集成光学课程第九章.ppt

IntegratedOpticalDetectors Detectorsforuseinintegrated opticapplicationsmusthavehighsensitivity shortresponsetime largequantumefficiencyandlowconsumption Inthischapter anumberofdifferentdetectorstructureshavingtheseperformancecharacteristicsarediscussed 9 1DepletionLayerPhotodiodes9 2SpecializedPhotodiodeStructure9 3TechniquesforModifyingSpectralResponse9 4FactorsLimitingPerformanceofIntegratedDetectors 本章学习各种探测器的工作原理 重点掌握耗尽层光电探测器的工作原理 9 1DepletionLayerPhotodiodes Themostcommontypeofsemiconductoropticaldetector usedinbothintegratedopticanddiscretedeviceapplications isthedepletion layerphotodiode Thedepletion layerphotodiodeisessentiallyareverse biasedsemiconductordiodeinwhichreversecurrentismodulatedbytheelectron holepairsproducedinornearthedepletionlayerbytheabsorptionofphotonslight Thediodeisgenerallyoperatedinthephotodiodemode withrelativelylargebiasvoltage 9 1 1ConventionalDiscretePhotodiode Thesimplesttypeofdepletionlayerphotodiodeisthep njunctiondiode Theenergybanddiagramforsuchadevice withreversebiasvoltageVaappliedisshowninFig 9 1 Thetotalcurrentofthedepletionlayerphotodiodeconsistsoftwocomponents adriftcomponentoriginatingfromcarriersgeneratedinregion b andadiffusioncomponentoriginatinginregions a and c Fig 9 1Energybanddiagramforap njunctiondiodeunderapplicationofareversebiasvoltageVa Holesandelectronsgeneratedinregion b areseparatedbythereversebiasfield withholesbeingsweptintothep region c andelectronsbeingsweptinton region a Holesgeneratedinthen regionorelectronsgeneratedinthep regionhaveacertainprobabilityofdiffusingtotheedgeofthedepletionregion b atwhichpointtheyaresweptacrossbythefield Majoritycarriers electronsin a orholesin c areheldintheirrespectiveregionsbythereversebiasvoltage andarenotsweptacrossthedepletionlayer Inordertominimizeseriesresistanceinapracticalphotodiodewhilestillmaintainingmaximumdepletionwidth usuallyoneregionismuchmoreheavilydopedthantheother Inthatcase thedepletionlayerformsalmostentirelyonthemorelightlydopedsideofthejunction asshowninFig 9 2 Suchadeviceiscalledahigh lowabruptjunction Fig 9 2Energybanddiagramforap n high low junctiondiodeunderapplicationofareversebiasvoltageVa InGaAsanditsternaryalloys electronmobilityisgenerallymuchlargerthanholemobility Thus thep regionisusuallymadethinnerandmuchmoreheavilydopedthanthen region sothatthedevicewillbeformedmostlyinn typematerial andthep regionthenservesessentiallyjustasacontactlayer Foradevicewiththehigh lowjunctiongeometryindicatedinFig 9 2 itcanbeshownthatthetotalcurrentdensityJtotisgivenby 9 1 1 where 0isthetotalphotonfluxinphotons cm2s Wisthewidthofthedepletionlayer qisthemagnitudeoftheelectroniccharge istheopticalinterbandabsorptioncoefficient Lpisthediffusionlengthforholes Dpisthediffusionconstantforholes andpn0istheequilibriumholedensity Thelasttermrepresentsthereverseleakagecurrent ordarkcurrent whichresultsfromthermallygeneratedholesinthen material Thisexplainswhythattermisnotproportionaltothephotonflux 0 Thefirsttermgivesthephotocurrent whichisproportionalto 0 andincludescurrentformboththedriftofcarriersgeneratedwithinthedepletionlayerandthediffusionanddriftofholesgeneratedwithinadiffusionlengthLpofthedepletionlayeredge Thequantumefficiencyofthedetector orthenumberofcarriersgeneratedperincidentphoton isgivenby 9 1 2 whichcanhaveanyvaluefromzerotoone Itshouldbenotedthat 9 1 1 and 9 1 2 arebasedonthetacitassumptionthatscatteringlossandfreecarrierabsorptionarenegligiblesmall Itcanbeseenfrom 9 1 2 that inordertomaximize q itisdesirabletomaketheproducts Wand Lpaslargeaspossible When Wand Lparelargeenoughsothat qisapproximatelyequaltoone