant experiments on 2D HgTe quantum wells and 3D strained HgTe will be<br />
discussed.<br />
[1] M. Z. Hasan and C. L. Kane Rev. Mod. Phys. 82 3045 (2010). X.-L. Qi, S.-C. Zhang<br />
arXiv:1008.2026 (2010).<br />
[2] B. Büttner, C. X. Liu, G. Tkachov, E. G. Novik, C. Brüne, H. Buhmann, E. M. Hankiewicz,<br />
P. Recher, B. Trauzettel, S. C. Zhang and L.W. Molenkamp Nature Physics 7, 418 (<strong>2011</strong>).<br />
[3] G. Tkachov and E. M. Hankiewicz Phys. Rev. Lett. 104, 166803 (2010).<br />
[4] G. Tkachov and E. M. Hankiewicz arXiv:1011.2756 (2010).<br />
[5] G. Tkachov, E. M. Hankiewicz arXiv:1102.4512 (<strong>2011</strong>).<br />
168
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors WeI3<br />
Progress in Nonpolar and Semipolar GaN Materials and Devices<br />
James S. Speck<br />
Materials Department, University <strong>of</strong> California, Santa Barbara, CA 93106<br />
speck@mrl.ucsb.edu<br />
Devices grown on c-plane GaN suffer from large internal electric fields due to discontinuities<br />
in spontaneous and piezoelectric polarization effects which cause charge separation between<br />
holes and electrons in quantum wells and limits the radiative recombination efficiency.<br />
Nonpolar GaN devices, such as in the m-plane (1100), are free from polarization related<br />
electric fields since the polar c-axis is parallel to any heterointerfaces. Semipolar GaN-based<br />
devices have reduced electric fields and in some cases, such as (1122), show a high propensity<br />
for Indium update for InGaN quantum wells.<br />
In this talk, we present work on outstanding materials issues including: morphological<br />
stability with special emphasis on the role <strong>of</strong> substrate orientation and growth conditions<br />
[1,2]; new results for dislocation-related strain relaxation in semipolar GaN-based<br />
heterostructures [3,4,5]; unambiguous determination <strong>of</strong> the polarization cross-over in<br />
semipolar InGaN/GaN heterostructures [6]; new detailed atom probe analysis <strong>of</strong> high<br />
performance GaN-based LEDs and laser diodes [7]. Additionally, we update progress on<br />
nonpolar electron devices and nonpolar and semipolar LEDs and LDs including the<br />
achievement <strong>of</strong> high performance true blue (� >450 nm) and true green (� >515 nm) lasers on<br />
m-plane and semipolar (namely, (2021)) GaN substrates [8,9]. Finally, we will present recent<br />
work on polarized light emission from m-plane LEDs [10] and demonstrate the first photonic<br />
crystal nonpolar LEDs.<br />
[1] R.M. Farrell et al, J. Cryst. Growth 313, 1 (2010).<br />
[2] R.M. Farrell et al, Appl. Phys. Lett. 96, 231113 (2010).<br />
[3] F. Wu et al. J. Appl. Phys. 109, 033505 (<strong>2011</strong>).<br />
[4] A.E. Romanov et al. J. Appl. Phys., accepted for publication (<strong>2011</strong>).<br />
[5] E.C. Young et al., Appl. Phys. Express, accepted for publication (<strong>2011</strong>).<br />
[6] H. Shen et al., phys. stat. solidi C7, 2378 (2010).<br />
[7] T. Prosa et al. Appl. Phys. Lett. accepted for publication (<strong>2011</strong>).<br />
[8] K.M. Kelchner et al, Appl. Phys. Express 3, 092103 (2010).<br />
[9] Y.D. Lin et al, Appl. Phys. Express 3, 082001 (2010).<br />
[10] S.E. Brinkley et al. Appl. Phys. Lett. 98, 011110 (<strong>2011</strong>).<br />
169
WeI4 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Recent Progress <strong>of</strong> DERI process for growth <strong>of</strong> InN and Related Alloys<br />
Y. Nanishi (1,2) * , T.Yamaguchi (3) , T.Araki (4) and E. Yoon (2)<br />
1 Global Innovation Research Organization, Ritsumeikan University, Kusatsu, 525-8577, Japan,<br />
2 Department <strong>of</strong> Materials Science and Engineering, Seoul National University, Seoul 151-744, Korea,<br />
3 Department <strong>of</strong> Information and Comunication Eng., KogakuinUniversity, Hachioji, 192-0015, Japan<br />
4 Department <strong>of</strong> Photonics, Ritsumeikan University, Kusatsu, 525-8577, Japan<br />
* Corresponding author email: nanishi@se.ritsumei.ac.jp<br />
Difficulties in growing high quality InN and In-rich group III-nitride alloys hinder device<br />
applications <strong>of</strong> these materials to IR light sources and detectors, very high efficiency solar cells<br />
and high frequency transistors. Main difficulty for growth comes from its low dissociation<br />
temperature and high equilibrium nitrogen vapor pressure during growth. In droplets which<br />
form on the growing surface give another essential problem for high quality InN growth.<br />
A new RF-MBE growth process called DERI (Droplet Elimination by Radical Beam<br />
Irradiation) was proposed very recently by our group. This growth method is composed <strong>of</strong> the<br />
two series <strong>of</strong> growth steps with In-rich growth step and consecutive nitrogen radical beam<br />
irradiation step. The method enables reproducible growth <strong>of</strong> high-quality InN film with flat<br />
surface. In-droplet formation and elimination processes can be monitored by both in situ<br />
RHEED intensity variation and in situ laser beam reflection. RHEED and laser beam reflection<br />
techniques provide important complementary information on the InN surface structure and<br />
Indium adsorption, droplet formation and desorption process, respectively.<br />
DERI method was also applied to the growth <strong>of</strong> InGaN alloys. Distinct segregation<br />
phenomena were observed in this case. It has been found that the effective segregation<br />
coefficient is dependent on the nitrogen radical beam intensity. Using this phenomenon,<br />
InN/InGaN MQW structure was successfully fabricated. Thick and uniform growth <strong>of</strong> InGaN<br />
alloys also became possible. Possible new approach to fabricate InN based nano-structure shall<br />
be proposed making use <strong>of</strong> this distinct segregation phenomenon. Series <strong>of</strong> Mg doping<br />
experiments on InN were also carried out. For DERI process, Mg supply during DEP(Droplet<br />
Elimination Process) is essential for successful p-type doping. Evidences for the existence <strong>of</strong><br />
free holes were successfully obtained using combinations <strong>of</strong> electrochemical capacitance<br />
voltage (ECV) pr<strong>of</strong>iling, thermo-power, XPS and electrical measurements.<br />
This work was supported by MEXT through Grant-in Aids for Scientific Research in Priority Areas<br />
“Optoelectronics Frontier by Nitride Semiconductor” #18069012 and Scientific Research (A) #21246004.<br />
This is also partly supported by WCU hybrid materials program <strong>of</strong> MSE at Seoul National University.<br />
170
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors WeO1<br />
Properties <strong>of</strong> TM pairs in the bulk and at the surface <strong>of</strong> GaN with and<br />
without Si or Mg codoping<br />
N. Gonzalez Szwacki 1 , J. A. Majewski 1 and T. Dietl 1,2<br />
1 Faculty <strong>of</strong> Physics, University <strong>of</strong> Warsaw, Warszawa, Poland<br />
2 Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences, Warszawa, Poland<br />
Novel nanocharacterization methods have allowed visualizing the nanoscale<br />
aggregation <strong>of</strong> transition metal (TM) cations in various semiconductors, either intrinsic or codoped<br />
by shallow impurities [1]. This opens up new possibilities for nanomagnetism and<br />
nanospintronics, provided the formation <strong>of</strong> buried ferro- and/or antiferromagnetic (FM and<br />
AFM, respectively) nanocrystals can be controlled at the nanometer scale, possibly by n-<br />
and/or p-type co-doping.<br />
We present results <strong>of</strong> first principles calculations (employing the GGA and GGA+U<br />
density functional theory schemes) for GaN doped with Cr, Mn, and Fe and simultaneously<br />
co-doped with Si or Mg. Our results confirm that TM cations have a strong tendency to<br />
aggregate, i.e., to form TM-rich nanocrystals [2]. For instance, the GGA pairing energies for<br />
Cr, Mn, and Fe in zb-GaN are 550, 614, and 291 meV, respectively (similar values have been<br />
obtained for wz-GaN). We find, however, that the gain in energy upon the formation <strong>of</strong> TM<br />
pairs at the (0001) wz-GaN gallium surface persists only for Fe [3]. This is shown in the Fig.1<br />
below, where we plot the pairing energy for XY (X,Y= Fe, Si or Mg) pairs in the zb-GaN bulk<br />
matrix as well as at the (0001) wz-GaN surface. We see that the formation <strong>of</strong> Fe-Si pairs is<br />
energetically favorable both in bulk and at the surface, whereas Fe-Mg pairs may form only in<br />
the bulk. Si-Si and Mg-Mg pairs could form neither in the bulk nor at the surface.<br />
Our studies <strong>of</strong> the nominally undoped systems have revealed that the magnetic<br />
interaction between magnetic ions pairs has different character in the bulk crystal than at the<br />
surface: for example, the Mn-Mn pairs are coupled ferromagnetically in the bulk but<br />
antiferromagnetically at the Ga surface. In the present paper, we also demonstrate how codoping<br />
with Si and Mg affects the formation <strong>of</strong> TM pairs and influences the magnetic<br />
coupling between TM ions. Finally, we present a detailed comparison <strong>of</strong> our theoretical<br />
results with available experimental findings.<br />
The work was supported by FunDMS Advanced Grant <strong>of</strong> ERC within the Ideas 7th FP<br />
<strong>of</strong> EC and InTechFun (Grant No. POIG.01.03.01-00-159/08).<br />
[1] A. Bonanni and T. Dietl, Chem. Soc. Rev. 39, 528 (2010).<br />
[2] K. Sato et al., Rev. Mod. Phys. 82, 1633 (2010).<br />
[3] N. Gonzalez Szwacki, J. A. Majewski, and T. Dietl, Phys. Rev. B (<strong>2011</strong>), in print,<br />
also arXiv:1011.5968v2.<br />
171
WeO2 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Experimental Probing <strong>of</strong> Exchange Interactions Between Localized Spins in<br />
a Dilute Magnetic Insulator (Ga,Mn)N�<br />
A. Bonanni 1 , W. Stefanowicz 2 , M. Sawicki 2 , T. Devillers 1 , B. Faina 1 , Tian Li 1 ,<br />
T. E. Winkler 1 , D. Sztenkiel 2 , A. Navarro-Quezada 1 , M. Rovezzi 1 , R. Jakieła 2 ,<br />
A. Meingast 4 , G. Kothleitner 4 , T. Dietl 2,3<br />
1 Institut für Halbleiter und Festkörperphysik, Johannes Kepler Universität, Linz, Austria<br />
2 Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences, Warszawa, Poland<br />
3 Faculty <strong>of</strong> Physics, University <strong>of</strong> Warsaw, Warszawa, Poland<br />
4 Institute <strong>of</strong> Electron Microscopy - FELMI, Graz University <strong>of</strong> Technology, Graz, Austria<br />
In view <strong>of</strong> the fact that most magnetic insulators are either antiferromagnets or<br />
ferrimagnets, particularly intriguing is the question whether ferromagnetism is at all possible<br />
in dilute magnetic insulators, where carriers remain strongly localized on parent impurities or<br />
defects. In order to shed some light on the character <strong>of</strong> the coupling in insulating III-V DMSs<br />
we study Ga1-xMnxN layers (x < 3%) grown by metalorganic vapor phase epitaxy. We have<br />
done a comprehensive nanoscale characterization, using high resolution transmission electron<br />
microscopy, synchrotron radiation diffraction and absorption methods as well as high<br />
precision SQUID magnetometry. Structural characterization gives no evidence <strong>of</strong> crystalline<br />
phases other than the wurtzite structure <strong>of</strong> the host GaN. The data show that the Mn ions are<br />
randomly distributed over the substitutional Ga sites.<br />
The magnetic properties as a function <strong>of</strong> temperature, magnetic field and its orientation<br />
with respect to the crystal c-axis also prove the single-phase <strong>of</strong> the studied layers for x < 1%.<br />
No ferromagnetic-like features are detected. The whole set <strong>of</strong> experimental data for x < 1% is<br />
quantitatively described by the group theory <strong>of</strong> a non-interacting paramagnetic Mn 3+ ions in a<br />
relevant crystal field [1,2].<br />
In the more concentrated case (1% < x < 3%) the sign, magnitude, and range <strong>of</strong> exchange<br />
couplings between pairs <strong>of</strong> Mn ions is estimated from Curie constant values analysis. Our<br />
findings confirm most <strong>of</strong> ab-initio predictions on the possibility <strong>of</strong> ferromagnetism in dilute<br />
magnetic insulators. We demonstrate that the coupling between neighboring Mn 3+ ions is<br />
ferromagnetic with Jnn > 10 meV and it changes to weak antiferromagnetic when the charge<br />
state <strong>of</strong> the Mn ions is reduced from 3+ to 2+. However the ferromagnetic coupling is found<br />
to be too short-ranged to lead to magnetic ordering above 1.85 K in the studied Mn<br />
concentration range up to 3% [3].<br />
The work was supported in part by the European Research Council through the FunDMS<br />
Advanced Grant within the "Ideas" 7th Framework Programme <strong>of</strong> the EC, EC Network<br />
SemiSpinNet (PITN-GA-2008-215368), and InTechFun (POIG.01.03.01-00-159/08) from EU.<br />
[1] J. Gosk et al., Phys. Rev. B 71, 094432 (2005).<br />
[2] W. Stefanowicz et al., Phys. Rev. B 81, 235210 (2010).<br />
[3] A. Bonanni et al., arXiv:1008.2083v1.<br />
172
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors WeO3<br />
Origin <strong>of</strong> uniaxial magnetic anisotropy in (Ga,Mn)As<br />
M. Birowska 1 , C. Śliwa 2 , K. Milowska 1 , J. A. Majewski 1 , and T. Dietl 1,2<br />
1 Faculty <strong>of</strong> Physics, University <strong>of</strong> Warsaw, ul. Hoża 69, 00-681 Warszawa, Poland<br />
2 Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences, al. Lotników 32/46, 02-668 Warszawa, Poland<br />
In this communication we present the results <strong>of</strong> theoretical studies that shed light on the long<br />
standing and intriguing problem <strong>of</strong> uniaxial magnetic anisotropy in (Ga,Mn)As [1,2]. In spite <strong>of</strong><br />
the fact that in zinc-blende structure crystallographic directions [1,1,0] and [1,-1,0] are<br />
equivalent, it turns out that the magnetization (M) in (Ga,Mn)As prefers to align along one <strong>of</strong><br />
these directions.<br />
We perform extensive studies <strong>of</strong> Mn ions pairing in GaAs based on the ab initio calculations<br />
in the framework <strong>of</strong> the density functional theory. We use relativistic pseudopotentials to account<br />
for the spin-orbit interaction and the local density approximation (LDA) for the exchangecorrelation<br />
density functional with the effects <strong>of</strong> non-collinear magnetism included. We use<br />
supercells with 64 atoms to incorporate a pair <strong>of</strong> Mn atoms in the cell (i.e., with 6.25% Mn<br />
concentration). Since the magnetic anisotropy is obtained from extremely small total energy<br />
differences for various orientations <strong>of</strong> M, we carry calculations using extraordinary high<br />
accuracy employing two different numerical packages (SIESTA and QUANTUM ESPRESSO).<br />
The results <strong>of</strong> calculations for a nearest-neighbor Mn-pair placed along [1-10] direction are<br />
shown in the figures below. It turns out that the system acquires maximal energy when M is<br />
oriented along [001] direction. The relative energy gain for M deviating from the [001] direction<br />
is depicted for M lying in the (001) and (1-10) planes in the first and second figure, respectively.<br />
It is clearly seen that the energetically most favorable direction <strong>of</strong> magnetization is along [1-10]<br />
one (angle 135 o ), i.e., when magnetization is parallel to the line with Mn pair.<br />
Further, we study the dependence the total energy on M orientation for other factors<br />
influencing the energetics, such as lattice relaxation <strong>of</strong> the Mn pairs, crystallographic orientation<br />
<strong>of</strong> Mn pairs, and stress. Further we analyze the energies <strong>of</strong> Mn pair formation and relate them to<br />
the magnetic anisotropy. We have just started calculations for Mn pairs at the GaAs surface to<br />
verify the role <strong>of</strong> the growth process. Finally, we attempt to analyze the anisotropy on the basis<br />
<strong>of</strong> the electronic structure changes induced by the substitution <strong>of</strong> the Mn pair into GaAs matrix.<br />
The work has been supported by FunDMS Advanced Grant <strong>of</strong> ERC within the Ideas 7th FP <strong>of</strong> EC,<br />
InTechFun (Grant No. POIG.01.03.01-00-159/08), and SemiSpinNet (Grant No. PITNGA-2008-215368).<br />
[1] D. Chiba, M. Sawicki, and Y.Nishitani, Nature Phys. 455, 07318 (2008).<br />
[2] M. Sawicki et al., Phys. Rev. B 71, 121302R (2005).<br />
173
WeO4 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
!<br />
" # $<br />
" % & '<br />
! ! " ! #$ % " &' & & ( !" ) * + , ,,+ - . / 0<br />
. ! 1 ! " / ! ! " % 2 3* %<br />
! " #$%<br />
! ! &<br />
' ( ! ' ) (') & % !<br />
"<br />
* & (<br />
* "<br />
+++,- & .<br />
!<br />
& !<br />
+ ) /% & 0<br />
1 &<br />
%<br />
! &<br />
! " #$% 2<br />
174<br />
(
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors WeO5<br />
Influence <strong>of</strong> composition and atomic arrangement on elastic properties <strong>of</strong><br />
wurtzite InGaN and InAlN alloys<br />
S. P. �epkowski and I. Gorczyca<br />
Institute <strong>of</strong> High Pressure Physics - Unipress, Polish Academy <strong>of</strong> Sciences, ul. Soko�owska 29,<br />
01-142 Warszawa, Poland<br />
We present ab-initio study <strong>of</strong> elastic properties <strong>of</strong> wurtzite InxGa1-xN and InxAl1-xN<br />
alloys. Our calculations reveal that a Vegard-like approximation, i.e., linear dependence on<br />
the alloy content, x, cannot describe in general, the composition dependences <strong>of</strong> elastic<br />
constants in these materials. Moreover, we demonstrate that in InxGa1-xN and InxAl1-xN, the<br />
elastic constants, and consequently, the Poisson ratio and the biaxial relaxation coefficient,<br />
depend significantly on arrangement <strong>of</strong> In atoms.<br />
The elastic properties <strong>of</strong> InxGa1-xN and InxAl1-xN alloys have been investigated using<br />
self-consistent ab-initio calculations within a supercell model. The indium concentrations, x =<br />
0.125, 0.25, 0.375, 0.5, 0.625, 0.75, and 0.875, have been realized by substituting 2, 4, 6, 8,<br />
10, 12, and 14 Al or Ga atoms by In in a 32-atom supercell [1]. Two different atomic<br />
arrangements have been considered for a given x, by either distributing the In atoms as<br />
uniformly as possible over the supercell or by clustering the In atoms together in a part <strong>of</strong> the<br />
supercell [1]. For each value <strong>of</strong> x, the structures <strong>of</strong> InxGa1-xN and InxAl1-xN have been<br />
optimized by minimization <strong>of</strong> the total energy with respect to volume and shape <strong>of</strong> the<br />
supercell. Then, the supercells have been subjected to test distortions grouped in five sets in<br />
order to determine the values <strong>of</strong> elastic constants for the wurtzite structure [2]. The total<br />
energy calculations have been performed using the VASP package which is a plane-wave<br />
pseudopotential implementation <strong>of</strong> the density-functional theory [3].<br />
Our study reveals that for alloys with a uniform distribution <strong>of</strong> In atoms, a Vegard-like<br />
approximation can describe the composition dependences <strong>of</strong> C12, C13, and C44 in InxGa1-xN,<br />
and C12, C13, and C33 in InxAl1-xN, whereas significant sublinear deviations are found for C11<br />
and C33 in InxGa1-xN and for C11 and C44 in InxAl1-xN. The effect <strong>of</strong> In clustering reduces C11,<br />
C12, and C44, and increases C33 in both InxGa1-xN and InxAl1-xN alloys. Consequently, C11, C12,<br />
and C44 depend sublinearly on x in clustered InxGa1-xN and InxAl1-xN alloys, whereas C33<br />
shows a linear and superlinear dependence on the composition in clustered InxGa1-xN and<br />
InxAl1-xN, respectively. The composition dependence <strong>of</strong> the bulk modulus is almost linear in<br />
InxGa1-xN alloys and shows a slightly sublinear character in InxAl1-xN alloys [4]. The influence<br />
<strong>of</strong> In clustering on the values <strong>of</strong> the bulk modulus is very small in both alloys [4]. On the<br />
other hand, the Poisson ratio and the biaxial relaxation coefficient depend significantly on<br />
arrangement <strong>of</strong> In atoms in these materials.<br />
This work was supported by the Polish State Committee for Scientific Research,<br />
Project No. NN202 010134.<br />
[1] I. Gorczyca, S. P. �epkowski, T. Suski, N. E. Christensen, and A. Svane, Phys. Rev. B 80,<br />
075202 (2009).<br />
[2] S. P. �epkowski and I. Gorczyca, (private communication).<br />
[3] G. Kresse and J. Furthmuller, Phys. Rev. B 54, 11169 (1996).<br />
[4] I. Gorczyca, A. Kami�ska, G. Staszczak, R. Czernecki, S. P. �epkowski, T. Suski, H. P.<br />
D. Schenk, M. Glauser, R. Butte, J. –F. Carlin, E. Feltin, N. Grandjean, N. E. Christensen, and<br />
A. Svane, Phys. Rev. B 81, 235206 (2010).<br />
175
WeO6 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
! "# $ %&' & ( $ )* %&+ & ! ( )$ %&+ & ! , - $ % & ,! ,. * + ! (! * $ %&+ &<br />
/! 0 $ 1 & ! %&+ & ! #* 2 & 3! 2 $ + & ! * %<br />
,! ($ * $ %&+<br />
! "# $ % & & '()*<br />
) $ + , - ,<br />
# & . / ! # &<br />
! "<br />
# $ %&&'%(& )<br />
' * + # , - *+#,- $<br />
+ $<br />
, . * / - ,.*/-<br />
') $<br />
*/- 0 ,.*/-$ *+#,-$<br />
) ' 1&&'(&&23<br />
*+#,-<br />
4 )<br />
,.*/-<br />
5 6 4 564 ! " 78& ' %&9<br />
*/-<br />
7%& ' 7:& ' ! " $ 9& ( '<br />
9& 8 '1 564 ! " 564$<br />
$ ; ) 564<br />
564 ) ' ) $ '<br />
0 ) 7:& ) ! " 564 4<br />
! "<br />
! "<br />
) 4 . $<br />
! " 5 )<br />
! " . ! " )<br />
7%&'%&& $<br />
) 7&% 4 ,.*/- )<br />
*+#,- ' )<br />
. ! & "" " 0 1 2<br />
# $ ! ) 3 2 " . 2 " + 4 / " 5<br />
/ / 2 6 - #7 7 7 8( 89 $- %2 " : 7<br />
< 3 = $ = , , $ , . 5 $<br />
5 > 1?@(8$ &9'971 4 $ , $ , A7B 11 B8:&(%1$ 0 A7B 11 B8:&(97$<br />
C<br />
176
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors WeP1<br />
Thermodynamic and thermoelectric properties <strong>of</strong> (Ga,Mn)As<br />
Cezary liwa 1 , Tomasz Dietl 1,2<br />
1 Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences, Warszawa, Poland<br />
2 Institute <strong>of</strong> Theoretical Physics, Faculty <strong>of</strong> Physics, University <strong>of</strong> Warsaw, Poland<br />
This work [1], by providing a theoretical analysis <strong>of</strong> the quantities which probe directly the<br />
thermodynamic density <strong>of</strong> states <strong>of</strong> carriers at the Fermi level, contributes to the ongoing<br />
discussion whether the holes in the canonical ferromagnetic semiconductor (Ga,Mn)As reside<br />
in the valence band or in an impurity band. In the latter case, one expects the thermodynamic<br />
DOS to be significantly enhanced.<br />
Available experimental results are analyzed assuming that holes occupy GaAs-like valence<br />
bands. Allowing for Gaussian fluctuations <strong>of</strong> magnetization, the p-d Zener model describes<br />
correctly a critical behaviour <strong>of</strong> magnetic specific heat found experimentally in (Ga,Mn)As<br />
near the Curie temperature TC [2]. The magnitudes <strong>of</strong> room temperature thermoelectric power,<br />
as measured for GaAs:Be and (Ga,Mn)As [3], are consistent with the employed model for the<br />
expected energy dependencies <strong>of</strong> the hole mobility. The same approach describes also<br />
temperature variations <strong>of</strong> conductance specific to the Anderson-Mott localization, found for<br />
various dimensionality (Ga,Mn)As nanostructures at subkelvin temperatures [4]. We conclude<br />
that the examined phenomena do not provide evidence for an enhancement <strong>of</strong> density <strong>of</strong> states<br />
by the presence <strong>of</strong> an impurity band at the Fermi energy in ferromagnetic (Ga,Mn)As.<br />
Furthermore, we present for (Ga,Mn)As expected values <strong>of</strong> both electronic specific heat at<br />
low temperatures T
WeP2 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Plasma-assisted Molecular Beam Epitaxy <strong>of</strong> GaN on Si (111) Substrates<br />
M. Sobanska, K. Klosek, Z.R. Zytkiewicz, J. Borysiuk, A. Wierzbicka,<br />
A. Reszka, and E. Lusakowska<br />
Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Science, Al. Lotnikow 32/46, 02 668 Warsaw, Poland<br />
High thermal conductivity, low price and availability <strong>of</strong> large diameter wafers make<br />
silicon promising substrate material for epitaxial structures <strong>of</strong> group III nitrides. However,<br />
epitaxy <strong>of</strong> GaN on Si is difficult due to a large lattice misfit. Moreover, large difference <strong>of</strong><br />
thermal expansion coefficients <strong>of</strong> GaN and Si (~116%) causes thermal strain that leads to<br />
cracking <strong>of</strong> GaN layers thicker than ~1 µm if strain compensating interlayers are not used.<br />
We report on growth <strong>of</strong> GaN layers on (111) silicon substrates by plasma assisted MBE<br />
(PAMBE). Riber Compact 21 system with elemental sources <strong>of</strong> Al, Ga, In, Si, and Mg was<br />
used for the growth. Active nitrogen was supplied from an Addon RF plasma source equipped<br />
with an optical Si sensor that measures the light intensity emitted by the nitrogen plasma in<br />
the wavelength range <strong>of</strong> 750 to 850 nm. We have shown that the sensor output signal is a<br />
direct measure <strong>of</strong> the amount <strong>of</strong> active nitrogen species produced by the RF source and<br />
available for growth. Thus, the use <strong>of</strong> the sensor assures precise control <strong>of</strong> the growth rate.<br />
In this work we show that before epitaxial growth <strong>of</strong> GaN an AlN buffer layer must be<br />
deposited on Si first. It helps to accommodate lattice mismatch with the substrate and protects<br />
the Si wafer against its contact with gallium that would lead to a catastrophic dissolution <strong>of</strong><br />
the substrate and to growth <strong>of</strong> polycrystalline material. Additional AlN interlayers are inserted<br />
into GaN to compensate large tensile thermal strain that appears in GaN upon post-growth<br />
cooling due to mismatch <strong>of</strong> thermal expansion coefficients <strong>of</strong> GaN and Si. They also<br />
efficiently block diffusion <strong>of</strong> Si and O impurities from the substrate into the layer.<br />
Optical microscopy, scanning electron microscopy, transmission electron microscopy<br />
photoluminescence, and high resolution X-ray diffraction were used to study how<br />
crystallographic perfection <strong>of</strong> the layers depends on growth parameters. In particular, we<br />
focused on preparation <strong>of</strong> the substrate (in-situ thermal oxide desorption vs. ex-situ chemical<br />
treatment), procedure <strong>of</strong> growth initiation (Al or N first), value <strong>of</strong> III/V ratio and an optimal<br />
design <strong>of</strong> the AlN/GaN stack.<br />
GaN<br />
AlN<br />
GaN<br />
AlN<br />
Si(111)<br />
3,2 3,3 3,4 3,5<br />
energy [eV]<br />
Cross-section <strong>of</strong> GaN on Si structure: SEM (a) and TEM (b) image; (c) PL <strong>of</strong> the sample.<br />
This work was partly supported by the European Union within European Regional<br />
Development Fund, through grant Innovative Economy (POIG. 01.03.01-00-159/08<br />
InTechFun).<br />
178<br />
intensity (Counts)<br />
3200<br />
3000<br />
2800<br />
2600<br />
2400<br />
2200<br />
2000<br />
1800<br />
1600<br />
1400<br />
1200<br />
1000<br />
800<br />
<strong>2011</strong>-02-23<br />
exc 300 nm<br />
T 15 K
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors WeP3<br />
Role <strong>of</strong> Interface Carrier Transport in Electrical Characteristics <strong>of</strong> Light<br />
Emitting Devices Based on Heterostructures<br />
Joanna Płaziak, Jakub Higersberger, and Jacek A. Majewski<br />
Institute <strong>of</strong> Theoretical Physics, Faculty <strong>of</strong> Physics, University <strong>of</strong> Warsaw, ul. Hoża 69,<br />
PL-00-681 Warszawa, Poland<br />
In this communication we present novel procedure to treat quantum effects in transport<br />
<strong>of</strong> carriers in optoelectronic devices based on heterostructures. Carrier transport in certain<br />
regions <strong>of</strong> Light Emitting Diodes (LEDs) and Laser Diodes (LDs) is treated quantum<br />
mechanically and these regions are smoothly embedded in the standard transport simulator for<br />
devices based on semi-classical drift-diffusion equations. Within the developed new scheme,<br />
we study the current-voltage characteristics <strong>of</strong> the typical arsenide and nitride structures.<br />
Correct modeling <strong>of</strong> carrier transport is a prerequisite <strong>of</strong> the understanding and ability<br />
to design new structures for optoelectronics. Traditional transport simulation tools use semiclassical<br />
drift-diffusion model to describe transport over the entire device, including the<br />
interfaces. This approach is based on the assumption that quasi-Fermi levels are continuous<br />
through the whole region <strong>of</strong> the device, also at the interfaces <strong>of</strong> heterojunctions. On the other<br />
hand, Monte Carlo transport simulations have demonstrated [1] that quasi-Fermi levels do not<br />
have to be continuous at interfaces and the transport<br />
over/through a barrier is not governed by physics<br />
included into drift-diffusion model. Therefore,<br />
quantum mechanical effects responsible for the<br />
physical mechanisms <strong>of</strong> transport in<br />
heterostructures have to be taken into account.<br />
However, the full quantum mechanical modeling <strong>of</strong><br />
transport in diffusive regime for whole device (i.e.,<br />
over the range <strong>of</strong> micrometers) is not possible yet.<br />
Therefore, in our studies we have followed previous<br />
suggestions to describe carrier transport quantum<br />
10<br />
0,0 0,2 0,4 0,6 0,8<br />
-5<br />
10 -4<br />
10 -3<br />
10 -2<br />
10 -1<br />
10 0<br />
10 1<br />
10 2<br />
10 3<br />
10 4<br />
10 5<br />
10 6<br />
10 7<br />
10 8<br />
10 9<br />
10 10<br />
mechanically only in the regions containing heterostructures and quantum wells [1]. We<br />
consider one <strong>of</strong> dominant transport mechanisms <strong>of</strong> quantum mechanical origin for<br />
heterostructures, namely, the tunneling <strong>of</strong> electrons through the narrow barriers <strong>of</strong> the<br />
quantum well systems. In other parts <strong>of</strong> the device, the semi-classical transport mechanism is<br />
