A strain relief model for the InxGa1-xAs/GaAs heteroepitaxial system and its application toward the growth of low-dislocation density large in content InxGa1-xAs

135 Pages·1992·4.7 MB·English

by  Krishnamoorthy, Viswanath, 1965-

ASTRAINRELIEFMODELFORTHEInxGai-xAs/GaAs
HETEROEPITAXIALSYSTEMAND ITSAPPLICATIONTOWARDTHE
GROWTHOFLOW-DISLOCATIONDENSITYLARGEInCONTENT
InxGai-xAs
By
VISWANATH KRISHNAMOORTHY
ADISSERTATIONPRESENTEDTOTHEGRADUATESCHOOL
OFTHEUNIVERSITYOFFLORIDAINPARTIALFULFILLMENT
OFTHEREQUIREMENTSFORTHEDEGREEOF
DOCTOROFPHILOSOPHY
UNIVERSITYOFFLORIDA
1992
ACKNOWLEDGEMENTS
Theauthorwishestothankhissupervisorycommitteechairman
Dr.R.M. Parkforhisguidanceandsupportduringtheresearchand
thewritingofthisdissertation. Theadviceandsupportoftheother
membersofthiscommittee, namely,Dr.K.S.Jones,Dr. P.H. Holloway,
Dr.R.T.DeHoffandDr.S.S.Li,arealsogratefullyacknowledged.
SpecialgratitudeisduetoDr.Y.W.LinandBruceLiufortheirtireless
andimpeccableworkingrowingaseeminglyendlesstiradeof
heterostructures. TheauthoralsowishestothankD.Venablesand
D.T.HoelzerfortheirintellectualandmoralsupportandC.Rouleau,
M.J.Troffer,L.CalhounandP.Ribasforputtingupwithhisnever
endingmonologues.
Theauthoralsowishestothankhisfamilyfortheirsupportand
understandingduringthesetryingtimesandhisfriendsforirkinghis
competitivespiritwithruderemarksabouthisintellectualcapacity.
ii
1
TABLEOFCONTENTS
page
ACKNOWLEDGEMENTS
ii
ABSTRACT.
CHAPTERS
1 INTRODUCTION 1
1.1 MotivationandObjectives 1
1.2 Background 3
1.2.1 CriticalThickness in Lattice-Mismatched
Systems 4
1.2.2 StrainReliefMechanisms 9
1.2.3 DislocationEvolutionintheInxGai-xAs/
GaAsMaterialSystem 14
1.2.4 CommonBufferStructuresUsedtoBlock
Dislocations 19
1.2.5 Summary 23
1.3 ScopeofthePresentWork 23
2 GROWTHANDCHARACTERIZATIONTECHNIQUES 25
2.1 MolecularBeamEpitaxy 25
2.2 CharacterizationTechniques 31
2.2.1 TransmissionElectronMicroscopy 3
2.2.1.1 Cross-sectionalTEM specimen
preparationtechnique 32
2.2.1.2 Plan-viewTEM specimen
preparationtechnique 33
2.2.2 EtchPitTechnique 33
2.2.3 ElectronMicroprobeAnalysis 34
2.2.4 HighResolutionX-RayDiffraction 34
2.2.4.1 Phillipshighresolutionx-ray
diffractometer 35
2.2.4.2 Optimizationprocedures 42
2.2.4.3 StrainanalysisusingHRXRD 46
iii
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3 STRAINRELIEFINInxGai-xAs/GaAsSINGLE
HETEROSTRUCTURES 50
3.1 Introduction 50
3.2 XTEManalysisofIn^cGalxAs/GaAsSingleLayer
Structures 51
3.3 DiscussionofResults 58
3.4 AForceBalanceMechanismforDislocation
Evolution 64
3.5 Summary 70
4 STRAINRELIEFINInxGai-xAs-BASED STRAINLAYER
SUPERLATOCESYSTEMS 71
4.1 Introduction 71
4.2 SingleStrainedLayerSuperlatticeBufferStructures..71
4.3 DesignandAnalysisofMultistageStrainRelief
SystemsContainingStrainedLayerSuperlattices 75
4.4 SummaryandConclusions 90
5 STRAINRELIEFINSTACKEDInxGai-xAs MULTILAYER
SYSTEMSNOTINVOLVINGSTRAINEDLAYER
SUFERLATTICES 91
5.1 Introduction 91
5.2 ExtentofResidualStraininSingleInxGai-xAs/GaAs
Heterostructures 92
5.3 StrainRelaxationinStackedInxGai-xAsMultilayer
Structures 102
6 CONCLUSIONSANDRECOMMENDATIONS 120
REFERENCES 123
BIOGRAPHICALSKETCH 127
iv
AbstractofDissertationPresentedtotheGraduateSchool
oftheUniversityofFloridainPartialFulfillmentofthe
RequirementsfortheDegree ofDoctorofPhilosophy
STRAINRELIEFMODELFORTHEInxGai-xAs/GaAsSYSTEMANDITS
APPLICATIONTOWARDTHEGROWTHOFLOWDISLOCATION
DENSITYInxGai-xAs
By
ViswanathKrishnamoorthy
August 1992
Chairman: Dr.Robert.M.Park
MajorDepartment: Materials Science and Engineering
StrainrelaxationoccurringintheInxGai-xAs/GaAs
heteroepitaxialsystemasafunctionofInxGai-xAscompositionwas
studied extensivelyusingtransmissionelectronmicroscopy, to
observedislocationevolution, andhighresolutionx-raydiffraction, to
determinetheextentofstrainrelaxation. BothsingleInxGai-xAs/
GaAsheterostructuresandstackedInxGai-xAsmultiplelayersystems
wereanalysed.