thediodecurrentisthenessentiallyproportionalto 0 becausethedarkcurrentisusuallynegligiblysmall Iftheinterbandabsorptioncoefficient istoosmallcomparedtoWandLp manyoftheincidentphotonswillpasscompletelythroughtheactivelayersofthediodeintothesubstrate asshowninFig 9 3 Onlythosephotonsabsorbedwithinthedepletionlayer ofthicknessW havemaximumeffectivenessincarriergeneration PhotonsabsorbedatdepthsuptoadiffusionlengthLpfromthedepletionlayeredgearesomewhateffectiveingeneratingphoto carriers inthatholescandiffuseintothedepletionlayer Fig 9 3Diagramofaconventionalmesa geometryphotodiodewithp ndopingprofileshowingphotonpenetration Photonsthatpenetratetoasdepthgreaterthat W Lp beforebeingabsorbedareessentiallylosttothephoto generationprocessbecausetheyhavesuchalowstatisticalprobabilityofproducingaholethatcanreachthedepletionlayerandbesweptacross Withinthesemiconductor thephotonfluxfallsoffexponentiallywithincreasingdepthxfromthesurface Thusif isnotlargeenough manyphotonswillpenetratetoodeeplybeforebeingabsorbed thusproducingcarriersthat onaverage willrecombinebeforediffusingfarenoughtoreachthedepletionlayer Interbandabsorptionisastrongfunctionofwavelengthinasemiconductor Thus itisimpossibletodesignadiodewithanidealWforallwavelengths Asidefromthereductionofquantumefficiencythatresultsfrompoormatchingof WandLp therearesomeotherlimitationstodepletionlayerphotodiodeperformancethatarealsoimportant SinceWisusuallyrelativelysmall intherangeof0 1to1 0 m junctioncapacitancecanlimithigh frequencyresponsethroughthefamiliarR Ctimeconstant Also thetimerequiredforcarrierstodiffusefromdepthsbetweenWand W Lp canlimitthehighfrequencyresponseofaconventionalphotodiode Thewaveguidedepletionlayerphotodiode whichisdiscussedinthenextsection significantlymitigatesmanyoftheseproblemsoftheconventionalphotodiode 9 1 2WaveguidePhotodiodes Ifthebasicdepletionlayerphotodiodeisincorporatedintoawaveguidestructure asshowninFig 9 4 anumberofimprovementsinperformancearerealized Inthiscase thelightisincidenttransverselyontheactivevolumeofthedetector ratherthanbeingnormaltothejunctionplane Thediodephotocurrentdensityisthengivenby 9 1 3 Fig 9 4Diagramofwaveguidedetector whereListhelengthofthedetectorinthedirectionoflightpropagation SinceWandLaretwoindependentparameters thecarrierconcentrationwithinthedetectorvolumeandthebasisvoltageVacanbechosensothattheLcanbemadeaslongasnecessarytomake L 1 Thus100 quantumefficiencycanbeobtainedforanyvalueof bymerelyadjustingthelengthL Forexample foramaterialwiththerelativelysmallvalueof 30cm 1 alengthofL 3mmwouldgive q 0 99988 Again ithasbeentacitlyassumedin 9 1 3 thatscatteringlossandfree carrierabsorptionarenegligible Becauseawaveguidedetectorcanbeformedinanarrowchannelwaveguide thecapacitancecanbeverysmall evenifLisrelativelylarge Thiscapacitanceisaboutafactoroftenlessthanthatoftypicalconventionalmesaphotodiode Hence thehighfrequencyresponsecanbeexpectedtobecorrespondinglyimproved Becausealloftheincidentphotonsareabsorbeddirectlywithinthedepletionlayerofawaveguidephotodetector notonly qisimproved butalsothetimedelayassociatedwiththediffusionofcarriersiseliminated Thisresultisafurtherimprovementinhighfrequencyresponse Duetothemanyimprovementsinperformanceinherentinthetransversestructureofthewaveguidedetector ascomparedtotheaxialgeometryoftheconventionalmesaphotodiode waveguidedetectorsshouldbeconsideredforuseindiscrete deviceapplicationsaswellasinopticalintegratedcircuits 9 2SpecializedPhotodiodeStructure Therearetwoveryusefulphotodiodestructuresthatcanbefabricatedineitherawaveguidingorconventional nonwaveguidingform ThesearetheSchottky