considered, as implemented in the state-<strong>of</strong>-the-art nextnano 3 code [2].<br />
The role <strong>of</strong> tunneling effects is illustrated for the GaAs/AlGaAs/GaAs system with a<br />
single barrier <strong>of</strong> two highs (0.1 and 0.3 eV) in the figure. For both structures, we have<br />
calculated the current-voltage [i.e., I(V)] characteristics (i) employing the drift diffusion<br />
approach [2], and (ii) calculating I(V) for tunneling process quantum mechanically [3]. For<br />
small barrier high, the drift diffusion current dominates over the tunneling current, whereas<br />
for higher barrier the situation is just the opposite, the transport through the barrier is<br />
completely determined by the tunneling process and neglecting it leads to totally wrong I(V)<br />
characteristics. The tunneling currents turn out to be also extremely important in structures<br />
with multiple barriers.<br />
[1] D. Schroeder, Modeling <strong>of</strong> Interface Carrier Transport for Device Simulation,<br />
Springer-Verlag, Wien, 1994.<br />
[2] http://www.nextnano.de<br />
[3] Y. Ando, and T. Itoh, J. Appl. Phys. 61, 1497 (1986).<br />
179<br />
Current Density (A/cm 2 )<br />
10 11<br />
Voltage (V)<br />
Drift-Diffusion model<br />
Tunneling effects<br />
DV = 0.1eV<br />
DV = 0.3eV<br />
DV
WeP4 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Influence <strong>of</strong> crystal structure and hole concentration on<br />
magnetic anisotropy <strong>of</strong> GeMnTe<br />
Andrzej ̷Lusakowski 1 , Piotr Bogus̷lawski 1,2 , Wojciech Kn<strong>of</strong>f 1 and<br />
Tomasz Story 1<br />
1 Institute <strong>of</strong> Physics, PAS, Al. Lotników 32/46, 02-668 Warszawa, Poland<br />
2 Institute <strong>of</strong> Physics, University <strong>of</strong> Bydgoszcz, ul. Chodkiewicza 30, 85-072 Bydgoszcz,<br />
Poland<br />
GeTe is a narrow gap IV-VI semiconductor crystallizing at high temperatures in the<br />
cubic NaCl structure. At temperatures below T0 = 670 K, a structural transition takes<br />
place to the rhombohedral phase with the angle characterizing the unit cell α ≈ 88.3 ◦ .<br />
In addition, the Ge and Te sublattices are displaced relative to each other along [111]<br />
direction by a vector τa0(111), where the lattice constant a0=5.987 ˚A and τ ≈ 0.02.<br />
This displacement causes GeTe to be ferroelectric. The above features characterize also<br />
Ge1−xMnxTe mixed crystals, with the difference that the critical temperature T0(x) decreases<br />
with the increasing manganese content x. Manganese ions introduce in GeMnTe<br />
local magnetic moments and this IV-VI diluted magnetic semiconductor exhibits ferromagnetic<br />
transition with the Curie temperature TC(x) up to 200 K. The simultaneous<br />
presence <strong>of</strong> ferromagnetic and ferroelectric order (ferroicity) makes this compound very<br />
interesting for possible applications.<br />
Recently, magnetization and ferromagnetic resonance (FMR) measurements were performed<br />
on monocrystalline layers <strong>of</strong> Ge1−xMnxTe grown on BaF2 (111) substrate as well<br />
as on polycrystalline, layered Ge0.9Mn0.1Te microstructures [1, 2, 3]. The experiments<br />
probed the magnetic anisotropy energy in the ferromagnetic state. One <strong>of</strong> the striking<br />
results is that in monocrystalline layers the easy axis <strong>of</strong> magnetization is perpendicular to<br />
the layer while the usual in-plane easy axis due to dipolar interactions (shape anisotropy)<br />
is observed in polycrystalline microstructures and is partially recovered in GeMnTe layers<br />
upon annealing. The analysis <strong>of</strong> the angle dependence <strong>of</strong> the FMR resonance field yielded<br />
for Ge0.85Mn0.15Te layer perpendicular magnetic anisotropy field HA=0.2 T.<br />
In this work we present the results <strong>of</strong> first principles calculations <strong>of</strong> magnetic anisotropy in<br />
distorted GeMnTe crystal and compare them with the experimental data for Ge1−xMnxTe<br />
layers. The calculations were done for an eight atom supercell containing three Ge, one<br />
Mn and four Te atoms, it means we consider Ge0.75Mn0.25Te mixed crystal. The magnetic<br />
anisotropy energy was assessed by calculating the total crystal energies for different orientations<br />
<strong>of</strong> magnetization relative to the crystal axes. The calculations were done for three<br />
rhombohedral angles α equal to 88 ◦ , 90 ◦ (cubic lattice) and 92 ◦ , and for the ferroelectric<br />
sublattice displacement 0 ≤ τ ≤ 0.025. Our model simulates the experimental situation<br />
<strong>of</strong> GeMnTe/BaF2 layer thermally strained along [111] growth direction. We studied also<br />
the influence <strong>of</strong> hole concentration, p, on the magnetic anisotropy energy.<br />
The results indicate a very strong dependence <strong>of</strong> anisotropy energy on α, τ and p. The<br />
main conclusion is that for the parameters characterizing the experimentally investigated<br />
samples the theoretical easy magnetic axis is perpendicular to the layer’s plane, in agreement<br />
with our measurements. Analysis <strong>of</strong> the underlying mechanism will be provided.<br />
We also discuss the role <strong>of</strong> single ion anisotropy.<br />
[1] W. Kn<strong>of</strong>f et al. Mat. Sci. Poland 25, 295 (2007), phys. st. sol. (b) <strong>2011</strong> - in print<br />
[2] Y. Fukuma et al. Appl. Phys. Lett. 93, 252502 (2008)<br />
[3] H. Przybylińska et al. Spintech V, Cracow, Poland (2009)<br />
180
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors WeP5<br />
High-resolution study <strong>of</strong> valence band maximum for (GaMn)As<br />
I. Ulfat 1,2,3 , L. Ilver 1 and J. Kanski 1<br />
1 Department <strong>of</strong> Applied Physics, Chalmers University <strong>of</strong> Technology, SE-41296 Goteborg,<br />
Sweden<br />
2 MAX-lab, Lund University, SE-22100 Lund, Sweden<br />
1 Department <strong>of</strong> Physics, University <strong>of</strong> Karachi, Karachi-75270, Pakistan<br />
The anticipation <strong>of</strong> utilizing (GaMn)As for constructing electronic devices where magnetic<br />
elements (for logic and/or memory) are integrated in the semiconductor structure has<br />
motivated intensive investigation <strong>of</strong> this system over the last two decades. Whether practical<br />
realization <strong>of</strong> spintronic devices based on this system will happen is still an open question<br />
because the ferromagnetic state is found only at temperatures below 200 K [1]. One issue <strong>of</strong><br />
recent debate [2] concerns the electronic states involved in the ferromagnetic ordering,<br />
specifically whether these states are holes in the GaAs valence band, or belong to an impurity<br />
band.<br />
In this report we present recent data on the electronic structure near the valence band<br />
maximum, obtained at the undulator beamline (BL-I3) at the synchrotron radiation laboratory<br />
MAX-lab. (Ga,Mn)As samples with varying Mn concentrations were prepared in a new online<br />
MBE system, and were transferred in UHV to the photoelectron spectrometer. Our results<br />
show that already at 0.5% Mn concentration a characteristic narrow band is formed near the<br />
center <strong>of</strong> the Brillouin zone. This band becomes increasingly accentuated with increasing Mn<br />
concentration, as shown in Fig. 1.<br />
a)<br />
b)<br />
Fig 1: VBM for clean GaAs(100 (a) and (Ga,Mn)As with 0.5% (b) and 1% Mn (c).<br />
Our results will be discussed in relation to previous valence band studies and to the different<br />
theoretical models <strong>of</strong> the valence band structure <strong>of</strong> (Ga,Mn)As.<br />
References:<br />
[1] L. Chen et al., Appl. Phys. Lett. 95, 182505, (2009)<br />
[2] see e.g. S. Ohya et al., Nat. Phys. 7, 342–347 (<strong>2011</strong>)<br />
181<br />
c)
WeP6 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Low temperature grown (Zn,Co)O studied in the band-gap spectral region<br />
J. Suffczyński 1 , K. Gałkowski 1 , P. Kaźmierczak 1 , J. Papierska 1 , M. Furman 1 ,<br />
W. Pacuski 1 , A. Golnik 1 , A. M. Witowski 1 , J. A. Gaj 1 , W. Stefanowicz 2 , M. Sawicki 2 ,<br />
M. Łukasiewicz 2 , E. Guziewicz 2 , M. Godlewski 2<br />
1 Institute <strong>of</strong> Experimental Physics, Faculty <strong>of</strong> Physics,University <strong>of</strong> Warsaw, Warsaw, Poland<br />
2 Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences, Warsaw, Poland<br />
Increasing interest in the low temperature grown semimagnetic semiconductors results<br />
from the perspective <strong>of</strong> their possible applications in spintronics and electronics, e. g. in a<br />
construction <strong>of</strong> 3D non-volatile memories.[1]<br />
The work presents the results <strong>of</strong> magnetooptical investigation <strong>of</strong> nonuniform<br />
(Zn,Co)O layers grown at temperatures in the range from 160 to 300 ºC by Atomic Layer<br />
Deposition (ALD) on Si buffer. The concentration <strong>of</strong> Co, determined by SIMS and EDS<br />
techniques, ranged up to 20 %. Rapid Thermal Annealing was performed on the selected<br />
samples at temperatures from 300 to 800 ºC. As shown by AFM and SEM characterization<br />
the samples exhibit polycrystalline structure.[2] Depending on the sample, either<br />
paramagnetic or ferromagnetic response was observed in magnetometry measurements.<br />
Reflectivity and photoluminescence (excitation at 400 nm) measurements are carried<br />
out in UV and visible spectral range at magnetic fields up to 7 T (Faraday configuration) at<br />
temperature 1.5 K and 300 K. Far- and mid-infrared reflectivity is also measured at 300 K.<br />
No distinct transitions and no meaningful splitting in magnetic field are evidenced in<br />
the near band gap spectral region <strong>of</strong> the reflectivity spectra. This indicates that the structures<br />
are highly inhomogeneous and that exchange interaction with individual Co ions does not<br />
induce splitting <strong>of</strong> bands. This observation shows that Co ions are aggregated in Co-rich<br />
regions and that concentration <strong>of</strong> individual Co ions is negligible, what is consistent with the<br />
lack <strong>of</strong> emission related to individual Co ions.<br />
Reflectivity spectra in the band gap region are strongly affected by Fabry-Perot<br />
interferences resulting from multiple reflections <strong>of</strong> the light in the thin film. The interferences<br />
are gradually damped in a wide spectral region when Co concentration increases. This<br />
indicates that absorption related to Co-rich precipitations increases and/or sample crystalline<br />
quality decreases with increasing Co doping. On application <strong>of</strong> the magnetic field the<br />
reflectivity spectra undergo a net splitting (0.25 meV over the range <strong>of</strong> 7 T) without an<br />
effective change <strong>of</strong> the spectrum shape. This is attributed to the field dependent contribution<br />
to the absorption <strong>of</strong> Co precipitates modifying the refractive index <strong>of</strong> (Zn,Co)O.<br />
The results <strong>of</strong> infrared reflectivity are modeled taking into account dynamical dielectric<br />
function including phonons and plasma <strong>of</strong> carriers. Carrier concentration is found to be<br />
typically in the range from 1 to 40 *10 16 cm -3 . Plasmonic absorption egde not observed in the<br />
case <strong>of</strong> some samples is recovered after annealing.<br />
Annealing <strong>of</strong> the samples recovers Fabry-Perot interferences, indicating that sample<br />
absorption is decreased and/or the crystal quality <strong>of</strong> the layers is improved. This results from<br />
Co-rich regions spatial redistribution, i.e. their diffusion towards the buffer-film interface, as<br />
confirmed by SIMS measurements. No emission from isolated Co ions even after annealing is<br />
observed indicating a negligible diffusion <strong>of</strong> the individual ions out <strong>of</strong> the Co precipitations.<br />
[1] E. Guziewicz et al., Acta Phys. Pol. 116, 814 (2009).<br />
[2] M. Łukasiewicz et al., Phys. Status Solidi B 247, 1666 (2010).<br />
The work was supported by FunDMS Advanced Grant <strong>of</strong> ERC within the Ideas 7th FP <strong>of</strong> EC, NCBiR<br />
project LIDER, and InTechFun (Grant No. POIG.01.03.01-00-159/08).<br />
182
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors WeP7<br />
# $<br />
! " # $ % & ' $ % ( ) *+ , + %<br />
# -! + % )! . /<br />
!" #<br />
$ % ! &'(() * "<br />
+ , $ % ! ' -& * "<br />
! """<br />
"""<br />
%<br />
& & '() * +<br />
',-.*<br />
" !<br />
'/- *<br />
()<br />
+ ! &<br />
& '%012* !<br />
3 %012 "<br />
4<br />
! 3 & 5 6 !<br />
/- &<br />
%' 3 78<br />
4 8 73 8 97 :4 8 73 8 ;7 7 4 :4 8 73 8 97 <<br />
'=
WeP8 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Symmetry <strong>of</strong> the top valence band states in GaN/AlGaN<br />
quantum wells and its influence on the polarization <strong>of</strong> emitted<br />
light<br />
W. Bardyszewski 1 and S.P. ̷Lepkowski 2<br />
1 Institute <strong>of</strong> Theoretical Physics, University <strong>of</strong> Warsaw, ul. Ho˚a 69, 00-681 Warszawa,<br />
Poland<br />
2 Institute <strong>of</strong> High Pressure Physics, “Unipress”, Polish Academy <strong>of</strong> Sciences,<br />
ul. Soko̷lowska 29/37, 01-142 Warszawa, Poland<br />
We show theoretically that for narrow GaN/AlGaN quantum wells, lattice matched<br />
to GaN substrate/buffer and grown along the c-crystallographic direction the topmost<br />
valence subband symmetry depends critically on such parameters as quantum well thickness<br />
and barrier composition. This effect determines polarization <strong>of</strong> the emitted light. It<br />
is noted that the symmetry <strong>of</strong> the topmost valence band level is sensitive to the values<br />
<strong>of</strong> the D3 and D4 deformation potentials and can be employed in verification <strong>of</strong> existing<br />
literature values <strong>of</strong> these parameters.<br />
Group III-N semiconductors crystallizing in the wurtzite structure have complex valence<br />
band structure in the vicinity <strong>of</strong> the Γ point consisting <strong>of</strong> three subbands corresponding<br />
to the heavy hole (HH), light hole (LH) and the crystal-field split-<strong>of</strong>f subband<br />
(CH). In the unstrained bulk GaN and InN the top subband is <strong>of</strong> HH character whereas in<br />
the bulk AlN the sequence <strong>of</strong> subbands is inverted with topmost light hole subband. This<br />
inversion obviously is present in AlGaN alloys. Combining two materials such as AlGaN<br />
and GaN in one quantum well system gives an opportunity to modify the structure <strong>of</strong><br />
the top <strong>of</strong> the valence band states. Quantum structures built with these semiconductor<br />
materials, conventionally grown along the c-crystallographic direction <strong>of</strong> the wurtzite<br />
structure, are used to produce ultraviolet light emitters and high power electronic devices<br />
[1]. In most cases the character <strong>of</strong> the topmost valence subband in GaN/AlGaN quantum<br />
wells is usually <strong>of</strong> HH type. However in very narrow quantum well a reordering <strong>of</strong> the top<br />
valence subbands occurs resulting in the LH subband at the top <strong>of</strong> the valence band [2].<br />
The description <strong>of</strong> this phenomenon requires taking into account the large biaxial strains<br />
and huge built-in electric fields (caused by piezoelectric and spontaneous polarizations)<br />
which are present in these structures and lead to dramatic changes in the electronic states<br />
<strong>of</strong> the conduction and valence bands [1]. We provide the quantitative description <strong>of</strong> this<br />
problem in the framework <strong>of</strong> the k ·p method demonstrating how sensitive is the ordering<br />
<strong>of</strong> the topmost valence levels to the exact values <strong>of</strong> relevant parameters, in particular<br />
the deformation potentials D3 and D4 [3]. In order to illustrate the effect <strong>of</strong> reordering<br />
<strong>of</strong> the topmost valence subbands on polarization <strong>of</strong> emitted light, we present theoretical<br />
excitonic absorption and emission spectra for various quantum well structures.<br />
This work was supported by the Polish State Committee for Scientific Research,<br />
Project No. NN202 010134.<br />
[1] H. Morkoc, Nitride semiconductors and devices, (Springer-Verlag, Berlin Heidelberg<br />
New York, 1999).<br />
[2] P.A. Shields, R.J. Nicholas, N. Grandjean, and J. Massies, Phys. Rev. B 63, 245319<br />
(2001).<br />
[3] Q. Yan, P. Rinke, M. Scheffler, and C.G.V. de Walle, Appl. Phys. Lett. 95, 121111<br />
(2009).<br />
184
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors WeP9<br />
Tunnelling effects in (Ga,Mn)As based heterostructures<br />
D. Sztenkiel 1 and T. Dietl 1,2<br />
1 Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences, Warszawa, Poland<br />
2 Faculty <strong>of</strong> Physics,Warsaw University,Warszawa, Poland<br />
There is considerable debate about whether the holes reside in narrow impurity-band<br />
orinaweaklyperturbedvalencebandinthedensityregimerelevanttotheferromagnetism<br />
<strong>of</strong> (Ga,Mn)As. For example, in a recent articles Ohya et al. [1,2] presented comprehensive<br />
investigations <strong>of</strong> tunnelling spectroscopy for a series <strong>of</strong> layer structures involving high<br />
quality Au/(Ga,Mn)As/AlAs/GaAs:Be tunnel junctions [1]. The oscillations observed in<br />
the d 2 I/dV 2 -V characteristics for negative bias at the (Ga,Mn)As side was attributed to<br />
resonant tunnelling involving valence band subbands in (Ga,Mn)As quantum well. The<br />
authors concluded, that the Fermi level in this compound exists in the band gap, weakly<br />
perturbedvalencebandremainsseparated<strong>of</strong>theMnimpuritybandanditsexchangesplitting<br />
is very small even in samples with high Curie temperatures. These findings suggest<br />
that the Zener double-exchange mechanism is probably applicable for GaMnAs. However,<br />
we would like to propose an alternative interpretation [3] assuming that pd Zener model<br />
can properly describe properties <strong>of</strong> ferromagnetism in p-type Mn-doped semiconductors.<br />
In the (Ga,Mn)As/AlAs/GaAs:Be heterostructures, owning to a modulation doping effect,<br />
an interfacial region in GaAs:Be is depleted <strong>of</strong> holes. By applying a negative bias, a<br />
flat band condition is reached at V ≈ 0.1 V, followed by hole accumulation in GaAs:Be.<br />
Due to a relatively low acceptor density and disorder, weakly broadened two-dimensional<br />
(2D) hole subbands are then formed in non-magnetic GaAs:Be. Accordingly, nonmonotonic<br />
dependence <strong>of</strong> d 2 I/dV 2 on V can be attributed to resonant levels created at the<br />
AlAs/GaAs:Be interface. In fact, formation <strong>of</strong> 2D hole subbands in the accumulation<br />
layer was already observed in p-type GaAs based heterostructures [4]. At the same time,<br />
the observation <strong>of</strong> resonant levels in (Ga,Mn)As by tunneling spectroscopy is hampered<br />
by high hole relaxation rates in this alloy. Thus, the proposed model allows explaining<br />
why the exchange splitting and a significant contribution <strong>of</strong> the impurity band were not<br />
observed in the tunnelling spectra.<br />
The work was supported by FunDMS Advanced Grant <strong>of</strong> ERC within the Ideas 7th<br />
FP <strong>of</strong> EC and InTechFun (Grant No. POIG.01.03.01-00-159/08)<br />
[1] S.Ohya, K. Takata and M. Tanaka, Nature Phys. (<strong>2011</strong>).<br />
[2] S. Ohya, I. Muneta, P. Nam Hai and M. Tanaka, Phys. Rev. Lett. 104, 167204 (2010).<br />
[3] T. Dietl and D. Sztenkiel, arXiv:1102.3267.<br />
[4] T. Hickmott, Sol. St. Comm. 76, 315 (1990); R. Hayden, et al., Phys. Rev. Lett.<br />
66, 1749 (1991).<br />
185
WeP10 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Modelling <strong>of</strong> Ordering Phenomena in Nitride Semiconducting Alloys<br />
Michał Łopuszyński 1 and Jacek A. Majewski 2<br />
1 Interdisciplinary Centre for Mathematical and Computational Modelling, University <strong>of</strong><br />
Warsaw, Pawińskiego 5A, 02-106 Warsaw, Poland<br />
2 Institute <strong>of</strong> Theoretical Physics, Faculty <strong>of</strong> Physics, University <strong>of</strong> Warsaw,<br />
Hoża 69, 00-681 Warsaw, Poland<br />
AlN, GaN and InN together with their alloys form important group <strong>of</strong> semiconducting<br />
materials, particularly well suited to applications in optoelectronics. This is mainly due to<br />
their electronic properties – all <strong>of</strong> them have direct band gaps, being respectively<br />
DEAlN = 6.3 eV, DEGaN = 3.5 eV, DEInN = 0.8 eV. This large spread <strong>of</strong> band gaps, in principle,<br />
enables for fabricating devices operating in very broad spectral range from UV through the<br />
whole visible region to the IR part. This, however, requires fabricating mixtures <strong>of</strong> AlN, GaN<br />
and InN. Unfortunately, physics <strong>of</strong> these alloys is currently not sufficiently well understood.<br />
Especially, the issue <strong>of</strong> In clustering attracted a lot <strong>of</strong> attention, as it is believed to have<br />
important consequences for luminescence <strong>of</strong> the samples [1]. The literature reports different<br />
experimental findings concerning occurrence <strong>of</strong> these phenomenon, e.g. transmission electron<br />
microscopy revealed considerable inhomogeneity <strong>of</strong> In concentration [2], whereas 3D atomic<br />
probe measurements show uniform distribution <strong>of</strong> In atoms in light emitting quantum wells<br />
[3]. Therefore, the modelling <strong>of</strong> structural properties <strong>of</strong> nitride alloys could provide here a<br />
valuable insight, useful both from scientific and practical viewpoint.<br />
In this work we present Monte Carlo modelling scheme based on the Keating model [4]<br />
and its recent parameterization for nitrides [5]. We carry out our simulations for the zinc<br />
blende phase. Our approach enables for studying the composition and temperature influence<br />
on homogeneity and ordering phenomena in the nitride alloy samples. To obtain quantitative<br />
picture Warren-Cowley short range order parameters G (i) are used, and their dependence on<br />
composition and temperature is calculated. Except the bulk alloys, we study also strained<br />
epitaxial layers within elastic accommodation <strong>of</strong> strain regime. This allows investigation how<br />
the degree <strong>of</strong> ordering is influenced by the presence <strong>of</strong> substrate and the associated misfit<br />
strain. It turns out that the presence <strong>of</strong> large misfit strains can facilitate the development <strong>of</strong><br />
long range order in the produced nitride layers. For examined substrates, we also estimate<br />
critical thickness as a function <strong>of</strong> layer alloy composition using Matthews and Blakeslee<br />
model [6].<br />
[1] S. Chichibu, T. Azuhata, T. Sota, and S. Nakamua, Appl. Phys. Lett. 70, 2822 (1997).<br />
[2] M. D. McCluskey, L. T. Romano, B. S. Krusor, D. P. Bour, N. M. Johnson, and S.<br />
Brennan, Appl. Phys. Lett. 72, 1730 (1998).<br />
[3] Mark J. Galtrey, Rachel A. Oliver, Menno J. Kappers, Colin J. Humphreys, Debbie J.<br />
Stokes, Peter H. Clifton, and A. Cerezo, Appl. Phys. Lett. 90, 061903 (2007).<br />
[4] P. N. Keating, Phys. Rev. 145, 637 (1966).<br />
[5] M. Łopuszyński and J. A. Majewski, J. Phys.Condens. Matter 22, 205801 (2010).<br />
[6] J. Matthews and A. Blakeslee, J. Cryst. Growth 27, 118 (1974).<br />
186
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors WeP11<br />
Infrared Spectroscopy <strong>of</strong> GaN Crystals Obtained by Ammonothermal<br />
Method<br />
M. Welna 1 , R. Kudrawiec 1 , M. Motyka 1 , J. Misiewicz 1 , R. Kucharski 2 , M. Zając 2 ,<br />
R. Doradziński 2 , and R. Dwiliński 2<br />
1 Institute <strong>of</strong> Physics, Wroclaw University <strong>of</strong> Technology, Wybrzeże Wyspiańskiego 27,<br />
50-370 Wrocław, Poland<br />
2 AMMONO sp. z.o.o., Czerwonego Krzyża 2/31, 00-377 Warsaw, Poland<br />
robert.kudrawiec@pwr.wroc.pl<br />
The recent progress in the growth <strong>of</strong> truly bulk GaN crystals by ammonothermal method [1]<br />
allows a mass production <strong>of</strong> GaN substrates with various crystallographic orientations<br />
including non-polar [2] and semi-polar substrates [3] <strong>of</strong> various conductivity. It was observed<br />
that the color and transparency <strong>of</strong> GaN substrates obtained by this method change<br />
significantly with the intentional doping. For many application the transparency <strong>of</strong> GaN<br />
substrates is a very important issue. Such a situation takes place for intersubband devices<br />
operating in mid-infrared spectral range where GaN templates grown on sapphire substrates<br />
can be less transparent. In this work we measured reflectance and transmittance for nominally<br />
undoped and intentionally doped GaN substrates in visible, infrared and mid-infrared spectral<br />
regions. It was clearly observed that in the mid-infrared spectral range the transparency <strong>of</strong><br />
undoped and slightly doped GaN substrates is excellent for applications in devices operating<br />
on intersubband transitions. In addition, it was observed that the color <strong>of</strong> GaN substrates<br />
results from the defect related absorption. The intensity <strong>of</strong> a given absorption band changes<br />
with the concentration <strong>of</strong> intentional doping. High transparency and no colors <strong>of</strong> GaN<br />
substrates can be achieved when the intentional doping is suitably tuned. In this work we will<br />
discuss the origin <strong>of</strong> light absorption in the transparency region <strong>of</strong> GaN crystals. In addition,<br />
we will discuss the phonon-related absorption in the mid-far infrared spectral region. Some<br />
examples <strong>of</strong> transmission spectra measured for various GaN substrates in the mid-far infrared<br />
spectral region are presented below.<br />
Light transmission, I/I 0<br />
1<br />
0.1<br />
0.01<br />
1E-3<br />
Increase <strong>of</strong> free<br />
electron concentration<br />
Absorption on free electrons<br />
n-type GaN<br />
SI GaN<br />
1E-4<br />
2 4 6 8 10 12 14 16 18<br />
Wavelength (mm)<br />
Light transmission, I/I 0<br />
1.0<br />
0.8<br />
0.6<br />
0.4<br />
0.2<br />
w TO w TO<br />
[1] R. Dwiliński, et al., J. Cryst. Growth 312, 2499 (2010).<br />
[2] R. Kucharski, et al., Appl. Phys. Lett. 95, 131119 (2009).<br />
[3] R. Kucharski, et al., Appl. Phys. Express 3, 101001 (2010).<br />
2w TO 2w TO<br />
AMMONO GaN<br />
HVPE GaN<br />
GaN/Al 2 O 3<br />
0.0<br />
0.05 0.10 0.15 0.20 0.25 0.30<br />
187<br />
Energy (eV)
WeP12 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Micro-photoluminescence <strong>of</strong> GaInNAs layers: Thermal quenching <strong>of</strong><br />
individual exciton lines<br />
M. Latkowska 1 , R. Kudrawiec 1 , G. Sęk 1 , J. Misiewicz 1 , J. Ibáñez 2 , M. Henini 3 ,<br />
and M. Hopkinson 4<br />
1 Institute <strong>of</strong> Physics, Wrocław University <strong>of</strong> Technology, Wybrzeze Wyspianskiego 27,<br />
50-370 Wrocław, Poland<br />
2 Institut Jaume Almera, Consell Superior d’Investigacions Científiques,<br />
08028 Barcelona, Spain<br />
3 School <strong>of</strong> Physics and Astronomy, University <strong>of</strong> Nottingham,<br />
Nottingham NG7 2RD, United Kingdom<br />
4 Department <strong>of</strong> Electronic and Electrical Engineering, University <strong>of</strong> Sheffield,<br />
S3 3JD Sheffield, United Kingdom<br />
GaInNAs (with ~2% nitrogen and ~20% indium content) is widely investigating due to their<br />
promising applications in optoelectronic devices including solar cells and GaAs-based lasers<br />
operating in the fiber optic telecommunication windows. Unfortunately, the structural quality<br />
<strong>of</strong> GaInNAs alloys deteriorates significantly after the nitrogen incorporation. This leads to a<br />
strong carrier localization at low temperatures. It is especially demanding to explore the origin<br />
<strong>of</strong> this phenomenon and determine its characteristic properties like the activation energy for<br />
the localized carriers. Currently it is widely accepted that broad PL band, which is attributed<br />
to recombination <strong>of</strong> localized excitons, is composed <strong>of</strong> sharp lines related to individual<br />
excitons. An evidence for this interpretation has been found in near-field PL [1] as well as in<br />
micro-PL [2] measurements. Unfortunately, the exact nature <strong>of</strong> this lines is still unknown and<br />
controversial. In this work the authors applied the temperature-dependent micro-PL to study<br />
the thermal quenching <strong>of</strong> individual exciton lines originating from GaInNAs layers grown by<br />
molecular beam epitaxy. It was observed that the localization energy varies from 0 to<br />
150meV, whereas the activation energy for each individual line is the same within the<br />
experimental uncertainty and equals ~6meV [3], see micro-PL spectra for a GaInNAs layer<br />
together with the determined activation energies in Fig. 1. This observation means that the<br />
main source <strong>of</strong> sharp lines are excitons localized on deep donor- (acceptor-) like states. At<br />
low temperatures these states are able to attend in the radiative recombination because <strong>of</strong><br />
coulomb attraction between electrons and holes, whereas at higher temperatures they do not<br />
participate in the radiative recombination but they can still trap carriers which recombine<br />
nonradiative through these states. In this work we will discuss the origin <strong>of</strong> deep donor-<br />
(acceptor-) like states in GaInNAs alloys.<br />
PL intensity (arb. u.)<br />
E a (meV)<br />
60<br />
40<br />
20<br />
0<br />
10<br />
5<br />
P ex. = 2mW<br />
5K<br />
8K<br />
12K<br />
16K<br />
20K<br />
24K<br />
Localized states<br />
(exciton emission)<br />
0<br />
0.86 0.88 0.90 0.92 0.94 0.96 0.98<br />
Energy (eV)<br />
188<br />
Fig.1 Micro-PL spectra measured at various<br />
temperatures together with activation<br />
energies determined for individual PL lines.<br />
The authors acknowledge support from the<br />
MNiSW (grant nos. N202 258339 and<br />
DPN/N125/COST/2009) and the COST<br />
Action MP0805.<br />
[1] A.M. Mintairov, et al., Phys. Rev. Lett.<br />
87, 277401 (2001).<br />
[2] R. Kudrawiec, et al., Appl. Phys. Lett.<br />
94, 011907 (2009).<br />
[3] M. Latkowska, et al., Appl. Phys. Lett.<br />
98, 131903 (<strong>2011</strong>).
