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INTEGRATIONOFAGaAs/AIGaAsSQWLASERAND ATAPEREDWAVEGUIDECOUPLER By SUNINGXIE ADISSERTATIONPRESENTEDTOTHEGRADUATESHOOL OFTHEUNIVERSITYOFFLORIDAINPARTIALFULFILLMENT OFTHEREQUIREMENTSFORTHEDEGREEOF DOCTOROFPHILOSOPHY UNIVERSITYOFFLORIDA 1996 Tomyparents ACKNOWLEDGMENTS First,IwanttothankDr.RamuV.Ramaswamy,myadvisor,forhissupportand guidancethroughoutthecourseofthiswork.Hishardworkandsuccessingenerating fundsforthePhotonicsResearchLaballowedmetocompletethisworkinaverywell equippedlab. IalsowanttothankDr.ShengS.Li,Dr.AmostNeugroschel,Dr.EwenThomson, andDr.DavidTannerfortheirparticipationonmysupervisorycommittee. ThanksalsogotomyfellowgroupmembersinthePhotonicsResearchLabfor theirdirectorindirecthelp,particularly,DavidMaring,Dr.ScottSamson,MarkSkowron- ski.Dr.RobertTavlykaev,Dr.WeidongWang,Dr.ChrisHussell,Dr.TizianaConese,Dr. SanjaiSinha,Dr.Sang-KookHanandDr.HyoonSooKim. Iwanttothankmyparents.Theyalwaysencouragemetoattainhighergoals.I knowthattheprogressinmyendeavor,suchasthecompletionofthisdissertation,isthe bestexpressionofgratitudetothem. iii TABLEOFCONTENTS page ACKNOLEDGMENTS iii ABSTRACT vi CHAPTERS 1 INTRODUCTION 1 1.1 Motivation 2 1.2 TheLaser-WaveguideIntegrationStructures 8 1.3 OrganizationoftheDissertation 16 2 NOVELINTEGRATIONOFANSQWLASERAND ATAPEREDCOUPLER 18 2.1 MultilayerOpticalWaveguide 18 2.2 QuantumWellLasers 24 2.3 TaperedWaveguideCoupler 30 2.4 TheSQWLaser-WaveguideIntegration 36 3 MULTIPLEQUANTUMWELLDISORDERING 39 3.1 Introduction 39 3.2 ZnDiffusionProfiles 46 3.3 CompositionalProfilesofDisorderedMQWs 56 3.4 RefractiveIndexProfilesofDisorderedMQWs 64 3.5 TheOpticalConfinementFactor 69 3.6 TheLasingThresholdCurrentDensity 77 4 GROOVEBETWEENTHELASERANDTHETAPER 84 4.1 TheCharacteristicMatrix 84 4.2 ReflectanceoftheGroove 88 IV 4.3 TheLasingThresholdVariation 93 4.4 ThePowerOutputVariation 94 5 FABRICATIONTECHNIQUESANDEXPERIMENTALRESULTS 98 5.1 MaterialGrowth 98 5.2 MQWDisorderingbyZnImplantationandAnnealing 101 5.3 GrooveEtchingByRIE 105 5.4 FabricationoftheTaperedCoupler 107 5.5 TheIntegration 114 6 SUMMARYANDCONCLUSIONS 120 REFERENCES 124 BIOGRAPHICALSKETCH 131 AbstractofDissertationPresentedtotheGraduateSchool oftheUniversityofFloridainPartialFulfillmentofthe RequirementsfortheDegreeofDoctorofPhilosophy IntegrationofaGaAs/AlGaAsSQWLaserand ataperedwaveguidecoupler By SuningXie December1996 Chairman:RamuV.Ramaswamy MajorDepartment:ElectricalandComputerEngineering Future high speedoptical fibercommunicationsandsignalprocessingsystems requirethemonolithicintegrationofalasersourceandotherpassiveaswellasactive devices.Keytointegrationisthecouplingoflightfromthelasersourcetoawaveguide circuitthatmakethelightavailableforotheropticalprocessingfunctionssuchasmodula- tion,switchingandrouting.Existingintegrationschemeseitherrequirecomplicatedand costlymaterialregrowthorimposeseveredesignandfabricationrestrictions.Thisdisser- tationstudiesanovelmonolithicintegrationofasinglequantumwell(SQW)laseranda taperedwaveguidecouplerintheGaAs/AlGaAsmaterialsystem.Theintegratedstructure consists of two sections: the laser and the tapered waveguide coupler. The SQW waveguideforthelaserandthemultiplequantumwell(MQW)waveguideastheoutput waveguideareverticallystacked.Inthelasersection,theMQWsaredisorderedbyZn VI implantationandannealingthusbecomingthecladdinglayeroftheSQWlaser.Inthe tapered waveguide coupler section, the MQWs are gradually disordered along the waveguidebynitrogenimplantationandannealing.Thispermitsthelaseroutputcouples totheMQW waveguide foroptical processing such as modulation.Agrooveetched betweenthelaserandthetapersectionsfunctionsasapartiallytransmissivemirrorpro- vidingthefeedbackforthelaser.Thetaperedwaveguidecoupleremploysanadiabatic