Carrier dynamics study of ZnSe and ZnSe based quantum well structures by using time-resolved spectroscopy

280 Pages·1996·8.9 MB·English

by  Wang, Li

CARRIERDYNAMICSSTUDYOFZnSeANDZnSeBASEDQUANTUMWELL
STRUCTURESBYUSINGTIME-RESOLVEDSPECTROSCOPY
BY
LIWANG
ADISSERTATIONPRESENTEDTOTHEGRADUATESCHOOLOFTHE
UNIVERSITYOFFLORIDAINPARTIALFULFILLMENTOFTHEREQUIREMENTS
FORTHEDEGREEOFDOCTOROFPHILOSOPHY
UNIVERSITYOFFLORIDA
1996
ACKNOWLEDGEMENTS
Firstofall,Iwouldliketothankmyresearchadvisor,Dr.JosephSimmons,
forallhisguidanceandsupportonthisworkinmanyways.Theexperienceof
workingwithhimhasbeenextremelyrewarding.
Secondly, Iwould liketo express mygratitudeto Dr. Robert Parkand
MinyongJeonforfabricatingtheMBEsamplesusedinthiswork.Thisworkwouldn't
bepossiblewithouttheirsamples.
Finally,Iwanttothankmyparents,mybrotherandsisterfortheirloveand
encouragementwhichhasalwaysbeenasourceofinspiration.
ii
TABLEOFCONTENTS
ACKNOWLEDGEMENTS
ii
ABSTRACT v
CHAPTERS
1 INTRODUCTIONTOCARRIERDYNAMICSANDITSAPPLICATION
TOSEMICONDUCTORRESEARCH 1
Overview 1
PhotoexcitedProcessesofCarriersInSemiconductors 4
OpticalProbeAsaTooltoStudy
CarrierDynamicalProcesses 6
ApplicationsofCarrierDynamicsStudyTo
HighFrequencyResponseDeviceDesign 8
FutureDevelopmentAndOutlook 10
2 COMMONEXPERIMENTALTECHNIQUESUSEDTO
STUDYCARRIERDYNAMICSINSEMICONDUCTORS
ANDTHEIRBASICPRINCIPLES 12
CoherentTime-resolvedSpectroscopy 13
IncoherentTime-resolvedSpectroscopy 30
SelectionCriteriaofSuitableTechniques 49
3 REVIEWONPASTPROGRESSONSEMICONDUCTORCARRIER
DYNAMICSSTUDY 52
SemiconductorBulkAndFilms 52
SemiconductorQuantumSizeStructures 65
SemiconductorDevices 75
4 CARRIERDYNAMICSINVESTIGATIONOFll-VISEMICONDUCTOR
FILMSANDQUANTUMSTRUCTURES(ZnSeANDCdZnSe/ZnSe
MULTIPLEQUANTUM WELLS) 80
ill
PropertiesAndApplicationsofll-VISemiconductors 80
MaterialStructuresStudied 85
FabricationByMolecularBeamEpitaxy 87
5 EXPERIMENTALTECHNIQUESUSEDANDTHEIRSETUPS 94
UltrafastTi:SapphireLaserSystemAsTheExcitationSource:
GeneralCharacteristicsAndOperatingPrinciples 94
PopulationMixing(orPicosecondExcitationCorrelation
Spectroscopy):PrincipleAndExperimentalSetup 116
TransientDifferentialAbsorption:PrincipleAnd
ExperimentalSetup 122
6 EXPERIMENTALRESULTSONQUANTUMWELLSAND
THEORETICALANALYSIS 129
QuantumMechanicalCalculationofBandStates 129
RoomTemperatureResults 150
QuantumMechanicalAnalysisOnRoomTemperatureResults...152
LowTemperatureResults:ExcitonAndBiexcitonDynamics 168
AnalysisOnLowTemperatureData 169
TemperatureDependenceStudyofCarrierLifetime 192
LasingMechanisminCdZnSe/ZnSeDiodeLasers 196
OpticalDegradationStudy 206
HotPhononEffect 212
CarrierTunnelingStudy 217
TransientDifferentialAbsorptionResults 222
7 EXPERIMENTALRESULTSONEPITAXIALFILMSAND
BULKCRYSTAL 231
RoomTemperatureResults:FreeCarrierDynamics 231
LowTemperatureResults:FreeAndBoundExcitonDynamics...237
8 CONCLUSIONSANDFUTUREWORK 251
ConclusionsofCarrierDynamicsStudy 251
FutureWork 265
REFERENCES 257
BIOGRAPHICALSKETCH 272
iv
AbstractofDissertationPresentedtotheGraduateSchool
oftheUniversityofFloridainPartialFulfillmentofthe
RequirementsfortheDegreeofDoctorofPhilosophy
CARRIERDYNAMICSSTUDYOFZnSeANDZnSeBASEDQUANTUMWELL
STRUCTURESBYUSINGTIME-RESOLVEDSPECTROSCOPY
BY
WANG
LI
AUGUST,1995
Chairman:Dr.Joseph.H.Simmons
MajorDepartment:MaterialsScienceandEngineering
ZnSeandZnSebasedquantumwellstructuresarepotentialcandidatesfor
blue/greenlightemittingdevicesandopticalswitches.Carrierdynamicsstudiesof
these structures are important in evaluating device performance as well as
understandingtheunderlyingphysicalprocesses.Inthiswork,acarrierdynamics
investigationwasconductedonZnSebulkandCd^Zni.^Se/ZnSe(x=0.2and0.25)
quantumwellstructuresgrownbymolecularbeamepitaxy(MBE)atbothroom