barrierphotodiodeandtheavalanchephotodiode APD 9 2 1Schottky BarrierPhotodiode TheSchottky barrierphotodiodeissimplyadepletionlayerphotodiodeinwhichthep njunctionisreplacedbyametal semiconductorrectifying blocking contact Forexample ifthep typelayersinthedevicesofFig 9 4werereplacedbyametalthatformsarectifyingcontacttothesemiconductor Schottky barrierphotodiodeswouldresult Thephotocurrentwouldstillbegivenby 9 1 1 and 9 1 3 andthedeviceswouldhaveessentiallythesameperformancecharacteristicsastheirp njunctioncounterparts TheenergybanddiagramsforaSchottky barrierdiode underzerobiasandunderreversebias aregiveninFig 9 5 Itcanbeseenthatthedepletionregionextendsintothen typematerialjustasinthecaseofap njunction Thebarrierheight Bdependsontheparticularmetal semiconductorcombinationthatisused Typicalvaluesfor Bareabout1V Fig 9 5a b EnergybanddiagramforaSchottky barrierdiode a zerobias b reversebiasedwithvoltageVa Inconventionalmesadevices athin opticallytransparentSchottky barriercontactisoftenused ratherthanap njunction toenhanceshort wave lengthresponse byeliminatingthestrongabsorptionofthesehigherenergyphotonsthatoccursintheplayer Inawaveguidephotodiode aSchottky barriercontactisnotneededforimprovedshort wavelengthresponsebecausethephotonsentertheactivevolumetransversely However easeoffabricationoftenmakestheSchottky barrierphotodiodethebestchoiceinintegratedapplications Forexample almostanymetal exceptforsilver producesarectifyingSchottky barrierwhenevaporatedontoGaAsatroomtemperature Gold aluminumorplatinumareoftenused Photodiodemaskingisadequatetodefinethelateraldimensionsduringevaporation andnocarefulcontroltimeandtemperatureisrequired asinthecaseofdiffusionofashallowp layer 9 2 2AvalanchePhotodiode Thegainofadepletionlayerphotodiode i e thequantumefficiency ofeitherthep njunctionorSchottky barrier canbeatmostequaltounity undernormalconditionsofreversebias However ifthedeviceisbiasedpreciselyatthepointofavalanchebreakdown carriermultiplicationduetoimpactionizationcanresultinsubstantialgainintermsofincreaseinthecarriertophotonratio Infact avalanchegainsashighas104arenotuncommon Typicalcurrent voltagecharacteristicsforanavalanchephotodiodeareshowninFig 9 6 Fig 9 6Responsecurvesforanavalanchephotodiode Theuppercurveisfordarkenedconditions whiletheloweroneshowstheeffectsofillumination Forrelativelylowreversebiasvoltage thediodeexhibitsasaturateddarkcurrentId0andasaturatedphoto currentIph0 However whenbiasedatthepointofavalanchebreakdown carriermultiplicationresultsinincreaseddarkcurrentId aswellasincreasephoto currentIph Itispossibletodefineaphoto multiplicationfactorMph givenby 9 2 1 andamultiplicationfactorM givenby 9 2 2 whereVbisthebreakdownvoltage andnisanempiricallydeterminedexponentdependingonthewavelengthoflight dopingconcentration and ofcourse thesemiconductormaterialfromwhichthediodeisfabricated Forthecaseoflargephoto currentIph0 Id0themultiplicationfactorisgivenby 9 2 3 whereIisthetotalcurrent givenby 9 2 4 Rbeingtheseriesresistanceofthediode includingspace chargeresistanceifsignificant Thederivationof 9 2 3 assumesthatIR Vb ForthecaseofId0andIdbeingnegligiblysmallcomparedtoIph0andIph itcanbeshownthatthemaximumattainablemultiplicationfactorisgivenby 9 2 5 Avalanchephotodiodesareveryusefuldetectors notonlybecausetheyarecapableofhighgain butalsobecausetheycanbeoperatedatfrequenciesinexcessof10GHz However noteveryp njunctionorSchottky barrierdiodecanbeoperatedintheavalanchemultiplicationmode biasednearavalanchebreakdown Forexample thefieldrequiredtoproduceavalanchebreakdowninGaAsisapproximately4x105V cm Hence foratypicaldepletionwidthof3 m