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors WeP13<br />
Nitride Based Sensors with Ga- and N-polarity<br />
Malte Fandrich, Gerd Kunert, Timo Aschenbrenner, Stephan Figge, Carsten<br />
Kruse, and Detlef Hommel<br />
Universität Bremen – Institut für Festkörperphysik, Otto-Hahn-Allee 1, 28359 Bremen,<br />
Germany<br />
Group-III nitrides have emerged as an advantageous material system for various chemical<br />
and biochemical sensing applications. Material specific properties like their chemical stability<br />
and large bandgap energies allow high temperature operation even in harsh environments.<br />
The large spontaneous and piezoelectric polarization along the polar c-direction in nitride<br />
based heterostructures originates a surface near two-dimensional electron gas (2DEG)<br />
with a high sheet carrier density. Hence, these structures are highly sensitive to interactions<br />
at the surface affecting the polarization-induced 2DEG. Using the 2DEG as a<br />
conductive channel between applied source- and drain-contacts, a manipulation <strong>of</strong> surface<br />
charges at the uncoated semiconductor surface causes a change in source-drain current,<br />
whereby for example polar liquids and gases can be detected.<br />
In order to investigate the influence <strong>of</strong> different polarity, GaN-buffer layers were grown on<br />
sapphire (Al2O3) using both metal-organic vapor-phase epitaxy (MOVPE) and molecularbeam<br />
epitaxy (MBE). Thus, Ga- (by MOVPE) and N-polar (by MBE) templates were<br />
obtained, which determine the polarity <strong>of</strong> subsequently grown layers and consequently the<br />
complete heterostructures [1]. To increase the sheet carrier density <strong>of</strong> the heterostructures<br />
AlGaN layers with different Al-content and Si doping concentration were grown on these<br />
templates. In addition AlInN/GaN-heterostructures were realized, since Al0.82In0.18N exhibits<br />
a higher spontaneous polarization than conventionally used Al0.2Ga0.8N [2] and is<br />
lattice matched to GaN. All epitaxial structures were characterized by high resolution<br />
X-ray diffraction (HRXRD), atomic force microscopy (AFM) and Hall measurements and<br />
optimized with respect to their structural and electrical properties.<br />
Based on the heterostructures open-gate sensors were processed. The sensing behavior <strong>of</strong><br />
these structures was investigated by exposure to polar fluids, where the molecular dipole<br />
moments in the liquid interact with the polar semiconductor surface [3]. The performances<br />
<strong>of</strong> the different devices will be compared and the influence <strong>of</strong> polarity will be discussed.<br />
All structures enable the detection and determination <strong>of</strong> polar liquids by means <strong>of</strong> a significant<br />
change in source-drain current. The electrical measurements demonstrated a good<br />
reproducibility after repeated coverage with the fluids.<br />
Additionally, the gas detection performance was analyzed by electrical measurements<br />
under different gas atmospheres, which prove the gas sensing capability <strong>of</strong> the devices.<br />
[1] M. Stutzmann, O. Ambacher, M. Eickh<strong>of</strong>f, U. Karrer, A. Lima Pimenta, R. Neuberger,<br />
J. Schalwig, R. Dimitrov, P. J. Schuck, and R. D. Grober, phys. stat. sol. (b) 228, 2<br />
(2001).<br />
[2] J. Kuzmik, IEEE Electron Device Lett. 22, 11 (2001).<br />
[3] M. Eickh<strong>of</strong>f, R. Neuberger, G. Steinh<strong>of</strong>f, O. Ambacher, G. Müller, and M. Stutzmann,<br />
phys. stat. sol. (b) 228, 2 (2001).<br />
189
WeP14 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Zinc vacancy induced ferromagnetic interaction in semimagnetic<br />
semiconductor (Zn,Mn)Te<br />
Le Van Khoi 1 , K. Swi tek 1 , M. Pawlowski 2 and R. R. Gał zka 1<br />
1 Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences. Al. Lotników 32/46, Warsaw, Poland.<br />
2 Institute <strong>of</strong> Electronic Materials Technology, ul. Wólczy ska 133, Warsaw, Poland.<br />
Ferromagnetic (FM) semiconductors are the subject <strong>of</strong> an intense research activity due<br />
to their potential applications in spintronics. The basic mechanism that governs<br />
ferromagnetism in the FM semiconductors is believed to be the carrier-mediated FM coupling<br />
among the Mn 2+ ions. The carriers are the holes reside in the valence or impurity bands. In<br />
this communication, we report on experimental evidence <strong>of</strong> the zinc vacancy mediated FM<br />
interaction in the (Zn,Mn)Te alloy. We have measured photoluminescence (PL) and electron<br />
paramagnetic resonance (EPR) on the undoped (Zn,Mn)Te crystals. We have observed a<br />
strong exchange interaction between the Mn 2+ ions and both the<br />
0<br />
−<br />
neutral donor ( D ) and single charged zinc vacancy ( V Zn ). That<br />
results in an exchange (Dexter) energy transfer channel from the<br />
−−<br />
recombinating donor-deep acceptor (<br />
V Zn ) pairs to the Mn 2+ ions<br />
at T 50 K. The EPR measurements on the annealed samples<br />
revealed positive Curie-Weiss temperature TCW indicating the<br />
presence <strong>of</strong> the FM coupling among the Mn 2+ ions.<br />
The undoped (Zn,Mn)Te crystals were grown by high<br />
pressure Bridgman method and annealed under a high pressure <strong>of</strong><br />
N2 gas. The PL measurements show that introduction <strong>of</strong> Mn into<br />
the ZnTe host results in a new 1.95 eV PL band originated from<br />
the intra-3d-shell transitions <strong>of</strong> the Mn 2+ ions. It was found that<br />
with the increasing Mn content the Mn emission intensity also<br />
0 − + −−<br />
increases, whereas the emission resulted from the D + VZn<br />
→ D + VZn<br />
+ hν<br />
deep transition is<br />
quenched, and the emission resulted from the<br />
0 0 + −<br />
D + VZn<br />
→ D + VZn<br />
+ hν<br />
shall transition<br />
remains unchanged. This indicates that Mn 2+ ions strongly interact with the paramagnetic D 0<br />
and V centers via the s,p-d exchange. As a result, an exchange energy transfer channel from<br />
−<br />
Zn<br />
the recombinating<br />
−−<br />
D −VZn 0<br />
pairs to the Mn 2+ ions is triggered. Further, the annealing <strong>of</strong> the<br />
(Zn,Mn)Te crystals increases the −<br />
VZn density and suppresses the compositional fluctuation in<br />
this material. The effect <strong>of</strong><br />
−<br />
VZn on the magnetic state <strong>of</strong> Mn 2+ in (Zn,Mn)Te was studied by<br />
performing the EPR measurements. We found that after the annealing, the shape <strong>of</strong> EPR<br />
spectrum exhibits a homogeneous broadening at T 10 K and its amplitude increases by a<br />
factor <strong>of</strong> ~4 (see fig. (a)). The T-dependence <strong>of</strong> the inverse EPR intensity (1/IEPR) is shown in<br />
−−<br />
figure (b). At T 50 K, major <strong>of</strong> the zinc vacancies are doubly charged ionized ( V Zn ), hence<br />
only the d-d superexchange interaction via the Te ions dominate and it is characterized by TCW<br />
~ −225 K. Bellow 50 K, the single ionized zinc vacancies V appear due to the localization<br />
<strong>of</strong> holes. The exchange interaction between the hole trapped at the<br />
−<br />
Zn<br />
3000 3200 3400 3600<br />
Magnetic field (Oe)<br />
−<br />
V Zn site and the Mn 2+ ions<br />
gives rise to the formation <strong>of</strong> the bound magnetic polarons (BMP). In the annealed sample<br />
there is a coalescence <strong>of</strong> BMP. As a result, the FM exchange contribution overcompensates<br />
the antiferromagnetic d-d superexchange. This makes the T-dependence <strong>of</strong> 1/IEPR to change its<br />
curvature at T 50 K and results in TCW ~ +2 K as shown in figure (b) and its inset.<br />
190<br />
10 8 / intensity (arb. u.)<br />
EPR signal (arb. u.)<br />
2.5<br />
2.0<br />
1.5<br />
1.0<br />
0.5<br />
(Zn,Mn)Te<br />
x=0.03<br />
x=0.001<br />
T=5.0 K<br />
2.5 K<br />
4.0 K<br />
5.6 K<br />
10 K<br />
As-grown Annealed<br />
(Zn,Mn)Te<br />
x=0.03<br />
T CW = -225 K<br />
10 9 / Intensity (arb. u.)<br />
(Zn,Mn)Te<br />
2.5<br />
x=0.03<br />
2.0<br />
As-grown<br />
1.5<br />
1.0<br />
0.5<br />
(a)<br />
(b)<br />
Annealed<br />
x=0.07<br />
Annealed<br />
0.0<br />
0 2 4 6<br />
T (K)<br />
8 10 12<br />
0.0<br />
0 50 100 150 200 250<br />
Temperature (K)
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors WeP15<br />
Magnetic Properties <strong>of</strong> (Ga,Mn)As near Metal-Isolator Transition<br />
S. Dobkowska 1 , W. Stefanowicz 1 , O. Proselkov 1 , R. Żuberek 1 , J. Sadowski 2 ,<br />
T. Dietl 1,3 , and M. Sawicki 1<br />
1 Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Science, Warsaw, Poland<br />
2 MAX-Lab, Lund University, Lund, Sweden<br />
3 Institute <strong>of</strong> Theoretical Physics, University <strong>of</strong> Warsaw, Warsaw, Poland<br />
Diluted magnetic semiconductors, especially (Ga,Mn)As, have been intensively<br />
studied for last decade since they bridge the physics <strong>of</strong> semiconductors and magnetism. As the<br />
ferromagnetic (FM) coupling among localized moments (solely Mn so far) is mediated by<br />
mobile carriers (holes) which concentration is limited either by the presence <strong>of</strong> Mn itself<br />
and/or by self-compensating defects in the host material, these materials are frequently found<br />
to be on the verge <strong>of</strong> localization <strong>of</strong> the electronic states. This can be regarded as detrimental<br />
for wide ranging spintronics applications, but on the other hand such materials serve as a<br />
powerful tool to study an interplay between magnetism and localization, or even to<br />
quantitatively investigate the very nature <strong>of</strong> the metal-insulator transition (MIT). In<br />
particular, due to qualitatively different magnetic signatures <strong>of</strong> regions with significantly<br />
different carrier density, an opportunity is given to trace the evolution <strong>of</strong> the character <strong>of</strong> the<br />
electronic states on crossing the MIT by using direct magnetometry. Therefore we expect to<br />
find (i) a long range FM order in metallic samples, (ii) percolating or not FM “bubbles” in<br />
weakly localizes materials, and (iii) uncoupled paramagnetic moments in purely insulating<br />
samples. But in practice various degrees <strong>of</strong> combinations <strong>of</strong> the above contributions are seen,<br />
so a firm ability <strong>of</strong> recognizing <strong>of</strong> these responses in (Ga,Mn)As is required if one wants to<br />
investigate the transition region.<br />
The complications start already in ultrathin metallic samples which show considerable<br />
deviations from the typical for (Ga,Mn)As behavior, as it is shown by Proselkov, et al. [1]. As<br />
one <strong>of</strong> the few possible explanation invokes a presence <strong>of</strong> sizable number <strong>of</strong> small FM<br />
volumes (the “bubbles”) created in the near-to-surface depletion zone [2] we perform a<br />
detailed and a wide-ranging magnetic studies <strong>of</strong> samples where magnetic response<br />
characteristic to the metallic phase is very small, or is even completely quenched. For this<br />
purpose we select a 600 nm thick layer with only 2% <strong>of</strong> Mn in which the proximity to MIT is<br />
solely assured by the low Mn concentration. Also, in order to link our findings with those <strong>of</strong><br />
[1] we investigate 5 and 7.5 % layers with thickness ranging from 4 to 8 nm. We perform both<br />
DC and AC SQUID magnetometry and an angle dependent FMR. All studied samples show a<br />
coexistence <strong>of</strong> a temperature reversible magnetization characteristic for a long-range-coupled<br />
magnetization as well as an irreversible one possessing characteristics similar to a blocked<br />
superparamagnet. The relative contribution <strong>of</strong> former to the latter scales down linearly with<br />
the inverse <strong>of</strong> the TC <strong>of</strong> the layer, confirming a gradual localization <strong>of</strong> the extended states<br />
already on the metallic side <strong>of</strong> MIT. The reach T-dependent AC magnetization compares well<br />
to the DC m(H) studies confirming a presence <strong>of</strong> both blocked superparamagnetic moment<br />
and a spin reorientation transition between uniaxial and biaxial magnetic anisotropy in the<br />
metallic part <strong>of</strong> the sample.<br />
The work was supported by EU FunDMS Advanced Grant <strong>of</strong> the European Research<br />
Council within the “Ideas” 7 th Framework Programme, EC Network SemiSpinNet (PITN-GA-<br />
2008-215368) and Polish MNiSW No. 2048/B/H03/2008/34 grant.<br />
[1] O. Proselkov, W. Stefanowicz, S. Dobkowska, J. Sadowski, T. Dietl, M. Sawicki,<br />
this conference.<br />
[2] M. Sawicki, et al., Nature Physics 6, 22 (2010).<br />
191
WeP16 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
!"#$ % & '( ) *+ +,-<br />
. / / 0<br />
+1$2 % *+3 #3-<br />
! " " " # $$ "<br />
% " $ $ " % "$ & % &<br />
# & " " " & "& " " " '<br />
# " & $ # & # ! " ! ' ' ' " "<br />
()))* & " " $ % &" " "" "" &<br />
+ % " # &$ " & &$ , & ##<br />
" % %<br />
& "<br />
.<br />
) - " " "<br />
&% & "& " " %<br />
/ $ & ' " $ & " (0 * " " # ! ' ' '<br />
" " ()))* & " &% & "& "<br />
" " 1 " + % & # 23 43 & 1<br />
5' & , & " " $ % 4 & %<br />
" " $ " # ) / +$ & " % )3<br />
& &$ + 263 & 244 &<br />
/ # 0 " & " "" )33 $" 1 " " ## +<br />
&% " 1 0 # & " & " " % # #<br />
$ " & & " & " 1 " $ $ "<br />
+$ " " & # & "" " &$ " ## & "<br />
/ " " & "" & " # & " # '# ' " "<br />
, & & " $ " " $<br />
" $ $ " % # &% " "<br />
& " & " 2<br />
) 7' 8 / 9 3 45/ *6%5 % 4/77/ % !" 3)2)3: (.3)3*<br />
. 0 1 ; #2"" 5/// 8597/ 7: 5 34% '8 7) $$ "" 0 *6%5 34 /;5/8 < #$ 3=4234 (.3)3*<br />
192
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors WeP17<br />
Wide range wavelength tuning <strong>of</strong> InGaAsP/InP laser diodes<br />
M. Bajda 1, 2 , F. Dybała 2 , A. Bercha 2 , W. Trzeciakowski 2 , and J. A. Majewski 1<br />
1 Institute <strong>of</strong> Theoretical Physics, Faculty <strong>of</strong> Physics, University <strong>of</strong> Warsaw, Poland<br />
2 Institute <strong>of</strong> High Pressure Physics Unipress, Polish Academy <strong>of</strong> Sciences, Poland<br />
The combination <strong>of</strong> pressure and temperature tuning with external-grating tuning [1] has<br />
been studied both theoretically and experimentally [2]. Bent-waveguide InGaAsP/InP laser<br />
diode (manufactured by Covega [3]) is tuned under pressure up to 2.2 GPa in the external<br />
cavity formed by diffraction grating. Due to bent waveguide, there is no reflection at the laser<br />
facet, independently <strong>of</strong> wavelength or the medium in which the laser is immersed. At each<br />
pressure the laser is tuned by the grating in the 60-120 nm range. The total tuning range that<br />
can be achieved with pressure and grating amounts to 390 nm (from 1610 nm down to 1220<br />
nm). Temperature tuning (in the 300K-100K range) has turned out to be less effective, mostly<br />
because <strong>of</strong> the strong tuning range reduction at low temperatures.<br />
In order to model the experimental results,<br />
we have performed calculations <strong>of</strong> gain as<br />
a function <strong>of</strong> pressure and temperature,<br />
employing an 8x8 k·p scheme [4] for electronic<br />
structure <strong>of</strong> the InGaAsP quantum well. The gain<br />
has been obtained as a function <strong>of</strong> carrier<br />
concentration and the concentration, in turn, has<br />
been related to current assuming two<br />
recombination channels: radiative and Auger<br />
ones. Since the radiative recombination<br />
dominates in temperatures up to 250K,<br />
Fig.1 Bent waveguide chip with highly<br />
reflective back mirror and anti-reflective<br />
coating on the front mirror.<br />
knowledge <strong>of</strong> the temperature dependence <strong>of</strong> the threshold currents allows for separation <strong>of</strong><br />
these two contributions. At each pressure/temperature we are able to calculate the threshold<br />
currents as a function <strong>of</strong> wavelength by requiring that the modal gain equals the total losses<br />
(mirror losses and the internal optical losses). We obtain fairly good agreement between the<br />
theory and experiment. This opens new perspectives <strong>of</strong> possible future developments <strong>of</strong> this<br />
new method for wave-length tuning <strong>of</strong> laser diodes.<br />
[1] C. Ye, Tunable external cavity diode lasers (World Scientific, Singapore, 2004).<br />
[2] F. Dybala, A. Bercha, P. Adamiec, R. Bohdan, and W. Trzeciakowski, Phys. Status Solidi<br />
B 244, 219 (2007).<br />
[3] http://www.covega.com/Products/pdfs/SAF%<strong>2011</strong>26%20Rev%20C.pdf<br />
[4] http://www.nextnano.de<br />
193
WeP18 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Transport properties <strong>of</strong> GaMnAs layers<br />
M. Gryglas-Borysiewicz 1 , J. Przybytek 1 , M. Baj 1 , A. Kwiatkowski 1 , P. Juszy�ski 1 ,<br />
D. Wasik 1 , P. Dziawa 2 and J. Sadowski 2,3<br />
1 Faculty <strong>of</strong> Physics, University <strong>of</strong> Warsaw, Ho�a 69, 00-681 Warsaw, Poland<br />
2 Institute <strong>of</strong> Physics, PAS, Al. Lotników 32/46, 02-668 Warsaw, Poland<br />
3 Max-Lab, Lund University, Lund, SE-221 00, Sweden<br />
(Ga, Mn)As is one <strong>of</strong> the most intensively investigated diluted magnetic semiconductors<br />
during last decades. The understanding <strong>of</strong> physical phenomena governing its magnetic<br />
properties is crucial for possible application <strong>of</strong> this material in spintronic devices. One <strong>of</strong> the<br />
issues that are currently addressed is the interplay between carrier localization and magnetic<br />
properties <strong>of</strong> the layers [1-3]. It was observed that for insulating sample a decrease <strong>of</strong> the<br />
interatomic distances leads to a decrease <strong>of</strong> the Curie temperature, in contrast to metallic<br />
sample, for which the Curie temperature was increased [3].<br />
In order to extend the studies presented in [3] a series <strong>of</strong> new samples was prepared.<br />
50nm-thick Ga1�xMnxAs layers were grown by molecular-beam epitaxy on (100) GaAs semiinsulating<br />
substrates. Three kinds <strong>of</strong> samples were studied: two with x = 2% and different<br />
compensations and one with x = 6%. The two former samples had Curie temperature TC about<br />
20 K and the latter one – 70 K. Standard Hall bars were prepared by means <strong>of</strong> optical<br />
lithography and six ohmic contacts were made with indium. The samples were investigated<br />
using magnetotransport measurements down to 4K, with magnetic field up to 7T.<br />
Figure 1 shows the temperature variation <strong>of</strong> resistivity. As planned, the samples<br />
have qualitatively different character: from metallic (lower curve) to highly insulating (top<br />
curve) one. The Hall and longitudinal voltages were measured as a function <strong>of</strong> magnetic field.<br />
The evolution <strong>of</strong> hysteresis observed in both �xx(B) and �xy(B) will be used to determine the<br />
Curie temperature and thus to analyze the localization <strong>of</strong> holes in the samples within the<br />
generalized alloy theory [1].<br />
� (cm *�)<br />
10<br />
1<br />
0.1<br />
10 20 30 40 50 60 70 80 90 100 110<br />
T (K)<br />
Figure 1. Resistivity <strong>of</strong> the studied samples as a function <strong>of</strong> temperature.<br />
[1] T. Dietl, Phys. Rev. B 77, 085208 (2008).<br />
[2] M. Sawicki et al., Nature Physics 6, 22 (2010)<br />
[3] M. Gryglas-Borysiewicz et al., Phys. Rev. B 82, 153204 (2010)<br />
194<br />
�
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors WeP19<br />
Group III-nitrides growth on N-polar substrates<br />
C. Chèze 1 , M. Sawicka 1,2 , M. Siekacz 1,2 , H. Turski 2 , A. Feduniewicz-Zmuda 2 ,<br />
G. Cywiński 2 , B. Grzywacz 2 , S. Grzanka 2 , I. Dziecielewski 2 , B. ̷Lucznik 1,2 , M.<br />
Boćkowski 1,2 , and C. Skierbiszewski 1,2<br />
1 TopGaN Ltd, Soko̷lowska 29/37, 01-142 Warszawa, Poland<br />
2 Institute <strong>of</strong> High Pressure Physics, PAS, Soko̷lowska 29/37, 01-142 Warszawa, Poland<br />
Thegrowth<strong>of</strong>nitrogen-polar(N-polar)groupIII-nitridesbyplasma-assistedmolecular<br />
beam epitaxy (PAMBE) is currently raising a lot <strong>of</strong> interest. One <strong>of</strong> the advantages <strong>of</strong><br />
the growth along the [000¯1] over the [0001] direction is the highest thermal dissociation<br />
limit <strong>of</strong> N-face InGaN allowing for a more efficient In incorporation in N-face InGaN<br />
[1]. Another benefit <strong>of</strong> the growth along the [000¯1] direction lies in the formation <strong>of</strong><br />
polarization-induced three dimensional hole gas at the interface <strong>of</strong> N-polar GaN-AlGaN<br />
with graded Al content. This method proposed by Simon et al [2] proves very efficient<br />
p-type doping. However, N-face III-nitrides (000¯1) usually suffer from poor morphology<br />
as no Ga bilayer can be stabilized at the growth front [3]. Therefore a new growth window<br />
under N-rich conditions as has been found for the Ga-face (0001) [4] and m-plane (10¯10)<br />
[5] would be highly desirable.<br />
In this work we study the growth <strong>of</strong> GaN, InGaN and AlGaN-GaN quantum wells<br />
on N-polar (000¯1) GaN by PAMBE. Similarly to the growth <strong>of</strong> nitrides on Ga-polar<br />
face (0001), we have achieved high quality step-flow growth <strong>of</strong> GaN on N-polar in the<br />
metal-rich regime. The surface <strong>of</strong> this layer is extremely smooth and we clearly observe<br />
single atomic steps flowing towards the [10¯10] direction when every second step edge is<br />
either straight or jagged. For Ga-polar GaN, a similar surface morphology was observed<br />
and explained by the growth anisotropy <strong>of</strong> two alternating types <strong>of</strong> step-edges [6]. In<br />
addition, the photoluminescence (PL) spectra at room temperature <strong>of</strong> the N-polar GaN<br />
layer exhibit a sharp peak at 363 nm with a full width at half maximum value <strong>of</strong> 4.53<br />
nm. In contrast, the N-rich regime results in a defect-mediated growth morphology, where<br />
stripes nevertheless covered by atomic steps meander between depressions and hillocks.<br />
We will discuss the effect <strong>of</strong> the III/V ratio on the growth mode <strong>of</strong> N-polar GaN. We will<br />
also compare the growth and characteristics <strong>of</strong> InGaN and AlGaN-GaN quantum wells<br />
grown in the metal-rich regime on N- and Ga-polar substrates. These preliminary results<br />
open up new possibilities for more efficient N-polar based deep ultra-violet and green<br />
optoelectronic applications.<br />
[1] J. Simon, V. Protasenko, C. Lian, H. Xing, and D. Jena, Science 327, 60 (2010)<br />
[2] D. N. Nath, E. Gr, S. A. Ringel, and S. Rajan, Appl. Phys. Lett. 97, 071903 (2010)<br />
[3] E. Monroy, E. Sarigiannidou, F. Fossard, N. Gogneau, E. Bellet-Amalric, J. L. Rouviere,<br />
S. Monnoye, H. Mank, and B. Daudin, App. Phys. Lett. 84, 2684 (2004).<br />
[4] G. Koblmuller, F. Wu, T. Mates T, J. S. Speck JS, S. Fernandez-Garrido, and E.<br />
Calleja, App. Phys. Lett. 91, 221905 (2007).<br />
[5] M. Sawicka, et al., J. Vac. Sci. Technol. B in press (<strong>2011</strong>).<br />
[6] M.H. Xie, S.M. Seutter, W.K. Zhu, L.X. Zheng, H. Wu, S.Y. Tong, Phys. Rev. Lett.<br />
82, 2749 (1999)<br />
This work was supported partially by the Polish Ministry <strong>of</strong> Science and Higher Education Grant No<br />
IT 13426 and the European Union within European Regional Development Fund, through grant Innovative<br />
Economy (POIG.01.01.02-00-008/08) and within FP7-PEOPLE-IAPP-2008 through grant SINOPLE<br />
230765.<br />
195
WeP20 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Absorption and Emission Properties <strong>of</strong> Light Emitting Diode Structures<br />
Containing GaInN/GaN QWs<br />
J. Binder 1 , K. P. Korona 1 , J. Borysiuk 1 , M. Kaminska 1 , M. Baeumler 2 , K. Köhler 2 ,<br />
L. Kirste 2<br />
1 Faculty <strong>of</strong> Physics, University <strong>of</strong> Warsaw, ul. Hoza 69, 00-681 Warsaw, Poland<br />
2 Fraunh<strong>of</strong>er Institute for Applied Solid State Physics, Tullastr. 72, 79108 Freiburg, Germany<br />
The technology <strong>of</strong> illumination is about to change towards an effective, energy saving<br />
future. A major role in this development is played by light emitting diodes (LEDs) which have<br />
the advantage <strong>of</strong> being a very effective and low cost light emitter. Structures based on GaN<br />
are playing an important role in the field <strong>of</strong> light emitting devices and a deeper understanding<br />
<strong>of</strong> the processes involved in light emission is therefore needed to further increase the<br />
capability <strong>of</strong> the produced structures.<br />
In this work we present measurements <strong>of</strong> 3 samples that have different quantum well<br />
(QW) widths (1.8 nm, 2.7 nm, 3.7 nm). The samples were grown on a sapphire substrate with<br />
a nucleation and a buffer layer <strong>of</strong> GaN, followed by a n-doped layer <strong>of</strong> GaN:Si. The active<br />
layer consists <strong>of</strong> 3 QWs made <strong>of</strong> GaN/Ga0.9In0.1N heterostructures followed by an<br />
AlxGa(1-x)N:Mg barrier (xAl about 0.04) and a p-doped layer <strong>of</strong> GaN:Mg. All layers were<br />
produced by MOCVD. Semitransparent Schottky contacts were evaporated on the surface.<br />
The ohmic contacts were made on the edges to connect the GaN:Si layer. The details <strong>of</strong> the<br />
structure were confirmed by high resolution electron microscopy.<br />
Numerical calculations <strong>of</strong> the electric field and potential pr<strong>of</strong>iles were performed for<br />
this structure. It has been found that due to the spontaneous polarization and the piezoelectric<br />
effect a strong electric field <strong>of</strong> the order <strong>of</strong> 1 MV/cm (100 mV/nm) was present in the GaInN<br />
QWs. This field has a reverse direction in comparison to the field generated by the p-n<br />
junction.<br />
In order to characterize the sample, measurements <strong>of</strong> photoluminescence (PL),<br />
electroluminescence (EL) and photocurrent (PC) were performed at room temperature. The<br />
emission was measured as a function <strong>of</strong> applied voltage revealing shifts caused by the<br />
quantum confined stark effect (QCSE).<br />
A strong emission <strong>of</strong> the QWs could be observed in EL at energies 3.07 eV, 3.11 eV and<br />
3.18 eV, for the QWs <strong>of</strong> width 3.7 nm, 2.7 nm and 1.8 nm, respectively. An additional peak<br />
from GaN was visible in PL at 3.4 eV. The different QW widths resulted in different emission<br />
energies, which can be explained by different quantization energies and the presence <strong>of</strong> the<br />
internal electric field caused by spontaneous and piezoelectric polarizations. The photocurrent<br />
measurements show the presence <strong>of</strong> spectral bands related to the GaInN QWs, GaN and<br />
AlGaN. The PC bands correspond strictly to the absorption bands at room temperature due to<br />
efficient delocalization <strong>of</strong> photoexcited carriers. The energy thresholds <strong>of</strong> the QW absorption<br />
bands decreased with increasing QW width from 3.19 eV for the 1.8 nm-QW to 3.11 eV for<br />
the 3.7 nm-QW, similarly to the case <strong>of</strong> emission.<br />
The comparison <strong>of</strong> the emission (EL and PL) and absorption (PC) on the same sample<br />
revealed a shift in energy, with the emission energy being significantly lower. This can be<br />
interpreted by a shift <strong>of</strong> the ground state energy caused by the QCSE. The shifts were higher<br />
for wider QWs, in agreement with even most simple theoretical calculations. More detailed<br />
numerical calculations were performed for a further study <strong>of</strong> the structure (taking into account<br />
the electric field inside and outside the sample). The obtained information is among other<br />
things helpful for the determination <strong>of</strong> radiative and nonradiative processes in light emitting<br />
diodes.<br />
196
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors WeP21<br />
Mn contribution to the valence band <strong>of</strong> Ga1−xMnxSb<br />
B.J. Kowalski 1 , R. Nietubyć 2 , and J. Sadowski 3,1<br />
1 Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences, Al. Lotników 32/46, 02-668 Warsaw,<br />
Poland<br />
2 The Andrzej Soltan Institute for Nuclear Studies, 05-400 Swierk/Otwock, Poland<br />
3 MAX-lab, Lund University, Box 118, SE-22100 Lund, Sweden<br />
The contribution <strong>of</strong> the Mn 3d states to the valence band <strong>of</strong> Ga1−xMnxSb, an important<br />
factor determining the properties <strong>of</strong> this system, has been revealed by photoelectron spectroscopy.<br />
The resonant photoemission experiment, carried out for photon energies close<br />
to the Mn 3d → 3p excitation, allowed us to identify the spectral feature corresponding<br />
to emission from the Mn 3d states. The scanning <strong>of</strong> the valence band along the [100] and<br />
[011] directions in the Brillouin zone, made by the angle-resolved photoemission experiment,<br />
showed that these states contributed to a dispersionless structure at the binding<br />
energy <strong>of</strong> 3.5 eV (with respect to the Fermi energy). Other bands were similar to those<br />
reported for GaSb [1].<br />
Ga1−xMnxSb is a III-V diluted magnetic semiconductor exhibiting ferromagnetic properties,<br />
although at relatively low temperatures (Tc=25 K [2]). It has been investigated<br />
much less than Ga1−xMnxAs but it attracts interest due to the opportunity to study the<br />
interactions <strong>of</strong> magnetic ions with charge carriers in a host with anions chemically different<br />
than in arsenides or due to the band structure <strong>of</strong> Ga1−xMnxSb particularly suitable<br />
for making a novel device (a ferromagnetic resonant interband tunneling diode [3]).<br />
The photoemission experiments were performed with use <strong>of</strong> the photoelectron spectrometer<br />
at the beamline 41 in the MAXlab synchrotron radiation laboratory <strong>of</strong> Lund<br />
University (Sweden). The sets <strong>of</strong> spectra were collected in the normal emission mode for<br />
the photon energy ranges from 30 to 51 eV (in order to observe the Mn 3p-3d resonance)<br />
and from 50 to 106 eV (to scan the band structure along the [100] direction) as well as in<br />
the <strong>of</strong>f-normal mode (to scan the band structure along the [011] direction).<br />
The Ga1−xMnxSb layers with Mn contents in the range <strong>of</strong> 1 to 3%, were grown on<br />
GaSb(100) substrates by molecular beam epitaxy (MBE) at low substrate temperature <strong>of</strong><br />
about 230 o C. The MBE growth was monitored by Reflection High Energy Electron Diffraction<br />
(RHEED).The 2-dimensional diffraction patterns (streaks) and distinct RHEED oscillations<br />
were observed throughout the growth <strong>of</strong> the Ga1−xMnxSb layers up to their<br />
final thicknesses (50 to 300 ˚A depending on the sample). No signs <strong>of</strong> secondary phases<br />
(segregated MnSb nanocrystals [4]) were detected on RHEED images after the growth.<br />
The (100) surfaces <strong>of</strong> the layers exhibited the Low Energy Electron Diffraction (LEED)<br />
patterns corresponding to the asymmetric (1x3) reconstruction. The absence <strong>of</strong> MnSb<br />
precipitates in the investigated samples was confirmed also by a comparative study (including<br />
the samples containing the precipitates) by scanning electron microscopy.<br />
This research has received funding from the European Community’s Seventh Framework<br />
Programme (FP7/2007-2013) under grant agreement no. 226716.<br />
[1] G.E. Franklin, D.H. Rich, A. Samsavar, E.S. Hirschorn, F.M. Leibsle, T. Miller,<br />
T.-C. Chiang, Phys. Rev. B 41, 12 619 (1990)<br />
[2] F. Matsukura, E. Abe, H. Ohno, J. Appl. Phys. 87, 6442 (2000)<br />
[3] I. Vurgaftman, J.R. Meyer, Appl. Phys. Lett. 82, 2296 (2003)<br />
[4] K. Lawniczak-Jablonska, A. Wolska, M.T. Klepka, S. Kret, J. Gosk, A. Twardowski,<br />
D. Wasik, A. Kwiatkowski, B. Kurowska, B.J. Kowalski, J. Sadowski, J. Appl. Phys.<br />
109, 074308 (<strong>2011</strong>).<br />
197
WeP22 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Time resolved photoluminescence studies for GaInNAsSb quantum wells<br />
emitting at 1.3 µm<br />
M. Baranowski 1 , M. Latkowska 1 , M. Syperek 1 , R. Kudrawiec 1 , J. Misiewicz 1 ,<br />
T. Sarmiento 2 , and J.S. Harris 2 ,<br />
1 Institute <strong>of</strong> Physics, Wroclaw University <strong>of</strong> Technology, Wybrzeze Wyspianskiego 27,<br />
50-370 Wrocław, Poland<br />
2 Solid State and Photonics Laboratory, Stanford University, Stanford, California 94305-4075<br />
Dilute nitrides (e.g., GaInNAs with a few percent <strong>of</strong> nitrogen atoms) have attracted much<br />
attention due to their potential use in low cost telecommunication laser based on GaAs<br />
operating at II and III telecommunication windows. However with increasing nitrogen mole<br />
fraction the optical quality <strong>of</strong> GaInNAs material strongly deteriorates due to point defects,<br />
nitrogen interstitials, vacancy-type defects as well the phase separation [1]. In the past few<br />
