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DEGRADATIONOFZnSPHOSPHORSDURINGELECTRONBEAM BOMBARDMENT By JosephSamuelSebastian ADISSERTATIONPRESENTEDTOTHEGRADUATESCHOOLOFTHE UNIVERSITYOFFLORIDAINPARTIALFULFILLMENTOFTHEREQUIREMENTS FORTHEDEGREEOFDOCTOROFPHILOSOPHY UNIVERSITYOFFLORIDA 1997 Copyright1997 by JosephSamuelSebastian ACKNOWLEDGMENTS FirstandforemostImustthankmyparentsSamandJoAnnwhoseloveand supportknownobounds. WithouttheirencouragementandguidanceIwouldsurely neverhavebeenabletoaccomplishthisgoal. IalsooweagreatdealtomyadvisorDr.PaulHolloway,amanofseemingly limitlessenergy,whodespitehishecticschedulealwaysfoundtimetoanswermy questionsandkeepmemovingforwardinmyresearch. Dr.HendrikSwartwhovisited ourresearchgroupforayearfromSouthAfricataughtmeagreatdealaboutAuger (repair),anddataanalysis. Dr.MarkDavidsonalwayscameupwithinnovativewaysto collectthedataIneeded,andhumoredmewhenItoldhimIneededittoday. Dr.W.BrockAlexanderlenthisexpertiseforinterpretationoftheHRXRDresults, andDr.MaggiePuga-LambertperformedtheSIMSanalysisandguidedmethroughthe datareduction. DeanEvansattheUniversityofGeorgiasacrificedpartofhisspring breaktocollectthelowvoltageefficiencydata. Dr.T.J.Kim,withwhomIsharean office,wasalwaystheretoencourageme,offersuggestionsandsharetheoccasional cupofcoffeeafteralongday. FinallyIwouldliketothankISHM(nowIMAPS)andthePhosphorTechnology CenterofExcellencefortheircontinuedfinancialsupportthroughoutmygraduate career. 261 TABLEOFCONTENTS ACKNOWLEDGMENTS iii ABSTRACT vi CHAPTERS INTRODUCTION 1 I 2 REVIEWOFTHELITERATURE II Luminescence II ConfigurationalCoordinateModel 1 EnergyFlowinthePhosphor 1 Cathodoluminescence 18 ZnS:MnLuminescence 23 PhosphorDegradation 24 TheoryofIsoelectronicTraps 28 EffectsofOxygeninZincSulfide 32 3 EXPERIMENTAL 49 SamplePreparation 49 AugerElectronSpectroscopy(AES) 50 ControloftheVacuumAmbient 5 AnalysisofLuminescence 52 HighResolutionX-RayDiffraction(HRXRD) 52 SecondaryIonMassSpectrometry(SIMS) 54 X-RayPhotoelectronSpectroscopy(XPS) 55 ActivatorEfficiencyMeasurments 56 Thermoluminescent(Glow)Experiments 57 4 RESULTS 65 AugerElectronSpectroscopy(AES)ofZnSThinFilms 65 ConcurrentAugerandCathodoluminescence 68 IV 58211 SputterExperimentsonZnSThinFilmandSingleCrystalSamples 71 EffectsofSputteronCathodoluminescentIntensity 7 AnalysisofZnOSurfaceLayerThickness 72 OpticalAbsorptioninZnOSurfaceLayer 73 HighResolutionX-RayDiffraction(HRXRD) 74 ActivatorEfficiencyMeasurements 75 SecondaryIonMassSpectroscopy(SIMS) 76 Thermoluminescent(Glow)Curves 77 5 DISCUSSION 112 SurfaceChemicalReactions I1 ModelingoftheElectronStimulatedSurfaceReaction I1 EffectoftheVacuumAmbient I1 EffectsofthePrimaryBeamEnergy 12 DependenceofCathodoluminescenceonSurfaceChemistry 122 SurfaceLayerEffects 123 X-RayPhotoelectronSpectroscopy(XPS) 124 EffectsofSurfaceLayeronCLBrightness 125 TheLuminescentCenterMn"^^ 128 ChangesintheZnSHostLattice 130 6 CONCLUSIONSANDFUTUREWORK 151 APPENDIXAOPTICALABSORPTIONCALCULATIONS 155 REFERENCES 157 BIOGRAPHICALSKETCH 165 V AbstractofDissertationPresentedtotheGraduateSchooloftheUniversityofFlorida inPartialFulfillmentoftheRequirementsfortheDegreeofDoctorofPhilosophy DEGRADATIONOFZnSPHOSPHORSDURINGELECTRONBEAM BOMBARDMENT By JosephSamuelSebastian August1997 Chairman:PaulH.Hollov/ay