temperatureandlowtemperatures.Atroomtemperature,thedynamicsoffree
carriersprovidesinsightsintotheband-to-bandtransitiondynamics;whileatlow
temperature,thedynamicsofexcitonicprocessesprovidesdetailedinformationon
carrierthermalizationprocessessuchascarrier-carrierinteractions,carrier-phonon
interactionsandplasmon-phononcoupling.Twotechniqueswereusedinthisstudy:
atime-resolvedphotoluminescencetechniquecalledpopulationmixing(alsocalled
V
excitationcorrelationspectroscopy),andafemtosecondpump-probetechnique
calledtransientdifferentialabsorption.BothdopedandundopedZnSefilmswere
studiedtodeterminetheroleofdopantsoncarrierdynamics.Multiplequantumwell
(MQW)structureswithdifferentbarrierwidths(50-200A)andwellwidths(25-1OOA)
werestudiedtodeterminetheeffectofcarrierlocalizationandtunnelingoncarrier
dynamics.Excitationoverawiderangeofcarrierdensities(10®-10^^/cm^)wasused
tostudyhotcarriereffectssuchashot-phononeffect,carrier-carriercollisionsand
excitonicmoleculeformation.Carrierrecombinationandthermalizationdynamics
inthree-dimensionalbulkandtwo-dimensionalquantumwellswerecomparedto
examinethedifferenceinfundamentalphysicsinvolved,suchasmuch-enhanced
oscillatorstrength,higheropticalnonlinearityandstrongerearnerscreeningof2D
structures.Themostimportantresultofstudiesonquantumwellsistheobservation
ofbiexcitons(orexcitonicmolecules).Biexcitonsareformedduetocondensation
ofahighdensityofexcitons,anditsemissionanddecaycharacteristicsareclosely
relatedtothoseoftheexcitons.Biexcitongainwasproposedasthemechanismof
stimulated emission in these quantum well structures based on the lasing
characteristicsofCdZnSe/ZnSeMQWlasersatcryogenictemperatures.Themost
importantresultofbulkstudiesistheidentificationofdifferentexcitonicprocesses,
suchasvariousdonorandacceptorboundexcitons,bytheircharacteristictemporal
decaybehaviors.Theinterestinstudyingthesedynamicprocessesarisesfromthe
currentdevelopmentofdiodelasersaswellasultrafasthotelectrontransistorsand
tunnelingdiodes.
vi
CHAPTER
1
INTRODUCTIONTOCARRIERDYNAMICSANDITSAPPLICATIONTO
SEMICONDUCTORRESEARCH
Overview
Investigation of carrier dynamics plays a central role in modern
semiconductorscience. Properties ofphotoexcited carriers are determined by
various interactions between carriers and other elementary excitations in the
semiconductor. Therefore, investigation of excited carrier properties provide
Informationaboutscattering processesthatareoffundamental interest inthe
physicsofsemiconductors. Furthermore, theseprocessesdeterminehigh-field
transportphenomenainsemiconductorsandthusformthebasisofoperationsof
manyultrafastelectronicandoptoelectronicdevices.Thefieldofcarrierdynamics
thusprovidealinkbetweenfundamentalsemiconductorphysicsandhigh-speed
devices.
Animportantmilestoneinthefieldofcarrierdynamicsinsemiconductorsis
thedevelopmentofultrafastopticalspectroscopy.Opticalexcitationcancreatehot
carriersandopticalspectroscopycanprovideinformationaboutthedistribution
functionofexcitedcarriers.Althoughtransportmeasurementsprovideconsiderable
information about various scattering processes in semiconductors, they are
averagedoverthecarrierdistributionfunctions.Incontrast,opticaltechniques,by
1
2
providingthebestmeansofdeterminingthecarrierdistributionfunctions,allowone
toinvestigatethemicroscopicscatteringprocesses.Amajoradvanceinthefieldof
opticalspectroscopycamerecentlywhenuitrafastlaserswithpulsewidthsasshort
as6fsbecameavailable.Theselasersallowtheinvestigationofthetimeevolution
ofcarrierdistributionsonultrashorttimescales.Sincedifferentprocessesoccuron
differenttimescales,itbecamepossibletoisolatevariousscatteringprocessesby
appropriatechoicesoftimewindows.
