激子與發(fā)光課件

課程內(nèi)容緒論經(jīng)典傳播帶間吸收激子發(fā)光半導(dǎo)體量子阱、自由電子、分子材料發(fā)光中心1課程內(nèi)容緒論122第四講ExcitonsTheconceptofexcitonsFreeexcitonsFreeexcitonsathighdensityFrenkelexcitons3第四講ExcitonsTheconceptofeTheconceptofexcitonsInsemiconductorsandinsulators:photonabsorptionelectronsintheconductionbandandholesinthevalenceband.Exciton:boundelectron–holepairbyCoulombinteractionObservedinsemiconductors;largeradius;delocalizedstates;movefreely;bindingenergy~0.01eVObservedininsulatorsandmolecularcrystals;smallerradius;localizedstates;lessmobileandhoping;bindingenergy~0.1-1eV.Stableexcitonswillonlybeformediftheattractiveenergy>>kBT(0.026eVatroomtemperature)Freeexcitonsarestableatcryogenictemperature.Tightboundexcitonsarestableatroomtemperature.Twotypesofexcitons:4Wannier激子（自由激子）Frenkel激子（束縛激子）:TheconceptofexcitonsInsemiTheconceptofexcitonsFreeexcitonsFreeexcitonsathighdensityFrenkelexcitons5第四講ExcitonsTheconceptofexcitons5第四講BindingenergyandradiusoffreeexcitonsFreeexcitons:weaklyboundelectron-holepair;ahydrogenicsystemApplyingtheBohrmodeltotheexciton,consideringdielectricconstantrofthemediumandthereducedmassofelectronandhole.

TheenergyofthenthlevelrelativetotheionizationlimitRHistheRydbergconstantofthehydrogenatom(13.6eV).RX:excitonRydbergconstant.Theradiusoftheelectron-holeorbit:aHistheBohrradiusofthehydrogenatom(5.2910-11m)andaxistheexcitonBohrradius.groundstatewithn=1hasthelargestbindingenergyandsmallestradius.n>1:lessstrongbindingenergyandlargerradius.Bidingenergytendstodecreaseandaxtoincreaseasrincrease.6BindingenergyandradiusoffRxtendstoincreaseandaxtodecreaseasEgincreases.Causes:

rtendstodecreaseandtoincreaseasthebandgapincreases.Ininsulatorswithbandgapsgreaterthanabout5eV,axbecomescomparabletotheunitcellsize,andtheWanniermodelisnolongervalid.narrowgapsemiconductors:RHissosmallthatexcitoneffectsishardtoobserve.(Eg=1-3eV,freeexcitonsbehaviorisbestobserved)7RxtendstoincreaseandaxtoExcitonabsorptionFreeexcitonsaretypicallyobservedindirectgapsemiconductors.(hardtoobserveintheabsorptionspectraofindirectsemiconductor)Thegroupvelocityofanelectronorholeinabandisgivenby:AttheBrillouimzonecentreofdirectsemiconductor:k=0andzerogradient.Eectron-holepairscreatedbydirecttransitionandhavethesamevelocities.Therefore,strongexcitonsoccurinthespectralregionclosetothefundamentalbandgap.Bandedgeabsorptionspectrumforadirectgapsemiconductorwithexcitoniceffectsincluded.Thedashedlineshowstheexpectedabsorptionwhentheexcitoniceffectsareignored.Freeexcitonscanonlybeobservedinverypuresamples.Impurities:screeningtheCoulombinteractionintheexcitonandtherebystronglyreducethebindingforces;generatingelectricfieldandtendtoionizeexcitons.8Excitoncreation:electron-holepairsandsamevelocities.Theenergyofexcitonabsorptionis:ExcitonabsorptionFreeexcitonExperimentaldataforfreeexcitonsinGaAsExcitonabsorptionofultrapureGaAsat1.2K.hydrogen-likeenergyspectrumoftheexcitoninthevicinityofthebandgap.E1=1.5149eV,E2=1.5180eV,E3=1.5187eVTheexperimentalRx=4.2meVisingoodagreementwiththecalculatedvalue.9Eg=l.5191eV,agreewithothermeasurements.ExperimentaldataforfreeexcTheconceptofexcitonsFreeexcitonsFreeexcitonsathighdensityFrenkelexcitons10第四講ExcitonsTheconceptofexcitons10第四講MortdensityNmott:thedensityatwhichtheexciton-excitondistanceisequaltotheexcitondiameter:Highdensityisachievablewithafocussedlaserbeam.11TheexcitondensitycanbecontrolledbytuningthelaserpowerLowdensity,theexciton-excitoninteractionsarenegligible;theexcitonwavefunctionsbegintooverlapathighdensityandtheinteractionwillbecomesignificant.MortdensityNmott:thedensit12DensityEffects:1.electron-holeplasmaweakeningandbroadeningoftheexcitonabsorptionlineisobserved(absorptionsaturation,nonlineareffects).2.Biexcitons(excitonmolecules)equivalentprocesstotheformationofanH2molecule;newfeaturelinecanbefound.3.electron-holedroplets4.Bose-Einsteincondensation(Stotal=0or1)Broadfeaturelineatlowerenergythanthefreeexciton12DensityEffects:1.electron-TheconceptofexcitonsFreeexcitonsFreeexcitonsathighdensityFrenkelexcitons13第四講ExcitonsTheconceptofexcitons13第四講Frenkelexcitonsoccurringinlargebandgapmaterialswithsmalldielectricconstantsandlargeeffectivemasses.smallradiiandlargebindingenergies,0.1eVtoseveraleV,stableatroomtemperature.

