Characterization of gallium antimonide crystals grown under microgravity conditions by the Liquid Encapsulated Melt Zone (LEMZ) technique

221 Pages·1999·8.4 MB·English

by  Mileham, Jeffrey Robert

CHARACTERIZATIONOFGALLIUMANTLMONIDECRYSTALS
GROWNUNDERMICROGRAVITYCONDITIONSBYTHE
LIQUIDENCAPSULATEDMELTZONE(LEMZ)TECHNIQUE
By
JEFFREYROBERTMILEHAM
ADISSERTATIONPRESENTEDTOTHEGRADUATESCHOOL
OFTHEUNIVERSITYOFFLORIDAINPARTIALFULFILLMENT
OFTHEREQUIREMENTSFORTHEDEGREEOF
DOCTOROFPHILOSOPHY
UNFVERSITYOFFLORIDA
1999
Tomywife,Beatriz
ACKNOWLEDGMENTS
Thisdissertationrepresentsnotonlytheworkoftheauthor,butalsotheeffortand
collaborationofalargeteamofpersonnelfrommanyfacets.Iwouldliketotakethis
opportunitytothankmyco-advisors,Dr.RezaAbbaschianandDr.StephenPearton,whose
encouragementandfaithinmyabilitynotonlyhelpedmecarryoutthisproject,butalso
allowedmyprofessionalcareertodevelop.IwouldliketogivespecialthankstoCarlos
LopezwhowasthekeyindividualinthisprojectbeforeIwasateammemberandwasthe
mainliaisonbetweentheUniversityofFloridaandNASA.Hiscontributionsinclude,butare
notlimitedto,severalprepossessingtasks(includingastronauttraining,ampoulefabrication,
andthemassesofpaperworkassociatedwithaNASAproject)andagreatportionofthe
experimentalprocessingeffortsthatwenttowardthemakingofthisdissertation.Thereare
severalpersonnelwhohelpedmewiththecharacterizationofthecrystalsincludingthe
NationalRenewableEnergyLaboratory(NREL)whoperformedthephotoluminescence
measurementsatnocostandGeorgeKimwhohelpedmewiththeHallmeasurements.
Finally,1wouldliketoacknowledgethecontributionsofNASAandtheCommercialFloat
ZoneFurnaceteam,includingtheCommercialDevelopmentandApplicationsofficeatthe
Marshall Space Flight Center who fiinded the project, the Canadian Space Agency
MicrogravitySciencesProgramOffice,theMcDonnellDouglasSPACEHABpersonnel,and
theastronautmissionspecialists,MarcGameauandAndyThomas.
iii
TABLEOFCONTENTS
ACKNOWLEDGMENTS
ABSTRACT ^'^
CHAPTERS
INTRODUCTION 1
ResearchMotivation 1
TheCommercialFloatZoneFurnaceandtheFloatingZoneTechnique 2
LiquidEncapsulatedMehZoneTechniqueonGaSb 4
LITERATUREREVIEW 6
GaSbMaterialSystem 6
BulkGrowthofGaSbCrystals 8
Segregation:ConvectionandGravity 14
GalliumAntimonide:NativePointDefects 18
HallEffectMeasurements 24
Photoluminescence 29
ExtendedDefects 32
EXPERIMENTAL 36
SampleProcessing 36
SampleCharacterization 44
PROCESSINGRESULTS 57
CrystalGrowth 57
EtchPitDensity 65
SIMSRESULTSANDDISCUSSION 90
MacroscopicSegregation 90
DepthProfilesandRadialSegregation 99
iv
HALLMEASUREMENTRESULTS 116
VariableTemperature 116
RoomTemperature 132
PHOTOLUMINESCENCERESULTS 157
p-typeSamples 157
n-typeSamples 169
SUMMARYANDCONCLUSIONS 191
APPENDIX
GaAsRESULTSANDDISCUSSION 197
LISTOFREFERENCES 202
BIOGRAPHICALSKETCH 212
V
AbstractofDissertationPresentedtotheGraduateSchool
oftheUniversityofFloridainPartialFulfillmentofthe
RequirementsfortheDegreeofDoctorofPhilosophy
CHARACTERIZATIONOFGALLIUMANTIMONIDECRYSTALS
GROWNUNDERMCROGRAVITYCONDITIONSBYTHE
LIQUIDENCAPSULATEDMELTZONE(LEMZ)TECHNIQUE
By
JeffreyRobertMileham
August1999
Chairperson:Dr.StephenPearton
Cochairperson:Dr.RezaAbbaschian
MajorDepartment:MaterialsScienceandEngineering
ALiquidEncapsulatedMehZone(LEMZ)techniquewasusedtogrow[100]GaSb
singlecrystalsundermicrogravityconditionsaboardtheSTS-77ShuttleEndeavormission.