years it has been found that the introduction <strong>of</strong> antimony, forming GaInNAsSb alloy, greatly<br />
improves the optical quality <strong>of</strong> this material system [1, 2]. This is because <strong>of</strong> reactive<br />
surfactant properties <strong>of</strong> antimony which reduces the group-III surface diffusion length thereby<br />
suppress the phase segregation in this material system [1]. This improvement is well<br />
documented by photoluminescence (PL) measurements as well as structural investigations [1].<br />
However, so far no time resolved photoluminescence (TRPL) studies on such QWs have been<br />
reported. In this work we applied TRPL spectroscopy to investigate the phenomenon <strong>of</strong><br />
carrier localization at low temperatures as well as the influence <strong>of</strong> non-radiative<br />
recombination on the efficiency <strong>of</strong> PL emission from the as-grown and annealed GaInNAsSb<br />
QWs. 7nm-wide Ga0.68In0.32N0.02As0.96Sb0.02/GaAs QWs were grown by molecular beam<br />
epitaxy and annealed at different temperatures varying from 680 to 800 °C. TRPL<br />
measurements were performed for different excitation powers and the spectra were measured<br />
at various temperatures. Some dispersions <strong>of</strong> PL decay time, which are related to carrier<br />
localization phenomenon, were observed at low temperatures but they disappear very quickly<br />
just above 40 K. In order to evaluate the contribution <strong>of</strong> non-radiative recombination, the PL<br />
decay time was measured at room temperature and compared for the as-grown and annealed<br />
samples. It was observed that the longest PL decay time (i.e., the best sample quality) is<br />
observed for samples annealed at ~720 °C. This time equals ~700 ps whereas for the as-grown<br />
sample this time equals ~100 ps. The measured PL decay times are the longest which were<br />
observed for this material system (dilute nitrides QWs) so far. This is an evidence <strong>of</strong> the high<br />
quality <strong>of</strong> GaInNAsSb QWs. In this work physical mechanisms, which are responsible for<br />
changes in the dynamic <strong>of</strong> photoluminescence as well as the origin <strong>of</strong> optical quality<br />
improvement upon annealing, will be discussed.<br />
The authors acknowledge support from the MNiSW (grant no. N202 258339).<br />
[1] J.S. Harris, et al., Physica Status Solidi (b) 244, 2707 (2007), and references therein.<br />
[2] X. Yang, et al., Appl. Phys. Lett. 75, 178 (1999).<br />
198
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors WeP23<br />
InGaN laser diodes with passive absorber section.<br />
A. Kafar, J. Goss, S. Stańczyk, R. Czernecki, M. Leszczynski, T. Suski, P. Wiśniewski,<br />
P . Perlin<br />
Institute <strong>of</strong> High Pressure Physics “Unipress”, Sokolowska 29/37, 01-142 Warsaw, Poland<br />
The laser diode cavities with the passive (non-injected) section form interesting system<br />
because <strong>of</strong> its possibility to generate self-pulsation via passive Q-switching effect. In this<br />
work we present the properties <strong>of</strong> two segmented laser diodes in which one section may<br />
remain unbiased (or negatively biased) during the operation. Such a system is thought to be<br />
useful as simple realization <strong>of</strong> the superluminescent diodes (SLEDs) since high absorption <strong>of</strong><br />
the passive segment prevents the system for lasing in the low-medium currents range. As a<br />
result such a SLED can demonstrate the optical powers ranging up to few miliwats. However,<br />
even high losses do not prevent system from lasing under high current regime. The lasing<br />
starts when the gain overcomes high cavity losses. By using Hakki-Paoli method we<br />
measured the gain and determined the losses <strong>of</strong> the passive part <strong>of</strong> the resonator. In the final<br />
part <strong>of</strong> this work we discuss: i) the properties <strong>of</strong> the system within the saturable absorber<br />
model and ii) possible applications for the realization <strong>of</strong> picoseconds lasers and<br />
superluminescent diodes.<br />
Figure 1. Optical power versus current curve showing superluminescent and lasing regimes.<br />
To the right a microphotograph <strong>of</strong> the two segment device.<br />
199
WeP24 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Evidence <strong>of</strong> a new magnetic order in short-period (Ga,Mn)As/GaAs SLs<br />
W. Szuszkiewicz 1* , F. Ott 2 , J.Z. Domagała 1 , E. Dynowska 1 , J. Sadowski 1,3<br />
1 Institute <strong>of</strong> Physics PAS, Al. Lotników 32/46, Warsaw, Poland<br />
2 CEA/CNRS IRAMIS, LaboratoireLéon Brillouin, CE Saclay, 91191 Gif-sur-Yvette, France<br />
3 MAX-Lab, Lund University, SE-22100, Lund, Sweden<br />
Short-period (Ga,Mn)As/GaAs superlattices (SLs) containing about 5% <strong>of</strong> Mn in<br />
(Ga,Mn)As layer were grown in a MBE system (KRYOVAK) equipped with an As2 valve<br />
cracker source. The number <strong>of</strong> repetitions was 100. The low-temperature growth <strong>of</strong> structures,<br />
performed at 200°C was preceded by a standard high-temperature growth <strong>of</strong> a 5000 thick<br />
GaAs buffer, deposited onto semi-insulating, (100)-oriented GaAs wafers. The Mn<br />
composition in (Ga,Mn)As layers was determined from the analysis <strong>of</strong> RHEED intensity<br />
oscillations during the growth. The structure quality <strong>of</strong> the SLs was checked by X-ray<br />
diffraction measurements using the high-resolution Philips diffractometer and Cu Kα1<br />
radiation. Magnetic properties <strong>of</strong> (Ga,Mn)As/GaAs SLs were characterized by means <strong>of</strong><br />
magnetization measurements using a SQUID, the TC values estimated on the basis <strong>of</strong> these<br />
measurements ranged from 30 K to 50 K.<br />
The samples were investigated by polarized neutron reflectometry on the spectrometer<br />
PRISM at the LLB. The presence <strong>of</strong> a ferromagnetic-like exchange interlayer coupling<br />
between the (Ga,Mn)As layers, observed long time ago for both long-period [1] and shortperiod<br />
[2] SLs was confirmed. Due to the lack <strong>of</strong> intentional, high p-type doping, no trace <strong>of</strong><br />
the interlayer coupling <strong>of</strong> antiferromagnetic type was detected (such coupling has been<br />
recently reported for SLs with Be-doped GaAs spacers [3]). Apart from the expected<br />
ferromagnetic order in the system under investigations the neutron spectra demonstrated a<br />
presence <strong>of</strong> a supplementary, incommensurate magnetic order, which takes place at low<br />
temperature (10K) and low magnetic fields (
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors WeP25<br />
Application <strong>of</strong> n-GaN layers grown by MBE for light- induced water<br />
splitting and hydrogen generation<br />
Z. Wiśniewski 1 , K. Izdebska 1 , P. Sybilski 1 , Z. R. Żytkiewicz 1 , M. Sobańska 1 , K.<br />
Kłosek 1 , A. Reszka 1 , B. J. Kowalski 1 , A. Suchocki 1,2<br />
1 Institute <strong>of</strong> Physics Polish Academy <strong>of</strong> Science Al. Lotników 32/46 02-668 Warsaw, Poland<br />
2 Institute <strong>of</strong> Physics, University <strong>of</strong> Bydgoszcz, Weyssenh<strong>of</strong>fa 11, 85-072 Bydgoszcz Poland<br />
GaN is a very promising material for environment friendly photovoltaic applications,<br />
especially for a hydrogen generation. Hydrogen is generated by photo – catalytic water<br />
decomposition carried out in electrochemical cells with one electrode made <strong>of</strong> semiconductor<br />
is exposed to the light and the second one made <strong>of</strong> metal. Electrical connection is provided by<br />
aqueous electrolyte which is simultaneously the source <strong>of</strong> the water. For practical application<br />
<strong>of</strong> this process, a semiconductor must satisfy the following conditions [1]: (i) a band-gap<br />
energy must be at least 1.7 – 1.9 eV, (ii) redox potential for water reduction must be below the<br />
conduction band energy and the water oxygenation redox potential must be above the valence<br />
band energy; (iii) the semiconductor must have high resistance to corrosion and photocorrosion<br />
processes. In order to satisfy the condition (ii) <strong>of</strong>ten it is necessary to apply external<br />
voltage. GaN-based materials can fulfill all <strong>of</strong> above requirements. The aim <strong>of</strong> this work was<br />
to check the influence <strong>of</strong> electrolyte on efficiency <strong>of</strong> water splitting reaction and hydrogen<br />
generation in photocells with n type-GaN.<br />
1 m thick n-GaN layers were grown on Si(111) substrates by plasma-assisted MBE<br />
technique. The morphology <strong>of</strong> GaN surfaces was investigated by SEM technique. They were<br />
used as photo-anodes in specially designed photocells. A counter electrode was made <strong>of</strong><br />
platinum. In this way the electrochemical chain n-GaN/electrolyte/ platinum was obtained.<br />
Na2SO4, NaCl, and KOH solutions <strong>of</strong> different concentrations were used as electrolytes. The<br />
n-GaN plates were illuminated by white light and by a He-Cd laser beam <strong>of</strong> 325 nm<br />
wavelength.<br />
Under the irradiation <strong>of</strong> UV beam and with presence <strong>of</strong> additional voltage water<br />
decomposition takes place, which is seen as hydrogen and oxygen bubbles formed on the<br />
surfaces <strong>of</strong> electrodes. The value <strong>of</strong> external bias needed for water decomposition decreases<br />
with a concentration <strong>of</strong> electrolyte. In the case <strong>of</strong> Na2SO4 required biases decrease from 1.4 V<br />
for 0.1 mol solution to 0.9 V for 0.5 mol solution. For NaCl solution required external bias is<br />
higher. I(V) characteristics <strong>of</strong> n-GaN/electrolyte/platinum systems were measured as a<br />
function <strong>of</strong> the intensity <strong>of</strong> light and electrolyte. We have also found that observed current<br />
intensity decreases with time due to the photo-corrosion <strong>of</strong> n-GaN, which might be related to<br />
high dislocation density commonly observed in GaN on Si structures. The effect <strong>of</strong> photocorrosion<br />
on the surface <strong>of</strong> n-GaN samples was studied with use <strong>of</strong> optical spectroscopy<br />
methods and by SEM. Design <strong>of</strong> special protective layers to reduce degradation effects in<br />
progress.<br />
Acknowledgment This work was partly supported by the European Union within European<br />
Regional Development Fund, through the Innovative Economy Grants (POIG. 01.01.02-00-<br />
108/2009/2 and POIG. 01.03.01-00-159/08)<br />
[1] T. Bak, J. Nowotny, M. Rekas, and C.C. Sorrel , Int. Jour. <strong>of</strong> Hyd. Energy. 27, 991 (2005).<br />
201
WeP26 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
! "<br />
# $ % & ' % ( ( )<br />
! " # $ %<br />
& '( ) ! " # $ %<br />
* + !, ,,- " # $ %<br />
!"#$<br />
% & '& "# &<br />
( ! )*+$<br />
"#<br />
,<br />
- . & '& /<br />
!+0)$ !1 2 3 4$<br />
- & + 5<br />
6 7 . !+8567$ 1 2 ( & '& +0)<br />
0) 1 3<br />
9 : 0)<br />
!1 4$ 9 :<br />
& ( 0) /<br />
0) ! 1 2$<br />
0)<br />
& ! $<br />
5 1 " +<br />
!51"+$ 1 2 . ! )*+$<br />
1 3 4 )*+ ;<br />
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors WeP27<br />
Microwave techniques investigations <strong>of</strong> (Zn,Co)O films grown by Atomic<br />
Layer Deposition<br />
M.I. Łukasiewicz 1 , A. Cabaj 2 , M. Godlewski 1,2 , E. Guziewicz 1 , A. Wittlin 1 ,<br />
M. Jaworski 1 , A. Woło 1 , Z. Wilamowski 1<br />
1 Institute <strong>of</strong> Physics PAS, Al. Lotników 32/46, 02-668 Warsaw, Poland<br />
2 Dept. Mathematics and Natural Sciences College <strong>of</strong> Sciences UKSW, Dewajtis 5,<br />
01-815 Warsaw, Poland<br />
Atomic Layer Deposition (ALD)-grown (Zn,Co)O films are studied in the present work. ALD<br />
technique enables us to control film uniformity <strong>of</strong> (Zn,Co)O, as demonstrated recently [1].<br />
The (Zn,Co)O films were grown at low temperature (the growth temperature < 300°C).<br />
Magnetic properties <strong>of</strong> these films are discussed in [1,2]. Magnetic investigations indicate that<br />
room temperature ferromagnetic (FM) response is observed in films with non-uniform Co<br />
distribution only [2]. The observed FM response is due to Co metal accumulations at the<br />
(Zn,Co)O/Si interface [2]. (Zn,Co)O films with the uniform Co distribution remain<br />
paramagnetic even at increased concentration <strong>of</strong> Co and intrinsic defects.<br />
In the present work we determine an inter-link between uniformity <strong>of</strong> Co distribution and<br />
electrical and magnetic response <strong>of</strong> the films. We employed two microwave techniques for<br />
these studies.<br />
The first is microwave AC conductivity. The microwave conductivity method is highly<br />
sensitive to detect small inclusions with high conductivity. Moreover, this is contactless<br />
method. Thus, we avoid problems with contact stability and measurements <strong>of</strong> highly resistive<br />
samples. Therefore, AC measurements allow us to investigate uniformity <strong>of</strong> Co-distribution in<br />
(Zn,Co)O films, since the presence <strong>of</strong> metal inclusions results in large deviations between DC<br />
and AC conductivity. Whereas the DC conductivity <strong>of</strong> our (Zn,Co)O films varies little among<br />
all studied samples, the AC one increases considerably for the layers with a substantial nonuniform<br />
Co-distribution and so strongly indicating an important, if not a dominant role <strong>of</strong><br />
metallic inclusions. We demonstrate direct correlation between sample uniformity, magnetic<br />
response and AC conductivity <strong>of</strong> our films.<br />
The second is electron spin resonance technique. These investigations were performed for the<br />
films with different Co fractions and different uniformity <strong>of</strong> Co distribution. They allow us to<br />
observe FM response at low temperature and determine magnitude <strong>of</strong> anisotropy fields,<br />
suggesting occurrence <strong>of</strong> magnetic order even in strongly diluted (Zn,Co)O samples.<br />
The research was partially supported by the EU within the European Regional Development<br />
Fund through grant Innovative Economy (POIG.01.01.02-00-008/08) and FunDMS Advanced<br />
Grant within the "Ideas" 7th Framework Programme <strong>of</strong> the EC.<br />
[1] M. Łukasiewicz et al., Acta Physica Polonica (A) (in press)<br />
[2] M. Godlewski et al., Phys. Status Solidi (b) (in press).<br />
203
WeP28 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Band-structure analysis from photoreflectance spectroscopy in (Ga,Mn)As<br />
O. Yastrubchak 1 , T. Andrearczyk 2 , J. Sadowski 2,3 , J. uk 1 and T. Wosi ski 2<br />
1 Institute <strong>of</strong> Physics, UMCS, Pl. M. Curie-Skłodowskiej 1, 20-031 Lublin, Poland<br />
2 Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences, 02-668 Warszawa, Poland<br />
3 MAX-Lab, Lund University, 22100 Lund, Sweden<br />
The ternary III-V semiconductor (Ga,Mn)As has recently drawn a lot <strong>of</strong> attention as the<br />
model diluted ferromagnetic semiconductor, combining semiconducting properties with<br />
magnetism. Intentionally undoped (Ga,Mn)As layers, grown by the low-temperature<br />
molecular-beam epitaxy (LT-MBE) technique, are p-type, where Mn atoms, substituting Ga<br />
atoms in the GaAs crystal lattice, supply both itinerant holes and magnetic moments. Below a<br />
magnetic transition temperature, TC, substitutional Mn 2+ ions are ferromagnetically ordered<br />
owing to interaction with spin-polarized holes. However, the character <strong>of</strong> electronic states<br />
near the Fermi energy and the valence-band structure in ferromagnetic (Ga,Mn)As are still a<br />
matter <strong>of</strong> controversy. There are two alternative models <strong>of</strong> the band structure in (Ga,Mn)As.<br />
The first one assumes mobile holes residing in the valence band <strong>of</strong> GaAs and the Fermi level<br />
position determined by the concentration <strong>of</strong> valence-band holes. The second one involves<br />
persistence <strong>of</strong> the narrow, Mn-related, impurity band in highly Mn-doped (Ga,Mn)As with<br />
metallic conduction. In this model the Fermi level exists in the impurity band within the band<br />
gap and the mobile holes retain the impurity band character.<br />
In this paper we have applied photoreflectance (PR) spectroscopy to study the bandstructure<br />
evolution in (Ga,Mn)As layers with increasing Mn content. Magnetic properties <strong>of</strong><br />
the layers were characterized with SQUID magnetometery. We investigated thick (230 – 300<br />
nm) (Ga,Mn)As layers with Mn content in the range from 1% to 6% and, as a reference,<br />
undoped GaAs layer, grown by LT-MBE on semi-insulating GaAs substrates. In addition, we<br />
investigated two thin (50 nm) (Ga,Mn)As layers (quantum wells) embedded in LT-GaAs.<br />
Importantly, the (Ga,Mn)As quantum wells displayed significantly higher TC with respect to<br />
that <strong>of</strong> thick (Ga,Mn)As layers <strong>of</strong> the same Mn content.<br />
The PR spectra measured in the photon-energy range from 1.3 to 1.7 eV revealed a rich,<br />
modulated structure containing peaks around the band-gap-transition energy and electricfield-induced<br />
Franz-Keldysh oscillations (FKO) at energies above the fundamental absorption<br />
edge. From analysis <strong>of</strong> the FKO periods we obtained the interband transition energies in<br />
(Ga,Mn)As layers with various Mn contents. In (Ga,Mn)As with a low (1% and 2%) Mn<br />
content this energy was slightly blue shifted with respect to that in the reference LT-GaAs<br />
layer, which was interpreted as a result <strong>of</strong> the Moss-Burstein shift <strong>of</strong> the absorption edge due<br />
to the Fermi level location below the top <strong>of</strong> GaAs valence band. On the other hand, a<br />
substantial red shift <strong>of</strong> the transition energy, revealed in (Ga,Mn)As with higher (4% and 6%)<br />
Mn content, was interpreted in terms <strong>of</strong> a disordered valence band, extended within the band<br />
gap, formed in highly Mn-doped (Ga,Mn)As from merging the Mn-related impurity band with<br />
the host GaAs valence band. These findings are consistent with our recent results obtained<br />
from the full-line-shape analysis <strong>of</strong> the PR spectra [1].<br />
O. Y. acknowledges financial support by the Foundation for Polish Science under Grant<br />
POMOST/2010-2/12. This work was also supported by the Polish Ministry <strong>of</strong> Science and<br />
Higher Education under Grant No. N N202 129339.<br />
[1] O. Yastrubchak, J. uk, H. Krzy anowska, J.Z. Domagała, T. Andrearczyk, J. Sadowski,<br />
T. Wosi ski, arXiv:1012.4760 (Phys. Rev. B, accepted)<br />
204
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors WeP29<br />
Influence <strong>of</strong> Nitrogen Plasma Parameters on Growth <strong>of</strong> GaN by Plasmaassisted<br />
Molecular Beam Epitaxy<br />
K. Klosek, M. Sobanska, Z.R. Zytkiewicz, H. Teisseyre, E. Lusakowska,<br />
A. Wierzbicka, P. Nowakowski, and L. Klopotowski<br />
Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Science, Al. Lotnikow 32/46, 02 668 Warsaw, Poland<br />
It is already well known that the best GaN layers obtained by plasma-assisted molecular<br />
beam epitaxy (PAMBE) are grown under Ga-rich conditions with a bilayer <strong>of</strong> gallium present<br />
on the surface. Since under such conditions the growth is limited by flux <strong>of</strong> active nitrogen<br />
fine tuning <strong>of</strong> nitrogen source must be used for precise growth. The aim <strong>of</strong> this work is to<br />
study how to control parameters <strong>of</strong> RF nitrogen plasma cell and what is their influence on<br />
properties <strong>of</strong> GaN layers.<br />
GaN layers were grown at temperature <strong>of</strong> 720 o C on commercially available<br />
GaN/sapphire templates using Riber Compact 21 system equipped with elemental sources <strong>of</strong><br />
Al, Ga, In, Si, and Mg. Active nitrogen was supplied from an Addon RF plasma source. An<br />
optical Si sensor was used to measure the light intensity emitted by the nitrogen source in the<br />
wavelength range <strong>of</strong> 750 to 850 nm. The sensor converts light emitted by the plasma into<br />
output voltage U.<br />
nitrogen flow [sccm]<br />
7<br />
6<br />
5<br />
4<br />
3<br />
2<br />
1<br />
U = 1.5 V<br />
U = 1.8 V<br />
U = 2.0 V<br />
U = 2.2 V<br />
U = 2.4 V<br />
U = 2.6 V<br />
300 350 400 450 500 550 600<br />
RF power [W]<br />
Fig.1: RF power and nitrogen flow<br />
values for a fixed sensor output U.<br />
growth rate [nm/min]<br />
5,5<br />
5,0<br />
4,5<br />
4,0<br />
3,5<br />
3,0<br />
2,5<br />
U = 1.5 V<br />
2,0<br />
1,0 1,5 2,0 2,5 3,0 3,5 4,0 4,5 5,0 5,5 6,0 6,5 7,0 7,5<br />
nitrogen flow [sccm]<br />
Fig. 2: GaN growth rate vs. nitrogen<br />
flow for U=1.5 V and U=2.4 V.<br />
1,4 1,5 1,6 1,7 1,8 1,9 2,0 2,1 2,2 2,3 2,4 2,5 2,6 2,7<br />
sensor output signal U [V]<br />
Fig. 3: GaN growth rate vs. sensor<br />
signal U for nitrogen flow <strong>of</strong> 2.5 sccm.<br />
Nitrogen plasma intensity, and so the optical sensor output voltage U, depends both on<br />
the RF power and the nitrogen gas flow. This is illustrated in Fig. 1 that shows broad range <strong>of</strong><br />
RF power and nitrogen flow parameters giving fixed value <strong>of</strong> sensor output U. In Fig. 2 the<br />
GaN growth rate vs. nitrogen flow is plotted for U=1.5 V and U=2.4 V. For each point the<br />
gallium flux was adjusted to keep the surface covered by 2ML <strong>of</strong> Ga. As seen, the growth rate<br />
does not change when the plasma parameters vary along the red (U=1.5 V) and the orange<br />
(U=2.4 V) curves in Fig. 1. On the contrary, the growth rate monotonically increases with<br />
intensity <strong>of</strong> light emitted by nitrogen plasma as shown in Fig. 3. Since under gallium-rich<br />
conditions growth is controlled by nitrogen flux, results shown in Figs. 2-3 clearly indicate<br />
that the optical sensor output signal is a direct measure <strong>of</strong> the amount <strong>of</strong> active nitrogen<br />
species available for growth.<br />
Optical spectra <strong>of</strong> the plasma were measured to check how intensity <strong>of</strong> light emitted by<br />
atomic and molecular nitrogen species depend on nitrogen flow and RF power <strong>of</strong> the cell.<br />
Finally, 0.7 µm thick GaN layers were grown under Ga-rich conditions with various nitrogen<br />
cell parameters to study if their properties, as seen by AFM, XRD, low temperature PL, and<br />
by electrical measurements, depend on the RF power and/or the nitrogen flow used during<br />
growth.<br />
This work was supported by the European Union within European Regional Development<br />
Fund, through grant Innovative Economy (POIG.01.01.02-00-008/08 NanoBiom).<br />
U = 2.4 V<br />
205<br />
growth rate [nm/min]<br />
6,5<br />
6,0<br />
5,5<br />
5,0<br />
4,5<br />
4,0<br />
3,5<br />
3,0<br />
2,5<br />
N 2 flow = 2.5 sccm
WeP30 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Deep defects in ZnO/GaN heterostructure analyzed by the admittance<br />
spectroscopy<br />
T. A. Krajewski 1 , P. Stallinga 2 , E. Zielony 3 , P. Kruszewski 1 , K. Go ci ski 1 ,<br />
Ł. Wachnicki 1 , S. Figge 4 , D. Hommel 4 , E. Guziewicz 1 1, 5<br />
, M. Godlewski<br />
1 Institute <strong>of</strong> Physics, Polish Acad. <strong>of</strong> Sciences, Al. Lotników 32/46, 02-668 Warsaw, Poland<br />
2 Universidade do Algarve, Campus de Gambelas, FCT, 8005-139 Faro, Portugal<br />
3 Institute <strong>of</strong> Physics, Wroclaw University <strong>of</strong> Technology,<br />
Wybrze e Wyspia skiego 27, 50-370 Wrocław, Poland<br />
4 University <strong>of</strong> Bremen, Institute <strong>of</strong> Solid State Physics,<br />
Kufsteiner Strasse 1, Bremen 28359, Germany<br />
5 Dept. <strong>of</strong> Mathematics and Natural Sciences College <strong>of</strong> Science<br />
Cardinal S. Wyszy ski University, ul. Dewajtis 5, 01-815 Warsaw, Poland<br />
Zinc oxide (ZnO) is currently widely investigated as a very promising II-VI<br />
semiconducting material for various electronic and optoelectronic purposes. For many <strong>of</strong> its<br />
modern applications it is useful to understand the physics behind the electrical properties <strong>of</strong><br />
this compound. This is strongly related to the knowledge <strong>of</strong> shallow and deep defect levels <strong>of</strong><br />
ZnO [1, 2].<br />
In this work an Ag/ZnO/GaN/Au device was analyzed with the admittance<br />
spectroscopy technique. It is shown, that to get useful information out <strong>of</strong> the device,<br />
admittance should be measured below 100 kHz. Above this cut-<strong>of</strong>f frequency the device will<br />
not respond and the obtained signals are meaningless. The analysis <strong>of</strong> measured capacitancevoltage<br />
characteristics allowed us to determine the doping concentration in ZnO at the level <strong>of</strong><br />
ND = 7.3×10 17 cm -3 , via the corresponding Mott-Schottky plot.<br />
Using capacitance transient measurements working at 10 kHz (performed with an HP<br />
4275A LCR bridge), four majority-carrier deep levels were identified, the most important one<br />
at approximately 0.5 eV below the conduction band edge with a density about two orders <strong>of</strong><br />
magnitude below the doping level (NT = 4×10 15 cm -3 ). The others, located at 0.2 eV, 0.66 eV<br />
and 0.75 eV, are about three orders <strong>of</strong> magnitude below the doping level (NT = 4–9×10 14 cm -3 ).<br />
[1] F. D. Auret, S. A. Goodman, M. Hayes, M. J. Legodi, H. A. van Laarhoven, D. C. Look,<br />
Appl. Phys. Lett. 79, 3074 (2001).<br />
[2] Y.-P. Wang, W.-I. Lee, T.-Y. Tseng, Appl. Phys. Lett. 69, 1807 (1996).<br />
Acknowledgements<br />
The research was partially supported by the European Union within European Regional<br />
Development Fund, through grant Innovative Economy (POIG.01.01.02-00-008/08) and by<br />
grant <strong>of</strong> the National Science Center <strong>of</strong> Poland (1669/B/H03/<strong>2011</strong>/40).<br />
206
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors WeP31<br />
Electrical characterization <strong>of</strong> the ZnO-based Schottky diodes for possible<br />
sensor applications<br />
Tomasz A. Krajewski 1 , Adam J. Zakrzewski 1 , Grzegorz Łuka 1 , Łukasz Wachnicki 1 ,<br />
Sylwia Gierałtowska 1 , Bartłomiej S. Witkowski 1 , Piotr Kruszewski 1 ,<br />
El bieta Łusakowska 1 , Rafał Jakieła 1 , El bieta Guziewicz 1 1, 2<br />
, Marek Godlewski<br />
1 Institute <strong>of</strong> Physics, Polish Acad. <strong>of</strong> Sciences, Al. Lotników 32/46, 02-668 Warsaw, Poland<br />
2 Dept. <strong>of</strong> Mathematics and Natural Sciences College <strong>of</strong> Science<br />
Cardinal S. Wyszy ski University, ul. Dewajtis 5, 01-815 Warsaw, Poland<br />
Zinc oxide is currently widely investigated as a very promising II-VI semiconducting<br />
material for various electronic and optoelectronic purposes. The ZnO applications include<br />
mainly solar cells, where it acts as a transparent electrode as well as the new generation <strong>of</strong> 3D<br />
memories built in the so-called cross-bar architecture, in which the ZnO-based Schottky diode<br />
plays a role <strong>of</strong> a selecting element. The ZnO-based junction can also be used as a gas or<br />
organic compounds’ sensor.<br />
In this work we report on fabrication and characterization <strong>of</strong> the ZnO-based Schottky<br />
diodes dedicated for the latter application. The junctions were fabricated using the low<br />
temperature Atomic Layer Deposition (ALD) process, in which the Si, ITO-covered glass and<br />
glass substrates were uniformly covered with the ZnO film in a double exchange reaction<br />
between diethylzinc and deionized water. Finally, on the obtained ZnO layer the Ag Schottky<br />
contact was evaporated.<br />
Interestingly, the examined diodes exhibit the reversible effect on their current-voltage<br />
characteristics under treatment with different organic liquids (acetone, isopropanol). As this<br />
behavior is not observed e.g. under their exposure to water, we deduce that the structures can<br />
be effectively used for sensing <strong>of</strong> the organic molecules.<br />
The obtained current-voltage characteristics were analyzed within the thermionicemission<br />
theory. In this way the values <strong>of</strong> some relevant diodes’ parameters were determined,<br />
including the ideality factor and Schottky barrier height. We found a satisfactory agreement <strong>of</strong><br />
experimental and theoretical results what will be demonstrated during the conference.<br />
Acknowledgements<br />
The research was partially supported by the European Union within European Regional<br />
Development Fund, through grant Innovative Economy (POIG.01.01.02-00-008/08) and by<br />
grant <strong>of</strong> the National Science Center <strong>of</strong> Poland (1669/B/H03/<strong>2011</strong>/40).<br />
207
WeP32 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Ab initio calculations <strong>of</strong> transition metal impurity levels in III-V<br />
semiconductors<br />
T. Zakrzewski and P. Bogusławski<br />
Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences, Al. Lotników32/46, 02-668 Warsaw, Poland<br />
Properties <strong>of</strong> transition metal impurities in semiconductors attract a considerable<br />
attention both from the fundamental point <strong>of</strong> view, and in the context <strong>of</strong> possible applications<br />
in fabrication <strong>of</strong> devices. In particular, from the theoretical point <strong>of</strong> view an important<br />
problem is the role and the correct description <strong>of</strong> the electron-electron interactions <strong>of</strong> delectrons.<br />
We have analyzed the electronic structure <strong>of</strong> Mn and Fe impurities in GaP and GaN.<br />
The calculations were performed within the density functional theory, using both the<br />
generalized gradient approximation and the Hubbard "+U "corrections, as implemented in the<br />
Quantum Espresso code. We have also employed the supercell approach, with supercells<br />
containing 128-atoms including the impurity. The obtained results are compared with the<br />
available experimental data, and a good agreement is obtained even without inclusion <strong>of</strong> the<br />
+U corrections. Moreover, we theoretically investigate the fulfillment <strong>of</strong> the Langer-Heinrich<br />
rule.<br />
208
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors WeP33<br />
Anisotropy <strong>of</strong> GaMnAs thin film. Planar and Anomalous Hall measurements.<br />
P. Juszyński 1 , D. Wasik 1 , M. Baj 1 , J. Przybytek 1 , M. Gryglas-Borysiewicz 1 , J. Sadowski 2,3<br />
1 Faculty <strong>of</strong> Physics, University <strong>of</strong> Warsaw, Hoża 69, 00-681 Warsaw, Poland<br />
2 Institute <strong>of</strong> Physics, PAS, Al. Lotników 32/46, 02-668 Warsaw, Poland<br />
3 Max-Lab, Lund University, Lund, SE-221 00, Sweden<br />
The search for appropriate semiconductor base materials for spintronic applications is<br />
a challenge <strong>of</strong> a last few years. Ga1-xMnxAs which reveals ferromagnetic behaviour at low<br />
temperatures is a natural candidate for such material. Its magnetic anisotropy is one <strong>of</strong> the<br />
most important features for possible applications <strong>of</strong> this material. It is well known that<br />
temperature [1] and strain [2] has influence on anisotropy. The aim <strong>of</strong> this paper was to study<br />
temperature dependence <strong>of</strong> the magnetic anisotropy <strong>of</strong> 50 nm thin film <strong>of</strong> Ga1-xMnxAs with<br />
epitaxial nearly critical strain by means <strong>of</strong> magnetotransport experimants. The Ga1-xMnxAs<br />
samples were grown by MBE method on GaAs substrate with InGaAs buffer.<br />
Standard and planar Hall measurements were<br />
performed as a function <strong>of</strong> magnetic field at various<br />
temperatures. Planar Hall hysteresis loops (Fig 1 up)<br />
and Anomalous Hall signal (Fig 1 down) were observed.<br />
We analyzed experimental data using single<br />
domain model [3], which appeared to be sufficient to<br />
understand magnetic and transport properties <strong>of</strong> the<br />
samples. Uniaxial and cubic anisotropy constants (Ku<br />
and Kc respectively) were treated as fitting parameters.<br />
We found that the shape <strong>of</strong> hysteresis loops strongly<br />
depends on the Ku , Kc parameters as well as on the<br />
hallbar alignment with respect to magnetic field.<br />
Our analysis allowed us to obtain anisotropy<br />
constants in the temperature range 14K – 60K and to<br />
find Tc ≈ 70K.<br />
[1] D. Y. Shin et al, Physical Review B 76, 035327 (2007)<br />
[2] H. Lee et al, Solid State Communication 149 (2009) 1300-1303<br />
[3] K. Pappert et al, New Journal <strong>of</strong> Physics 9 (2007) 354<br />
209<br />
RH [ ]<br />
RH [<br />
720<br />
710<br />
700<br />
690<br />
680<br />
670<br />
measurement<br />
fit<br />
660<br />
-0.2 -0.1 0.0 0.1 0.2<br />
150<br />
100<br />
50<br />
0<br />
-50<br />
-100<br />
-150<br />
Applied field [T]<br />
-0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4<br />
Applied field [T]<br />
figure 1<br />
Planar Hall hysteresis loop (upper panel)<br />
in temperature 14 K and Anomalous Hall<br />
Signal (lower panel).
