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1、Optoelectronics and PhotonicsPrinciples and PracticesYang JunPhotonics Research Center School of Science Harbin Engineering University2008.12专业词汇选编专业词汇选编Free electron hole pairs(EHPs):自由电子空穴对自由电子空穴对Photodiode:光探测器光探测器Pyroelectric pairoilectric detector:热探测器热探测器Acceptor：受主：受主 Donor:施主施主Antireflection
2、 coating:抗反射膜、增透膜抗反射膜、增透膜Depletion region:耗尽区耗尽区Space charge layer:空间电荷层空间电荷层Built-in voltage:内建电场内建电场Neutral regions:中性区中性区Photogenerate:光生光生Photocurrent:光电流光电流专业词汇选编专业词汇选编Drift velocity:漂移速度漂移速度Transit time:渡越时间渡越时间Upper cut-off wavelength:长波截至波长长波截至波长Absorption coefficient:吸收系数吸收系数Penetration dep
3、th:穿透深度穿透深度Direct bandgap:直接带隙直接带隙Indirect bandgap:间接带隙间接带隙Phonon Momentum:声子动量声子动量Lattice vibration:晶格振动晶格振动Quantum efficiency of the detector:探测器的量子效率探测器的量子效率External quantum efficiency:外量子效率外量子效率Responsivity:响应度响应度Spectral responsivity(radiant sensitivity)光谱响应度光谱响应度(辐射响应度辐射响应度)光电探测器(Photodetector
4、)光纤技术应用中，不可避免的会遇到将光辐射转换成易于测量和处理的电学量的问题，亦即光辐射的探测问题。光辐射探测技术是光纤技术的一个重要组成部分，而用于探测光辐射的器件通常称之为光探测器。在光纤技术的大多数应用中都需要将光辐射信号转换成电信号（或图像信息），光探测器是上述应用中实现光电转换的关键元件，光探测器性能的优劣将影响整个探测系统的性能。此外，利用将光辐射信号转换成电信号以进行显示或控制的功能，光探测器不仅可以代替人眼，而且由于其光谱响应范围宽，更是人眼的延伸。光纤技术中几种典型的光电探测器InGaAs-PIN光电二极管PIN-TIA接收组件Si-PIN光电二极管 光探测器之所以能探测光辐
5、射就是因为光辐射（即光频电磁波）传输能量。入射到光探测器上的光辐射使之产生光生载流子（或发射光电子）或使其本身的特性（如电阻、温度等）发生变化。根据上述光辐射响应方式或工作机理的不同，前者称之为光电效应，后者称之为光热效应，由此构成的光探测器分别称为光子探测器和热探测器。而光子探测器又分为：光电子发射探测器、光电导探测器、光伏探测器、光子牵引探测器；热探测器又分为：热探测器：测辐射热电偶、测辐射热计、热释电探测器、气动探测器。光电探测器(Photodetector)Photodetectors5.1 Principle of the pn Junction Photodiode5.2 Ramo
6、s theorem and external photocurrent5.3Absorption Coefficient and Photodiode Materials5.4 Quantum Efficiency and Responsivity5.5 The PIN Photodiode5.6 Avalanche Photodiode5.7 Heterojunction Photodiodes5.8 Phototransistors5.9 Noise in Photodetectors5.1 Principle of the pn junction photodiodePhotodetec
7、torsconvertalightsignaltoanelectricalsignalsuchasavoltageorcurrent.In photoconductors and photodiodes,thisconversionistypicallyachievedbythecreationoffree electron hole pairsbytheabsorptionofphotons.Inpyroelectric detectorstheenergyconversioninvolvesthegenerationofheatwhichincreasesthe temperatureof
8、thedevicewhichchangesitspolarizationandhenceitsrelative permittivity.pnjunctionbasedphotodiodetypedevicesonlyasthesedevicesaresmallandhavehighspeedandgoodsensitivityforuseinvariousoptoelectronics.Themostimportantapplicationisinoptical communications.Figure5.1(a)Aschematicdiagramofareversebiasedpn ju
9、nctionphotodiode.(b)Netspacechargeacrossthediodeinthedepletionregion.Nd andNa arethedonorandacceptorconcentrationsinthep andn sides.(c)Thefieldinthedepletionregion.The figure5.1(a)shows the simplified structure of a typical pn junction photodiode that has a p+n type of junction.The illuminated side
10、has a window,defined by an annular electrode,to allow photons to enter the device.There is an antireflection coating,typically Si3N4,to reduce light reflections.The p side is generally very thin(less than a micron)and is usually formed by planar diffusion into an n-type epitaxial layer.Figure5.2(b)s