Vbequals120V MostGaAsdiodeswillbreakdownatmuchlowervoltagesduetoothermechanisms suchasedgebreakdownormicroplasmsgenerationatlocalizeddefects thusneverreachingtheavalanchebreakdowncondition Inordertofabricateanavalanchephotodiode extremecaremustbetaken beginningwithadislocation freesubstratewaferofsemiconductormaterial 9 3TechniquesforModifyingSpectralResponse Thefundamentalproblemofwavelengthincompatibility whichwasencounteredpreviouslyinregardtothedesignandfabricationofmonolithiclaser waveguidesstructures isalsoverysignificantwithrespecttowaveguidedetectors Anidealwaveguideshouldhaveminimalabsorptionatthewavelengthbeingused Howeveradetectordependsoninterbandabsorptionforcarriergeneration Hence ifadetectorismonolithicallycoupledtoawaveguide somemeansmustbeprovidedforincreasingtheabsorptionofthephotonstransmittedbythewaveguidewithinthedetectorvolume Anumberofdifferenttechniqueshavebeenproveneffectiveinthisregard 9 3 1HybridStructures Oneofthemostdirectapproachestoobtainingwavelengthcompatibilityistouseahybridstructure inwhichadetectordiode formedinarelativelynarrowbandgapmaterial iscoupledtoawaveguidefabricatedinwiderbandgapmaterial Thetwomaterialsarechosensothatphotonsofthedesiredwavelengtharetransmittedfreelybythewaveguide butarestronglyabsorbedwithinthedetectormaterial Anexampleofthistypeofhybridwaveguide detectoristheglassonsiliconstructure asshowninFig 9 7 Fig 9 7Hybridwaveguidedetectorfeaturingaglasswaveguidecoupledtoasiliconphotodiode Thediodewasformedbyborondiffusiontoadepthofabout1 mintoann type 5cmsiliconsubstrate A1 mthicklayerofthermallygrownSiO2wasusedasadiffusionmask Theglasswaveguidewasthensputter depositedandsilverpaintelectrodeswereaddedasshown Totalguidelosswasmeasuredtobe0 8dB cm 10 forlightof632 8nmwavelength Theefficiencyofcouplingbetweenthewaveguideandthedetectorwas80 However becausethelightentersthediodeinthedirectionnormaltothejunctionplaneratherthanparalleltoit thisparticularwaveguidedetectorgeometrydoesnothavemanyoftheadvantagesdescribedinSect 9 1 2 Nevertheless goodhighfrequencyresponsecanbeexpected Thesediffuseddiodeshadacapacitanceofonly3x10 9F cm2whenbiasedwithVaequalto10V Thusadetectordiodeofapproximately10 mradius usedinconjunctionwitha50 loadresistance wouldhaveanRCtimeconstantofabout15ps implyingthatmodulationoffrequenciesinexcessof10GHzcouldbedetected Whilehybriddetectorsofferthepossibilityofchoosingthewaveguideanddetectormaterialsforoptimumabsorptioncharacteristics bettercouplingefficiencycanbeobtainedwithmonolithicfabricationtechniques Monolithicallyfabricatedwaveguidedetectorsalsohavetheadvantagesthatlightentersthedeviceintheplaneofthejunctionrathernormaltoit 9 3 2HeteroeptaxialGrowth Perhapsthemostpopularmethodofmonolithicallyintegratingawaveguideanddetectoristouseheteroepitaxialgrowthofarelativelynarrowbandgapsemiconductoratthelocationwhereadetectorisdesired AnexampleofthisapproachisgivenbytheInGaAsdetectorthathasbeenintegratedwithaGaAswavelguide asshowninFig 9 8 InInxGa 1 x As thebandgapcanbeadjustedtoproducestrongabsorptionoflightatwavelengthintherangefrom0 9to1 15 mbychangingtheatomicfractionxofindium Fig 9 8MonolithicallyintegratedInGaAsdetectorinaGaAswaveguide ThemonolithicallywaveguidedetectorstructureshowninFig 9 8combinesanepitaxiallygrowncarrier concentration reductiontypewaveguidewithaplatinumSchottkybarrierdetector A600nmthicklayerofpyrolyticallydepositedSiO2wasusedasamasktoetcha125 mdiameterwellintothe5to20 mthickwaveguide andthengrowtheInxGa 1 x Asdetectormaterial Aquantumefficiencyof60 wasmeasuredforthisdetectoratawavelengthof1 06 m forlowbiasvoltages Thelossinthewaveguidewaslessthan1dB