WeP34 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Interplay between internal and external electric field studied by<br />
photoluminescence in InGaN/GaN light emitting diodes.<br />
G. Staszczak 1 , A. Khachapuridze 1 , S. Grzanka 1,2 , R. Piotrzkowski 1 , R. Czernecki 1,2 ,<br />
P. Perlin 1,2 and T. Suski 1<br />
1 Institute <strong>of</strong> High Pressure Physics PAS, Sokołowska 29/37, 01-142 Warszawa, Poland<br />
2 TopGaN, Sokołowska 29/37, 01-142 Warszawa, Poland<br />
Efficient light emitting diodes (LEDs) and laser diodes (LDs) consist usually <strong>of</strong> multi<br />
quantum wells (MQWs)in their active region. In case <strong>of</strong> InxGa1-xN/GaN emitters, MQWs with<br />
identical or different content <strong>of</strong> In are employed. Due to the presence <strong>of</strong> internal electric field<br />
in polar structures and devices with InGaN/GaN QWs reduction in the overlap between<br />
electron and hole wave functions occur leading to a decrease <strong>of</strong> the emitted light intensity. In<br />
nitride LEDs and LDs an additional electric field originating from p-n junction contributes to<br />
the band edges pr<strong>of</strong>iles <strong>of</strong> the device structure and an involvement <strong>of</strong> individual QWs to the<br />
light emission can be tuned by externally applied voltage. This large number <strong>of</strong> factors<br />
influencing energy and light emission efficiency makes full understanding <strong>of</strong> the InGaN/GaN<br />
LEDs and LDs behavior very difficult. Capability <strong>of</strong> computer simulations is very limited<br />
also.<br />
To improve this unsatisfactory understanding <strong>of</strong> LEDS and LDs behavior we have<br />
studied series <strong>of</strong> polar InGaN/GaN LEDs consisting either blue or combination <strong>of</strong> blue and<br />
violet QWs (i.e., with intentionally introduced various content <strong>of</strong> Indium during metalorganic<br />
vapor deposition, MOVPE). Moreover, each type <strong>of</strong> the QW was placed closer or more<br />
distant from the p-type region <strong>of</strong> the particular LED. We found that in agreement with<br />
expectations electroluminescence induced by low currents characterizes light emission from<br />
QW more distant with respect to Mg-doped part <strong>of</strong> the structure. Micro photoluminescence<br />
was used in our studies as a probe supplying an information about the behavior <strong>of</strong> particular<br />
QWs subject to external polarization and excited by variable light intensity <strong>of</strong> He-Ne laser.<br />
We found that depending on external voltage applied to our LEDs as well as applied laser<br />
power we could modify drastically intensity <strong>of</strong> light emitted by our structures. In particular,<br />
we demonstrated: i) switching-on the observed light emission from one or two QWs, ii)<br />
evolution <strong>of</strong> the dominant PL signal intensity between violet and blue QWs. Screening <strong>of</strong><br />
electric field due to different laser intensity supplied an additional parameter capable to tune<br />
significantly properties <strong>of</strong> studied structures. Simulations performed by us are in qualitative<br />
agreement with our experiments and they give an inside into the dependence between<br />
modification <strong>of</strong> the bands pr<strong>of</strong>ile during applying external electric field and density <strong>of</strong><br />
optically injected carriers.<br />
210
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors WeP35<br />
Influence <strong>of</strong> antimony on the optical quality <strong>of</strong> GaInN:Sb multi quantum<br />
wells<br />
M. Baranowski 1 , M. Latkowska 1 , R. Kudrawiec 1 , M. Syperek 1 , J. Misiewicz 1<br />
G.S Karthikeyan 2 , S. Jong-In 3 , and K.L June 2<br />
1 Institute <strong>of</strong> Physics, Wrocław University <strong>of</strong> Technology, Wyspia skiego 27, 50-370 Wroclaw<br />
2 Interdisciplinary Program <strong>of</strong> Photonics Engineering, Chonnam National University,<br />
Gwangju 500-757, Republic <strong>of</strong> Korea<br />
3 Department <strong>of</strong> Electrical and Computer Engineering, Hanyang University, Ansan 426-971,<br />
Republic <strong>of</strong> Korea<br />
InGaN based semiconductors have attracted recently much attention, especially for<br />
their application in light emitting devices such as blue and green laser diodes and light<br />
emitting diodes. InGaN alloy with different In content is able to cover almost whole visible<br />
spectral range what is very important for the full color media applications. Concentration <strong>of</strong> In<br />
in InGaN alloy affects optical quality <strong>of</strong> this material, for low In content (few percentage)<br />
enhancement <strong>of</strong> radiative recombination is observed what is connected with carrier<br />
localization effects caused by small potential fluctuations. However further increase <strong>of</strong><br />
incorporated In strongly deteriorate optical quality <strong>of</strong> InGaN layers and QWs. It was shown<br />
that one <strong>of</strong> the reason <strong>of</strong> luminescence degradation is strong increase <strong>of</strong> QWs and layers<br />
inhomogeneity and formation <strong>of</strong> In-reach clusters. Because <strong>of</strong> that one <strong>of</strong> the main challenge<br />
for technologist is to improve growth method <strong>of</strong> InGaN alloy to increase homogeneity <strong>of</strong> this<br />
material.<br />
In the past few years it has been found that introduction <strong>of</strong> antimony, during growth <strong>of</strong><br />
GaInNAs and GaN alloy, improves optical quality <strong>of</strong> this materials. This is because <strong>of</strong><br />
reactive surfactant properties <strong>of</strong> antimony which reduces the group-III surface diffusion<br />
length thereby suppress phase segregation and roughening, improving homogeneity. The aim<br />
<strong>of</strong> this work is investigation if the presence <strong>of</strong> antimony in growth process <strong>of</strong> InGaN alloy<br />
improve its optical quality, similar as in case <strong>of</strong> GaInNAs alloy.<br />
Five InGaN multi-quantum wells (MQW) samples used in this study were grown on<br />
c-plane sapphire substrate by MOCVD method. QWs was grown on a n-doped GaN buffer<br />
layer. Structure consist <strong>of</strong> 5 InGaN/GaN QWs with ~25% <strong>of</strong> indium content. During the<br />
growth process <strong>of</strong> InGaN QW Sb flow with various (Sb)/((In)+(Ga)) ratios was introduced.<br />
The amount <strong>of</strong> incorporated antimony vary from 0-0.12%. More detail about samples can be<br />
found in ref. [1]. To investigate optical properties <strong>of</strong> samples we have measured<br />
photoluminescence and time-resolved photoluminescence at different temperatures. We have<br />
found that antimony has influence on the optical quality <strong>of</strong> samples. With initial increase <strong>of</strong><br />
antimony content the intensity <strong>of</strong> photoluminescence and the decay time <strong>of</strong><br />
photoluminescence increases and achieve maxim at about 0.05% <strong>of</strong> antimony content. Further<br />
increase <strong>of</strong> antimony content deteriorates optical quality <strong>of</strong> samples. Additionally we found<br />
that antimony does not affects the transition energy, all investigated samples emit on a 500nm<br />
wavelength. Obtained results suggest that properly used antimony can work as a surfactant for<br />
InGaN alloy and improve its optical quality.<br />
[1] Karthikeyan Giri Sadasivam, Jong-In Shim, and June Key Lee, Journal <strong>of</strong> Nanoscience<br />
and Nanotechnology Vol. 11, 1787–1790, (<strong>2011</strong>).<br />
211
WeP36 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Properties <strong>of</strong> Mn-doped GaAs Nanowires and GaAs/(Ga,Mn)As Core-Shell<br />
Nanowire Structures Grown by MBE on GaAs(111)B Substrates<br />
J. Sadowski 1,2 , A. Siusys 2 , P. Dziawa 2 , A. Reszka 2 , B. J. Kowalski 2<br />
1 MAX-Lab, Lund University, P.O. Box 118, 221 00 Lund, Sweden<br />
2 Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences, al. Lotników 32/46, 02-668 Warszawa,<br />
Poland<br />
During the last decade a huge research activity has been devoted to the bottom-up approach <strong>of</strong><br />
formation <strong>of</strong> nanostructures, in which the nanoscale structures are fabricated by self<br />
assembled growth. This is due to the remarkable progress in the growth methods <strong>of</strong> quasi-1dimensional<br />
structures - nanowires (NWs), which can now be grown from many different<br />
semiconducting materials. Usually the growth <strong>of</strong> NWs is initiated by nanodroplets <strong>of</strong> a<br />
catalyst (usually gold) deposited at the planar substrate used for the NWs deposition,<br />
however, depending on the material, also autocatalytic or catalyst-free NW growth can be<br />
achieved. The NWs can be grown with a use <strong>of</strong> methods applied so far for planar growth such<br />
as molecular beam epitaxy (MBE) or metal organic chemical vapor deposition (MOCVD),<br />
only slight modifications <strong>of</strong> the growth conditions and the special substrates (prepatterned<br />
and/or covered with a suitable catalyst) are needed to change the growth mode from 2dimensional<br />
(formation <strong>of</strong> layers) to 1-dimensional (formation <strong>of</strong> nanowires).<br />
Here we report on use <strong>of</strong> the NW growth techniques, namely gold catalyzed NW growth to<br />
formation <strong>of</strong> 1-D nanostructures implying GaMnAs ferromagnetic semiconductor. The NWs<br />
were grown by MBE on GaAs(111)B substrates covered with a thin (1 nm) gold layer. The Au<br />
layer is converted to the ensemble <strong>of</strong> nanodroplets by initial preheating <strong>of</strong> the substrate to<br />
about 600 o C in the MBE growth chamber, and then the NWs are grown at the substrate<br />
temperature <strong>of</strong> about 550 o C. Two types <strong>of</strong> NW structures were investigated: (i) GaAs NWs<br />
doped with Mn at high temperature (550 o C) MBE growth, (ii) core shell structures with high<br />
temperature grown GaAs core NWs and low temperature (250 o C) grown GaMnAs shell. The<br />
scanning electron microscopy images <strong>of</strong> the latter type <strong>of</strong> structure are shown in Fig.1.<br />
Figure 1. Scanning electron microscopy pictures <strong>of</strong> Au-catalysed GaAs-GaMnAs core-shell nanowires<br />
grown on GaAs(111)B substrate. The NWs grow along [111] direction perpendicular to the substrate.<br />
The Mn doping <strong>of</strong> GaAs NWs during high temperature MBE growth usually results in<br />
disordered NWs with branches and different orientations, caused by MnAs segregation and<br />
catalyzing properties <strong>of</strong> MnAs nanoislands. Structural and magnetic properties <strong>of</strong> both types<br />
<strong>of</strong> samples will be compared, in search <strong>of</strong> NWs with appropriate magnetic properties.<br />
This work has been partially supported by the EC Network SemiSpinNet (PITN-GA-2008-215368)<br />
212
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors WeP37<br />
Room temperature nano-magnetism <strong>of</strong> (Ga,Fe)N films:<br />
element specific spectroscopy and microscopy<br />
I.A. Kowalik 1 , M.A. Niño 2 , A. Locatelli 3 , T. Onur Menteş 3 , A. Navarro-Quezada 4 ,<br />
Mauro Rovezzi 4 , A. Bonanni 4 , T. Dietl 1,5 and D. Arvanitis 6<br />
1 Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences, Warsaw, Poland<br />
2 IMDEA, Facultad de Ciencias Módulo C-IX, Madrid, Spain<br />
3 Sincrotrone Trieste, Basovizza, Trieste, Italy<br />
4 Institut für Halbleiter-und-Festkörperphysik, J. Kepler University, Linz, Austria<br />
5 Institute <strong>of</strong> Theoretical Physics, University <strong>of</strong> Warsaw, Warsaw, Poland<br />
6 Department <strong>of</strong> Physics and Astronomy, Uppsala University, Uppsala, Sweden<br />
We present results <strong>of</strong> magnetic investigations at room temperature on (Ga,Fe)N films<br />
by means <strong>of</strong> s<strong>of</strong>t x-ray based element specific and spin sensitive spectroscopy [1]. The studied<br />
samples were grown by MOVPE and studied previously by SQUID magnetometry,<br />
transmission electron microscopy, synchrotron radiation based x-ray diffraction techniques,<br />
[2,3] as well as by EXAFS and XANES [4]. In order to establish a relationship between the<br />
nanocrystal structure and their magnetic properties we carried out laterally averaged<br />
measurements <strong>of</strong> XMCD and XLMD, and further spatially resolved by means <strong>of</strong><br />
photoemission microscopy (PEEM) with magnetic resolution (XMCD-PEEM) at the Fe Ledges<br />
and XMCD at the N-K edge.<br />
We complement these results by means <strong>of</strong> more systematic element specific and<br />
dichroic spectro-miscroscopy measurements, using synchrotron radiation s<strong>of</strong>t x-rays as the<br />
excitation source. The laterally resolved chemical characterization with X-PEEM at the Fe Ledges<br />
indicate that Fe is incorporated both in Ga substitutional sites in the GaN lattice, as well<br />
as in self-assembled FeNx nanocrystals. We can identify typical nanocrystal sizes from about<br />
30 nm to 150 nm, depending on the growth conditions. In samples grown under specific<br />
conditions, we observe also FeGa and GaFeN nanocrystals. This chemical imaging confirms<br />
that the Fe atoms are embedded in different local environments. By means <strong>of</strong> XMCD-PEEM<br />
we identify magnetic Fe-rich nanocrystals and can visualize the magnetic domain state <strong>of</strong><br />
each nanocrystal. A first analysis indicates that <strong>of</strong>ten the nanocrystal magnetic moments can<br />
order following a curling magnetic structure (vortex). Comparing chemical and magnetic<br />
micrographs within the same field <strong>of</strong> view, allows identifying the existence <strong>of</strong> both<br />
ferromagnetic and non-ferromagnetic nanocrystals. An XLMD signal is found for these<br />
samples, indicating that the existence <strong>of</strong> anti-ferromagnetic nanocrystals contributes to the<br />
absence <strong>of</strong> ferromagnetism in XMCD-PEEM. The present set <strong>of</strong> data illustrates the<br />
complementarity <strong>of</strong> X-PEEM and XMCD-PEEM, with angle dependent, XAS, XMCD and<br />
XLMD measurements without spatial resolution, on the same samples.<br />
Acknowledgments<br />
We acknowledge the EC support through the FunDMS Advanced Grant <strong>of</strong> the ERC within the<br />
"Ideas" 7th Framework Programme, the Austrian Fonds zur Förderung der wissenschaftlichen<br />
Forschung-FWF (P18942, P20065 and N107-NAN), the Swedish Research Council, the EC<br />
Seventh Framework Programme (FP7/2007-2013) under grant agreement Nr 226716 (ELISA)<br />
for access to MAX-lab and Elettra.<br />
[1] I.A. Kowalik et al., Phys. Rev. B, in press [arXiv: 1011.0847].<br />
[2] A. Bonanni et al., Phys. Rev. B 75, 125210 (2007); Phys. Rev. Lett. 101, 135502 (2008).<br />
[3] A. Navarro-Quezada et al., Phys. Rev. B 81, 205206 (2010).<br />
[4] M. Rovezzi et al., Phys. Rev. B 79, 195209 (2009).<br />
213
WeP38 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
! " #<br />
$ %& ' ( # ) $ * " + ! " #<br />
! " #$ % # & '( # #$ #$ ) # ' *+ , "<br />
#$ # - * % # & '( # # #$ # & - , # . . (<br />
- # ' $ ' " # # ' # #$ $ - % # & " # '<br />
+ # ' /. . % 0 "# ' #$ $ ' # " #$ $ - #$ %<br />
# & " # ' - #$ & - - ( - (%#$ $ -$# % # * %!,<br />
. $ /. . % 0 "# ' ' - #$ # #$ % # & '( # # (<br />
#$ $ . . # # #$ # # " # (<br />
# -$#" #$ " # & $ # # # ' $ '<br />
'. # # #$ . .$ ' * . % . " '( # #<br />
# # , 1 # # ' # ' $ # ( #$ ' # - $<br />
( # #$ # .$ ' 2 . # ' # ' ' .# . # " '<br />
! # - ' # 2 - . . # #$ & # 3 $ (# #<br />
# # (%#$ $ %! $ # # " #$ & #$ 3# # . # # ' #<br />
#$ % # & " # ' 1 "" # " '( # ( & # - .<br />
" #$ # ' . ' # - ' # $ . - # # # . # " ' #$<br />
# ( # " # ' # ' # # . #$ #$ . ' $ ' 1 #$ ( &<br />
' # 3. ' # . " ' # ' # - #$ & - - . ## '.#<br />
# "" # # ( # #$ "" # ( #$ " #$ % # & '( # #$<br />
# & # #$ +<br />
## 78 79 * 77:,<br />
! "#<br />
, - ( ) #<br />
# ) . " / #<br />
% % # " / -<br />
4 5 6 1.. $<br />
& /. . ;# 0 "# < 1.. $ & = > .. ?9 8%?9?: * :9 ,<br />
214
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors WeP39<br />
!" # $ % & " ' % & (" ) % & " ( ' * $ + & (" ) $ , & " - ./ , &<br />
#" ' % & " $ + & (" 0 ' * $ , & (" 0 * $ ,<br />
!<br />
"#$%!# & '<br />
( ) * %% # $+ + '<br />
% ,))-.- / / / / 0 1 2% ##$%!! '<br />
! " #<br />
" $ !<br />
!<br />
" % & ' (<br />
"<br />
# $$$ !<br />
" ) '<br />
*<br />
!<br />
+,-" .<br />
/ 012 / 0 ! "<br />
∆<br />
4 / / 5 3 3 0(5<br />
3 ' 6 .2, 6 . 7 $ '<br />
3$ 6 . '<br />
33 3 3 ' 869 $ '<br />
: (9 ; < ' =/<br />
*<br />
*<br />
3 / 0<br />
" $<br />
/ 012 / 0 ! "<br />
+,- " ( " 3" 2 " 4 " %12 567,58 65,, "<br />
/ 012 / 0 !<br />
* +,-" $<br />
/<br />
/ 012 / 0 !<br />
* / 012 / 0<br />
! / 0<br />
/ 012 / 0 !<br />
3 / 0 " #<br />
*<br />
'<br />
* !<br />
" .<br />
!<br />
" "<br />
!" * & "," $<br />
215
WeP40 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Spin-Polarized STM with Fe-Coated W Tips and Bulk Cr Tips<br />
E.P. Smakman, R. van Voornveld, J.G. Keizer, J.K. Garleff, P.M. Koenraad<br />
Our goal in this project is to use spin-polarized scanning tunneling microscopy (SP-STM) to study<br />
magnetic nano-structures in metals and semiconductors. As a pro<strong>of</strong> <strong>of</strong> concept, I present measurements<br />
on a metal surface which has been studied previously [1]: a W(110) crystal covered with 1.5 ML Fe. The<br />
crystal has step edges and plateaus <strong>of</strong> roughly 7 nm wide. A plateau is divided in a single layer wire (1<br />
ML) and a double layer wire <strong>of</strong> Fe (2 ML), because <strong>of</strong> the step flow growth method. By also coating the<br />
W tip in situ with several ML Fe, the tip is made spin sensitive. By means <strong>of</strong> a lock-in amplifier,<br />
differential conductivity data is obtained from the surface, which contains both electronic and magnetic<br />
information. The single and double layer wires <strong>of</strong> Fe on the surface show a different contrast in the data.<br />
Additionally, nearby single layer wires are shown to be antiferromagnetically ordered, which is a sign<br />
that our tip is sensitive for in-plane magnetization <strong>of</strong> the surface. With bulk Cr tips also spin-sensitive<br />
contrast is observed, showing double layers with an alternating magnetization. These spin-sensitive tips<br />
open up the possibility to study magnetic semiconductors on the atomic scale.<br />
216<br />
[1] O. Pietzsch, Phys. Rev. Lett. 84, 5212 (2000).
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors WeP41<br />
Cathodoluminescence measurements at liquid helium temperature<br />
<strong>of</strong> monocrystalline ZnO layers<br />
B.S. Witkowski 1 , Ł. Wachnicki 1 , E. Guziewicz 1 , M. Godlewski 1,2<br />
1 Institute <strong>of</strong> Physics, Polish Acad. <strong>of</strong> Sciences, Al. Lotników 32/46, 02-668 Warsaw, Poland<br />
2 Dept. <strong>of</strong> Mathematics and Natural Sciences College <strong>of</strong> Science<br />
Cardinal S. Wyszy ski University, Dewajtis 5, 01-815 Warsaw, Poland<br />
ZnO is a II-VI semiconductor which has a direct energy gap <strong>of</strong> 3,37eV at room temperature<br />
[1,2] and a high exciton binding energy (60meV). High transparency allows application <strong>of</strong><br />
ZnO as an electrode in solar cells [3], but also as an active part <strong>of</strong> transparent transistor, crossbar<br />
memories, etc. Due to these properties ZnO is an attractive material for photovoltaic,<br />
electronic and optoelectronic devices.<br />
In recent works we showed, that high quality ZnO layers can be obtained by the Atomic Layer<br />
Deposition at fairly low temperature <strong>of</strong> 300°C [4]. In the present work we discuss results <strong>of</strong> a<br />
Cathodoluminescence (CL) measurements <strong>of</strong> these films which were performed at<br />
temperature <strong>of</strong> 5K. CL is synchronized with Scanning Electron Microscopy (SEM), so we can<br />
image luminescence <strong>of</strong> the same area as in the SEM images. These measurements give<br />
information about uniformity <strong>of</strong> films and local potential fluctuations resulting in exciton<br />
localization. We searched for an inter-link between microstructure <strong>of</strong> the films and their<br />
optical properties. Role <strong>of</strong> localization effects will be demonstrated.<br />
The research was partially supported by the European Union within European Regional<br />
Development Fund, through grant Innovative Economy (POIG.01.01.02-00-008/08).<br />
References:<br />
[1] C. Klingshirn, Phys. Status Solidi, B 244 (9) (2007) 3027.<br />
[2] S.J. Pearton, D.P. Norton, K. Ip, Y.W. Heo, T. Steiner, Superlattices Microstruct. 34 (2003).<br />
[3] M. Godlewski, E. Guziewicz, G. Łuka, T. Krajewski, M. Łukasiewicz, Ł. Wachnicki,, A.<br />
Wachnicka, K. Kopalko, A. Sarem, B. Dalami, Thin Solid Films 518 (2009) 1145.<br />
[4] Ł. Wachnicki, T. Krajewski, G. Łuka, B. Witkowski, B. Kowalski, K. Kopalko, J.Z. Domagala, M.<br />
Guziewicz, M. Godlewski, E. Guziewicz, Thin Solid Films 518, 16, 4556-4559 (2010)<br />
217
WeP42 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Towards electrically controllable read-write devices based on<br />
ferromagnetic semiconductors<br />
T. Andrearczyk 1 , I. Krogulec 1,2 , T. Wosi ski 1 , T. Figielski 1 , A. M kosa 1 , Z. Tkaczyk 1 ,<br />
J. Wróbel 1 and J. Sadowski 1,3<br />
1 Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences, 02-668 Warsaw, Poland<br />
2 College <strong>of</strong> Science, Cardinal S. Wyszynski University, 01-815 Warsaw, Poland<br />
3 MAX-Lab, Lund University, 22100 Lund, Sweden<br />
The idea <strong>of</strong> writing and reading digital information in a single compact device, using<br />
electric current, is extremely attractive. A class <strong>of</strong> such devices can be based on the effect <strong>of</strong><br />
displacement <strong>of</strong> magnetic domain walls (DWs) caused by electric current in ferromagnetic<br />
semiconductors. The (Ga,Mn)As epitaxial layers, becoming ferromagnetic at low<br />
temperature, <strong>of</strong>fer a good model material for such devices. This is due to several merits,<br />
among which a relatively low critical current density for DW motion is one <strong>of</strong> the major<br />
importance. An essential ingredient <strong>of</strong> such devices is the magnetic anisotropy caused by<br />
patterning-induced strain relaxation, which introduces an additional degree <strong>of</strong> freedom that<br />
can be used in device operation. In our preliminary investigations we examined two types <strong>of</strong><br />
nanostructures fabricated from (Ga,Mn) As ferromagnetic layers. One <strong>of</strong> them has been the<br />
three-arm nanostructure [1], and the other – the cross-like nanostructure composed <strong>of</strong> two<br />
perpendicular nanostripes crossing in the middle <strong>of</strong> their lengths [2,3]. The latter structure is<br />
especially interesting as it allows, in principle, for full electrical control <strong>of</strong> the device. The<br />
underlying idea is that the magnetization in a nanostripe is forced to be directed along its long<br />
axis, while in the area <strong>of</strong> stripes intersection it is determined rather by the crystalline<br />
anisotropy <strong>of</strong> parent layer. Domain walls separating regions <strong>of</strong> different magnetization<br />
directions contribute an extra resistance to the total electrical resistance <strong>of</strong> a nanostripe in a<br />
rate depending on the spin misalignment. Manipulating the magnetization directions in the<br />
stripes by means <strong>of</strong> an external magnetic field, or electric current passing through the stripes,<br />
allows reaching either <strong>of</strong> two electrical states <strong>of</strong> each stripe that distinguish themselves by<br />
different resistances. In the present work we have studied those effects in more details using<br />
cross-like naostructures fabricated from Ga0.94Mn0.06As layers, grown by the low-temperature<br />
molecular-beam epitaxy method, and next patterned by the electron-beam lithography and<br />
chemical etching. We interpret the obtained results in terms <strong>of</strong> DWs rearrangement and also<br />
discuss the contribution <strong>of</strong> anisotropic magneto-resistance to the observed effects.<br />
This work has been partially supported by the Polish Ministry <strong>of</strong> Science and Higher<br />
Education under Grant No. N N202 129339.<br />
[1] T. Figielski, T. Wosi ski, A. Morawski, A. M kosa, J. Wróbel, and J. Sadowski, Appl.<br />
Phys. Lett. 90, 052108 (2007).<br />
[2] T. Andrearczyk, T. Wosi ski, T. Figielski, A. M kosa, J. Sadowski, Z. Tkaczyk, E.<br />
Łusakowska and J. Wróbel, Acta Phys. Pol. A 114, 1049 (2008).<br />
[3] T. Andrearczyk, T. Wosi ski, A. M kosa, T. Figielski, J. Wróbel, and J. Sadowski, Acta<br />
Phys. Pol. A 116, 901 (2009).<br />
218
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors WeP43<br />
Magnetic properties <strong>of</strong> Mn-ion implanted and plasma pulse treated Si<br />
J. B. Gosk 1, 2 , Z. Werner 3 , C. Pochrybniak 3 , M. Barlak 3 , J. Szczytko 1 and A.<br />
Twardowski 1<br />
1 Institute <strong>of</strong> Experimental Physics, Warsaw University, Ho a 69, 00-681<br />
Warsaw, Poland<br />
2 Faculty <strong>of</strong> Physics, Warsaw University <strong>of</strong> Technology, Koszykowa 75, 00-662<br />
Warsaw, Poland<br />
3 The Andrzej Soltan Institute for Nuclear Studies (IPJ)<br />
05-400 Swierk/Otwock, Poland<br />
The race for a room temperature ferromagnetic semiconductors for spintronic<br />
applications continues for about a decade. Although many different materials have been<br />
synthesized and studied so far, the highest Curie temperatures (TC) are is still below room<br />
temperature demanded for commercial applications. There is a hope that ion implantation<br />
methods, routinely used in manufacturing <strong>of</strong> integrated circuits, could overcame a problem <strong>of</strong><br />
low solubility limits <strong>of</strong> magnetic ions in magnetic semiconductors. In this context very<br />
promising seems to be the technique using plasma pulses allowing simultaneously both<br />
magnetic ion implantation and melting/regrowth <strong>of</strong> crystals.<br />
Here, we report on silicon implanted with Mn ions. Three types <strong>of</strong> samples were<br />
prepared basing on p-type Si(100) single-crystal wafers. The first type (denoted as Asamples)<br />
were implanted at room temperature with 2·10 +16 cm -2 <strong>of</strong> 190 keV Mn + ions. They<br />
were next treated with 4J/cm 2 <strong>of</strong> hydrogen plasma pulses <strong>of</strong> duration <strong>of</strong> about 1 s. The Mnion<br />
implanted Si but not treated by hydrogen plasma (denoted as B- samples) were used as<br />
reference samples. The third type <strong>of</strong> samples (denoted as C-type samples) were pulse Mn<br />
implanted in the Rod Plasma Injector (RPI) in the Deposition by Pulse Erosion (DPE) mode.<br />
The samples were RBSc analysed using 1.7 MeV He + beam at Rossendorf Van de<br />
Graaff accelerator. It appears that Mn implanted region recrystallizes as an epitaxial layer and<br />
that Mn atoms occupy interstitial positions, both after ion implantation and after pulse plasma<br />
treatment.<br />
Magnetic measurements <strong>of</strong> all the samples were performed using a SQUID<br />
magnetometer in the temperature range 2-300 K and magnetic fields (B) up to 7 T. In general<br />
magnetization (M) <strong>of</strong> all the samples consists <strong>of</strong> two components: a paramagnetic (PM) one<br />
and a ferromagnetic (FM) contribution. Similarly as in [1] FM contribution shows TC higher<br />
than room temperature. The relative abundance <strong>of</strong> FM contribution is the largest for Asamples<br />
(about one third <strong>of</strong> total magnetization at B=7 T), smaller for B-samples (about a<br />
quarter <strong>of</strong> total M at B=7 T) and the smallest for C-samples (about 4% <strong>of</strong> total M at B=7 T).<br />
PM contribution reveals typical Brillouin-type behavior: strong temperature dependence,<br />
pronounced tendency to saturation with magnetic field at low temperatures, and linear field<br />
dependence at high temperatures. For both A- and B-samples magnetization nearly saturates<br />
at B=7 T and T=2K, while for C-sample apparently 7 T is not enough to saturate M. All the<br />
samples show history dependence: there is a pronounced difference between Field Cooling<br />
and Zero Field Cooling protocols, different for A-, B- and C-samples. Such behavior suggests<br />
anisotropy effects present for manganese magnetic centers. These effects will be related to<br />
samples’ preparation procedures.<br />
[1] M. Bulduc at. all., Phys. Rev. B 71, 033302 (2005)<br />
219
WeP44 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Simulations <strong>of</strong> optical modes in InGaN based laser diodes operating at<br />
455 nm<br />
G. Muzioł 1,2 , M. Siekacz 2 , H. Turski 2 , C. Skierbiszewski 2<br />
1 Faculty <strong>of</strong> Applied Physics and Mathematics, Gdansk University <strong>of</strong> Technology,<br />
Gabriela Narutowicza 11/12, 80-233 Gdansk, Poland<br />
2 Institute <strong>of</strong> High Pressure Physics, Unipress, Sokolowska 29/37, 01-142 Warsaw,<br />
Poland<br />
Nitride based laser diodes (LDs) which cover the emission wavelengths from ultraviolet<br />
to the green region recently become a subject <strong>of</strong> intensive studies. The emission wavelength<br />
can be engineered by changing the indium composition in the quantum wells - one can shift<br />
the emission wavelength from ultra-violet up to green [1]. For a successful operation <strong>of</strong> LDs<br />
not only perfect structural quality <strong>of</strong> grown layers is crucial but also the reduction <strong>of</strong> the<br />
optical losses in the laser structure is <strong>of</strong> the great importance.<br />
In this work we study a new design for waveguide (WG) and cladding layers (CL) for<br />
nitride based true-blue LDs operating at 455 nm to reduce optical losses. We calculate the<br />
influence <strong>of</strong> composition and thickness <strong>of</strong> WG and CL on the confinement factor and internal<br />
losses <strong>of</strong> a LDs. A one-dimensional method developed by M. J. Bergmann and H. C. Casey Jr.<br />
[2] is used for numerical determination <strong>of</strong> the transverse electric field distribution. Parameters<br />
for calculations were taken from Ref [3]. We demonstrate how the thickness <strong>of</strong> upper cladding<br />
layer affects the internal losses. For too thin claddings the mode reaches the highly absorptive<br />
upper gold contact which causes unnecessary losses.<br />
By changing the composition <strong>of</strong> waveguide we can significantly modify the operating<br />
parameters <strong>of</strong> a LD. We show a comparison between two waveguide designs: a) 180 nm GaN<br />
waveguide, b) 180 nm GaN with internal 90 nm InGaN high refractive index waveguide. The<br />
second design not only gives a higher confinement factor but also lowers the internal losses<br />
caused by magnesium doped cladding and gold contact. In case <strong>of</strong> the waveguide without<br />
indium the mode leaks to the substrate which negatively affects the far-field pattern [4]. The<br />
design with an internal InGaN waveguide eliminates this phenomenon. In conclusion, we<br />
demonstrated that proper design <strong>of</strong> the waveguides and claddings reduces the optical losses in<br />
the contact layer and the substrate. We will compare the one dimensional model prediction<br />
with more general two-dimensional method developed by U. Schwarz et al. [5].<br />
Acknowledgements: This work was supported partially by the Polish Ministry <strong>of</strong><br />
Science and Higher Education Grant No IT 13426 and the European Union within European<br />
Regional Development Fund, through grant Innovative Economy (POIG.01.01.02-00-008/08).<br />
[1] J. Wu, W. Walukiewicz, K. M. Yu, J. W. Ager III, E. E. Haller, Hai Lu and William J.<br />
Schaff, Appl. Phys. Lett. 80, 4741 (2002).<br />
[2] M. J. Bergmann and H. C. Casey Jr., Journal <strong>of</strong> Applied Physics, 84(3):1196–1203,<br />
1998.<br />
[3] G. M. Laws, E. C. Larkins, I. Harrison, C. Molloy, and D. Somerford, J. Appl. Phys.,<br />
89(2):1108–1115, 2001.<br />
[4] L. A. Coldren and S. W. Corzine. Diode Lasers and Photonic Integrated Circuits.<br />
Willey Interscience, 1995.<br />
[5] U. T. Schwarz and B. Witzigmann, “Optical properties <strong>of</strong> edge-emitting lasers:<br />
measurement and simulation”, Nitride Semiconductor Devices, edited by J. Piprek, Wiley-<br />
VCH 2007.<br />
220
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors WeP45<br />
Monte-Carlo simulations and magnetic studies <strong>of</strong> ferromagnetic<br />
nanocomposites.<br />
Magdalena Woińska 1,2 , Karolina Madrak 2 , Jacek Szczytko 1 , Jacek Gosk 1,3 , Andrzej<br />
Majh<strong>of</strong>er 1 , Damian Pociecha 2 , Ewa Górecka 2 , Andrzej Twardowski 1<br />
1 Institute <strong>of</strong> Experimental Physics, Faculty <strong>of</strong> Physics, University <strong>of</strong> Warsaw,<br />
Hoża 69, 00-681 Warsaw, Poland<br />
2 Department <strong>of</strong> Chemistry, University <strong>of</strong> Warsaw, Pasteura 1, 02-093 Warsaw, Poland<br />
3 Faculty <strong>of</strong> Physics, Warsaw University <strong>of</strong> Technology,<br />
