Size-dependence of anisotropic exchange interaction in InAsG

A comprehensive study of the exchange interaction between charge carriers in self-organized InAs/GaAs quantum dots is presented. Single quantum-dot cathodoluminescence spectra of quantum dots of different sizes are analyzed. Special attention is paid to th

Size-dependenceofanisotropicexchangeinteractioninInAs/GaAsquantumdots

R.Seguin ,S.Rodt,A.Schliwa,K.P¨otschke,U.W.Pohl,andD.Bimberg

Institutf¨urFestk¨orperphysik,TechnischeUniversit¨atBerlin,Hardenbergstr.36,10623Berlin,Germany

Acomprehensivestudyoftheexchangeinteractionbetweenchargecarriersinself-organized

InAs/GaAsquantumdotsispresented.Singlequantum-dotcathodoluminescencespectraofquan-

tumdotsofdi erentsizesareanalyzed.Specialattentionispaidtotheenergeticstructureofthe

chargedexcitedexciton(hottrion).Avaryingdegreeofintermixingwithinthehottrionstatesleads

tovaryingdegreesofpolarizationofthecorrespondingemissionlines.Theemissioncharacteristics

changefromcircularlypolarizedforsmallquantumdotstoellipticallypolarizedforlargequantum

dots.The ndingsareexplainedbyachangeofmagnitudeoftheanisotropicexchangeinteraction

andcomparedtotherelatede ectof ne-structuresplittingintheneutralexcitonandbiexciton

emission.

I.INTRODUCTION

arXiv:cond-mat/0610460v1 [cond-mat.mtrl-sci] 17 Oct 2006Electron-holeexchangeinteractioninsemiconductorquantumdotshasbeenthesubjectofalivelydebateinrecentyears.Thefocuswasplacedonitsimpactontheexcitongroundstate[1,2,3].Insystemswithlowcon nementsymmetry(C2v)[4,5]theanisotropicexchangeinteractionliftsthetwofolddegeneracyoftheexcitonbrightstatesyieldingtheso-calledexciton ne-structuresplitting(FSS).IftheFSSislargerthantheexciton’shomogeneouslinewidth,itinhibitstheemissionofentangledphotonpairs[6].Thus,theFSSisthekeyparameterdeterminingthesuitabilityofquantumdotsforsingle-photonsourcesforquantumcryptography[7]andquantumcomputing[8].Thetwobrightexciton(X)statesyieldtwospectrallynarrowlinesatlowtemperaturewhicharecommonlylinearlypolarizedinthe[110]and[1¯10]crystaldirections,respectively.Sincethebiexciton(XX)netspinis0,theFSScanalsobeobservedintheXXtoXdecay(Fig.1(a))[9].Moreover,exchangeinteractionalterstheenergeticstructureoftheexcitedpositivetrion(X+ )whichcomprisesonesingletandthreetripletstatesallofwhicharetwofolddegenerate[10].ContrarytotheXcase,thisdegeneracyisnotliftedunderexchangeinteraction.However,itleadstoamixingofthetripletstates[10]whichcanbeprobedbyanalyzingtheluminescencelinesfromthepositivelychargedbiexciton(XX+)toX+ decay.Suchprobingispossiblesincethegroundstateoftheinitialstate,namelytheXX+,iscompletelydegenerate,analogoustotheXXcase.Thedegreeofpolarizationoftheselinesre ectsthedegreeofintermixingofthedi erentchargedtrionstatesandthereforethemagnitudeoftheanisotropicexchangeinteraction(Fig.1(b))[11].Understandingthise ectiscrucialfortheuseoftrionstatesinfuturespinmemorydevices[12].WedemonstrateherethatthesizeofthequantumdotshasastrongimpactonthemagnitudeoftheanisotropicexchangeinteractionbymonitoringthedegreeofpolarizationoftheXX+toX+ emissionlinesandcompareourresultstoapreviousstudyoftheexcitonicFSSobtainedonthesamesample[13].II.EXPERIMENTALThesampleinvestigatedwasgrownbymetalorganicchemicalvapordepositiononaGaAs(001)substrate.A300nmthickGaAsbu erlayerfollowedbya60nmAl0.6Ga0.4Asdi usionbarrierand90nmGaAsweregrown.FortheQDlayernominally1.9monolayersofInAsweredepositedfollowedbya5sgrowthinterruption.Antimonywasaddedduringthegrowthinterruptionactingasasurfactant.Subsequently,theQDswerecappedwith50nmofGaAs.Finally,a20nmAl0.33Ga0.67Asdi usionbarrieranda10nmGaAscappinglayerweredeposited.ThesamplewasexaminedwithaJEOLJSM840scanningelectronmicroscopeequippedwithacathodoluminescencesetupdescribedinRef.[14].ItwasmountedontoaHe owcryostat,whichprovidedtemperaturesaslowas6K.Theluminescencewasdispersedbya0.3mspectrometer.Thedetectionsystemconsistedofaliquid-nitrogencooledSi

charge-coupled-devicecameraandaliquid-nitrogencooledInGaAsdiodearray.TheFWHMasgivenbythesetupwas≈140µhttp://doc.guandang.netinglineshapeanalysiswecandeterminetheenergeticpositionofresolution-limitedemissionlinestoanaccuracyof≈20µeV.Metallicshadowmaskswith100nmapertureswereappliedinordertoreducethenumberofsimultaneouslyprobedquantumdots.

A comprehensive study of the exchange interaction between charge carriers in self-organized InAs/GaAs quantum dots is presented. Single quantum-dot cathodoluminescence spectra of quantum dots of different sizes are analyzed. Special attention is paid to th

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FIG.1:In uenceoftheanisotropicelectron-holeexchangeinteractiononthebiexciton→exciton→0(a)andthecharged-biexciton→excited-trion(b)cascades.|M givesthetotalangularmomentumofthecorrespondingexcitoniccomplex.Graycircles,arrows,andellipsesmarkthepolarizationofthecorrespondingemission.Dashedlinesmarkforbiddentransitions.TheX+ singletstatesareomittedforsimplicity.

Thequantumdotsexaminedformaseriesof

subensembles,eachrepresentingquantumdotswitha xedheight,rangingfromtwotomorethannineInAsmonolayers[15,16].Intheensemblespectra,thesubensemblesemitrathernarrowpeaks(FWHM≈30meV).Theysuperimposetogivethecompleteensemblespectrum,consistingofeightofsuchpeaks.Thisallowsforapreciseassignmentofagivenemissionenergytoacorrespondingquantumdotheight.

Energy (eV)

1.0491.0501.0511.0521.0531.0541.055

CL Intensity (arb. units)1.3221.3231.3241.3251.3261.3271.328

Energy (eV)

FIG.2:Linearlypolarizedspectrafortwodi erentquantumdotsemittingathighandlowenergiesareshown.Thedi erentFSSmagnitudesandthedi erentXX+degreesofpolarizationareclearlyvisible.NotethatbothX-axesaredrawntothesamescaleforeasiercomparison.

TwospectraofalargeandasmallsinglequantumdotemittingatlowandhighenergiesrespectivelyareshowninFig.2.Thedi erentexcitoniccomplexeswereidenti edaccordingtoRef.[17].WhiletheXX+emissionlinesfromthesmallquantumdotarecircularlypolarizedthelargequantumdotemitsellipticallypolarized …… 此处隐藏：7027字，全部文档内容请下载后查看。喜欢就下载吧 ……