MajorDepartment:MaterialsScienceandEngineering SamplesofZnS:Mnthinfilmsandsinglecrystalshavebeensubjecttoelectron bombardment(0.6to4KeV)overarangeofpressuresfrom IxlO'^to5x10®Torn Variousgasesincludinghydrogen,oxygen,andwatervaporwereintroducedintothe ambientduringbombardmenttoassesstheireffectsonthephosphorsurface. Auger ElectronSpectroscopydataindicatethatelectronbombardmentinthepresenceofO2 andH2Ocauseddepletionofsulfurandaccumulationofoxygenonthesurface. Hydrogenalsocauseddepletionofsulfur. Removalofsulfurwasshowntobe consistentwithelectronbeamdissociationofmolecularspeciestoatomichydrogen and/oroxygen,followedbyasurfacereactiontoformhighvaporpressuresulfur compounds(e.g.,SOxandH2S). Amodelforthiselectronstimulatedsurfacereaction (ESSR)hasbeenproposedthatcorrelateswellwiththeZnSdata. Concurrentincreases VI intheoxygenandzincAPPHduringbombardmentinoxygenandwatervaporand postdegradationanalysiswithX-RayPhotoelectronSpectroscopyshowtheconversion ofthesurfaceofZnStoZnO. Thesechangesinsurfacechemistry,specificallythe removalofsulfurandgrowthofthesurfaceoxide,arealsocorrelatedtoareductionin cathodoluminescent(CL)brightnessfromthesamples. Therearethreecomponentsof thedegradedphosphorthatcouldaccountforthislossofCLbrightness:theoxide layer,theactivatorion,andthehostlattice. Opticalabsorptionoflight,attenuationof theprimaryelectronbeambytheoxidelayerandchangesinactivatorionefficiency wereinvestigated,andthesefactorscouldaccountforatmost13%ofthebrightness loss. Theseresultsindicatethemajorityofthebrightnesslossmustbecausedbythe hostlattice,ZnS. BoththedegradedandreferenceareasofsinglecrystalZnSwere analyzedbySecondaryIonMassSpectrometry,whichshowedsignificantlymoreoxygen inthefirst51 nanometers(nm)ofthedegradedsampleascomparedtothereference. Additionalthermoluminescentexperimentsareconsistentwithoxygensubstitutionfor sulfurinthelatticetoformisoelectronicelectrontraps. Thedatasuggestthatthese non-radiativetrapsreducetheexcitationrateoftheMn^^ions,whichcausesreduced CLbrightness. VII CHAPTER INTRODUCTIION Itisdifficulttoimaginehowtheinformationagewouldhavedevelopedwithout thecathoderaytube(CRT)display(Figure1.1). Arguably,itmightnothavehappened atall. Withitshumblebeginningsinblackandwhitetelevision,theCRThasevolved intothefullcolorhighdefinitiondisplayswearefamiliarwithtoday. However, engineershavetriedtoreplacetheCRT,formanyreasons. CRTsarelargeandheavy, andtheyarefabricatedoutofglasswhichcanleadtoirreparabledamageduring shippingandhandling. Typically,theyoperateat20kilovolts(KV)ormorewhichcalls forhighvoltagepowersuppliesandshieldingtopreventshockhazard. Phosphordots arepatternedonthefaceplateofaCRTtubeandanelectronbeamisdeflectedby electromagneticfieldstoexcitethephosphordotslinebyline. Thereare,however, limitsontheamounttheelectronbeamcanbedeflected,whichrestrictsthemaximum diagonalsizeofaCRTdisplay. Theseandotherissueshavebeenaddressedbyavarietyofalternatedisplay technologies. Projectiontelevision[Haseetal. 1990]allowslargedisplayareasby projectingtheimagesfromthreeCRTdisplays(red,green,andblue)ontoalarger screenforviewing. Plasmadisplays[Mikoshiba1993]use,asthenamesuggests,a plasmadischargetoexciteluminescenceinthephosphors. Inexpensivedisplaysfor dashboardsandconsumerelectronicsareoftenofthevacuumfluorescentdisplay(VFD) I 2 type[Morimoto1982]. TheirfamiliarblueglowcomesfromtheZnOrZnphosphor commonlyusedbecauseofitslonglifeandhighbrightness. Electroluminescent(EL) displays[Hollowayetal. 