Scanning Gate Microscopy (SGM) of semiconductor ... - GDR Meso

Scanning Gate Microscopy (SGM)
of semiconductor nanostructures
H. Sellier, P. Liu, B. Sacépé, S. Huant
Dépt NANO, Institut NEEL, Grenoble, France
B. Hackens, F. Martins, V. Bayot
UCL, Louvain-la-Neuve, Belgique
M. Pala
IMEP, Minatec, Grenoble, France
L. Desplanque, X. Wallart
IEMN, Lille, France
GDR 2426 Physique Quantique Mésoscopique
Session thématique « Champ proche »
2 - 4 novembre 2010
1

Outline
1. Description of SGM technique
- context
- potential
- operation
2. Review of SGM experiments
- contributors
- microscopes
- quantum point contact
- quantum dot
- quantum Hall effect
- quantum ring
3. ANR project on electron interactions
- objectives
- strategy
2

Introduction to SGM
Local probe of electron properties in semiconductor heterostructures
where electrons are several tens of nanometers below the surface
thus not accessible by Scanning Tunneling Microscopy
2DEG
• Quantum Point Contact
• Quantum Wire
• Quantum Dot
• Quantum Ring
• Quantum Hall Effect
V
I
V
3

SGM versus STM
STM
Scanning Tunneling Microscopy
SGM
Scanning Gate Microscopy
I
e -
V
Φ
+ +
+
V tip
• conducting surface
• surfaces, nano-objects, defects
• tunneling current
• local density of state
I
V
• insulating surface
• high mobility 2DEG heterostructure
• conductance of device
• local gate effect
4

Tip induced scattering potential
Low density electron gas ⇒ imperfect screening of the tip potential
⇒ local potential change ⇒ modified electron scattering
V tip
< 0
equipotential lines - -
-
V contact
= 0
V 2DEG, local
< 0
Other ingredients :
Contact potential
Dielectric constants
Etched trenches
Surface gates
Charged defects
5

Tip induced scattering potential
Examples in the SGM literature :
Crook et al, Phys. Rev. B (2000)
Aidala et al, Nat. Phys. (2007)
Another model :
based on Krcmar et al, Phys. Rev. B (2002)
z 0
+ ε r1
ε r2
6

Tip induced scattering potential
SGM = scattering method
(STM = intrinsic LDOS)
E = E C
- e V
E = E C
- e V
E F
x tip
x
E F
x
Medium electron density (N ~ 10 12 cm -2 )
- small perturbation
x tip
Low electron density (N ~ 10 11 cm -2 )
- strong back-scattering
Leroy, 2003
PhD thesis
7

SGM operation
Device:
High mobility 2DEG
Device patterning
(surface gate and/or etching)
Instrument:
Low temperature AFM
( 4 He, 3 He, dilution)
with magnetic field
Positioning:
AFM topographic image
to locate the device
Scanning:
Tip scan at constant distance
with applied voltage V tip
while measuring conductance G
2DEG
doped barrier
undoped channel
buffer layer
substrate
I
V
Result:
Image of the local gate effect
on the global device conductance
8

Outline
1. Description of SGM technique
- context
- potential
- operation
2. Review of SGM experiments
- contributors
- microscopes
- quantum point contact
- quantum dot
- quantum Hall effect
- quantum ring
3. ANR project on electron interactions
- objectives
- strategy
9

SGM around the world
Start Place Group 2DEG
1996 US - Harvard Westervelt, Eriksson, Topinka,
Leroy, Bleszinski, Aidala,...
US - Santa-Barbara
1999 US - Berkeley McEuen, Bachtold, Woodside,... US - Stanford
US - Santa-Barbara
2000 UK - Cambridge Ritchie, Smith, Crook,... UK - Cambridge
2004 CH - Zürich Ensslin, Ihn, Pioda, Gildemeister,
Baumgartner,...
D - Regensburg
US - Santa-Barbara
2005 US - Arizona Ferry, Aoki, DaCunha,... JAP ? (InGaAs)
2006 F - Grenoble Huant, Bayot, Hackens, Martins,
Sellier,...
2007 US - Stanford Goldhaber-Gordon, Jura,
Topinka,...
F - IEMN (InGaAs)
US - Bell Labs
2010 I - Pisa Heun, Paradiso,... US - Bell Labs
2010 B - Louvain Bayot, Hackens, Martins,... F - IEMN (InGaAs)
10

SGM tips
Piezoresistive AFM cantilevers
Tortonese, APL (1993)
ThermoMicroscopes, CA
not produced any more ...
Piezoelectric quartz tuning fork
Karrai (1995) Giessibl (1996)
SNOM, AFM, STM, SGM
Example :
Harvard
Example :
Grenoble
11