propagatingthroughthecrystalbyhopping.Localizedontheatomsite,maythereforebeconsideredasexcitedstatesoftheindividualatomsormolecules,especiallyforn=1excitonenergy

.

TheoreticaltreatmentofFrenkelexcitonsismorecomplicated.14FrenkelexcitonsoccurringinRaregascrystalsCrystallizeatcryogenictemperatures.Largebandgap,Neonhasthelargestbandgapinnature.ExcitontransitionsalloccurinthevacuumultravioletspectralrangeBindingenergiesareverylarge.Aclosecorrespondencebetweenthen=1excitonenergiesinthecrystalsandtheopticaltransitionsoftheisolatedatoms(ForXenon5p65p56s).

Theradiusincreaseswithn,delocalized,correspondencegetsweaker.15RaregascrystalsCrystallizeAlkalihalides

Largedirectbandgaps(5.9eV13.7eV)LiFhasthewidestbandgapofanypracticalopticalmaterial.Egandexcitonbindingenergytendstoincreasewithdecreasinganionandcationsize.Theexcitonsarelocalizedatthenegative(halogen)ions.StrongexcitoneffectsatRTbecauseoflargebindingenergy(0.8eVand1.9eV)16AlkalihalidesLargedirectbaPrinciplesofluminescenceInterbandluminescencePhotoluminescenceElectroluminescent第五講InterbandLuminescence17Principlesofluminescence第五講發(fā)光的定義固體中的電子受到外界能量的激發(fā)，從基態(tài)躍遷到激發(fā)態(tài)，這是一種非平衡態(tài)。處于激發(fā)態(tài)的電子具有一定的壽命，以一定幾率回落到基態(tài)，并把多余的能量以各種形式釋放出來。如果以光能的形式釋放，稱為發(fā)光過程。任何物體在一定溫度下均有熱輻射（熱發(fā)光）。為了區(qū)分其它發(fā)光形式和熱發(fā)光，嚴(yán)格的固體發(fā)光概念不包含熱發(fā)光。發(fā)光現(xiàn)象有兩個(gè)主要特征：發(fā)光為固體吸收外界能量后，所發(fā)出總輻射超出熱發(fā)射的部分。（發(fā)光的定義，指出了與熱輻射的區(qū)別）外界激發(fā)源對(duì)物體的作用停止后，發(fā)光現(xiàn)象會(huì)持續(xù)一段時(shí)間。（發(fā)光與散射、反射等現(xiàn)象的區(qū)別）18發(fā)光的定義固體中的電子受到外界能量的激發(fā)，從基態(tài)躍遷到激發(fā)態(tài)發(fā)光的分類依據(jù)激發(fā)方式不同，固體發(fā)光可分為以下幾種形式：光致發(fā)光：如熒光燈，PDP。電致發(fā)光：如LED。陰極射線發(fā)光：CRT。高能射線或粒子（X射線，射線，粒子等）發(fā)光：如醫(yī)學(xué)胸透?；瘜W(xué)發(fā)光：如熒光棒。生物發(fā)光：如螢火蟲。機(jī)械發(fā)光：摩擦發(fā)光。19發(fā)光的分類依據(jù)激發(fā)方式不同，固體發(fā)光可分為以下幾種形式：光LightemissioninsolidsR=1/A，radiativelifetimeoftransitionInjectedelectronorholeRelaxtotheminimumenergystateThephotonisemittedwhenanelectroninanexcitedstatedropsdownintoanemptystateinthegroundstatebandLuminescence.IftheupperlevelhasapopulationNattimet,theradiativeemissionrateisgivenby:A:Einsteincoefficient.20LightemissioninsolidsR=1/A21PhotonabsorptionPhotonemissiontransitionswhichhavelargeabsorptioncoefficientsalsohavehighemissionprobabilitiesandshortradiativelifetimes.photonscanbeabsorbedtoanystatewithintheexcitedstateband,nomatterhowfaritisabovethebottomoftheband.Electronsandholesrelaxrapidlytothelowestlevelsofexcitedstate,andthelightwillthereforeonlybeemittedwithinanarrowenergyrangefromthelowestlevelsintheexcitedstateband.Normally,theabsorptionandemissionspectraarenotsame.21PhotonabsorptionPhotonemisTheluminescentintensityatfrequency:ThematrixelementM:Fermi’sgoldenruleThejointdensityofstateg(h)theoccupancyfactorsgivetheprobabilitiesthattherelevantupperlevelisoccupiedandthelowerlevelisempty.22Theluminescentintensityatf光致發(fā)光的效率輻射躍遷并不是激發(fā)態(tài)電子回到基態(tài)的唯一途徑。另一途徑：無輻射躍遷，發(fā)射聲子（吸收光能轉(zhuǎn)變?yōu)闊幔?。消弱發(fā)光。設(shè)無輻射躍遷壽命為NR，同時(shí)考慮輻射躍遷和無輻射躍遷，激發(fā)態(tài)電子數(shù)變化速率：發(fā)光效率R定義為輻射躍遷速率/總躍遷速率：高效發(fā)光材料要求輻射躍遷壽命R遠(yuǎn)小于無輻射躍遷壽命NR