11、hows the net space charge distribution across the p+n junction.These charges are in the depletion region,or in the space charge layer,and represent the exposed negatively charged acceptors in the p+side and exposed positively charged donors in the n-side.The depletion region extends almost entirely
12、into the lightly doped n-side and,it is a few microns.Principle of the pn junction photodiodeThephotodiodeisnormallyreverse biased,theappliedreversebiasVr dropsacrossthehighlyresistivedepletionlayerwidthWandmakesthevoltageacrossW equaltoVo+VrwhereVo is the built-in voltage.Thefieldisfoundbytheintegr
13、ationofthenetspacechargedensitynetacrossWsubjecttoavoltagedifferenceofVo+Vr.Thefieldonlyexistsinthedepletion regionandisnot uniform.Itvariesacrosspenetratesintothen-side.Theregionsoutsidethedepletionlayeraretheneutral regionsinwhichtherearemajoritycarriers.Itissometimesconvenienttotreattheseneutralr
14、egionssimplyasresistiveextensionsofelectrodestothedepletionlayer.WhenaphotonwithanenergygreaterthanthebandgapEgisincident,itbecomesabsorbedtophotogenerate a free EHP.Usually,thephotogenerationtakesplaceinthedepletion layer.ThefieldEinthedepletionlayerseparatestheEHPanddriftstheminoppositedirectionsu
15、ntiltheyreachtheneutralregions.Driftingcarriersgenerateacurrent,calledphotocurrentIph,intheexternalcircuitthatprovidestheelectricalsignal.ThephotocurrentIphdependsonthe number ofEHPsphotogeneratedandthedrift velocitiesofthecarrierswhiletheyaretransitingthedepletionlayer.Thephotocurrentintheexternalc
16、ircuitisduetotheflowofelectrons,nottobothelectronsandholes.Photodetectors5.1 Principle of the pn Junction Photodiode5.2 Ramos theorem and external photocurrent5.3Absorption Coefficient and Photodiode Materials5.4 Quantum Efficiency and Responsivity5.5 The pin Photodiode5.6 Avalanche Photodiode5.7 He
17、terojunction Photodiodes5.8 Phototransistors5.9 Noise In Photodetectors5.2 Ramos theorem and external photocurrentConsiderasemiconductormaterialwithanegligibledarkconductivity.Theelectrodesdonotinjectcarriersbutallowexcesscarriersinthesampletoleaveandbecomecollectedbythebattery.ThefieldEinthesamplei
18、suniformanditisV/L.Figure 5.2(a)AnEHPisphotogeneratedatx=l.Theelectronandtheholedriftinoppositedirectionswithdriftvelocitiesvh andve.Supposethatasinglephotonisabsorbedstapositionx=l fromtheleftelectrodeandinstantlycreatesanelectronholepair.Transit time:isthetimeittakesforacarriertodriftfromitsgenera
19、tionpointtothecollectingelectrode.Figure 5.2(b)Theelectronarrivesattimete=(L l)/ve andtheholearrivesattimeth=l/vh.Considerfirstonlythedriftingelectron.Supposethattheexternalphotocurrentduetothemotionofthiselectronisie(t).Workdone=eEdx=Vie(t)dtUsingE=V/Landve=dx/dtwefindtheelectronphotocurrentThecurr
20、entcontinuestoflowaslongastheelectronisdrifting.Itlastsforadurationteattheendofwhichtheelectronreachesthebattery.Thusalthoughtheelectronhasbeenphotogeneratedinstantaneously,theexternalphotocurrentisnotinstantaneousandhasatime spread.ElectronphotocurrentHolephotocurrentThetotalexternalcurrentwillbeth