InthecaseofstackedmultiplelayerInxGai-xAs/GaAssystems,
itwasdiscoveredthatlow-dislocationdensityInxGai-xAstoplayers
(layersontopofthestackedstructures) canbegrownprovidedthat
thecompositionaldifferencebetweentwo adjacentInxGai-xAslayers
inthe stackismaintainedbelowadetermined criticalcompositional
difference ofAx=0.18, takingpartial lattice-relaxation properlyinto
account. Forthiscaseitwasdeterminedthatdislocationevolutionis
confinedtotheGaAssubstrateandtheInxGai-xAslayersunderlying
v
thetopInxGai-xAslayerinthestack, thetopInxGai-xAslayerbeing
essentiallydislocation-free. Thisphenomenonisattributedtoa
monotonicincreaseintheyieldstrengthofInxGai-xAsatthe
appropriategrowthtemperatureswithincreasingvaluesofx. Such
behaviourappearstopersistuptoanInxGai-xAscompositionof
approximatelyx=0.5, whereuponafurtherincreaseincomposition
(increaseinxvalue)resultsindislocationevolutioninthetoplayerof
thestack. ItispostulatedthattheyieldstrengthofInxGai-xAs
decreases atelevated temperatures (growth temperatures) with
increasingvaluesofxbeyondx=0.5.
Itwas determined thatextremelylow-dislocation density
InxGai-xAsmaterialcanbegrownonGaAsusingthestackedstructure
approachasevidencedbyetchpitanalysis. Forexample, dislocation
densities of 1-2x10)3/cm2 ancj 5-6x1O^/cm^ were recorded from
In0.35Gao.65AsandIno.48Gao.52Astoplayers,respectively. Such
In^ai-xAsalloyswouldbepotentiallysuitableforthefabricationof
photonic devices operatingat 1.3pm (x=0.35) and 1.55pm (x=0.48).
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CHAPTER 1
INTRODUCTION
1.1 MotivationandObjectives
The incorporation ofsemiconductorheterostructures into
electronicandphotonicdeviceshasinrecentyearsledtodramatic
improvementsindevice operatingcharacteristics. Forinstance,
heteroemitterbipolartransistorsdisplayhighercurrentgainthan
conventionalhomojunctionbipolartransistors, modulation-doped
field-effect transistors or high-electron mobility transistors (HEMTs)
operateathigherspeedsthandoconventionalfield-effecttransistors
anddouble-heterostructurediodelasershavegreatlyreduced
threshold current densities compared to theirhomojunction
counterparts.
High performance electronic and photonic devices that
incorporateheterostructures, suchasthosementionedabove, have
beenrealized duetorecentadvancesinepitaxialgrowthtechniques
suchasmolecularbeamepitaxy(MBE)andmetal-organicchemical
vapordeposition (MOCVD). Theseadvanced epitaxialgrowth
techniquesarelow-temperatureinnatureandpermitatomiclevel
controloverthe depositionprocessenablingheterostructures tobe
grownhavingextremelyabruptinterfaces.
1
2
Aparticularadvantageofmodemepitaxialgrowthtechniquesis
thattheycanbeemployedtointegratematerialshavingdifferent
propertiesinamonolithicfashionwithinlimitsimposedbyfactors
suchaslattice-mismatchandthermalstability. Anexampleof
materialsintegrationwouldbetheGaAs/Siepitaxialsystem,where
photonicdevicesfabricatedinGaAscouldbeintegratedmonolithically
withSielectronicdevices. Asecond example ofmaterialsintegration
concernsthesubjectofthisdissertation,namelytheInxGai-xAs/GaAs
epitaxialsystem.