。23光致發(fā)光的效率輻射躍遷并不是激發(fā)態(tài)電子回到基態(tài)的唯一途徑。另PrinciplesofluminescenceInterbandluminescencePhotoluminescenceElectroluminescent24第五講InterbandLuminescencePrinciplesofluminescence24第五InterbandluminescenceInterbandluminescence:inasemiconductor,anelectronthathasbeenexcitedintotheconductionbanddropsbacktothevalencebandbytheemissionofaphoton.Correspondingtotheannihilationofanelectron-holepair(electron-holerecombination).1.DirectgapmaterialsTheopticaltransitionsaredipole-allowedandhavelargematrixelements.radiativelifetime:10-8-10-9s;luminescentefficiencyishigh.injectedelectronsandholesrelaxveryrapidlytothelowestenergystates.electronandholethatrecombinemusthavethesamekvector,downwardverticalarrow.Nomatterhowweexcitetheelectronsandholesinthefirstplace,luminescenceatenergiesclosetothebandgapisalwaysobtained.25InterbandluminescenceInterbanLuminescencespectrumandabsorptionofaGaNepilayerat4K.Thephotoluminescence(PL)wasexcitedbyabsorptionof4.9eVphotons.Theemissionspectrumconsistofanarrowemissionlineat3.5eVclosetothebandgapenergy,whiletheabsorptionshowstheusualthresholdatEgwithcontinuousabsorptionfor?>Eg.Theemissionandabsorptionspectraarenotthesame,eventhoughtheyaredeterminedbythesamematrixelement.Thebandgapcorrespondstothethresholdforopticalabsorption,buttotheenergyoftheopticalemission.26Luminescencespectrumandabso2.IndirectgapmaterialsRequiringemittingbothaphononandaphotonduringthetransition.asecond-orderprocess,withasmalltransitionprobability.longerradiativelifetime,smallerluminescentefficiency.Theindirectgapmaterialssuchassiliconandgermaniumaregenerallybadlightemitters.272.IndirectgapmaterialsRequiPrinciplesofluminescenceInterbandluminescencePhotoluminescenceElectroluminescent28第五講InterbandLuminescencePrinciplesofluminescence28第五Photoluminescenceinadirectgapsemiconductor:interbandluminescenceexcitedbyaphotonwithenergygreaterthanEg.ExcitationandrelaxationPhotonsabsorptionfromanexcitationsource(laserorlamp),electrons(inconductionband)andholes(invalenceband)arecreated.hvL>EgTheelectronsandholesrapidlyrelaxtothebottomoftheirbandsbyphononemission(~10-13s)beforerecombiningbyemittingaphoton(~10-9s).occupancyfactorsshownbytheshadingcanbecalculatedbyapplyingstatisticalphysicstotheelectronandholedistributions.(a)SchematicdiagramoftheprocessesoccurringduringPLinadirectgapsemiconductorafterexcitationatfrequencyL.