21、esumofie(t)andih(t).EvaluatethecollectedchargeQcollectedFigure5.2(d)Thisresultcanbeverifiedbyevaluatingtheareaundertheiph(t)curveinFigure5.2(d).RamostheoremPhotodetectors5.1 Principle of the pn Junction Photodiode5.2 Ramos theorem and external photocurrent5.3Absorption Coefficient and Photodiode Mat
22、erials5.4 Quantum Efficiency and Responsivity5.5 The pin Photodiode5.6 Avalanche Photodiode5.7 Heterojunction Photodiodes5.8 Phototransistors5.9 Noise In Photodetectors5.3 Absorption coefficient and photodiode materialsThephotonabsorptionprocessforphotogeneration,thatisthecreationofEHPs,requiresthep
23、hotonenergytobeatleastequaltothebandgapenergyEg ofthesemiconductormaterialtoexciteanelectronfromthevalenceband(VB)totheconductionband(CB).Theuppercut-offwavelength(orthethresholdwavelength)gforphhotogenerativeabsorptionisthereforedeterminedbythebandgapenergyEg ofthesemiconductorsothatorForexample:Si
24、Eg=1.12eV,gis1.11m;GeEg=0.66eV,gis1.87m;Fromabove,itiscleartatSiphotodiodescannotbeusedinopticalcommunicationsat1.3and1.55mwhereasGephotodiodesarecommerciallyavailableforuseatthesewavelengths.D70.18InSbD3.50.35InAsI1.870.66GeD1.640.75In0.53Ga0.47AsD1.40.89In0.7Ga0.3As0.64P0.36I1.111.12SiD1.081.15GaA
25、s0.88Sb0.12D0.911.35InPTypeg(m)Eg(eV)SemiconductorTABLE5.1BandgapenergyEgat300K,cutoffwavelengthg andtypeofbandgap(D=DirectandI=Indirect)forsomephotodetectormaterials.Incidentphotonswithwavelengthsshorterthangbecomeabsorbedastheytravelinthesemiconductorandthelightintensity,whichisproportionaltothenu
26、mberofphotons,decaysexponentiallywithdistanceintothesemiconductor.ThelightintensityIatadistancex fromthesemiconductorsurfaceisgivenbyAbsorptioncoefficientWhereIoistheintensityoftheincidentradiationandistheabsorptioncoefficientthatdependsonthephotonenergyorwavelength.Absorptioncoefficient isamaterial
27、property.Mostofthephotonabsorption(63%)occursoveradistance1/and1/calledthepenetration depth.Figure 5.3Theabsorptioncoefficient()vs.wavelength()forvarioussemiconductorsIndirect bandgapsemiconductorssuchasIII-Vsemiconductors(e.g.GaAs,InAs,InP,GaSb)andinmanyoftheiralloys(e.g.InGaAs,GaAsSb)thephotonabso
28、rptionprocessisadirectprocessthatrequiresnoassistancefromlatticevibrations.Thephotonisabsorbedandtheelectronisexciteddirectlyfromthevalancebandtotheconductionbandwithoutachangeinitsk-vectorinasmuchasthephotonmomentumisverysmall.ThechangeintheelectronmomentumfromthevalencetotheconductionbandThisproce
29、sscorrespondstoaverticaltransitionontheE-kdiagraminFigure5.4(a).ECBVBkkDirect BandgapEgPhotonEcEv(a)GaAs(Directbandgap)Ekk(b)Si(Indirectbandgap)VBCBEcEvIndirect Bandgap,EgPhotonPhonon(a)Photonabsorptioninadirectbandgapsemiconductor.(b)Photonabsorptioninanindirectbandgapsemiconductor(VB,valenceband;C
30、B,conductionband)Inindirect bandgapsemiconductorssuchasSiandGe,thephotonabsorptionnearEgrequirestheabsorptionandemissionoflatticevibrations,thatisphonons,duringtheabsorptionprocessasshowninFigure5.4(b).IfK isthewavevectorofalatticewave(latticevibrationstravelinthecrystal),thenKisaphonon momentum.Whe
