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MOLECULARBEAMEPITAXIALGROWTHOF HOMOEPITAXIALZINCSELENIDE By MINHYON JEON ADISSERTATIONPRESENTEDTOTHEGRADUATESCHOOLOFTHE UNIVERSITYOFFLORIDAINPARTIALFULFILLMENTOFTHE REQUIREMENTSFORTHEDEGREEOFDOCTOROFPHILOSOPHY UNIVERSITYOFFLORIDA 1995 ® M S OFP_oR®<a®* universe TOMYPARENTS ACKNOWLEDGMENTS Theauthor’ssincereandutmostappreciationisbestowedonDr.RobertM.Park, who is his supervisorandchairman ofhis committee, forhis guidance, supportand encouragementduringtheresearchandthewritingofthisdissertation. Theadviceand supportofthe othermembers ofhis committee, namely, Dr. R.T. DeHoff, Dr. P.H. Holloway,Dr.S.S.LiandDr.R.K.Singharealsogratefullyacknowledged. TheauthorwishestothankLynnCalhounandBrentGila,hisfellowgraduate students,fortheirhelpandfriendship. Theperformanceofetch-pitdensitymeasurementsbyGregHaugenofthe3M Company(St.Paul,MN)isgratefullyacknowledged. Also,financialsupportfromthe 3MCo.madethisprojectpossible. ThephotoluminescencemeasurementswereperformedbyDr.LudwigofProf. Hummel’s group and the author gratefully acknowledge the valuable assistance and friendshipofDr.Ludwig. The author would like to express his love and gratitude to his lovely wife, SoonJeong, and adorable daughter, HyoJoo, for their sacrifices, prayer and encouragementwithhisheart. Finally, the author wishes to thank his parents, a brother, YongJin, and two sisters,namelyHeeJungandEunKyoung, fortheirendlesssupportandunderstanding duringthesehardtimes. IV TABLEOFCONTENTS Page ACKNOWLEDGMENTS iii ABSTRACT vii CHAPTER 1 INTRODUCTION 1 MotivationandObjectives 1 LiteratureReview 4 Blue-GreenLaserDiodeDevelopmentHistory 4 Wide-GapII-VI/GaAsLaserDiodeFailureMechanisms 7 HomoepitaxialZnSe 9 MigrationEnhancedEpitaxy 13 ScopeoftheDissertation 17 2 MBEGROWTHSYSTEMANDCHARACTERIZATION TECHNIQUES 18 Introduction 18 MBESystemandSampleGrowth 18 CharacterizationTechniques 20 HighResolutionX-RayDiffraction 20 TransmissionElectronMicroscopy 22 Etch-PitDensityMeasurement 24 3CHARACTERIZATIONOFSUMITOMO ZINCSELENIDEWAFERS 28 SumitomoZnSeWafers 28 Characterization 29 HRXRDAnalysis 29 AFMAnalysis 34 PhotoluminescenceAnalysis 39 TEMAnalysis 40 Summary 41 v 4 WAFERPREPARATIONANDCONVENTIONALZnSe HOMOEPITAXIALGROWTH 81 5 Ex-situPreparation 81 Introduction 81 UV-ozonetreatmentofZnSesubstrates 83 InSituOxideReduction 84 MBEGrowthofHomoepitaxialZnSe 85 ExperimentalResultsandDiscussion 86 HRXRDAnalysis 86 PLAnalysis 90 6 Summary 92 INVESTIGATIONOFANOVELZnSeGROWTHMODE 132 Introduction 132 GrowthProcedures 134 CharacterizationResults 135 Low-TemperaturePLAnalysis 135 HRXRDAnalysis 136 TEMAnalysis 139 Etch-PitDensityAnalysis 141 AConceptofSurfaceRelaxationControlledEpitaxy 142 Introduction 142 ZnSeSurfaceStructures 143 RHEEDObservations 144 ModelforSeDesorption 146 Summary 148 CONCLUSIONSANDRECOMMENDATIONS 192 APPENDIX:ATOMICFORCEMICROSCOPY 195 REFERENCELIST 202 BIOGRAPHICALSKETCH 211 vi AbstractofDissertationPresentedtotheGraduateSchool oftheUniversityofFloridainPartialFulfillmentofthe RequirementsoftheDegreeofDoctorofPhilosophy MOLECULARBEAMEPITAXIALGROWTHOF HOMOEPITAXIALZINCSELENIDE By MinhyonJeon December,1995 Chairman:Dr.RobertM.Park MajorDepartment:MaterialsScienceandEngineering Thefocusofthis dissertationconcerns thesubjectofZnSehomoepitaxy. Of primary concern in the study was ZnSe wafer quality and extensive analysis was performedonwaferspurchasedfromtheSumitomoCompanyofJapanthatweretobe employedassubstrates. The analysis of substrate material, which involved, high-resolution x-ray