(b)Densityofstatesandleveloccupanciesfortheelectronsandholesafteropticalexcitation.29PhotoluminescenceinadirectLowcarrierdensitiesAtlowcarrierdensities,theoccupancyofthelevelsissmalland+1factorinfe(E)canbeignored.Theelectronandholedistributionwillbedescribedbyclassicalstatistics.FermiBoltzmanndistribution:Theluminescentintensityatfrequency:Assumingthatthematrixelementisindependentoffrequency.ArisingfromthejointdensityofstatesArisingfromtheBoltzmannstatisticsoftheelectronsandholes.30LowcarrierdensitiesAtlowcPLspectrumofGaAsat100K.Theexcitationsourcewasaheliumneonlaseroperatingat632.8nm(1.96eV).ThespectrumshowsasharpriseatEgduetothe(h-Eg

)1/2factor.ThenfallsoffexponentiallyduetotheBoltzmannfactor.Thefullwidthathalfmaximumoftheemissionlineisverycloseto~kBT

31PLspectrumofGaAsat100K.Photoluminescencespectroscopy

Photoluminescence(PL)spectra:

Thesampleisexcitedwithalaserorlampwithphotonenergygreaterthanthebandgap.Thespectrumisobtainedbyrecordingtheemissionasafunctionofwavelength.Photoluminescenceexcitationspectroscopy(PLE):Theluminescenceintensityatthepeakoftheemissionismeasuredastheexcitationwavelengthisscanned.32Photoluminescencespectroscopy33GaN:Zn:excitationandemissionspectra33GaN:Zn:excitationandemissPrinciplesofluminescenceInterbandluminescencePhotoluminescenceElectroluminescent34第五講InterbandLuminescencePrinciplesofluminescence34第五35GeneralprinciplesofelectroluminescentdevicesElectroluminescenceistheprocessbywhichluminescenceisgeneratedwhileanelectricalcurrentflowsthroughanoptoelectronicdevice.Twomaintypesofdevices:

LightEmittingDiodes(LED)andLaserDiode(LD).Structure:epitaxiallayer;p-andn-typeregion;activeregion.Mechanism:operatedinforwardbias;electronsandholesinjectionandrecombinationinactiveregion.Besameasthephotoluminescenceandbandgapdeterminingtheemissionspectra(lineemissionatEgwithbandwidthof~kT).Commercialelectroluminescentdevicesarethereforemadefromdirectgapcompounds.Threefactorsforthechoiceofmaterials:1.bandgapsize;2.latticematching;3.p-typedoping.35Generalprinciplesofelectr36BandgapofselectedIII-Vsemiconductorsvslatticeconstant“l(fā)atticematching”betweentheepitaxiallayersandthesubstrate:ifnot,theformeddislocationwilldegradetheopticalquality.Nitride?AlxGa1-xAs:630-870nm,redandinfraredLED;perfectlatticematching.GaxInl-xAsyPl-y:0.92~1.65m,aslightsourceforopticalfibercommunication(operatedat1.55and1.3m)