Koszykowa 75, 00-662 Warsaw, Poland<br />
It has been reported that thermal annealing <strong>of</strong> Ga1-xMnxAs layers leads to formation <strong>of</strong><br />
MnAs dots [1] and that in MOVPE-grown (Ga,Fe)N ferromagnetic FeNx nanocrystals aggregate<br />
by precipitations like GaxMny, MnxNy or FexNy [2]. Usually these precipitations form<br />
(nano)crystals <strong>of</strong> different size and orientation embedded in semiconductor matrix. Such<br />
nanocomposites are promising candidates for information storage and spin electronics<br />
applications [3]. The analysis <strong>of</strong> magnetic properties <strong>of</strong> such semiconductor-based<br />
nanocomposites is a complex task because <strong>of</strong> difficulties in the control over the size <strong>of</strong><br />
ferromagnetic nanoclusters and the distance between them. Usually these nanoprecipitations have<br />
a broad distribution <strong>of</strong> theirs diameters and one cannot change and control independently size and<br />
concentration <strong>of</strong> them. They consist <strong>of</strong> small and usually single domain (superparamagnetc) and<br />
large multiple domain (ferromagnetic) nanocrystals. Thus, in order to test theoretical models and<br />
numerical approaches, one may look for systems where at least crystals’ size distribution is<br />
narrow. Having a theoretical approach which describes well such model magnetism one can adapt<br />
it to the more complex semiconductor-based materials<br />
Ferromagnetic cobalt nanoparticles immersed in a solid organic medium can be a model<br />
system for such investigations. In this work we present a Monte-Carlo study <strong>of</strong> such structures.<br />
The zero field cooling – field cooling (ZFC/FC) sequences were measured as a function <strong>of</strong> the<br />
size (6 nm and 11 nm) and concentration <strong>of</strong> nanoparticles. The experimental results are compared<br />
with Monte-Carlo simulations <strong>of</strong> magnetism <strong>of</strong> such nanocomposites. The comparison between<br />
the magnetism <strong>of</strong>: isolated, interacting and aggregated nanoparticles will be discussed.<br />
[1] J. Sadowski, et al., Appl. Phys. Lett. 87, 263114 (2005)<br />
[2] A. Bonanni, et al,. Phys. Rev. B 75, 125210 (2007), Phys. Rev. Lett. 101, 135502 (2008).<br />
[3] T. Dietl et al., Phys. Rev B 63, 195205 (2001)<br />
221
WeP46 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Di-TEMPO amine for molecular spintronics – model <strong>of</strong> p-shell magnetism.<br />
Jacek Szczytko 1 , Jadwiga Szydlowska 2 , Nevill Gonzalez Szwacki 3 , Konrad<br />
Dziatkowski 1 , Paweł Giziński 1 , Andrzej Kaim 2 , Andrzej Twardowski 1<br />
1 Institute <strong>of</strong> Experimental Physics, Faculty <strong>of</strong> Physics, University <strong>of</strong> Warsaw,<br />
Hoża 69, 00-681 Warsaw, Poland<br />
2 Department <strong>of</strong> Chemistry, University <strong>of</strong> Warsaw,<br />
Al. Żwirki i Wigury 101, 02-089 Warsaw, Poland<br />
3 Institute <strong>of</strong> Theroretical Physics, Faculty <strong>of</strong> Physics, University <strong>of</strong> Warsaw,<br />
Hoża 69, 00-681 Warsaw, Poland<br />
Growing interest in spintronics and spin control on the atomic level arises from predicted<br />
superiority <strong>of</strong> the spin <strong>of</strong> the electron over the electron charge in terms <strong>of</strong> energy and speed<br />
efficiency. Advances in this field require a new approach to the concept <strong>of</strong> spin manipulation<br />
and new magnetic materials. Most <strong>of</strong> the solution are based on magnetic metals and<br />
semiconductors with localized magnetic moments on the d-shell. However one can induce<br />
magnetic properties also on the p-shell which is occupied by single electron forming a free<br />
radical or two p-shell electrons in triplet state. It is probable that p-shell magnetism is<br />
responsible for a high magnetic moment in Gd-focused ion-beam-implanted GaN [1].<br />
The bottom-up strategy for spintronics can only be realized by extensive studies <strong>of</strong> spin<br />
interactions in well defined clusters <strong>of</strong> atoms or molecules. In this communication we present<br />
the study <strong>of</strong> antiferromagnetic coupling between two free radicals localized in di-TEMPO<br />
amine structure.<br />
We present the results <strong>of</strong> electronic spin resonance (EPR), SQUID magnetometry, X-ray<br />
diffraction and theoretical investigations carried out using the NWChem code suite [2]. Our<br />
model compound – di-TEMPO possess the property <strong>of</strong> adjustable lattice structure, depending<br />
on the type <strong>of</strong> solvent in which molecular crystal was precipitated. Still, within the accuracy<br />
<strong>of</strong> measurements, modifications <strong>of</strong> the external structure <strong>of</strong> the compounds don’t seem to<br />
affect in a significant manner the magnetic properties <strong>of</strong> the di-TEMPO complex, which seem<br />
to depend solely on the internal structure the compounds. For the other hand theoretical<br />
prediction <strong>of</strong> exchange interaction gives two order <strong>of</strong> magnitude greater value <strong>of</strong> this<br />
interaction than observed by SQUID magnetometry, which suggest that p-p interaction is<br />
diminished by the interactions with neighboring atoms in a crystallized sample. The<br />
interaction between magnetic single occupied electronics orbitals (SOMO) in the crystal<br />
lattice will be discussed and compared with experimental results <strong>of</strong> spin resonance and spinspin<br />
interaction.<br />
[1] S. Dhar, T. Kammermeier, A. Ney, L. Pérez, K. H. Ploog, A. Melnikov, and A. D. Wieck, Ferromagnetism<br />
and colossal magnetic moment in Gd-focused ion-beam-implanted GaN Appl. Phys. Lett. 89, 062503 (2006);<br />
[2] E. J. Bylaska et al., NWChem, A Computational Chemistry Package for Parallel Computers, Version 5.1.1<br />
2008).<br />
222
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors WeP47<br />
Hopping excitons in GaInNAs alloys: Radiative versus non-radiative<br />
recombination at various temperatures<br />
M. Baranowski, M. Latkowska, R. Kudrawiec, and J. Misiewicz<br />
Institute <strong>of</strong> Physics, Wrocław University <strong>of</strong> Technology, Wybrze e Wyspia skiego 27,<br />
50-370 Wroclaw, Poland<br />
Incorporation <strong>of</strong> a few percents <strong>of</strong> nitrogen atoms into Ga(In)As host (i.e., forming dilute<br />
nitrides) strongly affects its band structure and optical properties, i.e., the energy gap is<br />
strongly reduced and the electron effective mass is increased. In addition, nitrogen atoms<br />
deteriorate optical properties <strong>of</strong> Ga(In)As host and some characteristic features appear for this<br />
material in photoluminescence (PL) spectra at low temperatures. Usually the spectra are very<br />
broad and asymmetric. The temperature dependence <strong>of</strong> PL peak energy exhibits a deviation<br />
from Varshni’s formula, i.e., so called S-shape behavior. These features are usually explained<br />
by the recombination <strong>of</strong> excitons localized on some potential fluctuations. In 1998<br />
Baranovskii at al. have proposed a model <strong>of</strong> hopping excitons in semiconductors [1] and<br />
recently this model has been applied to dilute nitrides [2]. However the main spectral motive<br />
observed in near-field PL and micro-PL spectra (i.e., the sharp PL lines) was not simulated so<br />
far by this model. Recently we have introduced some changes to this model and applied such<br />
a modified model to explain the origin <strong>of</strong> sharp lines observed in micro-PL spectra <strong>of</strong><br />
GaInNAs layers and their changes with the excitation power and temperature [3]. Instead <strong>of</strong><br />
two types <strong>of</strong> recombination centres (radiative and non-radiative centres) introduced by<br />
Baranovskii we have proposed one kind <strong>of</strong> localization centres with the radiative and nonradiative<br />
rates. Such a modification is justifiable due to our recent experimental observations<br />
for GaInNAs alloys [4] and allows us to explain the fast thermal quenching <strong>of</strong> localized<br />
emission from this alloy. Our simulations clearly show that the individual sharp PL lines<br />
observed at low temperatures appear for this material due to exciton hopping between<br />
localization centres. Taking into account saturation effects and exciton dissociation<br />
phenomenon it has been shown that the observed changes in power- and temperaturedependent<br />
-PL spectra can be excellently reproduced by the modified model. In this work we<br />
will focus on simulations <strong>of</strong> PL spectra obtained at various temperatures for different<br />
excitation powers for GaInNAs alloys with a different concentration <strong>of</strong> localizing centers<br />
which are related to deep donor(acceptor)-like states. These simulations clearly show that<br />
these states are responsible for weak PL intensity from GaInNAs alloys at room temperatures.<br />
The authors acknowledge support from the MNiSW (grant no. N202 258339).<br />
[1] S. D. Baranovskii, et al., Phys. Rev. B 58, 13081 (1998).<br />
[2] O. Rubel, et al., Phys. Rev. B 73, 233201 (2006).<br />
[3] M. Baranowski, et al., J. Physics: Cond. Matter, in press (<strong>2011</strong>).<br />
[4] M. Latkowska, et al., Appl. Phys. Lett. 98, 131903 (<strong>2011</strong>).<br />
223
WeP48 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
! " # $ % & $ ' ! # # $ ( ) $<br />
!" ! ! #<br />
$ % " " ! & ' !" ! ! #<br />
& & & $ + &<br />
! "#<br />
& ' () ()# (( *)#<br />
& $ ,<br />
& & &<br />
& & +-."# (( *)# & / 0 $ % & *<br />
( / 0 & &&<br />
+-." $ 1 & $<br />
2 / 0 3 & & $ 4<br />
& )))(#<br />
& & & & $ 2<br />
& +-." & (( *)# & &<br />
& $ &<br />
5 6 . & 5 . 78 .<br />
& & & # $<br />
& 5 858 # & &<br />
9:) ()) ; * # & & &<br />
$ 1 ,<br />
)$= ( * &<br />
& $ & &<br />
())@$ A<br />
?)))(@ & ?(<br />
& $ 1 & && "B " &<br />
8 & , % " .C%/$)($)($)* )) ))=D)=#<br />
& 0 0*)* )()(E3 . 7 + " $<br />
224<br />
$ %
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors WeP49<br />
!" # $ % & ' # ( ) * +<br />
, !& # ( - # ( . # , / 0 #<br />
!" "<br />
# $ % &" " '$ & ( ( ) *+ , -.<br />
/<br />
" / ' ++0 1 2 "<br />
* /3 !<br />
! ! ! "<br />
! ! # $<br />
! #<br />
%<br />
&<br />
' # #<br />
# " (<br />
) # "<br />
& "<br />
! " # # !<br />
' & * !<br />
" ! # ! #<br />
& !<br />
+ +, " " -<br />
! . # / " "<br />
#<br />
! # 0 # " .<br />
! +, 1<br />
" # ! - " 234 5<br />
6& ),&6,5 ,<br />
2 # 0 # 0 7 8&7 9 : &. .<br />
# . " 7 77 ; 7 7 " # .<br />
. +, A B = " " .<br />
! . # . " !<br />
( C 0 # ! 1 + 2<br />
+( (2'D 7 7& D ' # 0 # ! 2)> + E F F F 7<br />
9 9<br />
A B ( C ! 0 G H 0 ) @ I # 0 G * 5 J ' 0 G 2 # 0<br />
' ? K 0 6 "L 7
WeP50 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Admittance spectroscopy in GaNp-n junction<br />
P. Kamyczek 1 , E. Popko 1 , Z. Gumienny 1 , E. Zielony 1 , S.Grzanka 2,3 , R. Czernecki 2,3 ,<br />
T. Suski 3<br />
1 Institute <strong>of</strong> Physics, Wroclaw University <strong>of</strong> Technology, WybrzezeWyspianskiego 27,<br />
50-370 Wroclaw, Poland<br />
2 TopGaN Ltd., Sokolowska 29/37, 01-142 Warszawa, Poland<br />
3 Institute <strong>of</strong> High Pressure Physics ‘UNIPRESS’, Polish Academy <strong>of</strong> Sciences, Sokolowska<br />
29/37, 01-142 Warszawa, Poland<br />
P-n GaN junctions grown by metal-organic vapor-phase epitaxy technique (MOVPE)<br />
on Ga-polarity (0001) surface <strong>of</strong> GaN substrates were investigated using frequency-dependent<br />
capacitance–voltage measurements and admittance spectroscopy. Admittance spectroscopy<br />
(AS) is frequently used to characterize deep impurities in semiconductors. It indeed gives<br />
direct access to the emission–capture processes occurring between an impurity level and the<br />
conduction or the valence band. A defect state in the band gap leads to a decrease <strong>of</strong><br />
capacitance and an appearance <strong>of</strong> the peak in the curves <strong>of</strong> conductance/frequency versus<br />
frequency. Through the temperature dependence <strong>of</strong> the position <strong>of</strong> this peak, the energy<br />
activation level can in principle be deduced from corresponding Arrhenius plot. This<br />
technique is an alternative method to the deep level transient spectroscopy (DLTS)<br />
extensively exploited in characterization <strong>of</strong> defects in semiconductors. However if a deep<br />
level cannot follow the high frequency voltage modulation (DLTS is usually based on a 1<br />
MHz capacitance bridge meter) this technique fails.<br />
For the samples under study the capacitance versus bias (C-V) and AS measurements<br />
were performed at temperatures ranging from 300K down to 20K and for frequencies between<br />
10 2 and 3x10 6 Hz. The room-temperature C-V measurements yield apparent free carrier<br />
concentration NC-V in the order <strong>of</strong> 2.6 x10 18 cm -3 irrespective to the applied frequency. On the<br />
other hand, the capacitance vs frequency (C-f) curve exhibits strong frequency dependence,<br />
implying the presence <strong>of</strong> traps <strong>of</strong> concentration . Within the temperatures<br />
ranging from 300K down to 140K, a single relaxation maximum is visible on the<br />
conductance/frequency versus frequency plot. At lower temperatures another peak emerges.<br />
Additionally it was found that the external bias does not affect the plots. Thus it may be<br />
assumed that the peaks are related to point defects. Activation energies for the peaks obtained<br />
from corresponding Arrhenius plot are equal to 86±5 meV and 5±0.5meV respectively for the<br />
high and low temperature peak. The high temperature peak is presumably related to Mg deep<br />
acceptor state whereas the low-temperature peak to carrier freeze-out. The energy <strong>of</strong> Mg<br />
related state is lower than found elsewhere [1,2] however this discrepancy may be linked with<br />
the Poole-Frenkel effect [3]. Presence <strong>of</strong> internal field related to p-n junction significantly<br />
lowers trap energy obtained from AS in the case <strong>of</strong> high doping level, like in the case <strong>of</strong> the<br />
studied samples. Obtained results are complementary to the DLTS measurements run by us on<br />
the same sample [4] yielding additional source <strong>of</strong> information on defects in the studied p-n<br />
GaN junctions.<br />
[1] E. Monroy et al., Appl. Phys. Lett.74, 1171 (1999).<br />
[2] J. W. Huang et al., Appl. Phys. Lett.68, 2392 (1996).<br />
[3] D. J. Kim, J. Appl.Phys. 88, 1929 (2000).<br />
[4] E. Płaczek-Popko et al., Physica B: Physics <strong>of</strong> Condensed Matter, 404, 4889 (2009).<br />
226
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors WeP51<br />
Role <strong>of</strong> hydrogen in the ammonia based growth <strong>of</strong> GaN and<br />
InGaN - ab initio study<br />
Pawel Kempisty 1 , Pawel Strak 1 , and Stanislaw Krukowski 1,2<br />
1 Institute <strong>of</strong> High Pressure Physics, Polish Academy <strong>of</strong> Sciences, Sokolowska 29/37,<br />
01-142 Warsaw, Poland<br />
2 Interdisciplinary Centre for Materials Modeling, Warsaw University, Pawinskiego 5a,<br />
02-106 Warsaw, Poland<br />
Growth <strong>of</strong> GaN and GaInN nitrides by ammonia based method, such as hydride vapor<br />
phase epitaxy (HVPE) and metal organic vapor phase epitaxy (MOVPE) relies on<br />
the ammonia decomposition providing active nitrogen and also hydrogen at the crystal<br />
surface. Using ab intio density functional theory simulations, a basic scenario <strong>of</strong> molecular<br />
processes was derived. It is shown that adsorption <strong>of</strong> molecular hydrogen on bare<br />
GaN(0001) surface leads to its dissociation and location <strong>of</strong> H atoms in the sites, above<br />
the Ga surface atoms. The process strongly depends on the coverage, being strongly<br />
exothermic for coverage lower and endothermic for higher than 0.75 ML. It is shown that<br />
adsorption <strong>of</strong> ammonia on bare or hydrogen covered surface, is barrierless and occurs<br />
in the molecular form leading to mixed NH2 and NH3 coverage, as some <strong>of</strong> the excess<br />
hydrogen is desorbed in the form <strong>of</strong> H2 molecules. This coverage can be shifted towards<br />
ammonia by additional flux <strong>of</strong> molecular hydrogen. It is calculated that molecular hydrogen<br />
overcomes adsorption barrier <strong>of</strong> about 1 eV, dissociating at the surface and converting<br />
NH2 radicals into NH3 admolecules. Thus the ammonia coverage may be dominant in the<br />
hydrogen rich ambient. The adsorption <strong>of</strong> metal adatoms, Ga and In, strongly depends<br />
on the surface coverage type, being stable for the coverage dominated by NH2 radicals<br />
and unstable for coverage dominated by NH3 admolecules.<br />
The research was supported by the European Union within European Regional Development<br />
Fund, through grant Innovative Economy (POIG.01.01.02-00-008/08).<br />
227
WeP52 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Effect <strong>of</strong> the built-in strain on the in-plane optical anisotropy <strong>of</strong><br />
m-plane GaN/AlGaN quantum wells<br />
S.P. ̷Lepkowski 1 , W. Bardyszewski 2 , and H. Teisseyre 3<br />
1 Institute <strong>of</strong> High Pressure Physics, “Unipress”, Polish Academy <strong>of</strong> Sciences,<br />
ul. Soko̷lowska 29/37, 01-142 Warszawa, Poland<br />
2 Institute <strong>of</strong> Theoretical Physics, University <strong>of</strong> Warsaw, ul. Hoza 69, 00-681 Warszawa,<br />
Poland<br />
3 Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences, Al. Lotników 32/46,<br />
02-668 Warszawa, Poland<br />
Group III-nitrides heterostructures grown along nonpolar directions <strong>of</strong> the wurtzite<br />
structure (corresponding to a-plane or m-plane) are attractive for applications and basic<br />
research due to the lack <strong>of</strong> built-in electric fields and the presence <strong>of</strong> large in-plane optical<br />
anisotropy [1]. The built-in strain caused by the lattice mismatch between the quantum<br />
well and the substrate may further enhance the anisotropy <strong>of</strong> the emitted light polarization.<br />
This effect may be exploited as a tool for studying the light - matter coupling in<br />
semiconductor lasers and microcavities.<br />
In the present study we investigate theoretically the influence <strong>of</strong> the built-in strain<br />
on the electronic structure and optical transition oscillator strengths for the m-plane<br />
GaN/AlGaN quantum wells grown on AlGaN substrates with different aluminum contents.<br />
The anisotropic built-in strain is calculated by applying the linear theory <strong>of</strong> elasticity<br />
and assuming that the structure is pseudomorphic. Energy levels and electronic<br />
wave functions are obtained by using the finite element method to diagonalize the k · p<br />
Rashba - Sheka - Pikus Hamiltonian for the valence band. The conduction band levels<br />
were obtained in the parabolic approximation. The parameters for AlxGa1−xN are estimated<br />
using the linear interpolation between binaries except for the energy gap and<br />
the spontaneous polarization for which bowing is taken into account as in Ref. 2. The<br />
calculations were performed for a 5 nm GaN/Al0.2Ga0.8N quantum well grown on the<br />
AlxGa1−xN substrate with x less than 0.5.<br />
Inthecase<strong>of</strong>unstrainedquantumwells, i.e. forx = 0, thelowestenergytransition(c1v1)<br />
corresponds to the valence band level with the Γ9 symmetry and the next transition<br />
(c1-v2) is associated with Γ7 symmetry. Therefore, the oscillator strength <strong>of</strong> the c1-v1<br />
transition is large for the in-plane polarization perpendicular to the c axis and much<br />
smaller for the polarization parallel to the c-axis. On the other hand, the next transition<br />
c1-v2 is dominant for the polarization parallel to the c axis and almost negligible for the<br />
polarization perpendicular to the c axis.<br />
With increasing Al content x in the substrate all transition energies increase and so<br />
does the energy separation between the c1-v1 and the c1-v2 transitions. At the same<br />
time the oscillator strength for the c1-v1 transition decreases for the in-plane polarization<br />
perpendicular to the c axis and increases for the polarization parallel to the c-axis. The<br />
opposite occurs for the c1-v2 transition. As the result, by increasing the concentration <strong>of</strong><br />
Al in the substrate from x = 0 to x = 0.5 one can change the polarization <strong>of</strong> the emitted<br />
light with respect to the c-axis by 90 degrees.<br />
[1] P. Misra, O. Brandt, H. T. Grahn, H. Teisseyre, M. Siekacz, C. Skierbiszewski, and<br />
B. ̷Lucznik, Appl. Phys. Lett. 91, 141903 (2007).<br />
[2] S. P. ̷Lepkowski, J. A. Majewski, and G. Jurczak, Phys. Rev. B 72, 245201 (2005).<br />
228
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors WeP53<br />
Magnetooptical properties <strong>of</strong> (Ga,Fe)N layers<br />
J. Papierska 1 , J.-G. Rousset 1 , A. Golnik 1 , W. Pacuski 1 , M. Nawrocki 1 , J. A. Gaj 1 ,<br />
J. Suffczy ski 1 , I. Kowalik 2 , W. Stefanowicz 2 , M. Sawicki 2 , T. Dietl 1,2 , A. Navarro-Quezada 3 ,<br />
B. Faina 3 , Tian Li 3 , A. Bonanni 3<br />
1 Faculty <strong>of</strong> Physics, University <strong>of</strong> Warsaw, Warsaw, Poland<br />
2 Institute <strong>of</strong> Physics Polish Academy <strong>of</strong> Sciences, Warsaw, Poland<br />
3 J. Kepler University <strong>of</strong> Linz, Austria<br />
Numerous works on diluted magnetic semiconductors carried out over the last decade<br />
have aimed in examining intricate interplay <strong>of</strong> ferromagnetic and semiconducting properties<br />
[1]. Wide band gap semiconductors such as GaN and ZnO doped with magnetic ions are<br />
particularly interesting in view <strong>of</strong> their wide ranging applications. Here we present results <strong>of</strong><br />
magnetic circular dichroism (MCD) and Magneto-optical Kerr effect (MOKE) measurements<br />
in the excitonic spectral region performed on (Ga,Fe)N layers. In contrast to previous<br />
magnetooptical studies [2], we focus here on possible magnetoptical signatures <strong>of</strong> Fe-rich<br />
nanocrystals that appear in (Ga,Fe)N films obtained under suitable growth conditions [3].<br />
We compare magnetoptical properties <strong>of</strong> two layers with similar concentrations <strong>of</strong><br />
paramagnetic Fe ions (3×10 19 cm -3 and 2×10 19 cm -3 , respectively), one containing Fe-rich<br />
nanocrystals, the other not, according to extensive SQUID, XRD, and HRTEM<br />
characterization. As found in SQUID measurements, the sample with Fe-rich nanocrystals<br />
reveals a ferromagnetic response persisting up to above room temperature for two orientations<br />
<strong>of</strong> the magnetic field in respect to the c-axis. Reflectivity and MOKE measurements are<br />
performed in the Faraday configuration with the c-axis parallel to the incident beam in the<br />
magnetic field up to 7 T at 2 K. The sample is illuminated by a Xe lamp.<br />
We find that the field dependent magnitudes <strong>of</strong> MCD and Kerr rotation angle in the<br />
excitonic region are well descried by the paramagnetic Brillouin function in both samples.<br />
This means that ferromagnetic features, clearly seen in SQUID magnetometry, are not<br />
contributing to the magnetooptical response. We conclude that either character <strong>of</strong> nanocrystal<br />
magnetoptical properties or the nanocrystal distribution and density are not optimized to result<br />
in sizable magnetoptical phenomena. Instead, the presence on Fe-rich nanocrystals reduces<br />
the magnitude <strong>of</strong> the paramagnetic-like magnetooptical signals. In line with this observation,<br />
we find that a large magnetooptical response can be recovered by Ar sputtering <strong>of</strong> the sample<br />
surface, the finding consistent with the location <strong>of</strong> nanocrystals near the film surface [3].<br />
[1] T. Dietl et al., Science 287, 1019 (2000); Nature Mater. 9, 965 (2010).<br />
[2] W. Pacuski et al., Phys. Rev. Lett. 100 (2008) 037204<br />
[3] A. Bonanni et al., Phys. Rev. Lett. 101, 135502 (2008); A. Navarro-Quezada et al., Phys.<br />
Rev. B 81, 205206 (2010).<br />
The work was supported by FunDMS Advanced Grant <strong>of</strong> ERC within the Ideas 7th FP <strong>of</strong> EC,<br />
NCBiR project LIDER, and InTechFun (Grant No. POIG.01.03.01-00-159/08).<br />
229
WeP54 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Gas Phase Reactions during GaN Growth by MOVPE Method – Ab initio<br />
Study<br />
Maria Ptasinska 1 , Jacek Piechota 1 , Jakub Sołtys 1 , and Stanisław Krukowski 1,2<br />
1 Interdisciplinary Centre for Materials Modeling, University <strong>of</strong> Warsaw, Pawi skiego 5a, 02-<br />
106 Warsaw, Poland<br />
2 Institute <strong>of</strong> High Pressure Physics, Polish Academy <strong>of</strong> Sciences, Sokołowska 29/37, 01-142<br />
Warsaw, Poland<br />
Ab initio density functional theory (DFT) simulations were used to determine basic reaction<br />
during growth <strong>of</strong> gallium nitride by Metal Organic Vapor Phase Epitaxy (MOVPE) method,<br />
using trimethylgallium (TMG - Ga(CH3)3) and ammonia (NH3) as active sources <strong>of</strong> gallium<br />
and nitrogen, respectively. DFT based commercial codes were used: for the simulations <strong>of</strong><br />
molecular reaction in the vapor – Dmol3 [1,2] and for reactions at the solid surfaces -VASP<br />
[3,4]. The decomposition <strong>of</strong> the TMG and the liberation <strong>of</strong> methyl groups are necessary steps<br />
for incorporation <strong>of</strong> gallium into GaN. The two possible scenarios <strong>of</strong> these processes were<br />
taken into consideration, either assuming reaction with ammonia in the vapor, or TMG<br />
adsorption and consecutive reaction. Calculations showed that the creation <strong>of</strong> GaN:NH3<br />
compound is energetically possible in the gas phase and energy needed to create that<br />
compound was calculated. Also the behavior <strong>of</strong> the TMG in hydrogen environment as well as<br />
energy needed for direct release the CH3 radials from TMG molecule was obtained.<br />
Simulation <strong>of</strong> the GaN surface, either covered by hydrogen or by ammonia admolecules or by<br />
NH2 radicals, indicate that TMG shows remarkable durability at GaN(0001) surface, which is<br />
enhanced by the change <strong>of</strong> electrostatic charge at the surface. Therefore, the decomposition <strong>of</strong><br />
TMG at the surface is difficult, which suggest that dominant role is played by the vapor phase<br />
reactions, either by thermal decomposition <strong>of</strong> TMG or by reaction with ammonia leading to<br />
creation <strong>of</strong> the complex molecules which could enhance liberation <strong>of</strong> methyl groups and<br />
incorporation <strong>of</strong> gallim into GaN crystal.<br />
The research was supported by the European Union within European Regional Development<br />
Fund, through grant Innovative Economy (POIG.01.01.02-00-008/08 and POIG.01.03.01-14-<br />
155/09).<br />
[1] B. Delley, J. Chem. Phys. 92, 508 (1990).<br />
[2] B. Delley, . J. Chem. Phys. 113, 7756 (2000).<br />
[3] G. Kresse, and J. Hafner, Phys. Rev. B 47, 558 (1993); ibid. 49, 14 251 (1994).<br />
[4] G. Kresse, and J. Furthmüller, Comput. Mat. Sci. 6, 15(1996).<br />
[5] G. Kresse, and J. Furthmüller, Phys. Rev. B 54, 11169 (1996).<br />
230
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors WeP55<br />
#<br />
! "<br />
!"#"$<br />
& ' (<br />
%<br />
(<br />
"!- %<br />
) * % + , ! #<br />
. & ( ' & ( ) * %<br />
/ ! #! %<br />
! "<br />
#<br />
$ % &<br />
' ( )<br />
*!(+" ,<br />
- . /<br />
) ) ( 0<br />
) %& / & $) - ./ $<br />
1 +<br />
2 / / (<br />
" 3 " %<br />
0 / ' " 4+!' 5 5 5 55 556756 +!' 5 5 5 8<br />
9975:;<br />
- . , ( /<br />
< / :68<br />
- . , * & $)/ = * > / * + > / 000 1 2 0 2 . *2 $/<br />
4 :: ;<br />
231
WeP56 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Density Functional Theory (DFT) simulations <strong>of</strong> the physical properties <strong>of</strong><br />
AlN/GaN multiple quantum wells (MQWs)<br />
Pawel Strak 1 , Pawel Kempisty 1 , Konrad Sakowski 1 , and Stanislaw Krukowski 1,2 *<br />
1 Institute <strong>of</strong> High Pressure Physics, Polish Academy <strong>of</strong> Sciences, Sokołowska 29/37, 01-142<br />
Warsaw, Poland<br />
2 Interdisciplinary Centre for Materials Modeling, Warsaw University, Pawi skiego 5a, 02-<br />
106 Warsaw, Poland<br />
Ab initio DFT simulations <strong>of</strong> AlN/GaN multiquantum wells (MQW) were used to obtain<br />
physical properties <strong>of</strong> the system. By appropriate averaging procedure an electric potential<br />
pr<strong>of</strong>ile was obtained showing the existence <strong>of</strong> strong electric field in the wells and barriers<br />
and additionally revealing a new phenomenon <strong>of</strong> electric potential jumps at the interfaces.<br />
The fields and the potential jumps were used to obtain density <strong>of</strong> the polarization charges and<br />
the magnitude <strong>of</strong> the dipole layer at AlN/GaN heterointerfaces, respectively. It was shown<br />
that polarization dipoles are confined within single double atomic layers, proving that they are<br />
<strong>of</strong> different nature from the dipole layers emerging at the semiconductor surfaces or within pn<br />
junctions. Band structure analysis is used to obtain the change <strong>of</strong> the quantum states energy<br />
in electric field confirming existence <strong>of</strong> Quantum Confined Stark Effect (QCSE). The<br />
mechanical stress influence was accounted for by systematic changes <strong>of</strong> the lattice constant<br />
from AlN to GaN, assuming that the system is conformal with the substrate. It is shown that<br />
the stress affects strongly not only polarization fields but also the magnitude <strong>of</strong> dipole layers<br />
at AlN/GaN heterointerfaces.<br />
The research was supported by the European Union within European Regional Development<br />
Fund, through grant Innovative Economy (POIG.01.01.02-00-008/08 and POIG.01.03.01-14-<br />
155/09).<br />
232
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors WeP57<br />
Plasma-assisted MBE growth and characterization <strong>of</strong> GaN nanocolumns on<br />
Si (111) Substrates<br />
Z.R. Zytkiewicz, P. Dluzewski, J. Borysiuk, M. Sobanska, K. Klosek,<br />
B. Witkowski, P. Nowakowski, and J. Dabrowski<br />
Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences, Al. Lotnikow 32/46, 02 668 Warsaw, Poland<br />
There is current interest in the development <strong>of</strong> electronic devices based on nanowires or<br />
nanocolumns made <strong>of</strong> wide bandgap semiconductors. Due to possibility <strong>of</strong> charge localization<br />
by quantum confinement and low density <strong>of</strong> structural defects use <strong>of</strong> nanocolumns is<br />
promising for optoelectronic applications. Moreover, a high surface to volume ratio makes<br />
crystalline nanostructures a topic <strong>of</strong> intense research in sensor applications.<br />
In this work we report on growth <strong>of</strong> GaN nanocolumns (NCs) on (111) silicon substrates<br />
by plasma assisted MBE (PAMBE). Riber Compact 21 system equipped with elemental<br />
source <strong>of</strong> Ga and an Addon RF plasma source <strong>of</strong> active nitrogen was used for the growth. The<br />
growth started by deoxidation <strong>of</strong> Si (111) substrate followed by its exposure to a nitrogen flux<br />
at temperature <strong>of</strong> 790 o C. Then, self-organized growth <strong>of</strong> nanocolumns started at ~750 o C<br />
under highly nitrogen-rich conditions. No catalyst was used to induce nucleation <strong>of</strong> NCs.<br />
Fig. 1 shows SEM birds-view image <strong>of</strong> the sample grown for 90 minutes. An ensemble<br />
<strong>of</strong> GaN NCs is clearly visible. Typically, they are ~350 nm long with diameter <strong>of</strong> 20 – 30 nm.<br />
It is noteworthy that the nanocolumns are homogenously distributed and well oriented with<br />
the c-axis being perpendicular to the substrate. Fig. 2 shows cross-sectional TEM image <strong>of</strong> the<br />
sample while Fig. 3 presents high-resolution TEM image <strong>of</strong> the interface between Si(111) and<br />
the GaN nanocolumn (magnification <strong>of</strong> the rectangular area in Fig. 2). Presence <strong>of</strong> ~2 nm<br />
thick film <strong>of</strong> amorphous SixNy between GaN and the substrate is clearly visible. Despite that<br />
GaN is perfectly epitaxially aligned to the Si(111) substrate. Moreover, no wetting layer is<br />
seen that indicates that NCs are grown spontaneously by Volmer-Weber mechanism [1]. Low<br />
temperature photoluminescence and cathodoluminescence were used to study optical<br />
properties <strong>of</strong> GaN NCs.<br />
GaN NCs<br />
Si (111)<br />
Fig.1: SEM image <strong>of</strong> GaN NCs sample<br />
taken under a declination angle <strong>of</strong> 20 o .<br />
[1] J. Ristic et al. J. Crystal Growth 310 (2008) 4035.<br />
Si<br />
GaN<br />
20 nm<br />
This work was partly supported by the European Union within European Regional<br />
Development Fund, through grant Innovative Economy (POIG.01.01.02-00-008/08<br />
NanoBiom).<br />
GaN<br />
Fig. 2: TEM image <strong>of</strong> GaN<br />
nanocolumns.<br />
233<br />
Fig. 3: TEM image <strong>of</strong> the interface<br />
between Si(111) and GaN nanocolumn.