The emergence of semiconductor quantum size structures grown by
molecularbeamepitaxyinlate1970shaschangedthedirectionofthefieldof
carrierdynamicsconsiderably.Quantumwell(QW)heterostructuresconsistingof
thinlayersofsemiconductorswithdifferentbandgapshavemanyuniqueproperties,
suchasresonanttunnelingandballistictransport.Thesepropertieshaveattracted
muchresearchattentionduetopotentialnoveldeviceapplicationsbesidesthe
fundamentalinterest.Infact,progressinunderstandingthesenanostructureshas
ledtothedevelopmentofnewfastresponsedevicessuchasmodulation-doped
field-effecttransistorsandresonanttunnelingdiodes.
Electronicstatesinaquantumconfinedsystemaredifferentfromthosein
abulksemiconductor.Theconductionandvalencebandsbreakupintovarious
subbandsasaresultofconfinement.Thewavefunctionsoftheconfinedstates
penetrateintothebarrierforfinitebarrierheightsbutvanishattheboundaryfor
infinitelyhighbarriers. Forthickbarriers, eachwell inamultiplequantumwell
structurecanbetreatedasindependentofotherwells.Withdecreasingbarrier
3
thickness,thewavefunctionsofadjacentwellsoverlapwitheachotherandleadto
theformationofminibands,withinterestingtransportproperties.Whiletheelectron
statesaresimple,theholestatesareextremelycomplicatedinquasi-2Dstructures.
The valencebands in bulkzincblendesemiconductorsare nonparabolicand
anisotropic.Inclusionofquantizationeffectsleadstoverycomplicatedstructures
forheavyandlightholesubbands. Adetailed understandingofholebandsis
clearlynecessaryinordertounderstandcarrierdynamicsinvolved.
Fundamental carrier dynamic processes of interest in semiconductor
nanostructures include electron-phonon interactions, many-body physics, hot-
phononeffects,carriertunneling,andquantumtransport.Amongtheseprocesses,
thelasttwoarepossessedonlybynanostructures.Opticalspectroscopiessuchas
ultrafast luminescence, femtosecond pump-and-probe transmission, Raman
scattering, and coherent four-wave mixing are very valuable tools for the
investigation ofthese processes. Each ofthese techniques provides detailed
informationaboutaspecificprocess,forexample, luminescencereflectscarrier
recombinationdynamics,pump-probetransmissionreflectscarrier-carrierscattering
dynamics, Raman scattering reflects phonon dynamics, andfour-wave mixing
reflectscoherentdynamics,therefore,thecombinationofthesetechniqueswill
provideuswithawholepictureofcarrierrelaxationdynamics.
MostresearcheffortsinthepasthavebeenfocusingonGaAsandGaAs
based quantumwell structures. Tremendousprogress has been madeand is
reflectedthroughthedevelopmentofanewclassofhighfrequencyresponse
4
devicesandnewadvancesintheoreticalphysics. NewmaterialssuchasZnSe
haveattractedmuchattentionrecentlyduetotheirpotentialapplicationsaslight
emittingdevices(lasersandlightemittingdiodes)andopticalswitches.Asaresult,
a fundamental understanding of these materials is of immediate practical
importancebesidesfundamentalphysicsinterest.Thiscarrierdynamicsstudyon
ZnSeandZnSebasedquantumstructureswillprovideuswithinsightsintothe
propertiesofthismaterial.
PhotoexcitedCarrierProcessesinSemiconductors
Excitation. Whenatypicallll-Vorll-VIsemiconductorisexcitedbyultrafast
laserpulseswithanenergyequaltoorlargerthanthesemiconductorbandgap,
electronandholepairsarecreatedthroughinterbandabsorption.Ingeneral,one
excites electrons from all three valence bands in such semiconductors. The
distributionsofphotoexcitedelectronsandholesaredeterminedbythefrequency
andspectralwidthofthelaserpulseandthedispersionoftheconductionandthe
valencebands.
Thermalization. The photoexcited semiconductorgoesthrough several
nonequilibriumstatesbeforeitreturnstothermalequilibrium.Immediatelyfollowing
photoexcitation,thephotopopulatedelectronicstatesstillretaincoherenceamong
themselvesandwiththeelectromagneticradiation.Ifthedephasingtime,orthe
timetolosethecoherence,isfasterthananyotherrelaxationprocess,thenthe
carrierdistributionfunction(DF)isnonthermalevenafterthephotopopulatedstates
have losttheircoherence. Subsequently, the energy in the carriersystem is