GaxIn1-xN:Theemissionwavelengthvariedfrom360to650nm,greenandblueLED;Latticematchingandp-dopingproblems;p-typedopinginwidebandgapsemiconductorwillresultindeepacceptorlevels.Andthenthelowholedensitygivesthelayersahighresistivity,whichcausesohmicheatingwhenthecurrentflowsandhencedevicefailure.Thisacommonproblemformostofwidegapsemiconductors.36BandgapofselectedIII-Vs37LightEmittingDiodes(LED)HeavilydopingproducesdegeneratedistributionsofholesandelectronsInthermalequilibriumatzerobias,theFermienergymustbesameeverywhere.depletionregionformedelectronandholeseperatedandnolightemissionAtforwardbias:Thedepletionregionshrinks;createsaregionatthejunctionwherebothelectronsandholesarepresent.Theelectronsrecombinewiththeholes,emittingphotonsatenergyEgbyinterbandluminescence.37LightEmittingDiodes(LED)H38ElectroluminescencespectrumofaGaAsLEDatroomtemperature.Acurrentof1mAflowingthroughthedevice.Emissioninthenear-infraredaround870nm.(GaAs,Eg=1.42eV)Thefullwidthathalfmaximum(FWHM)oftheemissionlineis58meV,whichisabouttwicekBTat293K.38Electroluminescencespectrum39作業(yè)：Question4.11）Calculatethebindingenergyandradiusofthen=1andn=2freeexcitonsinzincsulphide(ZnS)whichhasme*=0.28m0,mh*=0.5m0andr=7.9.Wouldyouexpecttheseexcitonstobestableatroomtemperature.thebandgapofZnSisabout3.76eV,calculatedthewavelengthofthephotoncorrespondingtothen=1excitonabsorptionpeak.Excitonscanabsorbphotonsbymakingtransitionstoexcitedstatesinexactlythesamewaythathydrogenatomsdo.CalculatethewavelengthofthephotonrequiredtopromoteanexcitoninZnSfromthen=1tothen=2state..39作業(yè)：Question4.1402.Page112,question5.4402.Page112,question5.4課程內(nèi)容緒論經(jīng)典傳播帶間吸收激子發(fā)光半導(dǎo)體量子阱、自由電子、分子材料發(fā)光中心41課程內(nèi)容緒論1422第四講ExcitonsTheconceptofexcitonsFreeexcitonsFreeexcitonsathighdensityFrenkelexcitons43第四講ExcitonsTheconceptofeTheconceptofexcitonsInsemiconductorsandinsulators:photonabsorptionelectronsintheconductionbandandholesinthevalenceband.Exciton:boundelectron–holepairbyCoulombinteractionObservedinsemiconductors;largeradius;delocalizedstates;movefreely;bindingenergy~0.01eVObservedininsulatorsandmolecularcrystals;smallerradius;localizedstates;lessmobileandhoping;bindingenergy~0.1-1eV.Stableexcitonswillonlybeformediftheattractiveenergy>>kBT(0.026eVatroomtemperature)Freeexcitonsarestableatcryogenictemperature.Tightboundexcitonsarestableatroomtemperature.Twotypesofexcitons:44Wannier激子（自由激子）Frenkel激子（束縛激子）:TheconceptofexcitonsInsemiTheconceptofexcitonsFreeexcitonsFreeexcitonsathighdensityFrenkelexcitons45第四講ExcitonsTheconceptofexcitons5第四講BindingenergyandradiusoffreeexcitonsFreeexcitons:weaklyboundelectron-holepair;ahydrogenicsystemApplyingtheBohrmodeltotheexciton,consideringdielectricconstantrofthemediumandthereducedmassofelectronandhole.