Onetellurium-dopedandtwoundopedsinglecrystals,encapsulatedinaeutecticmixtureof
sodium-chlorideandpotassium-chloride,wereregrownfrom16mmdiametersinglecrystal
rods.Thequalityofthecrystalswascomparedtothosegrownongroundusingthesame
technique.Itwasfoundthatthemicrogravity-growncrystals,particularlytheTe-doped
crystal,exhibitedlowerdislocationdensitiesthanthosegrownonground.Thedislocation
densitywasalsofoundtodependonthesolid/liquidinterfaceshapewithaplanarinterface
resultinginalowerdensity.Reductionofrotationalandnonrotationalstriationswasobtained
vi
withtheencapsulationtechniqueinallcrystals.Moreover,theuseoftheencapsulantallowed
forlongerandmorestablemeltzonesforthosesamplesprocessedonground.
Macrosegregationofthetelluriumdopantrevealedthatconvective-controlledgrowth
occurredinbothmicrogravityandterrestrialenvironments.Theuseofanencapsulanthelped
controlbutdidnoteliminateMarangoniconvectionand/orconvectioncausedbyvariations
inthegravitylevel.Irregularradialdopantdistributioninthemicrogravitysamplewas
attributedtolocalizedconvectiveflowsatthegrowthinterfacecausedbyunevenheating
whenthesamplewaspushedoffitsgrowthaxisbyanargonbubbleintheampoule.
Hallandphotoluminescencemeasurementsconfirmedthehighqualityofthecrystals,
butrevealedthat growth underdifferent gravity environmentsdidnot influencetheir
electricaloropticalproperties.However,severalresuhsfromthisstudywerehighlyuseful
inhelpingtounderstandthelesserstudiedgalliumantimonidematerialsystemitself.
VariabletemperatureHallandphotoluminescencemeasurementsintheundopedcrystals
foundtwodominantacceptorsat12.5and33meVbelowthebandgapedge.Thetemperature
variationofthelatterpeakcomparedtothebandgapedgewasexplainedbyeitheratrue
decreaseintheionizationenergyofthislevelorbyavariationinthephotoabsorptioncapture
crosssectionwhichisdifferentthanmostothersemiconductors.Inthen-typesamples,Hall
measurementswereusedtocalculatetheenergylevelofthesubsidiaryconductionbandand
itstemperaturevariation.
vii
INTRODUCTION
ResearchMotivation
Semiconductorsinglecrystalsarekeycomponentmaterialsinalargevarietyof
technical products. Unfortunately, all currently available growth methods suffer from
inherent limitations and, as a result, the crystals contain growth-related defects and
undesirable imperfections. Presently, established growth procedures are not able to
adequatelymeetofthedegreeofchemicalandcrystallineperfectionrequiredforadvanced
applications,especiallyincompoundsemiconductors.Complicatingthisproblem,compound
semiconductorgrowthishinderedinboththeresearchandindustrialenvironmentbypoor
reproducibilityundersimilargrowthconditions,sometimesevenusingthesameequipment.
AsoutlinedbyMuUer[95MUL],amajorreasonfortheseproblemsistheindustrial
developmentofgrowthprocessesbasedonexperimentationandscientificintuitionrather
thanfundamentalunderstandingofgrowth-relatedphenomenon.Eventoday,theprimaryaim
ofindustrialresearch,achievementofcompetitiveyields,usuallytakesprecedenceoverthe
developmentofnewgrowthtechniquesortheadvancementofknowledgeincurrentmethods
ofcrystalgrowth.Fundamentalresearchoncrystalgrowthfromthemehhasbeenvery
limited and has not been carried out at a scale proportional to the importance of
semiconductorsinglecrystalstocommercialapplications.