31、nanelectroninthevalencebandisexcitedtothecondu-ctionbandthereisachangeinitsmomentuminthecrystalandthischangeinthemomentumcannotbesuppliedbythemo-mentumoftheincidentphotonwhichisverysmall.Thus,themomentumdifferencemustbebalancedbyaphononmomen-tum:Theabsorptionprocessissaidtobeindirectasitdependsonlat
32、ticevibrationswhichinturndependonthetemperature.Sincetheinteractionofaphotonwithavalenceelectronneedsathirdbody,alatticevibration,theprobabilityofphotonabsorptionisnotashighasinadirecttransition.Furthermore,thecut-offwavelengthisnotassharpasfordirectbandgapsemiconductors.Duringtheabsorptionprocess,a
33、phononmaybeabsorbedoremitted.Ifisthefrequencyofthelatticevibrationsthenthephononenergyish.Thephotonenergyis where isthephotonfrequency.ConservationofenergyrequiresthatThus,theonsetofabsorptiondoesnotexactlycoincidewithEg,buttypicallyitisveryclosetoEginasmuchas issmall(EgpIp heh+AbsorptionregionAvala
34、ncheregion(a)(b)(c)Electroden+Figure 5.9 (a)A schematic illustration of the structure of an avalanche photodiode(APD)biased for avalanche gain.(b)The net space charge density across the photodiode.(c)The field across the diode and the identification of absorption and multiplication regions.Theabsorp
35、tionofphotonsandhencephotogenerationtakesplacemainlyinthelong-layer.Thenearlyuniformfieldhereseparatestheelectronholepairs(EHPs)anddriftsthematvelocitiesnearsaturationtowardsthen+andp+sidesrespectively.Whenthedriftingelectronsreachthep-layer,theyexperienceevengreaterfieldsandthereforeacquiresufficie
36、ntkineticenergytoimpact-ionizesomeoftheSicovalentbondsandreleaseEHPsasillustratedinFigure5.10.ThesegeneratedEHPsthemselvescanalsobeacceleratedbythehighfieldsinthisregiontosufficientlylargekineticenergiestofurthercauseimpactionizationandreleasemoreEHPswhichleadstoanavalancheofimpact ionization proces
37、ses.h+En+peAvalancheregioneh+EcEv(a)(b)EFigure 5.10 (a)A pictorial view of impact ionization processes releasing EHPs and the resulting avalanche multiplication.(b)Impact of an energetic conduction electron with crystal vibrations transfers the electrons kinetic energy to a valence electron and ther
38、eby excites it to the conduction band.Fromasingleelectronenteringthep-layeronecangeneratealargenumberofEHPsallofwhichcontributetotheobservedphotocurrent.Thephotodiodepossessesaninternal gain mechanisminthatasinglephotonabsorptionleadstoalargenumberofEHPsgenerated.ThephotocurrentintheAPDinthepresence
39、ofavalanchemultiplicationthereforecorrespondstoaneffectivequantumefficiencyinexcessofunity.Thereasonforkeepingthephotogenerationwithinthe-regionandreasonablyseparatefromtheavalanchep-regioninFigure5.9(a)isthatavalanchemultiplicationisastatisticalprocessandhenceleadstocarriergenerationfluctuationwhic
40、hleadstoexcess noiseintheavalanchemultipliedphotocurrent.ThemultiplicationofcarriersintheavalancheregiondependsontheprobabilityofimpactionizationwhichdependsstronglyonthefieldinthisregionandhenceonthereversebiasVr.TheoveralloreffectiveavalanchemultiplicationfactorMofanAPDisdefinedasWhereIphistheAPDp