TheenergyofthenthlevelrelativetotheionizationlimitRHistheRydbergconstantofthehydrogenatom(13.6eV).RX:excitonRydbergconstant.Theradiusoftheelectron-holeorbit:aHistheBohrradiusofthehydrogenatom(5.2910-11m)andaxistheexcitonBohrradius.groundstatewithn=1hasthelargestbindingenergyandsmallestradius.n>1:lessstrongbindingenergyandlargerradius.Bidingenergytendstodecreaseandaxtoincreaseasrincrease.46BindingenergyandradiusoffRxtendstoincreaseandaxtodecreaseasEgincreases.Causes:

rtendstodecreaseandtoincreaseasthebandgapincreases.Ininsulatorswithbandgapsgreaterthanabout5eV,axbecomescomparabletotheunitcellsize,andtheWanniermodelisnolongervalid.narrowgapsemiconductors:RHissosmallthatexcitoneffectsishardtoobserve.(Eg=1-3eV,freeexcitonsbehaviorisbestobserved)47RxtendstoincreaseandaxtoExcitonabsorptionFreeexcitonsaretypicallyobservedindirectgapsemiconductors.(hardtoobserveintheabsorptionspectraofindirectsemiconductor)Thegroupvelocityofanelectronorholeinabandisgivenby:AttheBrillouimzonecentreofdirectsemiconductor:k=0andzerogradient.Eectron-holepairscreatedbydirecttransitionandhavethesamevelocities.Therefore,strongexcitonsoccurinthespectralregionclosetothefundamentalbandgap.Bandedgeabsorptionspectrumforadirectgapsemiconductorwithexcitoniceffectsincluded.Thedashedlineshowstheexpectedabsorptionwhentheexcitoniceffectsareignored.Freeexcitonscanonlybeobservedinverypuresamples.Impurities:screeningtheCoulombinteractionintheexcitonandtherebystronglyreducethebindingforces;generatingelectricfieldandtendtoionizeexcitons.48Excitoncreation:electron-holepairsandsamevelocities.Theenergyofexcitonabsorptionis:ExcitonabsorptionFreeexcitonExperimentaldataforfreeexcitonsinGaAsExcitonabsorptionofultrapureGaAsat1.2K.hydrogen-likeenergyspectrumoftheexcitoninthevicinityofthebandgap.E1=1.5149eV,E2=1.5180eV,E3=1.5187eVTheexperimentalRx=4.2meVisingoodagreementwiththecalculatedvalue.49Eg=l.5191eV,agreewithothermeasurements.ExperimentaldataforfreeexcTheconceptofexcitonsFreeexcitonsFreeexcitonsathighdensityFrenkelexcitons50第四講ExcitonsTheconceptofexcitons10第四講MortdensityNmott:thedensityatwhichtheexciton-excitondistanceisequaltotheexcitondiameter:Highdensityisachievablewithafocussedlaserbeam.51TheexcitondensitycanbecontrolledbytuningthelaserpowerLowdensity,theexciton-excitoninteractionsarenegligible;theexcitonwavefunctionsbegintooverlapathighdensityandtheinteractionwillbecomesignificant.MortdensityNmott:thedensit52DensityEffects:1.electron-holeplasmaweakeningandbroadeningoftheexcitonabsorptionlineisobserved(absorptionsaturation,nonlineareffects).2.Biexcitons(excitonmolecules)equivalentprocesstotheformationofanH2molecule;newfeaturelinecanbefound.3.electron-holedroplets4.Bose-Einsteincondensation(Stotal=0or1)Broadfeaturelineatlowerenergythanthefreeexciton12DensityEffects:1.electron-TheconceptofexcitonsFreeexcitonsFreeexcitonsathighdensityFrenkelexcitons53第四講ExcitonsTheconceptofexcitons13第四講Frenkelexcitonsoccurringinlargebandgapmaterialswithsmalldielectricconstantsandlargeeffectivemasses.smallradiiandlargebindingenergies,0.1eVtoseveraleV,stableatroomtemperature.

propagatingthroughthecrystalbyhopping.Localizedontheatomsite,maythereforebeconsideredasexcitedstatesoftheindividualatomsormolecules,especiallyforn=1excitonenergy

.