1994]firstinmonochromeyellowandrecentlyinfullcolor, provideaveryrugged,solid-state,althoughexpensivedisplay. Thesetechnologieshave providedsolutionsformanydisplayapplications,buttheyhavefailedtoaddressoneof thelargestandmostrapidlyexpandingareas. Technologicaladvancesincomputerpowerandcompactnesshavefueled developmentofnewdisplaytechnologies. Forthenotebookcomputertobecomea realitylargearea,thin,lightweightdisplayswererequired. Inaddition,toallowfor sufficientbatterylife,thepowerconsumptionhadtobecontrolled. Itwasunderthese demandingconditionsthatliquidcrystaldisplays(LCDs)[Doane1995]emergedasthe technologyofchoice. LCDtechnologyhasdevelopedfrommonochrometofullcolor, andarenowavailableinsizescomparableto14”diagonalCRTcomputermonitors. EarlyproblemswithlineaddressedorpassiveLCDswereeliminatedbythe introductionofthethinfilmtransistor(TFT)oractivematrixLCD(AMLCD) [Brody1973]. Inthisdesigneachpixelisindividuallyaddressedbyitsownswitching transistor. TheseadvanceshaveledtothedominanceoftheLCDinportablecomputer design,withmorethana95%marketshareworldwide. WhiletheTFT-LCDprovidesgoodcolorrenderingandeliminatestheghosting foundinpassiveLCDs,itsuffersfromseveralmajordrawbacks. First,itisenormously complextomanufacture,typicallyrequiringmorethan150separateprocesssteps (Figure1.2)whichleadstolowproductionyieldsandhighcost[Matsueda1989, Nakajima1989]. Second,itsuffersfromslowrefreshrateswhichmakesitunsuitable 3 forhighresolutionfullmotionvideo. Third,theLCDisalightvalvewhichfilterslight passingthroughitfromawhitelightsourcelocatedbehindthepanel. Thislimitsthe powerefficiencyofthedisplay,becausemuchofthelightintensityfromthebacklightis lostwhenitpassesthroughtheLCDpanel. Obtainingsufficientviewingcontrastand brightnessindirectsunlightwiththistypeofdisplayisdifficult. Finally,LCDstendtobe fragile,becausetheyarepatternedonthinglasssubstratesandaresensitiveto temperaturevariationswhichdrasticallyalterthedisplay’scontrast,bothattributeslimit theirserviceenvironment. Theselimitationshaveprovidedincentivesforthe developmentofcompetingdisplay technologies. Oftheseemergingtechnologies,thefieldemissiondisplay(FED)[Curtin 1991] (Figure1.3)isprojectedtobethemostlikelytoreplacetheLCD(Tablel.l). InaFED muchasinaCRTphosphordotsdepositedbyelectrophoresisarepatternedonaglass faceplatethathasbeencoatedwithaclearconductivelayer(ITO). Thisfaceplateis supportedabovethebaseplatebyinsulatingspacerspositionedperiodicallybetween thephosphordots. Thebackplate,whichistypicallyinsulating,ispatternedwithmetal linesthatserveastherowelectrodesforaddressingtheindividualpixels. Emittertips aredeposited[Spindt1968]ontherowelectrodesandaninsulatinglayerisdeposited aroundthesetips. Thecolumnelectrodesaredepositedontopofthisstructureand individualpixelsareaddressedbyapplyingcurrenttotheappropriaterowandcolumn electrodes. Underhighelectricfields(i.e., 10*V/cm)electronsareemittedfromthese tipsandexciteluminescencefromthephosphors. BoththepowerconsumptionandthicknessofCRTscanbereducedbyutilizing multipleelectronemittersperpixel,asopposedtoasinglescannedelectrongun. This