diffraction(HRXRD)photoluminescence(PL),transmissionelectronmicroscopy(TEM), atomicforcemicroscopy(AFM)andetch-pitdensity(EPD)measurements,revealedthe bestZnSewaferssuppliedbySumitomotobesuitableforaserioushomoepitaxystudy. VII Forinstance,x-rayco rockingcurvelinewidths(using7-crystalopticsuystem)wereon theorderof8 arcsec. anddefectdensities wereestimatedinthe 105 cm2range. In addition,asurfaceroughnessontheorderof0.16nmwasassessedwhichiscomparable tothesurfaceroughnessofcommercialGaAswafers. Prior to conducting the detailed homoepitaxy study, ex-situ and in-situ ZnSe wafertreatmentprocesseswereoptimized. Anex-situtreatmentprocessinvolvingasix minuteUV-ozoneexposurewasfoundtobeoptimumwithregardtosurfacemorphology andresultantepilayercrystallographicquality. And, intermsofin-situ processing, a combinedhydrogenatom/thermal treatment was foundtoresult intheeliminationof surfaceoxidesattemperaturesaslowas300°C(comparedwith500°Cforconventional thermaltreatment). ZnSe homoepitaxial layers were grown on suitably prepared substrates by conventionalmolecularbeamepiatxy(MBE)andusinganovelgrowthmodewhichwe term“SurfaceRelaxationControlledEpiatxy”(SRCE). Inbothcases,followingin-situ wafercleaning,theZnSesubstrateswereexposedtoaSefluxforabouttwominutes duringwhichtimeaSe-stabilizedsurfacedevelopedasevidencedbythedevelopmentof a(2x1)surfacereconstruction. Thereafter,inthecaseofSRCE,thesubstratewasalternatelyexposedto ZnandSe,ineachcasethe“shutteropen”orexposuretimewasthesame,namely30 seconds. Thedelaytime,however,thetimeallowedtoelapsebetweenclosingtheZn shutterandopeningtheSeshutter,orvice-versa,wassystematicallyvariedfromrun-to- runovertherange,0to60seconds. viii Inallcases,thisgrowthmodewasineffectfor10cycles(10Znexposuresand10 Seexposures)beyondwhichconventionalMBEgrowthwasperformed(bothZnandSe shutterspermanentlyopen)until2(tmofZnSehadbeendeposited. ZnSehomoepitaxial layers grownusing the SRCEgrowthmode atthe initial stages of the growth were characterized by a number of techniques including low- temperaturePL,HRXRD,TEMandetch-pitdensitymeasurementsanditwasfoundthat thedelaytimestronglyaffectedthedefectdensityinthefilms. Anoptimumdelaytime wasfoundfortheSesurfacewhichresultedinthelowestdefectdensityfilmsanda modelispresentedtoexplainthisphenomenon. IX CHAPTER1 INTRODUCTION MotivationandObjectives ZincselenideandrelatedII-VIcompoundsemiconductorshaveforseveralyears beenthesubjectofintenseresearchbecauseoftheirpotentialforuseinthefabricationof optoelectronic devices operating in the blue-green region of the electromagnetic spectrum,specifically,blue-greenlightemittingdiodes(LEDs)andlaserdiodes(LDs). Traditionally,epitaxialgrowthofZnSeandrelatedII-VIcompoundsemiconductorshas beenconductedonGaAssubstratesbecauseofthesmalllatticemismatchbetweenZnSe andGaAs(0.27%atroomtemperature)andbecauseofthecommercialavailabilityof very high quality, reasonably priced GaAs wafers. In addition, surface preparation procedures for subsequent epitaxial growth on GaAs substrates are well established. However, despite these advantages, there are some drawbacks associated with using GaAsasasubstratematerialforwide-gapII-VIbasedinjectiondevices. Firstly,epitaxialZnSeonGaAsexperiencessignificantbiaxialstrainwhichcan cause the formation of misfit dislocations in the epilayer. The biaxial strain is a consequenceofthesmallbutsignificantlatticemismatchofthissystem,andthedifferent thermalexpansioncoefficientsexhibitedbythesetwomaterials[1].Thelatticemismatch problemcanbeovercomebydepositingternaryorquaternaryalloyssuchasZnSSeor 1