WeP58 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Crossing Size Limits <strong>of</strong> Bulk III-V Crystals Feasible by Liquid Phase<br />
Electroepitaxy<br />
Zbigniew R. Zytkiewicz 1 , Pawel Strak 2 , and Stanislaw Krukowski 2<br />
1 Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Sciences, Al. Lotnikow 32/46, 02 668 Warsaw, Poland<br />
2 Institute <strong>of</strong> High Pressure Physics, Polish Academy <strong>of</strong> Sciences, ul. Sokolowska 29/37,<br />
01 142 Warsaw, Poland<br />
Liquid phase electroepitaxy (LPEE) is a crystal growth method in which the DC electric<br />
current passing through the solution-substrate interface is the driving force for crystallization<br />
while temperature <strong>of</strong> the system is kept constant. LPEE has been found to be very effective<br />
for the growth <strong>of</strong> bulk crystals <strong>of</strong> multicomponent semiconductors. In particular, the LPEE<br />
grown crystals show compositional uniformity that is not attainable by other solution or melt<br />
growth methods (see [1] for a review). Unfortunately, as shown experimentally [2] and<br />
theoretically [3, 4] thickness <strong>of</strong> crystals feasible by LPEE is limited to ~3-4 mm. This is due<br />
to Joule heating in the body <strong>of</strong> growing crystal, which finally leads to complete termination <strong>of</strong><br />
the growth. In this work we present a new design for the growth <strong>of</strong> bulk III-V crystals by<br />
electroepitaxy which avoids severe drawbacks <strong>of</strong> standard LPEE. In the technique, that we<br />
name contactless LPEE (CLPEE), the electric current still being the main mechanism <strong>of</strong><br />
solute transport, does not flow through the solution/seed interface. By proper arrangement <strong>of</strong><br />
the current flow, the Joule heating <strong>of</strong> the growing crystal is eliminated. Thus the main obstacle<br />
on the road to long duration processes necessary to obtain bulk multicomponent substrate<br />
crystals, paving the route to new method <strong>of</strong> the growth <strong>of</strong> crystals from solution is proposed.<br />
CLPEE method <strong>of</strong> growth from the liquid solution is modeled using finite volume (FV)<br />
commercial Fluent code. This allowed us to study how distributions <strong>of</strong> temperature, electric<br />
potential, solute concentration and growth rate depend on geometry <strong>of</strong> the growth cell, time<br />
and electric current density. We consider the case <strong>of</strong> CLPEE <strong>of</strong> GaAs as an example <strong>of</strong> the<br />
growth <strong>of</strong> III-V semiconductors. This is because for the Ga-As/GaAs system values <strong>of</strong> many<br />
material parameters required by the model are easily available. It should be noted, however,<br />
that the growth technique proposed can be used for other crystals too and extension <strong>of</strong> the<br />
model for other materials is straightforward if values <strong>of</strong> these parameters are determined.<br />
[1] T. Bryskiewicz, Prog. Crystal Growth and Characterization 12 (1986) 29.<br />
[2] Z.R. Zytkiewicz, J. Crystal Growth 146 (1995) 283.<br />
[3] Z. R. Zytkiewicz, J. Cryst. Growth 172 (1997) 259.<br />
[4] P. Strak, Z.R. Zytkiewicz, K. Sakowski, and S. Krukowski, Cryst. Res. Technol. 45 (2010)<br />
1290.<br />
This work was partially supported by the Polish Ministry <strong>of</strong> Science and Higher Education<br />
(grant N N507 280636) and by the European Union within European Regional Development<br />
Fund, through grant Innovative Economy (POIG.01.01.02-00-008/08 NanoBiom).<br />
234
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors ThI1<br />
Quantum Cascade Lasers<br />
Maciej Bugajski 1,2 , Kamil Kosiel 1 , Anna Szerling 1 , Piotr Borowik 2 ,<br />
Leszek Adamowicz 2 , Grzegorz Hałda 3 , Andrzej Kolek 3<br />
1 Institute <strong>of</strong> Electron Technology, Al. Lotników32/4, 02 668 Warsaw, Poland<br />
2 Faculty <strong>of</strong> Physics, Warsaw University <strong>of</strong> Technology, Koszykowa 75,<br />
00 662 Warsaw, Poland<br />
3 Department <strong>of</strong> Electronics, Rzeszów University <strong>of</strong> Technology, W. Pola 2,<br />
35 959 Rzeszów, Poland<br />
The quantum cascade lasers (QCLs) are unipolar devices based on tunneling and<br />
intersubband transitions, in which the electronic states, wavefunctions and lifetimes <strong>of</strong><br />
relevant states are engineered through the quantum mechanical confinement imposed by a<br />
complex multilayer structure. The second main feature <strong>of</strong> this type <strong>of</strong> lasers is the cascading<br />
scheme <strong>of</strong> carriers route through the laser active region. For QCLs to work, the extremely<br />
precise tailoring <strong>of</strong> energy levels <strong>of</strong> quantum states, optical dipole matrix elements, tunneling<br />
times and scattering rates <strong>of</strong> carriers is required.<br />
Thus the principle <strong>of</strong> operation <strong>of</strong> QCL structures places stringent requirements on the<br />
individual layer thickness and composition as well as the overall periodicity <strong>of</strong> the whole<br />
structure. The laser operation is possible only when the designed structure is strictly realized,<br />
with the extreme technological precision concerning geometrical and doping features.<br />
Another crucial problem <strong>of</strong> QCLs’ operation are the heating effects, which are distinctly<br />
larger than in the state-<strong>of</strong>-the-art bipolar lasers. This is connected with the processes <strong>of</strong><br />
depopulation <strong>of</strong> the lower laser level and carrier thermalization in the injector, by scattering<br />
with the optical or acoustic phonons. The heating results in the higher threshold and operation<br />
currents <strong>of</strong> the lasers, and all this in turn results in the necessity <strong>of</strong> the effective heat<br />
extraction. The heat dissipation in QCLs is strongly hampered because <strong>of</strong> the nature <strong>of</strong> their<br />
active regions containing many interfaces, and layers with thickness similar to the mean free<br />
path <strong>of</strong> phonons.<br />
In this lecture we will present the development <strong>of</strong> mid-infrared GaAs/AlGaAs QCLs<br />
technology and discuss basic characteristics <strong>of</strong> lasers fabricated at the Department <strong>of</strong><br />
Photonics at the Institute <strong>of</strong> Electron Technology [1-4]. We will also show that reliable<br />
simulation methods which can deal with the complicated physical phenomena involved in the<br />
quantum cascade lasers operation are necessary tools to predict the behavior <strong>of</strong> new structures<br />
and optimize their performance. We will briefly discuss Monte Carlo approach to modelling<br />
electron transport in QCLs [5,6] and nonequlibrium Green’s function formalism [7].<br />
[1] K. Kosiel, M. Bugajski, A. Szerling, Photonics Letters <strong>of</strong> Poland, 1, 16 (2009)<br />
[2] M. Bugajski, K. Kosiel, A. Szerling, Bulletin <strong>of</strong> the Polish Academy <strong>of</strong> Sciences;<br />
Technical Sciences 58, 471 (2010)<br />
[3] K. Pier ci ski, D. Pier ci ska, K. Kosiel, A. Szerling, M. Bugajski, Journal <strong>of</strong> Electronic<br />
Materials, 39, 630 (2010)<br />
[4] K. Kosiel, M. Bugajski, A. Szerling, Terahertz and Mid Infrared Radiation, NATO Science<br />
for Peace and Security Series B: Physics and Biophysics, Chapter 13, Springer (<strong>2011</strong>)<br />
[5] P. Borowik, J.L. Thobel, M. Bugajski, L. Adamowicz, Acta Physica Polonica, 116, 49<br />
(2009)<br />
[6] P. Borowik, J.L. Thobel, Leszek Adamowicz, Journal <strong>of</strong> Applied Physics, 108, 073106<br />
(2010)<br />
[7] G.Hałda , A. Kolek, M. Bugajski, Optoelectronics Review, in print (<strong>2011</strong>)<br />
235
ThI2 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
New Concepts and Materials for Solar power Conversion Devices<br />
Wladek Walukiewicz*<br />
1 Materials Sciences Division Lawrence Berkeley National Laboratory**, 1 Cyclotron Road,<br />
Berkeley, CA 94720, USA<br />
In the presentation I will introduce various new concepts for high efficiency conversion <strong>of</strong><br />
solar energy. The core presentation will focus on two research areas: group III-nitrides for<br />
high efficiency multijunction solar cells and highly mismatched alloys for multiband solar and<br />
photoelectrochemical cells.<br />
The discovery <strong>of</strong> the low band gap <strong>of</strong> InN greatly expanded the range <strong>of</strong> the direct gaps <strong>of</strong><br />
group III-nitride alloys from 0.64 in InN to 6.1 eV in AlN. Therefore In1-xGaxN and In1-yAlyN<br />
alloys are promising materials for high efficiency solar cells, as their band gaps are<br />
continuously tunable across the solar spectrum. The potential <strong>of</strong> In1-xGaxN as a solar cell<br />
material has been well established and there has been a significant worldwide effort aimed at<br />
practical application <strong>of</strong> this material system for photovoltaic devices. The large electron<br />
affinity (5.8 eV) <strong>of</strong> InN <strong>of</strong>fers a unique opportunity <strong>of</strong> matching the conduction band edge <strong>of</strong><br />
group III-In-nitride alloys to the valence band <strong>of</strong> standard semiconductors such as Si and Ge.<br />
Thus n-type In1-xGaxN with x=0.55 or In1-yAlyN with y=0.3 form “ideal” low resistance<br />
junctions with p-type Si. Calculations show that two junction InGaN/Si or AlInN/Si tandem<br />
cells could have practical power conversion efficiencies exceeding 35%. The low resistance<br />
contact between n-InGaN and p-Si has been demonstrated for InGaN films grown on Si with<br />
GaN as well as Al thin buffer layers. Latest results on practical realization <strong>of</strong> InGaN/Si<br />
tandem cells will be presented and remaining challenges will be discussed.<br />
The second part <strong>of</strong> the presentation will be devoted to recent progress in synthesis <strong>of</strong><br />
highly mismatched alloys (HMAs). Such alloys exhibit unusual optical and electrical<br />
properties and with a proper choice <strong>of</strong> component materials allow for an independent<br />
engineering <strong>of</strong> band gaps and band <strong>of</strong>fsets. We have synthesized group II-VI dilute oxides<br />
and group III-V dilute nitride HMAs. The alloys have a unique band structure with a narrow<br />
intermediate band in the band gap <strong>of</strong> the host material. The band serves as a “stepping stone”<br />
allowing pairs <strong>of</strong> sub-bandgap photons to contribute to the solar light induced photo-current,<br />
leading to better utilization <strong>of</strong> the full solar spectrum. Ongoing efforts to produce simple,<br />
single junction high efficiency cells based on these alloys will be presented. I will also<br />
present most recent results on successful synthesis <strong>of</strong> GaNAs alloys in the whole composition<br />
range and discuss potential use <strong>of</strong> these materials in photoelectrochemical cells for solar water<br />
dissociation.<br />
*In collaboration with Solar Energy Materials Research Group (http://emat-solar.lbl.gov/)<br />
**Supported by the Division <strong>of</strong> Materials Science and Engineering, US DOE<br />
236
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors ThI3<br />
ZnO biosensing<br />
Danek Elbaum<br />
Institute <strong>of</strong> Physics, Polish Academy <strong>of</strong> Science, Warsaw, Poland<br />
ZnO nanocrystals have recently attracted a lot <strong>of</strong> attention as promising candidates for<br />
novel devices, due to a possibility <strong>of</strong> continuous tuning <strong>of</strong> optical and electronic properties by<br />
varying the particle sizes. They are also <strong>of</strong> interest for pharmaceutical industry, medicine<br />
and/or biology.<br />
ZnO nanostructures were obtained using two diverse methods: sol-gel and<br />
electrospinning. We have explored nanocrystals <strong>of</strong> ZnO/MgO as potential intracellular<br />
sensors. Synthesized nanocrystals were characterized structurally by AFM, TEM, X-ray<br />
diffraction and optically by absorption and emission. Addition <strong>of</strong> MgO shell resulting in a<br />
more intense and stable visible emission that is characteristic <strong>of</strong> nanocrystalline ZnO. XRD<br />
patterns <strong>of</strong> powdered ZnO/MgO nanocrystals and TEM data proved wurtzite crystalline<br />
structure. A temperature sensitive Fluorescence Resonance Transfer (FRET) was observed<br />
between ZnO/MgO (donors) and Nile Red (acceptor) when linked by beta-carboxymethyl<br />
cyclodextrin molecules.<br />
Electrospinning is a method capable to produce fibers with diameters ranging from<br />
tens <strong>of</strong> nanometer to microns, possessing interesting photoelectrical properties.. Electrospun<br />
nan<strong>of</strong>ibrous scaffolds have great potential in several biomedical applications, such as wound<br />
dressing, enzyme immobilization, drug delivery, tissue engineering, and they can serve as<br />
materials for extracellular biosensors. We constructed a FET detector taking advantage <strong>of</strong><br />
the electric conduction properties <strong>of</strong> ZnO nan<strong>of</strong>ibers.<br />
237
ThI4 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Nitride laser diodes<br />
Piotr Perlin, Katarzyna Holc<br />
Institute <strong>of</strong> High Presure Physics PAS<br />
TopGaN, Ltd.<br />
InGaAlN laser diodes have been first realized more than 15 years ago and their<br />
technology acquired enough maturity to make possible the development <strong>of</strong> new, advanced<br />
applications. I would like to mention here full color display technologies, including laser TV<br />
and movie theater projectors, printing, medical and biosensing applications. The process <strong>of</strong><br />
matching nitride laser diodes parameters to these expected by the new application produces a<br />
strong bias for even stronger effort to understand the basic limitation and advantages <strong>of</strong> a<br />
complicated physical systems formed by III-N semiconductors. During this presentation I will<br />
briefly review the recent advancement in the field focusing specially on couple <strong>of</strong> specific<br />
problems like:<br />
Substrates for laser diodes epitaxy<br />
Design and fabrication <strong>of</strong> laser diode waveguide<br />
InGaN quantum wells from UV to green<br />
Novel devices, new concepts<br />
238
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors ThI5<br />
Application <strong>of</strong> CdSe quantum dots for single photon emitters<br />
at room temperature<br />
D. Hommel 1 , C. Kruse 1 , T. Kümmell 2 , O. Fedorych 2 , G. Bacher 2<br />
1 Institute <strong>of</strong> Solid State Physics, University <strong>of</strong> Bremen, Germany<br />
2 CeNIDE, University Duisburg-Essen, Germany<br />
Solid state single photon sources are central devices for quantum information<br />
technology. A variety <strong>of</strong> concepts has come up during the last years, mostly based on colloidal<br />
quantum dots, color centers and on self organized semiconductor quantum dots (QDs). The<br />
latter are especially attractive, as they can be easily integrated into electrically driven devices.<br />
But a drawback has to be noted: While electrically driven sources <strong>of</strong> single and even<br />
entangled photons have been implemented, these devices are based mainly on the InGaAs or<br />
the InGaP material system, and therefore the operation temperature is limited to temperatures<br />
below 100 K.<br />
In contrast, for operation under ambient conditions wide-bandgap II-VI quantum are<br />
expected to be ideally suited: They inherently provide a much better carrier confinement and<br />
higher exciton binding energies compared to Ga(In)As. In addition it could be shown that<br />
electrically driven CdSe-QD laser diodes under pulsed excitation have a sufficient lifetime<br />
[1]. Therefore this system should be be a good candidate for single photon emission at room<br />
temperature (RT). Optimizing the carrier confinement in CdSe QDs by adding thin MgS<br />
barriers and ZnSSe <strong>of</strong> high sulfur concentration highly efficient RT emission from single<br />
quantum dots has been reported by us both under optical [2] and electrical operation [3].<br />
We now report for the first time on single photon emission from a single, epitaxially<br />
grown quantum dot up to room temperature. For the devices, self organized CdSe quantum<br />
dots grown by molecular beam epitaxy are formed in a similar structure within ZnSSe and<br />
2nm MgS barriers to provide high quantum efficiency at elevated temperatures. Nanoapertures<br />
down to 150 nm allow for addressing single quantum dots.<br />
In photoluminescence, we can track the single quantum dot emission up to room<br />
temperature, with the intensity <strong>of</strong> the emission decreasing by a factor <strong>of</strong> only 3 between 4 K<br />
and 300 K. In accordance with earlier findings [2], we detect a significant broadening <strong>of</strong> the<br />
emission lines up to nearly 28 meV due to exciton-phonon interaction. Fine structure effects<br />
have been studied as well [4].<br />
Photon correlation measurements were performed using a Hanbury-Brown-Twiss-<br />
Setup. Even under continuous wave excitation, a striking antibunching behavior is found up to<br />
T = 300 K. Second-order correlation measurements reveal a surprisingly low value <strong>of</strong> g (2) ( ) =<br />
0.15 for zero time delay even at room temperature, confirming the high potential <strong>of</strong> these<br />
quantum dots for future single photon emitting devices operating under ambient conditions.<br />
[1] A. Gust et al., phys. stat. sol (c) 2, 1098 (2005)<br />
[2] R. Arians el al., Appl. Phys. Lett. 90, 101114 (2007)<br />
[3] R. Arians el al., Appl. Phys. Lett. 93, 173506 (2008)<br />
[4] T. Kümmell et al., Phys. Rev. B 81, 241306 (2010)<br />
239
ThI6 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Terahertz Emitters Based on GaN/AlGaN HEMTs<br />
W. Knap, N.Dyakonova, D. Coquillat, F. Teppe<br />
TERALAB - Université Montpellier 2 and CNRS,Montpellier, 34090, FRANCE<br />
The channel <strong>of</strong> High Electron Mobility Transistor can act as a resonator for the<br />
plasma waves propagating in 2D electron gas. The plasma frequency increases with reduction<br />
<strong>of</strong> the channel length and can reach the Terahertz range for nanometre size transistors. As it<br />
was predicted by Dyakonov and Shur 1 , when a current flows through a field effect transistor,<br />
the steady state can become unstable against generation <strong>of</strong> plasma waves leading to the<br />
emission <strong>of</strong> an electromagnetic radiation at the plasma wave frequencies. It was also<br />
predicted 2 that nonlinear properties <strong>of</strong> the 2D plasma in the transistor channel can be used for<br />
resonant and voltage tuneable detection 2 <strong>of</strong> THz radiation. In this work we present an<br />
overview <strong>of</strong> experimental results on THz detection and emission by GaN/AlGaN<br />
nanotransistors. An overview <strong>of</strong> the subject can be found in Ref [3,4]<br />
We will present recent results on THz emission obtained in different types <strong>of</strong><br />
GaN/AlGaN nanometric high electron mobility transistors [5]. We show that depending on<br />
the transistor geometry different THz emission mechanism play role. In field plate HEMTs in<br />
agreement with the theoretical predictions i) the emission frequencies correspond to the<br />
estimated characteristic plasma wave frequencies and ii) the emission appears once the drain<br />
current exceeds a certain well defined threshold value. First results have been obtained at<br />
cryogenic temperatures , however recently it was shown that from nanometer gate length<br />
GaN/AlGaN transistors one can also obtain an efficient room temperature THz emission [5].<br />
Possible aplications <strong>of</strong> GaN HEMTs as THz sources will be discussed.<br />
[l] M. I. Dyakonov, M. S. Shur, “Shallow water analogy for a ballistic field effect transistor:<br />
New mechanism <strong>of</strong> plasma wave generation by dc current”, Phys. Rev. Lett. 71, 2465 (1993).<br />
[2] M. I. Dyakonov, M. S. Shur, “ Plasma wave electronics: novel terahertz devices using two<br />
dimensional electron fluid,” IEEE Trans. Electron Devices 43, 380 (1996).<br />
[3] W. Knap, F. Teppe, N. Dyakonova, D. Coquillat, J Łusakowski, ”Plasma wave<br />
oscillations in nanometer field effect transistors for terahertz detection and emission”, J.<br />
Phys.: Condens. Matter 20, 384205 (2008).<br />
[4] W. Knap, M. Dyakonov, D. Coquillat, F. Teppe, N. Dyakonova, J. Łusakowski, K.<br />
Karpierz, G. Valusis, D. Seliuta, I. Kasalynas, A. El Fatimy, T. Otsuji, “Field Effect<br />
Transistors for Terahertz Detection: Physics and First Imaging Applications”, J. Infrared Milli<br />
Terahz Waves 30, 1319 (2009).<br />
[5] A. El Fatimy, N. Dyakonova, Y. Meziani, T. Otsuji, W. Knap, S. Vandenbrouk, K.<br />
Madjour, D.Théron, C. Gaquiere, M. A. Poisson, S. Delage, P. Pristawko, C. Skierbiszewski,<br />
AlGaN/GaN high electron mobility transistors as a voltage-tunable room temperature terahertz<br />
sources J. <strong>of</strong> Appl. Phys. 107, 024504 (2010).<br />
240
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors ThI7<br />
Spin transfer torque in TMR and GMR nanostructures for spintronic<br />
devices<br />
Tomasz Stobiecki and Witold Skowro ski<br />
AGH University <strong>of</strong> Science and Technology, Department <strong>of</strong> Electronics, al. Mickiewicza 30,<br />
30-059 Kraków, Poland<br />
First widely used spintronic device was the Giant Magnetoresistive Spin-Valve (GMR-SV)<br />
sensor which exhibits large changes in electrical resistance in the presence <strong>of</strong> the magnetic<br />
field. The GMR-SV consist <strong>of</strong> two ferromagnetic layers separated by a thin metallic spacer<br />
layer. By designing the structure so that even a faint external magnetic field would change the<br />
relative magnetic orientations <strong>of</strong> the ferromagnetic layers, the GMR devices became a very<br />
sensitive magnetic-field sensor used as read-head in a new generation hard disc drives.<br />
More prospective multilayer system is so called Magnetic Tunnel Junction (MTJ) which<br />
consists <strong>of</strong> two metal ferromagnetic electrodes separated by an ultra thin layer <strong>of</strong> insulator<br />
(about 1-2 nm typically, amorphous Al-O or crystalline MgO). Many groups have been<br />
steadily improving the properties <strong>of</strong> Al-O and MgO barrier MTJs, so that the tunnel<br />
magnetoresistance (TMR) ratio has been increasing year by year. Recently the TMR ratio<br />
measured at room temperature has reached 700% in MgO based MTJs.<br />
By passing high density spin-polarized current through GMR-SV or MTJ nanopillars the Spin<br />
Transfer Torque (STT) effect is observed. This effect can induced precession <strong>of</strong> magnetization<br />
vector or switch the magnetization <strong>of</strong> the free layer. Spin transfer torque magnetic RAM<br />
(STT-RAM) has the advantages <strong>of</strong> lower power consumption and better scalability over<br />
conventional MRAM. Crucial issues <strong>of</strong> STT in MTJs are a reduction <strong>of</strong> the critical current<br />
density, which is necessary for switching the junction and a reduction <strong>of</strong> the resistance area<br />
(RA) product. Making the ultra thin MTJ tunnel barrier, below 1 nm, is one <strong>of</strong> the approaches.<br />
In this talk the influence <strong>of</strong> the MgO barrier thickness on the STT effect in MTJs with a MgO<br />
wedge barrier will be discussed. The typical multilayer stack <strong>of</strong> MTJ for STT-RAM<br />
applications is composed <strong>of</strong>: Si/Si-ox substrate/ buffer layers/ PtMn(16) antiferromagnet/<br />
Co70Fe30(2)/Ru(0.9) synthetic anti-ferrimagnet/ Co40Fe40B20(2.3) bottom electrode (reference<br />
layer) / MgO(0.6 - 1) tunnel barrier/ Co40Fe40B20(2.3) top electrode (free layer) / protection<br />
layers (thickness in nm). Elliptical shaped nanopillars with RA product ranging from 2 to 10<br />
m 2 , sizes <strong>of</strong> 120 nm × 230 nm and TMR values <strong>of</strong> up to 160% were prepared by electron<br />
beam lithography, ion beam milling and lift-<strong>of</strong>f. Current induced magnetization switching <strong>of</strong><br />
the free layer (top electrode) was observed. The critical current density was 10 6 A/cm 2 order<br />
<strong>of</strong> magnitude.<br />
The application <strong>of</strong> RF current to a nanosized MTJ generate DC voltage across the device,<br />
when the frequency is in resonance with resistance oscillations, arising from the STT. Such<br />
spin torque ferromagnetic resonance (ST-FMR) excitation in a MTJ nanopillar, as well as an<br />
inverse effect, i.e., generation <strong>of</strong> the RF signal, when MTJ is supplied with DC current,<br />
provide potential application in the telecommunications technology.<br />
Project supported by the Foundation for Polish Science MPD Programme, co-financed by the EU European<br />
Regional Development Fund, SPINSWITCH MRTN-CT-2006-035327 and Ministry <strong>of</strong> Science and Higher<br />
Education, grants (NN 515544538 and IP 2010037970).<br />
241
ThI8 _______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Si-based MEMS devices<br />
Jan Dziuban<br />
Wrocław University <strong>of</strong> Technology, Poland<br />
242
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors ThI9<br />
Abstract<br />
Giant photoconversion on silicon derived materials<br />
Z.T. Kuznicki<br />
Photonic Systems Laboratory, Boulevard Sébastien Brant, BP 10413, 67400 Illkirch, France<br />
tel: +(33) 3 90 24 46 07, fax: +(33) 3 90 24 46 19, email: zbigniew.kuznicki@lsp.u-strasbg.fr<br />
Invited/oral<br />
One <strong>of</strong> the most challenging topics <strong>of</strong> today’s research and development concerns the efficiency <strong>of</strong> the light-toelectricity<br />
conversion, and this obviously on Si-derived devices. The best <strong>of</strong> the possible/imaginable solutions<br />
has to allow overcoming the indirect Si bandgap constraints. This aim becomes realizable by transforming the<br />
hard photon-matter interaction by a s<strong>of</strong>t photon-electron-electron one due to additional new low-energy<br />
mechanisms allowing a multistage conversion cycle. Such effects have been observed by us within new Si<br />
derived metamaterials obtained by multiple transformations leading to a nanoscale Si-layered system. In such<br />
systems, a giant photoconversion could be observed for the first time due to hot electron interactions with active<br />
interfaces and conditioned crystalline defects representing tectons (unities <strong>of</strong> the Si metamaterial). Today it<br />
seems to be the best way to overcome conversion shortages <strong>of</strong> the bulk, <strong>of</strong> the thin-film or <strong>of</strong> any Si based<br />
converter. We present in this work a background and experimental demonstrations <strong>of</strong> giant photoconversion.<br />
Keywords: giant photoconversion, Si derived metamaterial, light-to-electricity conversion cycle, multiinterface<br />
device, nanoscale Si-layered system, photon-carrier and photon-carrier-carrier interactions, low-energy<br />
generation and multiplication, multistage conversion, collection efficiency exceeding unity.<br />
CE (%)<br />
100<br />
80<br />
60<br />
40<br />
20<br />
400 nm<br />
W00-C10<br />
W02-C11<br />
W04-C10<br />
W14-C10<br />
W14-C11 i-01<br />
W14-C12<br />
LP-06-C2<br />
LP-07-C6<br />
LP-08-C4<br />
W11-C2<br />
W11-C3<br />
W11-C5<br />
W11-C6<br />
0<br />
1E+11 1E+12 1E+13 1E+14 1E+15<br />
TRANSMITED PHOTON NUMBER<br />
IQE (%)<br />
160<br />
100<br />
135 %<br />
transmited @ 400 nm<br />
W09-C10<br />
40<br />
1E+12 1E+13 1E+14 1E+15<br />
AVERAGE TRANSMITED PHOTON NUMBER (cm-2)<br />
Examples <strong>of</strong> non-linear CE (IQE) <strong>of</strong> test devices containing a metamaterial in their superficial nanostratum. The<br />
curves result from the 400 nm band IQE peak variation versus incident light intensity in different devices (a).<br />
Saturation appears under the weak photon flux <strong>of</strong> about 8x10 13 photons/s/cm 2 (b). This is an example <strong>of</strong> the<br />
record PV conversion on the Si-based material with 1.35 electrons per incident photon. The average reflection is<br />
on the order <strong>of</strong> X%. The whole low energy generation/multiplication effect appears exclusively within the<br />
superficial stratum.<br />
243<br />
Multistage conversion: step-like difference between<br />
measured and simulated conventional CE (yellow<br />
diamonds) after optical confinement correction. The<br />
conventional CE has been simulated using the<br />
experimental data. The step-like green line results<br />
from the simulation <strong>of</strong> carrier multiplication with the<br />
probability p = 0.8.