Theworktobedescribedwasmotivatedbytheideaof
monolithicallyintegratingphotonicdevicesfabricatedinInxGai-xAs
withGaAsintegratedcircuits (ICs). Suchaschemeisinterestingin
principleduetothefollowingreasons: Firstly, GaAsICtechnologyis
rapidlyadvancingtothepoint, forexample, thathigh-speed GaAslogic
elementshavebeenincorporatedinCraycomputers. Andsecondly,
theInjcGai-xAsternarysystemhasadirect-bandgaprangefrom
1.42eV(x=0) to 0.36eV (x=l). Thisbandgap rangewould correspond
to photonic devices operatingin the, 0.87pm (x=0) to 3.4pm (x=l),
wavelengthrange. Suchawavelengthregimeencompassesseveral
importantwavelengths. Forinstance,wavelengthsof1.3and 1.55pm
representthe dispersionand absorptionminima, respectively, in the
caseofsilicaglassfibersandalsofluoride-basedfibersappeartohave
anattenuationminimumaround2pm.
HinderingthegrowthofdevicequalityInxGai-xAsalloyson
GaAs, however, arethelargelattice-mismatchstrainsassociatedwith
thismaterialsystemwhichleadtodislocationformationandthusthe
degradationofphotonicdevices. Controloverdislocationpropagation.
3
consequently, isofparamountimportanceand, hence, fundamental
studiesconcerningthissubjectarerequiredsuchthatthe
InxGai-xAs/GaAsmaterialintegrationcanbecomeareality.
The presentlydescribedworkhad twoprincipal objectives.
Firstly, themechanismsresponsibleforstrainreliefinthe
InxGai-xAs/GaAsheteroepitaxialsystemweretobeidentifiedasa
functionofInxGai-xAscomposition, coveringaswideacomposition
rangeaspossible. Secondly, acompositionrangewastobeidentified
overwhichlow-dislocationdensityInxGai-xAsmaterialcouldbe
growneitherdirectlyorindirectlyonGaAs.
Priorto outliningthescopeofthepresentwork, adiscussion
sectionfollowsontheliteratureasitpertainstothisdissertation.
1.2 Background
Lattice-mismatchstrainpresentsthegreatestobstacletowards
thegrowthofhighqualityInxGai-xAsonGaAs. Thelatticeparameter
ofInxGai-xAsislargerthanthatofGaAswiththemismatchstrain
reaching7%whenx=l [1], Therefore,intheInxGai-xAs/GaAs
system, the epilayerexperiences in-planebiaxialcompressionsince
theepilayerlatticetriestoconformtothatofthesubstrateuntilan
elasticlimitisreachedandstrainrelaxationoccurs. Thefollowing
sectionreviewsthepublishedliteratureonstrain, strainreliefand
dislocation evolutioninlattice-mismatched systemsingeneral, with
particularreferencetotheInxGai-xAs/GaAsmaterialsystem. Also,
variousbufferstructuresusedtoblockdislocationsinsuchlattice-
mismatched systems are reviewed.
4
1.2.1 CriticalThicknessinLattice-Mismatched Systems
Inthecase ofheteroepitaxialsystemsinwhichthelattice-
mismatchissmall, thefirstatomiclayersthataredepositedduring
epitaxialgrowthwillbestrainedtomatchthesubstratelattice
(pseudomorphic growth) and a coherentinterface is formed.
However, asthethicknessoftheepilayerincreases, thehomogeneous
strainenergyintheepilayerbecomeslargeenoughtoallowthe
nucleation ofmisfitdislocations attheheterointerface. Thethickness
oftheepilayeratwhichmisfitdislocationsarenucleated corresponds
tothecriticalthicknessforthatepilayer/substratematerialsystem. In
this sectionseveralmodelsusedtopredictcriticalthicknessare
discussed, andpublishedexperimentaldataoncriticalthicknessesin
theInxGai-xAs/GaAsmaterialsystemarepresented.
Van derMerwe [21 proposed amodel to predictthe critical
thicknessofalattice-mismatchedepilayer/ substratesystemby
equatingthearealstrainenergydensityintheepilayertothe
interfacial energyperunitlengthbetweentheepilayerandthe
substratearisingasaresultofthegenerationofmisfitdislocations.
TheinterfacialenergywasevaluatedusingthePeierls-Nabarro
potentialto representtheinteratomicforcesattheinterface. Sucha
calculationoftheinterfacialenergy, however, becomessignificantly
inaccuratewhenthemisfitdislocationspacingisgreaterthantwice
theepilayerthicknessleadingtoaconservativeestimate ofthecritical
thickness.