SGM microscopes
Grenoble
4
He 4K
9T
Heun
3
He 400mK
9T
12

SGM microscopes
Westervelt
3
He 400mK
7T
Ensslin
3
He 4 He 100mK
8T
13

Quantum Point Contact
Harvard (Westervelt)
Imaging electron flow
Creates interferences
Topinka et al, Nature (2001)
1.7 K
n = 4.5 x 10 11 cm -2
µ = 1 000 000 cm 2 /Vs
2DEG 57 nm below surface
tip at 13 nm
V tip
= -3 V
14

Quantum Point Contact
Harvard (Westervelt)
Imaging electron flow
and cyclotron orbit
under magnetic field
Aidala et al, Nat. Phys. (2007)
4.2 K
n = 3.8 x 10 11 cm -2
µ = 500 000 cm 2 /Vs
2DEG 47 nm below surface
15

Quantum Point Contact
Arizona (Ferry)
Universal Conductance Fluctuations
DaCunha, Aoki, et al, Appl. Phys. Lett. (2006)
280 mK
16

Quantum Point Contact
Cambridge (Ritchie)
Tuning QPC conductance
« 0.7 anomaly »
Erasable Electrostatic Lithography
Crook et al, Science (2006)
150 mK
n = 3.1 x 10 11 cm -2
µ = 5 000 000 cm 2 /Vs
2DEG 97 nm below surface
17

ETH Zürich (Ensslin)
Coulomb blockade resonances
Analysis of tip potential
(AFM nanolithography)
Quantum Dot
Pioda et al, Phys. Rev. Lett. (2004)
300 mK
n = 5 x 10 11 cm -2
µ = 450 000 cm 2 /Vs
2DEG 34 nm below surface
18

Quantum Dot
Harvard (Westervelt)
Spectroscopy of single electron dot
Tip potential width >> dot size...
Fallahi et al, NanoLetters (2005)
1.7 K
n = 3.8 x 10 11 cm -2
µ = 470 000 cm 2 /Vs
2DEG 52 nm below surface
19

Quantum Dot
Harvard (Westervelt)
Image Coulomb blockade centers
InAs nanowire with Ti/Al contacts
Bleszinski et al, NanoLetters (2007)
4.2 K
20

Quantum Dot
Berkeley (McEuen)
Carbon nanotube with kinks
Image Coulomb blockade centers
+ Single Electron Force Microscopy
Woodside et al, Science (2002)
SGM @ 6 K
EFM @ 0.6 K
21

Quantum Hall Effect
SGM at high magnetic field :
- Berkeley (McEuen)
- ETH Zürich (Ensslin)
Transmission by edge states : no back scattering
⇒ SGM images only at the transition between plateaus
See talk by B. Hackens, Louvain-la-Neuve (Belgium)
22

Quantum Rings
Grenoble 2 + Louvain + Lille
SGM experiments (NEEL)
MBE growth (IEMN)
E-beam lithography (UCL)
600 nm
Theory and simulation (IMEP)
N s
~ 2 x 10 12 cm -2
µ ~ 100 000 cm 2 /Vs [4K]
L e
~ 2 µm [4K] ballistic
L φ
≥ µm [4K] coherent
23

Quantum Rings
Aharonov-Bohm interferences by SGM
B. Hackens et al., Nature Physics (2006)
Dephasing by tip potential
electrostatic A-B effect
e
= 2π ∫ ( V1
− V ) dt
h
Δϕ
2
Dephasing by magnetic field
magnetic A-B effect
iso-phase lines
=
information
on electron
wave function
interferences
24

Quantum Rings
Experiment
Local Density of State by SGM
Simulation (Marco Pala, IMEP, Grenoble)
F. Martins et al, Phys. Rev. Lett. (2007)
Influence of defects and magnetic field :
Analytical model for single channel :
M. Pala et al., Phys. Rev. B. (2008), Nanotechnology (2009)
25

Outline
1. Description of SGM technique
- context
- potential
- operation
2. Review of SGM experiments
- contributors
- microscopes
- quantum point contact
- quantum dot
- quantum Hall effect
- quantum ring
3. ANR project on electron interactions
- objectives
- strategy
26

0.7 anomaly in QPC
27

ANR ITEM
ITEM = Interaction et Transport à l'Echelle Mésoscopique
ITEM-Th (2008)
J.L. Pichard, CEA, Saclay
R. Jalabert, D. Weinmann, IPCMS, Strasbourg
1D chain
U = 0 U = 1.7
Freyn et al, Phys. Rev. Lett. (2008)
28

ITEM = Interaction et Transport à l'Echelle Mésoscopique
ITEM-Exp (2010)
H. Sellier et al, Néel, Grenoble
M. Sanquer et al, CEA, Grenoble
A. Ouerghi et al, LPN, Marcoussis
ANR ITEM
29