____________________ NOTES ____________________<br />
244
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
INDEX<br />
A<br />
Abdussalam W. TuP14<br />
Abram<strong>of</strong> E. MoP24<br />
Abstreiter G. SaI2<br />
Adam S. TuI2<br />
Adamowicz L. ThI1<br />
Aizpurua J. TuP04<br />
Albert S. WeO4<br />
Aleszkiewicz M. MoP01, TuP06<br />
Alves E. MoP35<br />
Andrearczyk T. WeP28,WeP42,<br />
WeP49<br />
Andrzejewski J. TuP16<br />
Anisimovas E. MoP05, TuP27<br />
Antonowicz J. MoP18<br />
Araki T. WeI4<br />
Arcade P. TuP50<br />
Arvanitis D. WeP37<br />
Aschenbrenner T. TuP34, WeP13<br />
Atrashchenko A.V. MoP55<br />
B<br />
Babi ski A. TuP29,TuP30,TuP32<br />
Bacewicz R. MoP18<br />
Bacher G. ThI5<br />
Baeumler M. WeP20<br />
Baj M. TuP40, TuP44,<br />
WeP18, WeP33<br />
Bajda M. WeP17<br />
Balakauskas S. MoP53<br />
Ballester A. TuP33<br />
Baranowski J.M. TuP03, TuP42,<br />
TuP44, TuP45<br />
Baranowski M. WeP22,WeP35,<br />
WeP47<br />
Bara ska A. TuP50<br />
Bara ski J. TuP21<br />
Barbagini F. WeO4<br />
Bardyszewski W. WeP08, WeP52<br />
Barlak M. WeP43<br />
Bartsch G. TuP36<br />
Basta M.Ł. MoP49<br />
Baturin V.A. MoP42, MoP43<br />
Bauer G. MoP01<br />
Bayer M. TuP36<br />
Bednarski H. MoP36, MoP54,<br />
TuP22<br />
Bengoechea-Encabo A. WeO4<br />
Bercha A. WeP17<br />
245<br />
Bercha D.M. MoP33<br />
Bercha S.A. MoP33<br />
Berthing T. TuP53<br />
Białek M. TuP39<br />
Biermann K. TuI4<br />
Binder J. WeP20<br />
Birowska M. MoP10, TuO2,WeO3<br />
Błaszczyk J.A. TuP45<br />
Bo kowski M. MoP35, WeP07,<br />
WeP19,WeP38,<br />
WeP48<br />
Bogusławski P. WeP04, WeP32<br />
Bonanni A. WeO2, WeP37,<br />
WeP53<br />
Bonde S. TuP53<br />
Borowik P. ThI1<br />
Borysiewicz M.A. MoP16<br />
Borysiuk J. TuP29, TuP44,<br />
WeP02, WeP20,<br />
WeP57<br />
Boukari H. TuP46<br />
Bracker A.S. TuI1<br />
Bradley R. TuI4<br />
Bragar I. TuP26<br />
Brik M.G. MoP21, MoP35<br />
Brus V.V. MoP13, MoP26<br />
Bryant G.W. TuP04<br />
Bryja L. TuP36, TuP43<br />
Buczko R. MoO3, MoP02,<br />
MoP07, TuP10<br />
Bugaiova M.E. MoP32<br />
Bugajski M. MoP20, ThI1<br />
Bujko B. TuP43<br />
Bukała M. MoP02, MoP07<br />
Butte R. WeP07<br />
Buzatu P. TuP38<br />
Bykov O.I. MoP42, MoP43<br />
C<br />
Cabaj A. WeP27<br />
Calleja E. WeO4<br />
Cambel V. MoP15<br />
Campbell P.M. TuP58<br />
Carter S.G. TuI1<br />
Cerda-Méndez E. TuI4<br />
Chegodaev A.D. TuP47<br />
Chen M.S. MoP48<br />
Chenaud B. TuP41<br />
Chèze C. WeP19, WeO6<br />
Chitta V.A. MoP24<br />
Chwastek M. MoP36<br />
Chwastyk M. MoP09<br />
Consejo C. TuP37, TuP41
_______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Coquillat D. MoO4, ThI6, TuP38,<br />
TuP41<br />
Cozan V. MoP54<br />
Cui K. WeP16<br />
Cywi ski G. WeO6, WeP19<br />
Cywi ski Ł. MoP01, TuP01<br />
Czajkowski G. TuP17<br />
Czapkiewicz M. TuP06, TuP13,<br />
TuP39<br />
Czernecki R. WeP23, WeP34,<br />
WeP38, WeP50<br />
D<br />
Dabrowski J. WeP57<br />
Dalecki W. MoP08<br />
DeJaeger B. TuP52<br />
Devillers T. WeO2<br />
Di Carlo A. SuI1<br />
Diduszko R. MoP08<br />
Dietl T. MoP01, TuO3, WeI2,<br />
WeO1, WeO2,<br />
WeO3, WeP01,<br />
WeP09, WeP15,<br />
WeP37, WeP53<br />
Dluzewski P. WeP57<br />
Dmitriev A.I. MoP34<br />
Dobkowska S. TuO3, WeP15<br />
Dobosz D. MoP04, MoP46,<br />
MoP50<br />
Domagała J.Z. WeP24, WeP48,<br />
WeP49<br />
Doma ski M. MoP36, MoP54<br />
Doma ski T. MoP58, TuP21<br />
Dominiak A. TuP45<br />
Domukhovski V. MoP02<br />
Doradzi ski R. WeP11, WeP39<br />
Drabi ska A. MoP03, TuP45<br />
Droba A. MoP46, MoP50<br />
Drobiazg T. MoP18<br />
Drozd V. MoP04<br />
Drozdziel A. TuP55<br />
Drube W. MoP47<br />
Duda H. MoP11, MoP31,<br />
MoP40, MoP44,<br />
MoP45, MoP56<br />
Durygin A. MoP04<br />
Dusheyko M.G. MoP41<br />
Dussaigne A. WeP07<br />
Duzynska A. MoP04<br />
Dwili ski R. WeP11, WeP39<br />
Dyakonova N. ThI6, TuP37, TuP38,<br />
TuP41<br />
Dybała F. WeP17<br />
246<br />
Dybko K. MoP57<br />
Dyczewski J. MoP16, MoP19<br />
Dynowska E. MoP16, MoP19,<br />
MoP51, WeP24<br />
Dziatkowski K. TuP12, WeP46<br />
Dziawa P. MoP02, WeP18,<br />
WeP36<br />
Dziawa S.P. MoP47<br />
Dziecielewski I. WeP19<br />
Dziuban J. ThI8<br />
E<br />
Ebeling J. TuP34<br />
Eddy Jr. C.R. TuP58<br />
El Fatimy A. TuP38, TuP41<br />
Elbaum D. ThI3<br />
Litwin-Staszewska E. WeP55<br />
Escartín J.M. TuP33<br />
Evtikhiev V.P. MoP55<br />
Faina B. WeO2, WeP53<br />
Fandrich M. WeP13<br />
Fedorych O. ThI5<br />
Feduniewicz-Zmuda A. WeP19<br />
Ferone R. MoO1<br />
Fidelus J.D. MoP04<br />
Figge S. TuP34, WeP13,<br />
WeP30<br />
Figielski T. WeP42<br />
Fobelets K. TuP52<br />
Forchel A. TuP16<br />
Fronc K. TuP08, TuP13,<br />
TuP39<br />
Furman M. WeP06<br />
F<br />
G<br />
Gaj J.A. TuO6, TuP02,<br />
TuP09, WeP06,<br />
WeP53<br />
Galazka R.R. WeP14<br />
GalickaM. MoO3<br />
GałkowskiK. WeP06<br />
Gammon D. TuI1<br />
Gao Y. TuP12<br />
Garleff J.K. WeP40<br />
GasK. MoP47, MoP51<br />
Gaska R. TuO1
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Gaskill D.K. TuP58<br />
Gauthier N. WeP16<br />
Gawarecki K. TuP25, tuP26<br />
Gaži S. MoP15<br />
G gor A. MoP44, MoP45<br />
Gelczuk Ł. TuP56<br />
Gierałtowska S. MoP12, MoP19,<br />
MoP27, WeP31<br />
Giffard B. MoO4<br />
Gizi ski P. WeP46<br />
Gladysiewicz M. WeP39<br />
Gluba Ł. WeP49<br />
Glukhov K.E. MoP33<br />
Godlewski M. MoP12, MoP14,<br />
MoP27, MoP49,<br />
WeP06, WeP27,<br />
WeP30, WeP31,<br />
WeP41<br />
Golnik A. TuO5, TuO6, TuP02,<br />
TuP09, TuP54,<br />
WeP06, WeP53<br />
Golub L.E. TuP24<br />
Gołasa K. TuP29, TuP30,<br />
TuP32<br />
Goncharuk G.A. MoO6<br />
Gonzalez-Szwacki N. WeO1, WeP46,<br />
MoP59<br />
Gorczyca I. WeO5<br />
Goryca M. MoO5, TuO4, TuO5,<br />
TuP01, TuP09<br />
Gosk J.B. WeP43, WeP45<br />
Goss K. WeP23, WeP26<br />
Go ci ski K. WeP30<br />
Górecka E. WeP45<br />
Grabecki G. MoP01<br />
Grabiec P. TuP37<br />
Grandjean N. WeP07<br />
Gregušová D. MoP15<br />
Greilich A. TuI1<br />
Grigelionis I. TuP52<br />
Grodecki K. TuP03, TuP42,<br />
TuP45<br />
Gro T. MoP11, MoP28,<br />
MoP31, MoP40,<br />
MoP44, MoP45,<br />
MoP56<br />
Gryglas-Borysiewicz M. TuP40,<br />
TuP44, WeP18,<br />
WeP33<br />
Grynberg M. TuO1, TuP39,<br />
TuP46<br />
Grzanka S. WeO6, WeP19,<br />
WeP34, WeP38,<br />
WeP50, WeP55<br />
Grzegory I. WeO6, WeP07,<br />
247<br />
WeP48<br />
Grzywacz B. WeP19<br />
Guda K. TuI4<br />
Gumienny Z. TuP20, TuP35,<br />
WeP50<br />
GuziewiczE. MoP12, MoP27,<br />
WeP06, WeP27,<br />
WeP30, WeP31,<br />
WeP41<br />
Gwarek W. MoO4, TuP46<br />
H<br />
Hajduk B. MoP36<br />
Hałda G. ThI1<br />
Hankiewicz E.M. MoI3<br />
Harris J.S. WeP22<br />
Hartsfield T. TuP12<br />
Henini M. WeP12<br />
Henrykowski A. TuP20<br />
Hey R. TuI4<br />
Higersberger J. WeP03<br />
Hilmer H. TuO6<br />
Hoefling S. TuP16<br />
Holc K. ThI4<br />
Hołyst R. MoP51<br />
Hommel D. SaI1, ThI5, TuO6,<br />
TuP02, TuP34,<br />
TuP54, WeP13,<br />
WeP30<br />
Hopkinson M. WeP12<br />
Horvath Z.J. MoP41<br />
Hosatte M. MoP49<br />
Hruban A. MoP03, MoP08<br />
Hrubiak R. MoP04<br />
Hwang W.S. TuP58<br />
Ibáñez J. WeP12<br />
Ievtushenko A.I. MoP41, MoP42,<br />
MoP43<br />
Ilaschuk M.I. MoP13<br />
IlverL. WeP05<br />
Ivanov S.V. TuP24<br />
Iwi ska M. MoP20<br />
Izdebska K. WeP25<br />
I
_______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
J<br />
Jacak J. TuP20<br />
Piechota J. WeP54<br />
Jadczak J. TuP36, TuP43<br />
Jahn U. WeO4<br />
Jakieła R. WeO2, WeP31<br />
Sołtys J. WeP54<br />
Jakubczyk T. TuO6, TuP02,<br />
TuP54<br />
Janicki L. WeP39<br />
Janik E. MoO5, MoP12,<br />
MoP51<br />
Jarimavi i t -Žvalionien R. MoP37<br />
Jaskolski W. TuP04<br />
Jaworski D. TuP52<br />
Jaworski M. WeP27<br />
Jena D. SuI2, TuP58<br />
Nygard J. TuP53<br />
Jie M. MoP18<br />
Johnson R.L. MoP17, MoP53<br />
Jong-In S. WeP35<br />
Jonson M. MoO1<br />
June K.L. WeP35<br />
Jungwirth T. WeI1<br />
Jurkiewicz-Wegner E. MoP08<br />
Jurusik J. MoP36<br />
Juszy ski P. WeP18, WeP33<br />
K<br />
Kacman P. MoO3, MoP02,<br />
MoP07<br />
Kaczmarkiewicz P. TuP11<br />
Kafar A. WeP23<br />
Kaim A. WeP46<br />
Kaminskien Ž. MoP37<br />
Kami ska A. MoP04, MoP21,<br />
MoP35, MoP51,<br />
TuP19<br />
Kami ska E. MoP16, MoP51<br />
Kami ska M. MoP03, MoP48,<br />
WeP20<br />
Kamyczek P. TuP20, TuP56,<br />
WeP50<br />
Karachevtseva L.A. MoP22<br />
Karbownik P. MoP20<br />
Karczewski G. MoO5, MoP30,<br />
MoP57, TuP08,<br />
TuP13, TuP19,<br />
TuP20, TuP35<br />
Karpenko A.Y. MoP42, MoP43<br />
Karpierz K. TuO1<br />
248<br />
Karpyna V.A. MoP23<br />
Karthikeyan G.S. WeP35<br />
Karwat P. TuP18<br />
Katrunov K. MoP29<br />
Kazimierczuk T. MoO5, TuO4, TuO5,<br />
TuP02, TuP08,<br />
TuP09, TuP32<br />
Ka mierczak P. WeP06<br />
Keizer J.G. WeP40<br />
Kempisty P. WeP51, WeP56<br />
K pisty G. TuP42<br />
Khachapuridze A. WeP34<br />
Khomyak V.V. MoP13, MoP25<br />
Khyzhun O. Yu. MoP25, MoP42<br />
Kim D. TuI1<br />
Kirste L. WeP20<br />
Klochkov L.A. MoP25<br />
Klochkov L.O. MoP42, MoP43<br />
Klosek K. WeP02, WeP29,<br />
WeP57<br />
Kłopotowski Ł. MoI2, TuP08,<br />
TuP13, WeP29<br />
Kłosek K. WeP25<br />
Knap W. MoO4, ThI6, TuO1,<br />
TuP37, TuP38,<br />
TuP41<br />
Kn<strong>of</strong>f W. MoP02, MoP17,<br />
MoP32, WeP04<br />
Koba M. MoP60<br />
Kobak J. TuP02<br />
Koenraad P.M. WeP40<br />
Köhler K. WeP20<br />
Kolek A. ThI1<br />
Kolkovsky V. MoP16, MoP30,<br />
TuP06<br />
Kolwas K.A. MoP01<br />
Koperski M. TuP09<br />
Kopyt P. MoO4, TuP46<br />
Korbecka A. MoP06<br />
Korona K.P. MoP48, WeP20<br />
Korwin-Mikke K. MoP19<br />
Korzekwa K. TuP15<br />
Kosiel K. ThI1<br />
Kossacki P. MoO5, TuO4, TuO5,<br />
TuP01, TuP02,<br />
TuP09, TuP54<br />
Kossut J. TuP13<br />
Kothleitner G. WeO2<br />
Kovalyuk Z.D. MoP13, MoP26<br />
Kowalczyk L. MoP02<br />
Kowalik I. WeP53<br />
Kowalik I.A. WeP37<br />
Kowalski B. MoP25, WeP26<br />
Kowalski B.J. WeP21, WeP25,<br />
WeP36
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Kowalski P. TuP28<br />
Kozanecki A. MoP04, MoP35,<br />
MoP46, MoP50<br />
Koziarskyi D.P. MoP38<br />
Koziarskyi I.P. MoP38<br />
Kozłowski A. TuP23<br />
Kozub M. TuP07, TuP31<br />
Kozubal M. TuP44<br />
Krajewski T.A MoP27, WeP30,<br />
WeP31<br />
Kret S. MoO5, MoP51<br />
Krive I.V. MoO1<br />
Krizhanovskii D. TuI4<br />
Krogulec I. WeP42<br />
Krok-Kowalski J. MoP11, MoP28,<br />
MoP40, MoP44,<br />
MoP56<br />
Krukowski S. WeP51, WeP54,<br />
WeP55, WeP56,<br />
WeP58<br />
Kruse C. ThI5, TuO6, TuP02,<br />
TuP54, WeP13<br />
Krushynskaya L.A. MoP32<br />
Kruszewski P. WeP30, WeP31<br />
Krzy anowska H. WeP49<br />
Krzy osiak M. TuP10<br />
Kubisa M. TuP43, TuP49<br />
Kucharski R. WeP11, WeP39<br />
Kuchmii S.Ya. MoP22<br />
Kúdela R. MoP15<br />
Kudrawiec R. WeP11, WeP12,<br />
WeP22, WeP35,<br />
WeP39, WeP47<br />
Kukli ski K. TuP13<br />
Kukuła Z. MoP31<br />
Kümmell T. ThI5<br />
Kunert G. WeP13<br />
Kutsay O.M. MoP43<br />
Ku nicki Z.T. MoP49, ThI9<br />
Kwiatkowski A. WeP18<br />
Lapinskas S. MoP37<br />
Lashkarev G.V. MoP23, MoP25,<br />
MoP32, MoP41,<br />
MoP42, MoP43<br />
Latkowska M. WeP12, WeP22,<br />
WeP35, WeP47<br />
Lazorenko V.I. MoP25, MoP32,<br />
MoP41, MoP42,<br />
MoP43<br />
Le V.Kh. WeP14<br />
Lefebvre P. WeO4<br />
L<br />
249<br />
Leonelli R. WeP16<br />
Leszczynski M. WeP23, WeP26,<br />
WeP38<br />
Marcinkowski L. WeP55<br />
Levrat J. WeP07<br />
Li T. WeO2,WeP53<br />
Li X. TuP12<br />
Locatelli A. WeP37<br />
Loeffler A. TuP16<br />
Loginov D.K. TuP47<br />
Lopuszynski M. WeP10<br />
Lucznik B. WeP19, WeP48<br />
Lukasik P. MoO4<br />
Luna E. WeO4<br />
Lytvynenko O.A. MoP22<br />
Łach P. MoP21, TuP19<br />
Łaszcz A. MoP19<br />
Łepkowski S.P. WeO5, WeP08,<br />
WeP52<br />
Łuka G. WeP31<br />
Łukasiewicz M. WeP06<br />
Łukasiewicz M.I. WeP27<br />
Łusakowska E. MoP02, WeP02,<br />
WeP29, WeP31<br />
Łusakowski A. WeP04<br />
Łusakowski J. TuO1, TuP30,<br />
TuP32, TuP37,<br />
TuP39, TuP46,<br />
TuP50, TuP51,<br />
TuP52<br />
Ł<br />
M<br />
Ma C.G. MoP35<br />
Machnikowski P. TuP07, TuP10,<br />
TuP11, TuP14,<br />
TuP15, TuP18,<br />
TuP25, TuP26,<br />
TuP28<br />
Maci ek E. MoP28<br />
Madrak K. WeP45<br />
Madsen M.H. TuP53<br />
Maistruk E.V. MoP38<br />
Majewski J.A. MoP06, MoP10,<br />
MoP59, TuO2,<br />
WeO1, WeO3,<br />
WeP03, WeP10,<br />
WeP17<br />
Majh<strong>of</strong>er A. WeP45
_______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Malicka E. MoP11, MoP44,<br />
MoP45, MoP56<br />
Malkova N. TuP04<br />
Marcinowski Ł. TuP10<br />
Marczewski J. TuP37<br />
D browska-Szata M. TuP56<br />
Marianchuk P.D. MoP38<br />
Mariette H. TuP46<br />
Marinchio H. TuP38<br />
Marona L. WeP38<br />
Martinez K.L. TuP53<br />
Maryanchuk P.D. MoP26<br />
Marynski A. TuP16<br />
Materna A. MoP03, MoP08<br />
Maude D.K. MoO6, MoP24<br />
M kosa A. MoP30, WeP42<br />
Meingast A. WeO2<br />
Meltser B.Y. TuP24<br />
Meziani Y. TuP38<br />
Mi Z. WeP16<br />
Mickevi ius J. MoP29<br />
Mickievicius S. MoP47, MoP53<br />
Milowska K. MoP10, TuO2,<br />
WeO3<br />
Misiewicz J. TuP11, TuP16,<br />
TuP31, TuP43,<br />
TuP49, TuP57,<br />
WeP11, WeP12,<br />
WeP22, WeP35,<br />
WeP39, WeP47<br />
Mitard J. TuP52<br />
Molas M. TuP29, TuP30,<br />
TuP32<br />
Molenkamp L.W. MoP57<br />
Monthioux M. MoO1<br />
Morhange J.F. MoP51<br />
Mostowski J. TuP10<br />
Motyka M. WeP11<br />
Movilla J.L. TuP33<br />
Mroczy ski R. MoP19<br />
Mukhin M.S. TuP24<br />
Musiał A. TuP11, TuP16,<br />
TuP31<br />
Muzioł G. WeO6, WeP44<br />
Mydlarz T. MoP28<br />
Myers-Ward R.L. TuP58<br />
N<br />
Najda S.P. WeP38<br />
Nakwaski W. MoP52<br />
Nanishi Y. WeI4<br />
Narkiewicz U. MoP14<br />
Navarro-Quezada A. WeO2, WeP37,<br />
250<br />
WeP53<br />
Nawrocki M. TuP02, TuP09,<br />
WeP53<br />
Nestoklon M.O. TuP24<br />
Nguyen H. WeP16<br />
Nietuby R. WeP21<br />
Nikiforov K. MoP40<br />
Niño M.A. WeP37<br />
Noe L. MoO1<br />
Nogajewski K. TuO1, TuP46<br />
Nötzel R. MoO2<br />
Nouvel P. TuP38<br />
Novotný T. TuP05<br />
Nowakowski P. WeP29, WeP57<br />
Nowicki P. TuP06<br />
Nygård J. MoO1<br />
O<br />
Olender K. MoP30<br />
Oliveira Jr. N.F. MoP24<br />
OnurMente T. WeP37<br />
Orlita M. MoO6<br />
Orłowski B.A. MoP17, MoP47,<br />
MoP53<br />
Orłowski W. MoP08<br />
Ott F. WeP24<br />
Pacuski W. TuO6, TuP02,<br />
TuP54, WeP06,<br />
WeP53<br />
Pacyna A.W. MoP11, MoP28,<br />
MoP40, MoP56<br />
Papierska J. WeP06, WeP53<br />
Papis E. TuP50<br />
Parfenyuk O.A. MoP13<br />
Pasternak J. MoP19, MoP51<br />
Pawlowski M. WeP14<br />
Pawłowska J. WeO6<br />
Pełech I. MoP14<br />
Peres M.L. MoP24<br />
Perlin P. ThI4, WeO6,<br />
WeP23, WeP26,<br />
WeP34, WeP38<br />
Petrouchik A. MoO5<br />
Pi M. TuP33<br />
Piersa M. MoP03, MoP08<br />
Pier ci ska D. MoP20<br />
Pier ci ski K. MoP20<br />
Pietnoczka A. MoP18<br />
P
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Pietrzyk M.A. MoP17, MoP46,<br />
MoP50, MoP53<br />
Pi tka B. TuP50, TuP51<br />
Piot B.A. MoO6<br />
Piotrowska A. MoP16, MoP19<br />
Piotrzkowski R. WeP34<br />
Placzek-Popko E. TuP56<br />
Planelles J. TuP33<br />
Plaziak J. WeP03<br />
Plochocka P. TuO4<br />
Pochrybniak C. WeP43<br />
Pociecha D. WeP45<br />
Podemski P. TuP11, TuP16<br />
Popko E. TuP20, TuP35,<br />
WeP50<br />
Potemski M. MoO6, TuO4, TuP36<br />
Proselkov O. TuO3, WeP15<br />
Prosy evas I. MoP37<br />
Przezdziecka E. MoP04, MoP46,<br />
MoP50<br />
Przybytek J. TuP40, TuP44,<br />
WeP18, WeP33<br />
Ptasi ska M. WeP54<br />
Pyszniak K. TuP55<br />
R<br />
Radchenko M.V. MoP32<br />
Radzvilavicius A. MoP05<br />
Rancova O. TuP27<br />
Rappl P.H. MoP24<br />
Ratchford D. TuP12<br />
Rawski M. TuP55<br />
Rduch P. MoP40, MoP44,<br />
MoP45<br />
Reithmaier J.P. TuP16, TuP31<br />
Reitzenstein S. TuP16<br />
Reszka A. MoP02, MoP25,<br />
MoP47, MoP53,<br />
TuP19, WeP02,<br />
WeP25, WeP26,<br />
WeP36<br />
Reuter D. TuP57<br />
Romaniec M. MoP08<br />
Roshko V.Ya. TuP48<br />
Rossbach G. WeP07<br />
Roszak K. TuP05, TuP10<br />
Rousset J.-G. WeP53<br />
Rovezzi M. WeO2, WeP37<br />
Ró a ski P.T. MoP09<br />
Rudzi ski M. WeP39<br />
Ryczko K. TuP57<br />
Ryzhikov V. MoP29<br />
251<br />
Sadowski J. TuO3, WeP05,<br />
WeP15, WeP18,<br />
WeP21, WeP24,<br />
WeP28, WeP33,<br />
WeP36, WeP42,<br />
WeP49<br />
Sajkowski J.M. MoP46, MoP50<br />
Sakowicz M. WeP16<br />
Sakowski K. WeP55, WeP56<br />
Sanchez-García M.A. WeO4<br />
Sankowski P. MoP07<br />
Santos P.V. TuI4<br />
Sarker D. TuI4<br />
Sarmiento T. WeP22<br />
Sarzynski M. WeP26<br />
Sawicka M. WeO6, WeP19<br />
Sawicki M. TuO3, WeO2,<br />
WeP06, WeP15,<br />
WeP53<br />
Š epka T. MoP15<br />
Schillak P. TuP17<br />
Schmidt-Grund R. TuO6<br />
Schneider J.M. MoO6<br />
Schreyeck S. MoP57<br />
Schumacher C. MoP57<br />
Schuster F. MoO4<br />
Sciesiek M. TuP54<br />
Seabaugh A. TuP58<br />
Semenov A.N. TuP24<br />
S k G. MoI4, TuP11,<br />
TTuP16, uP31,<br />
WeP12<br />
Sheikin I. MoO6<br />
Shekhter R.I. MoO1<br />
Shevchenko D. MoP29<br />
Shtepliuk I.I. MoP25<br />
Shur M.S. TuO1<br />
Sibera D. MoP14<br />
Sichkovskyi V.I. MoP32<br />
Siekacz M. WeO6, WeP19,<br />
WeP44<br />
Sildos I. MoP21<br />
Silva C. WeP16<br />
Simoen E. TuP52<br />
Sims J. TuP04<br />
Sitarek P. TuP57<br />
Sitek A. TuP18<br />
Sitko R. MoP44<br />
Sitnikova A.A. TuP24<br />
Sitek A. TuP14<br />
Siusys A. WeP36<br />
Sizov F.F. MoP22<br />
S
_______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Skierbiszewski C. WeO6, WeP19,<br />
WeP44<br />
Skolnick M.S. TuI4<br />
Skowro ski W. ThI7<br />
Słupi ski T. MoP18<br />
Smajek E. MoP02<br />
Smakman E.P. WeP40<br />
Smole ski T. TuP01<br />
Smrcka L. MoO6<br />
Soba ska M. WeP02, WeP25,<br />
WeP29, WeP57<br />
Sochacki M. MoP19, TuP45<br />
Solignac P. TuP41<br />
Sorensen C.B. TuP53<br />
Spałek J. TuP22<br />
Speck J.S. WeI3<br />
Springholz G. MoP01<br />
Stachowicz M. MoP46, MoP50<br />
Stallinga P. WeP30<br />
Sta czyk S. WeP23<br />
Starik S.P. MoP43<br />
Starzhinskiy N. MoP29<br />
Staszczak G. WeP34<br />
Stefanowicz W. TuO3, WeO2,<br />
WeP06, WeP15,<br />
WeP53<br />
Stelmakh Y.A. MoP32<br />
St pniewski R. TuP03, TuP42,<br />
TuP44, TuP45<br />
Stobiecki T. ThI7<br />
Story T. MoP02, MoP17,<br />
MoP32, MoP57,<br />
WeP04<br />
Strak P. WeP51, WeP56,<br />
WeP58<br />
Stronska O.J. MoP22<br />
Stroyuk A.L. MoP22<br />
Strupi ski W. TuP03, TuP42,<br />
TuP44, TuP45,<br />
WeP39<br />
Strzelecka G. MoP03, MoP08<br />
Suchocki A. MoP04, MoP21,<br />
MoP35, TuP19,<br />
WeP25<br />
Suffczy ski J. TuP02, WeP06,<br />
WeP53<br />
Suski T. WeP07, WeP23,<br />
WeP26, WeP34,<br />
WeP38, WeP50<br />
Svoboda P. MoO6<br />
Swiatek K. WeP14<br />
Sybilski P. WeP25<br />
Syperek M. TuP43, WeP22,<br />
WeP35<br />
Szczepa ski P. MoP60<br />
252<br />
Szczerbakow A. MoP53<br />
Szczytko J. TuP50, TuP51,<br />
WeP43, WeP45,<br />
WeP46<br />
Szerling A. MoP20, ThI1<br />
Sznajder M. MoP33<br />
Sznajder P. TuP51<br />
Szot M. MoP02, MoP57<br />
Sztenkiel D. WeO2, WeP09<br />
Szuszkiewicz W. MoP51, WeP24<br />
Szydlowska J. WeP46<br />
Szymura M. TuP08<br />
liwa C. WeO3, WeP01<br />
T<br />
Tahy K. TuP58<br />
Taliashvili B. MoP02, MoP17<br />
Tamulaitis G. MoP29<br />
Tang Z. TuP12<br />
Taube A. MoP19<br />
Tedesco J.L. TuP58<br />
Teisseyre H. MoP04, WeP07,<br />
WeP29, WeP48,<br />
WeP52<br />
Teppe F. MoO4, ThI6, TuP37,<br />
TuP38, TuP41<br />
Thiess S. MoP47<br />
Tim<strong>of</strong>eeva I.I. MoP25<br />
Tkach V.M. MoP42<br />
Tkaczyk Z. WeP42<br />
Tomaszewicz E. MoP31<br />
Tomaszewski D. TuP37<br />
Toropov A.A. TuP24<br />
Torres J. TuP38, TuP41<br />
Trampert A. WeO4<br />
Trushkin S. MoP01<br />
Trzeciakowski W. WeP17<br />
Turek M. TuP55<br />
Turski H. WeO6, WeP19,<br />
WeP44<br />
Twardowski A. WeP43, WeP45,<br />
WeP46<br />
U<br />
Ueta A.Y. MoP24<br />
Ulfat I. WeP05<br />
Ulin V.P. MoP55<br />
Umansky V. TuP39<br />
Urbanowicz P. MoP31<br />
Urba czyk A. MoO2
40th _______________________________________________________________<br />
"Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Utko P. MoO1,TuP53<br />
V<br />
vanOtten F.W.M. MoO2<br />
vanVoornveld R. WeP40<br />
Varani L. TuP38<br />
Vasek P. MoO6<br />
Videlier H. TuP37<br />
Vincent B. TuP52<br />
Vyborny Z. MoO6<br />
W<br />
Wachnicki Ł. MoP12, MoP27,<br />
WeP30, WeP31,<br />
WeP41<br />
Walukiewicz W. ThI2<br />
Wasik D. WeP18, WeP33<br />
Wasilewski Z. WeO6<br />
Wasilewski Z.R. TuP29, TuP41<br />
Wei S.H. MoP18<br />
Welna M. WeP11<br />
WernerZ. WeP43<br />
Weszka J. MoP36, MoP54<br />
Wiater M. MoP02, MoP51,<br />
TuP06<br />
Wieck A. TuP57<br />
Wierzbicka A. MoP46, MoP50,<br />
WeP02, WeP29<br />
Wilamowski Z. MoP03, WeP27<br />
Winkler T.E. WeO2<br />
Wisniewski P. WeP38<br />
Wi niewski P. WeP23<br />
Wi niewski Z. WeP25<br />
Witkowski B. WeP57<br />
Witkowski B.S. MoP12, MoP14,<br />
MoP27, MoP49,<br />
WeP31, WeP41<br />
Witowski A.M. WeP06<br />
Wittlin A. WeP27<br />
Władarz G. MoP40<br />
Woi ska M. WeP45<br />
Wojciechowski T. TuP13<br />
Wojnar P. MoO5, TuO4, TuO5,<br />
TuP08, TuP09,<br />
TuP13, TuP19<br />
253<br />
Wojtowicz T. MoO5, MoP02,<br />
MoP51, TuP06,<br />
TuP08, TuP13,<br />
TuP19, TuP30,<br />
TuP32, TuP46<br />
Woło A. MoP03<br />
Wolska E.A. MoP14<br />
Woło A. WeP27<br />
Wosi ski T. MoP30, WeP28,<br />
WeP42, WeP49<br />
Wouters M. TuI4<br />
Wójs A. TuI3, TuP36<br />
Wrachtrup J. MoI1<br />
Wróbel J. TuP06, TuP39,<br />
WeP42<br />
Wu S.H. MoP48<br />
Wysmołek A. TuP03, TuP42,<br />
TuP44, TuP45<br />
X<br />
Xing H. TuP58<br />
Yakovlev D.R. TuP36<br />
Yamaguchi T. WeI4<br />
Yang J. TuO1<br />
Yaremko A.M. MoP23<br />
Yastrubchak O. WeP28, WeP49<br />
Yatsunenko S.A. MoP14<br />
Yoon E. WeI4<br />
Yurtsenyuk N.S. MoP39<br />
Zaj c M. WeP11, WeP39<br />
Zakrzewski A.J. WeP31<br />
Zakrzewski T. WeP32<br />
Zaleszczyk W. MoP51<br />
Zału ny M. TuP23<br />
Zapalska M. MoP58<br />
Zdrojek M. MoP19<br />
Zhang S. WeP16<br />
Zholudev M. TuP41<br />
Zieli ski M. MoP09, TuP04<br />
Zielony E. TuP20, TuP35,<br />
WeP30, WeP50<br />
Ziółkowska D. MoP48<br />
Y<br />
Z
_______________________________________________________________<br />
40th "Jaszowiec" <strong>2011</strong> International School and Conference on the Physics <strong>of</strong> Semiconductors<br />
Zürbig V. TuP31<br />
Zychowska M. MoP19<br />
ak D. MoP52<br />
uberek R. WeP15<br />
ytkiewicz Z.R. MoP04, WeP02,<br />
WeP25, WeP29,<br />
WeP57, WeP58<br />
uk J. TuP55, WeP28,<br />
WeP49<br />
254
____________________ NOTES ____________________
____________________ NOTES ____________________
____________________ NOTES ____________________
____________________ NOTES ____________________
����������������������<br />
������������������������������������<br />
���������������������������������<br />
����������������������<br />
������� ��� �������� �� �������<br />
������������������������<br />
�<br />
Addendum to the <strong>Book</strong> <strong>of</strong> Abstracts<br />
�<br />
������������������������������������<br />
�������������
QUANTUM SEMICONDUCTOR NANOSTRUCTURES<br />
FOR APPLICATIONS IN BIOLOGY AND MEDICINE – DEVELOPMENT<br />
AND COMMERCIALISATION OF NEW GENERATION DEVICES FOR<br />
MOLECULAR DIAGNOSTICS ON THE BASIS OF NEW POLISH<br />
SEMICONDUCTOR DEVICES<br />
Participants <strong>of</strong> the project:<br />
1. Institute <strong>of</strong> Physics <strong>of</strong> the Polish Academy <strong>of</strong> Sciences – coordinator<br />
2. Institute <strong>of</strong> High Pressure Physics <strong>of</strong> the Polish Academy <strong>of</strong> Sciences<br />
3. Institute <strong>of</strong> Physical Chemistry <strong>of</strong> the Polish Academy <strong>of</strong> Sciences<br />
4. Wrocław University <strong>of</strong> Technology, Faculty <strong>of</strong> Microsystems Electronics and<br />
Photonics<br />
5. Institute <strong>of</strong> Electron Technology<br />
6. Nencki Institute <strong>of</strong> Experimental Biology <strong>of</strong> the Polish Academy <strong>of</strong> Sciences<br />
7. University <strong>of</strong> Warsaw, Interdisciplinary Centre for Mathematical and<br />
Computational Modelling<br />
The project aims at development <strong>of</strong>:<br />
1. modern GaN, ZnO and related materials nano- and semiconductor structures<br />
for diagnostic devices for applications in biology, medicine and environmental<br />
protection,<br />
2. materials technology for sensors and molecular diagnostic applications,<br />
3. scientific community and increase <strong>of</strong> employment.<br />
Core-Shell ZnTe/Zno nanowires manufactured GaN 2 inch substrate grown by Hydrogen<br />
at the Institute <strong>of</strong> Physics PAS Vapor Phase Epitaxy (HVPE) method<br />
at the Institute <strong>of</strong> High Pressure Physics PAS<br />
Project financed by the European Union within the European Regional Development Fund<br />
from Operating Program Innovative Economy<br />
website: http://info.ifpan.edu.pl/nanobiom
MODERN MATERIALS AND INNOVATIVE METHODS<br />
FOR PROCESSING AND MONITORING THE ENERGY (MIME)<br />
Participants <strong>of</strong> the project:<br />
1. Institute <strong>of</strong> Physics <strong>of</strong> the Polish Academy <strong>of</strong> Sciences – coordinator<br />
2. Institute <strong>of</strong> Electron Technology<br />
3. AGH University <strong>of</strong> Science and Technology, Faculty <strong>of</strong> Energy and Fuels<br />
Project is devoted to innovative solutions aimed at the production, storing, and<br />
saving the electric energy:<br />
1. converting waste heat energy into useful electricity by using the thermoelectric<br />
effect in materials on the PbTe basis<br />
2. storing the energy in lithium batteries, using modified LiFePO4;<br />
3. creating photovoltaics <strong>of</strong> the III generation on II-VI nanostructurized<br />
semiconducting materials on the basis <strong>of</strong> CdTe;<br />
4. hydrogen photogenerators with wide band-gap semiconducting and organic<br />
electrodes;<br />
5. devices based on conventional and high-temperature superconducting<br />
materials;<br />
6. detectors <strong>of</strong> the radiation and dosimeters for applications in the nuclear power<br />
industry.<br />
Project financed by the European Union within the European Regional Development Fund<br />
from Operating Program Innovative Economy<br />
website: http://info.ifpan.edu.pl/MIME
COMEF Scientific and Research Instruments<br />
2 Gda�ska St. 40-719 Katowice, Poland; Phone: +48322034149; Fax.:+48322035823 ext. 30<br />
www.comef.com.pl; comef@comef.com.pl;<br />
COMEF Division Warsaw, 36/37 Rakowiecka Street , 02-532 Warszawa<br />
Phone/Fax: +48 22 606 36 20, warszawa@comef.com.pl<br />
COMEF is a distributor <strong>of</strong> world leading manufacturers in the field <strong>of</strong> scientific<br />
instruments. Since 1993 we perform sales, high quality service and application trainings for<br />
our customers. In year 2000 we have started our business in other Central Europe countries:<br />
Czech Republic, Slovak Republic, Ukraine, Lithuania.<br />
COMEF’s manager is Mr. Andrzej Wi�niewski.<br />
Product Specialty:<br />
1. ALCATEL-ADIXEN: vacuum pumps, leak detectors, vacuum gauges, plasma sensors,<br />
valves, flanges and fitting;<br />
2. ALLIANCE CONCEPT: Sputtering, thin film coating systems and leak test equipments.<br />
3. JPK nano-analytical instruments;<br />
4. ANNEAL SYS: Rapid Thermal Processing / Annealing (RTP / RTA) and Chemical Vapor<br />
Deposition CVD, MOCVD systems, Spray CVD and LPCVD furnaces);<br />
5. Cameca: Magnetic sector SIMS and electron microprobe (EPMA), OTAP;<br />
6. CRESSINGTON: coating systems for TEM/SEM;<br />
7. HGH: the development <strong>of</strong> optronic and infrared systems for security, industrial and civil<br />
applications;<br />
8. Hitachi Electron microscopes: SEM, TEM, STEM;<br />
9. JANIS: Cryosystems;<br />
10. Horiba Jobin Yvon: Raman Spectrometers, Emission Spectrometers: ICP, GD-OES,<br />
Monochromators, Diffraction Gratings, Spectroscopic Ellispometer;<br />
11. Newport: Photonics, Vibration and Motion Control, Optics and Opto-Mechanics, Oriel<br />
Light Sources, Spectroscopy Instruments, Detectors and Detection Systems, Optical Filters,<br />
Hilger Crystals, Diffraction Gratings;<br />
12. ORIEL: Monochromators and Spectrographs, Light Sources, Solar Simulators, FT-IR<br />
Spectrometers;<br />
13. Riber: MBE;<br />
14. SETARAM: Calorimeters (CALVET, DSC), high temperatures dilatometers; thermal<br />
analyzers (TG, TG-DTA, TG-DSC, TGA-MS, TG-FTIR);<br />
15. Spectra-Physics: lasers;<br />
16. Thermo Fisher Scientific: AUGER and ESCA Spectrometers , EDS and WDS<br />
Spectrometers for Electron Microscopy;<br />
17. THERMAL TECHNOLOGY: implementation <strong>of</strong> vacuum furnace and high temperature<br />
furnace applications;<br />
18. TRIBOTECHNIC: standardisation <strong>of</strong> tribologicals and scratch tests for coatings.<br />
Our clients: Universities <strong>of</strong> technology, universities, medical academies, agricultural<br />
academies, departments <strong>of</strong> Polish Academy <strong>of</strong> Sciences, scientific and research organizations,<br />
industrial and environmental laboratories, private and governmental companies, foundations.<br />
