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APPLIED PI-IYS1C:S LETTERS VOLUME 79, NUMBER 8 20 AlJGUST 2001 Observation of an anomalously large blueshift of the photoluminescence peak and evidence of band-gap renormalization in InP/lnAs/lnP quantum wires Xiaodong Mu, loulia B. Zotova, Yujie J. ~ ing,~) Haeyeon Yang, and Gregory J. Salamo Department of Physics, UniversiT). ofArkansn.s, Fayetteville. Arkarisas 72701 (Rcceivcd 17 November 2000; acccpted for publication 5 Junc 200 1) We havc investigated polarization-dependent pl~otolu~nincscence InPllnAslI~iP quantum wires directly fonned on the top of InP substrates. With excitation laser intensity we have observed an anon~alously large blueshift of the photolun~incscence pcak using a cw lascr with cxtremcly low intensities. We have also observed evidence of band-gap renormalization. In addition, we have measured two-photon luminescence spectra and confirmed their dependence on photoluminescence polarization. O 2001 American Institute of' Physics. [DOI: 10.106311 .I 3904831 Quantum wires can be formed by the T intersection of set of 460 meV. For simplicity, we have neglected strain two GaAs quantum wells.' It has been reported that near- anisotropy and excitonic binding energies. Our rough estiinfrared optical study of single GaAs quantum wires grown mates of the peak transition energies have confinned the avon a patterned (311)A GaAs surface can reveal interesting erage height and width of the quantum wires directly mcaeft'ects.here are also some efforts for obtaining vertically sured by STM. As one can see, depending on the PI; stacked quailtun1 wircs fabricated 011 GaAs(311)A pattcmcd polarizatioil the PL intensity is differcnt. Figurc 2 shows the substrates.~ecei~tly, high-density Ids nanowires embedded in an In0,52A10.48A~ matrix were fabricated in situ by molecular-bean1 epitaxy (MBE) on a (100) InP substrate." Most recently, it was demonstrated that InP buffer layers grown by MBE produced quantum-wired str~ctures.~ In both Refs. 4 and 5, the photoluminescence (PL) intensity is strongly polarized along onc dircction. Quantum wires can also be formed on As-diffused structures." In this letter, we present our results of polarizationdependent PL studies on InPIInAslInP quantum wires. In addition, we have also observed polarization-dependent twophoton lun~inescence. Furthemlore, we have observed an anomalously large blueshift of the apparent PL peak and evidence of band-gap renoinlalization using a continuous-wave (CW) laser bcan~. Our quantiun-wire sample was grown using a Riber 32 MBE system. A 0.3 pln InP buffer layer and then 3.8 ML of InAs were directly grown on a (100) InP substrate at the ternperahre of 480 "C and a growth rate of 0.3 MLIs. A 300 A InP cap layer was grown without interruption on the top of the InAs layer. Our in situ scanning-tunnelingnlicroscope (STM) picture (as shown in Fig. 1) reveals that quantum wires are fomled with the average height of about 23 A, width of 150 A, and length of about I pin. Our recent work7 has demonstrated that if quantum wires are annealed at 480 "C: without As overpressure quantum dots could be formed. We have used a cw Ti:sapphire laser beam as a pump beam to measure the PL spectra shown in Fig. 2. The peak wavelength and linewidth are determined to be 1.4345 pm and 55 meV, respectively, based on Fig. 2. Wc have calculated the peak transition energies by using a free-particle model. In our calculations. we have used a valence-band off- "'Electronic mail: yding@ua].k.edu FIG. 1. STM pichlre of quanhlrn wires 0003-6951 /2001/79(8)11091131$18.00 1091 O 2001 American Institute of Physics Downloaded 09 Mar 2008 to 130.184.237.6. Redistribution subject to AIP license or copyright; see http:llapl.aip.orglapllcopyright.jsp
JOURNAL OF APPLIED PHYSICS VOLUME 89, NUMBER I2 15 JUNE 2001 Formation of quantum wires and dots on lnP(001) by AsIP exchange Haeyeon ~ang,~) P. Ballet, and G. J. Salamo Depi~rlmenl of Physics, Universiy of Arkansas. Fflj~etteville, Arkclnsas 72701 (Received 18 September 2000; accepted for publication 27 March 2001) We report on the use of in situ scanning tunneling microscopy to study ASP exchange on InP(001) surfaces by inolecular beam cpitaxy. Results demonstrate that the exchange process can be controlled to selectively produce either quantum wires or quantum dots. 15 nm wide self-assembled nanowires are observed, and they are elongated along the dimer row direction of the lnP(001)-2 X 4 surface with a length of over 1 pin and flat top 2 X 4 surfaces. In addition, when the nanowires arc annealcd with no arsenic ovelpressure, the surfacc reconstruction transforms from 2 X4 to 4 X 2 and the llanowires transfornl into dots with a rectangular base and flat top. O 2001 American hntihite qf Physics. [DOI: 