1
2
Sinceestablishedgrowthproceduresforonematerialsystembaseduponempirical
knowledgearenotfullytransferabletoothermaterialsystems,propertyrequirementsfor
many advanced applications cannot be adequately met by current growth methods.
Modificationsofgrowthprocedureshastobedeterminedbytimeconsumingandexpensive
trial-and-errorexperimentation.Complicatingmattersfurther,thedevelopmentofthecurrent
techniqueshasadvancedsofarthatisbecomingmoredifficulttooptimizeonegrowth
parameterwithoutadverselyaffectingotherparameters.Tostepfromempiricismtoscience
incrystalgrowthrequiresthatweobtaintheknowledgetodescribethecorrelationbetween
propertiesofcrystalsandtheparametersbywhichtheyweregrown.
Tothismeans,thefollowingwilldescribetheeffectsofgravityasavariablein
crystalgrowth.Traditionally,growthprocessesrarelyconsideredgravityasacomponentin
crystalgrowththatcouldbealtered.Ononeextreme,surprisinglylittleresearchhasbeen
donewithenhancedgravity(forareviewofthis,seeanotherpublicationbyMullerin
[90MUL]).Withtherelativelyrecentemergenceofspaceflight,theotherextremeof
microgravity(^g)canbereached.Adiscussionoftheeffectofgravityandotherparameters
upontraditionalandstate-of-the-artcrystalgrowthprocessessuchasthatusedinthisstudy
willbedetailedintheliteraturereview.
TheCommercialFloatZoneFurnaceandtheFloatingZoneTechnique
TheexperimentsforthisworkwereprocessedintheCommercialFloatZoneFurnace
(CFZF)payloadwhichwaslaunchedonMay19, 1996,intheSPACEHAB-04Module,
aboardtheShuttleEndeavor,aspartoftheSTS-77missionoperations.Theobjectiveforthis
payload,managedbytheCanadianSpaceAgency(CSA),wastogrowsinglecrystalsof
technologicallyimportantsemiconductormaterialsinamicrogravityenvironmentbymeans
ofthefloating-zone(FZ)technique.Fourinstitutionscomprisedthescientificteams,each
oneprocessingdifferentmaterials.TheUniversityofFlorida,fundedbytheMarshallSpace
FlightCenter(MSFC),usedtheLiquidEncapsulatedMeltZone(LEMZ)techniqueto
processundopedandTellurium-dopedGalliumAntimonide(GaSb),encapsulatedinan
eutecticmixtureofNaClandKCl.TheUniversityofFreiburg,sponsoredbytheDeutsche
AgenturfurRaumfahrtangelegenheitenGmbH(DARA),processedunencapsulatedundoped
GalliumAntimonide(GaSb).TheUniversityofDalhousie,fundedbyCSA,experimented
withencapsulatedCadmiumGermaniumArsenide(CdGeAs2).TheCanadianCenterfor
MineralandEnergyTechnology(CANMET),alsofundedbyCSA,processedsamplesof
BismuthGermaniumOxide(BGO).
Thefloating-zonetechnique,whichwasutilizedbyallfourgroupstogrowthe
crystals,makesuseofthephysicalpropertyknownassurfacetensiontomaintainacolumn
ofmoltenmaterialattachedtosectionsofunmoltenmaterialaboveandbelowit.Themolten
section,knownasthefloatzone,istranslatedalongtheaxisofthecrystaltoeffectthecrystal
growthandthusconvertthefeedstockintoanewlygrownmaterial.Thefiirnaceestablished
thisfloatzonebymeansofaParaboloid-EllipsoidMirrorFurnace(ELLI-FM)manufactured
andoperatedbytheDomierDeutscheAerospaceCompany,Friedrichshafen,Germany.The
fiimacefocusesradiation,emittedbyahalogenlamp,ontoanarrowbandofsamplematerial
tomeltthisandthusestablishafloatzone.Thesample,enclosedinhermeticallysealed
quartzampoules,thentranslatedalongitsaxistoeffectthedesiredcrystalgrowth.The