powertransferprocessandallowsforindependentoptimizationofindividualcomponents tobeintegrated.Theverticalconfiguration ofthetwowaveguidesandthecontrolled impurity-induceddisorderingeliminatetheregrowthrequirementswithoutsacrificingthe devicedesignandfabricationflexibility. Inthisdissertation,thekeytechnologiesfortheintegrationoftheSQWlaserand thetaperedwaveguidecouplerhavebeeninvestigatedtheoreticallyandexperimentally. TheMQWsinthelasersectionaredisorderedbyZnimplantationandsubsequentanneal- ing.Afinitedifferenceapproachhasbeendevelopedtodeterminetheconcentration(Zn) dependentinterdiffusionandthemodelleadstoapredictionofasharpreductioninthresh- oldcurrentdensityduetothedisorderingoftheMQWsinthelasersection,whichagreed wellwithourexperimentalresults.Inaddition,dependenceofthelasingconditiondueto thegrooveareinvestigated.ThefabricationtechniquesincludingtheMQWsdisordering byimplantationandannealingandthereactiveionetching(RIE)forthegroovehavebeen developed. Utilizingthesetechniques,theintegrationofanSQWlaserandatapered waveguidecouplerintheGaAs/AlGaAsmaterialsystemhasbeenaccomplished. vii CHAPTER 1 INTRODUCTION Thebreakthroughinsemiconductordiodelasersinthe1960sandthedevelopment oflowlossopticalfiberinthe1970swereresponsiblefortheexplosioninthefiberoptical communicationandopticalsignalprocessingsystems.Theprogressintheseopticalsys- tems,inturn,hasstimulatedthegrowthofanewclassofpassiveandactiveopticalcom- ponentsinthindielectriclayersusingguidedwaveoptics.Thehighlypromisingand sophisticatedtechnologyresponsibleforthenewclassofdevicesisknownasintegrated optics.Inadditiontotheadvantagesofgreaterbandwidthandimmunityfromelectromag- neticinterference,integratedopticdevicesincorporateincreasedcomplexcircuitryand functionalityalongwithhighreliability. Successfulfuturedeploymentofopticalfiber communicationsandopticalsignalprocessingsystemsreliesonthemonolithicintegration ofalasersourcewithvariouspassiveandactivedevices.Oneofthekeyintegrationsisthe couplingoflightfromthelasersourcetoawaveguidecircuitinvolvingotheropticalpro- cessingsuchasmodulation,switchingandrouting.Inthisdissertation,wetheoretically andexperimentallyinvestigateanovellaser-taperedwaveguidecouplerintegration;inpar- ticular,wedevelopthekeyfabricationtechniques,anddemonstratethemonolithicintegra- tionintheAlGaAs/GaAsmaterialsystem.Thefirstsectionofthisintroductorychapter 1 2 describesthemotivationforthiswork.Insection1.2,wecomparevariousexistinglaser- waveguideintegrationstructuresandintroducethenovelstructureweproposed.Theorga- nizationoftheremainingchaptersofthisdissertationispresentedinsection1.3. 1.1Motivation Opticalfiberofferstransmissioncapacityofmanyordersofmagnitudegreatthan thatofconventionaltransmissionlines.Theoverallbandwidththatisavailableforoptical transmissionisintheorderofTHz.Thisenormouscapacitycanbeaccessedbyusing techniquessuchaswavelengthdivisionmultiplex(WDM);itallowsfortheimplementa- tionofmanynewservicessuchasHDTVdistribution,broadbanddatahighwaysand WDM videotelephony.Ina system,theoveralldatastreamisdividedintoasetofsub- streamseachhavingadifferentwavelength.Theopticalsignalsaremultiplexedbyan opticalpowercombinerandthendemultiplexedbyopticalfiltersatthereceiverends.Fig- ure 1 shows amultiwavelengthopticalbroadcastnetwork.Thenetworkbasedonthe LAMBDANETarchitecture[1]iscomposedofaclusterofNcommunicationnodes.Each nodetransmitsitsinformationatauniquewavelengthandeachnodereceivesalltheinfor- mationfromtheothernodes.Thenodesareopticallycoupledtogetherusingatransmis- siveopticalstarcoupleratahublocationwhichbroadcastsinformationatallwavelengths toallnodes. 3 NODE#2 Figure1.1TheLAMBDANETmultiwavelengtharchitecture.