cm Atbiasvoltagegreaterthanabout40V avalanchemultiplicationwasobserved withmultiplicationfactorsashighas50 Optimumperformancewasobtainedatawavelengthof1 06 mforanInconcentrationofx 0 2 Thefactthatquantumefficiencyinthisdevicesdidnotapproach100 morecloselywasmostlikelycausedbylessthanoptimumdepletionwidthintheSchottky barrierdiode ThecarrierconcentrationinthewaveguidemustbeverycarefullycontrolledinordertomakeWequaltothewaveguidethickness DefectcentersassociatedwithstressattheGaAs GaInAsinterfacemayhavealsoplayedaroleinreducing q Ingeneral theIII Vcompoundsemiconductorsandtheirassociatedternary andquaternary alloysofferthedevicedesignerawiderangeofbandgaps andcorrespondingabsorptionedgewavelengths Therelationshipsbetweenbandgaps absorptionedgewavelengthsandlatticeconstantsareshowninFig 9 9 Dottedportionsofthecurvescorrespondtorangesofcompositionforwhichthebandgapisindirect Fig 9 9DependenceofabsorptionedgewavelengthandlatticeconstantoncompositionforselectedIII Valloys Directbandgapmaterialsgenerallyhaveinterbandabsorptioncoefficientsgreaterthan104cm 1forwavelengthsshorterthantheabsorptionedge while maybeseveralordersofmagnitudelessinindirectgapmaterials Neverthelesseffectivedetectorscanbemadeinindirectbandgapmaterials especiallywhenthewaveguidedetectorgeometryisused sothatthelengthLcanbeadjustedtocompensateforsmall Themostpopularmaterialforthefabricationofdetectorsinthe1 0to1 6 mwavelengthrangeisGaInAsP 9 3 3ProtonBombardment InChap 4 protonbombardmentwasdescribedasamethodforproducingopticalwaveguidesinasemiconductorbygeneratingcarrier trappingdefectcentersthatresultedinreducedcarrierconcentrationandincreasedindexofrefraction Inthatcase thewaveguideswerealwayssufficientlyannealedafterprotonbombardmenttoremovetheopticalabsorptionassociatedwiththetrappingcenters However oneofthemechanismsresponsibleforthisabsorptionistheexcitationofcarriersoutofthetraps freeingthemtocontributetotheflowofphoton current Thus aphotodiodecanbefabricatedbyformingaSchottky barrierjunctionovertheimplantedregion asshowninFig 9 10 Ashallowp njunctioncouldalsobeused Fig 9 10Diagramofaproton implantedopticaldetector Photon currentflowswhenthejunctionisreversebiased becausecarriersliberatedbyphoton excitationwithinthedepletionlayerofthediodearesweptacrossitbythefield Sinceasubstantialnumberofthetrappingcentershaveenergylevelslyingwithintheforbiddengap theeffectivebandgapofthesemiconductorisdecreased sothatphotonshavingless than bandgapenergycanbeabsorbedandtakepartinthecarriergenerationprocess Thusaproton bombardedphotodiode madeinagivensemiconductor canberespectivetophotonsthatwouldordinarilynotbeabsorbedinthematerial Forexample Stolletal havemadeadetectorinGaAsthatissensitiveto1 15 mwavelengthradiation Theopticalwaveguidestructureconsistedofa3 5 mthickn typeepitaxiallayer S doped n 1016cm 3 grownonadegeneratelydopedn typesubstrate n 1 25x1018cm 3 Priortoprotonimplantationtheopticalattenuationat1 15 mwasmeasuredtobe1 3cm 1 butafterimplantationwithadoseof2x1015cm 2300keVprotonsintheregionwhereadetectorwasdesired increasedtoover300cm 1 Apartialannealingofdamageat500 Cfor30minwasperformedtoreduce to15cm 1inordertoallowsomeopticaltransmissionthroughtheentirelengthoftheimplantedregion Then anAlSchottky barriercontactwasevaporatedontopoftheimplantedregiontocompletethedevice Therelativephoto responseoftheprotonimplanteddetectorasafunctionofwavelengthisshowninFig 9 11 alongwiththecorrespondingcurveforasimilardetectorformedinunimplantedGaAs OnlynegligibleresponseoccursintheunimplantedGaAsatwavelengthslongerthan900nm butasubstantialabsorptiontailisobservedfortheprotonbombardedde