10.10631 1,13726221 INTRODUCTION Driven by the promise for different devices,' there has been great interest in the growth of nanostructures. The growth of heteroepitaxial nanostructures involves several key issues such as strain due to a lattice mismatch, or material exchange due to diffusion between different layers. Both of these effects, usually considered detrimental to quality interfaces, can play significant roles in nanostructure formation. For example, self-assenlbled three dimensional (3D) nanostructures are realized by utilizing the strain between materials with different lattice constants such as InAs quantum dots on GaAs substrates. In addition, coupled with the formation of nanostructures, it is well known2 that when an 11~4s layer is fornled on GaAs, the Ga atoms and In atoms diffuse and exchange. This inteinlixing affects the composition of the nanostructure and plays a significant role in the nature of the confined electronic state^.^ Other quantum confined systems which are driven by a lattice mismatch, such as, InAs dots on InP s~bstrates~,~ and GaSb on GaAs substrates,%ave been studied to exploit their electronic propcrties. Growths of these hetcrostructurcs also involves active exchange of group V elements. The exchange of group V elements has been studied in many systems such as AS IS^,^ s~R,' and ASP.".'^' The focus of these studies tcnds to be on the effccts of cxchange on interface quality for heteroepitaxial growth. While such exchange may be viewed as a problem, like strain, it can also be a useful tool. For example, exchange of Asp has been used to remove the oxide layer on InP s~rfaces,"~'~ where the resultant interface leads to epilayers of InGaAs with excellent electrical properties. In this article, we present results that dcmonstrate that intertnixing can also be a useful tool in the formation of self-assembled 3D nanostructures. Previously reported studies of InAs nanostructures on planar InP(001) surfaces are on quantum dots formed either by exchange4 or by direct deposition.'3~'4 Although quantum wire formation has also been reported recently by direct deposition of InAs on InP(001) s~bstrates,'~,'~ there are no "Electronic mail: hayan~(irjcomp.uark.cdu 0021 -8979/2001/89(12)/787114/$18.00 comesponding reports on nanowire formation by an cxchange process. Here we have demonstrated the foimation of nanowires on an In(001) surface by simple exchange of As/P, cxtending the possibility of using an exchange process to fabricate devices based on quantum wires in heteroepitaxial growths. We also report that by simple control of the As overpressure, and the corresponding surface reconstniction, either nanowires or dots can be selected for growth. The experiment is carried out in a molecular beam epitaxy (MBE) chamber with solid sourccs of arscnic and phosphorous which are equipped with valves to provide control over fluxes. The substrate temperature is measured using optical transmission thcrnlomctry for reproducibility and absolute measurement to within 2"~.'~ After loading a conlmercial n-type planar (miscut within 0.05") InP(001) wafer into the MBE chamber, the oxide layer on the wafer is removed by annealing the wafer above 480 "C under a cracked (950"C) phosphorous (P2) beam equivalent pressure (BEP! of lo-' Torr. The resultant surface yields 2 X4 patterns in reflection high energy electroil diffraction (RHEED), similar to those of 2 X4 patterns of a GaAs(001) surface.'' After oxide removal, a 0.3 pm thick buffer layer of InP is grown on the substrate at 470°C. To have a smooth InP(001)-2 X4 surface, the substrate is annealed at 500 "C under a P, pressure of 1 X Torr for 15 min and then cooled to 480 OC. At 480 "C, thc valvc for thc P2 flux is closed and the residual phosphorous is pumped out for about 3 min. During this pcriod the RHEED continuously indicates a stable 2 X 4 surface reconstruction that is independent of the P2 ovcrpressurc. After preparing an InP surface, the sample is rapidly cooled down below 250 "C and transferred to the scanning tunneling microscope (STM) chamber through the ultrahigh vacuum modutrack. STM images are takcn for fillcd states (-3 V on the sample) with tunneling current around 100 PA. The STM images of the starting surface of InP(001), just before deposition of InAs, are shown in Figs. l(a) and l(b). The images show terraces, vacancy islands, monolayer (ML) high steps and 2 X 4 surface reconstruction with dimer rows [the inset in Fig. I (b)] which are along the [Mo] direction. The STM images indicatc a beautiful lnP(00 1) surfacc even '871 O 2002 American Institute of Physics Downloaded 09 Mar 2008 to 130.184.237.6. Redistribution subject to AIP license or copyright; see http://jap.aip.org/japlcopyright.jsp
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