41、hotocurrentthathasbeenmultipliedandIphoistheprimaryorunmultiplied photocurrent,thephotocurrentthatismeasuredintheabsenceofmultiplication,forexample,underasmallreversebiasVr.ThemultiplicationMisastrongfunctionofthereversebiasandalsothetemperature.MultiplicationFactorThemultiplicationMcanempiricallybe
42、expressedasWhereVbrisaparametercalledtheavalanchebreakdownvoltagenisacharacteristicindexthatprovidesthatthebestfittotheexperimentaldata.M isastrongfunctionofbothreversebiasvoltageandtemperature.M 100(SiAPD),M 10(GeAPDs).structure of practical Si APDSiO2GuardringElectrodeAntireflectioncoatingnnn+p+pS
43、ubstrateElectroden+p+pSubstrateElectrodeAvalanchebreakdown(a)(b)Figure 5.11(a)A Si APD structure without a guard ring.(b)A schematic illustration of the structure of a more practical Si APD.Oneofthedrawbacksofthesimplereach-throughAPDstructureisthatthefieldaroundthen+pjunctionperipheraledgereachesav
44、alanchebreakdownbeforethen+pregionsundertheilluminatedareaasillustratedinFigure5.11(a).Ideallytheavalanchemultiplicationshouldoccuruniformlyintheilluminatedregiontoencouragetheavalanchemultiplicationoftheprimaryphotocurrentratherthanthemultiplicationofthedarkcurrent.InapracticalSiAPD,ann-typedopedre
45、gionactingasaguard ringsurroundsthecentraln+regionasshownFigure5.11(b)sothatthebreakdownvoltagearoundtheperipheryisnowhigherandavalancheisconfinedmoretoilluminatedregion.Typical Characteristics of Different PDsPhotodetectors5.1 Principle of the pn Junction Photodiode5.2 Ramos theorem and external ph
46、otocurrent5.3Absorption Coefficient and Photodiode Materials5.4 Quantum Efficiency and Responsivity5.5 The pin Photodiode5.6 Avalanche Photodiode5.7 Heterojunction Photodiodes5.8 Phototransistors5.9 Noise In Photodetectors5.7 Heterojunction PhotodiodesIII-VcompoundbasedAPDshavebeendevelopedforuseatt
47、hecommunicationswave-lengths1.3mand1.55m.Asinthereach-throughSiAPD,theabsorptionorphotogeneration regionisseparatedfromtheavalanche ormultiplication regionwhichallowsthemultiplicationtobeinitiatedbyonetypeofcarrier.Figure5.12isasimplifiedschematicdiagramofthestructureofanInGaAs-InPAPDwithaseparate a
48、bsorption and multiplication(SAM).A.Separate Absorption and Multiplication(SAM)APDFigure 5.12Simplifiedschematicdiagramofaseparateabsorptionandmultiplication(SAM)APDusingaheterostructurebasedonInGaAs-InP.P andN refertop andn-typewider-bandgapsemiconductor.Thereareanumberofpracticalfeaturesthatarenot
49、showninthehighlysimplifieddiagraminFigure5.12.Photogeneratedholesdriftingfromn-InGaAstoN-InPbecometrappedattheinterfacebecausethereisasharpincreaseinthebandgapandasharpchangeEvinEv(valencebandedge)betweenthetwosemiconductorsandholescannoteasilysurmountthepotentialenergybarrierEvasdepictedinFigure5.1
50、3(a).Thisproblemisovercomebyusingthinlayersofn-typeInGaAstoInPasdepictedinFigure5.13(b).InPInGaAsh+e EEcEvEcEvInPInGaAsEvEvInGaAsPgradinglayerh+DEv(a)EnergybanddiagramforaSAMheterojunctionAPDwherethereisavalencebandstepDEvfromInGaAstoInPthatslowsholeentryintotheInPlayer.(b)Aninterposinggradinglayer(