TheoreticaltreatmentofFrenkelexcitonsismorecomplicated.54FrenkelexcitonsoccurringinRaregascrystalsCrystallizeatcryogenictemperatures.Largebandgap,Neonhasthelargestbandgapinnature.ExcitontransitionsalloccurinthevacuumultravioletspectralrangeBindingenergiesareverylarge.Aclosecorrespondencebetweenthen=1excitonenergiesinthecrystalsandtheopticaltransitionsoftheisolatedatoms(ForXenon5p65p56s).

Theradiusincreaseswithn,delocalized,correspondencegetsweaker.55RaregascrystalsCrystallizeAlkalihalides

Largedirectbandgaps(5.9eV13.7eV)LiFhasthewidestbandgapofanypracticalopticalmaterial.Egandexcitonbindingenergytendstoincreasewithdecreasinganionandcationsize.Theexcitonsarelocalizedatthenegative(halogen)ions.StrongexcitoneffectsatRTbecauseoflargebindingenergy(0.8eVand1.9eV)56AlkalihalidesLargedirectbaPrinciplesofluminescenceInterbandluminescencePhotoluminescenceElectroluminescent第五講InterbandLuminescence57Principlesofluminescence第五講發(fā)光的定義固體中的電子受到外界能量的激發(fā)，從基態(tài)躍遷到激發(fā)態(tài)，這是一種非平衡態(tài)。處于激發(fā)態(tài)的電子具有一定的壽命，以一定幾率回落到基態(tài)，并把多余的能量以各種形式釋放出來。如果以光能的形式釋放，稱為發(fā)光過程。任何物體在一定溫度下均有熱輻射（熱發(fā)光）。為了區(qū)分其它發(fā)光形式和熱發(fā)光，嚴(yán)格的固體發(fā)光概念不包含熱發(fā)光。發(fā)光現(xiàn)象有兩個(gè)主要特征：發(fā)光為固體吸收外界能量后，所發(fā)出總輻射超出熱發(fā)射的部分。（發(fā)光的定義，指出了與熱輻射的區(qū)別）外界激發(fā)源對(duì)物體的作用停止后，發(fā)光現(xiàn)象會(huì)持續(xù)一段時(shí)間。（發(fā)光與散射、反射等現(xiàn)象的區(qū)別）58發(fā)光的定義固體中的電子受到外界能量的激發(fā)，從基態(tài)躍遷到激發(fā)態(tài)發(fā)光的分類依據(jù)激發(fā)方式不同，固體發(fā)光可分為以下幾種形式：光致發(fā)光：如熒光燈，PDP。電致發(fā)光：如LED。陰極射線發(fā)光：CRT。高能射線或粒子（X射線，射線，粒子等）發(fā)光：如醫(yī)學(xué)胸透?；瘜W(xué)發(fā)光：如熒光棒。生物發(fā)光：如螢火蟲。機(jī)械發(fā)光：摩擦發(fā)光。59發(fā)光的分類依據(jù)激發(fā)方式不同，固體發(fā)光可分為以下幾種形式：光LightemissioninsolidsR=1/A，radiativelifetimeoftransitionInjectedelectronorholeRelaxtotheminimumenergystateThephotonisemittedwhenanelectroninanexcitedstatedropsdownintoanemptystateinthegroundstatebandLuminescence.IftheupperlevelhasapopulationNattimet,theradiativeemissionrateisgivenby:A:Einsteincoefficient.60LightemissioninsolidsR=1/A61PhotonabsorptionPhotonemissiontransitionswhichhavelargeabsorptioncoefficientsalsohavehighemissionprobabilitiesandshortradiativelifetimes.photonscanbeabsorbedtoanystatewithintheexcitedstateband,nomatterhowfaritisabovethebottomoftheband.Electronsandholesrelaxrapidlytothelowestlevelsofexcitedstate,andthelightwillthereforeonlybeemittedwithinanarrowenergyrangefromthelowestlevelsintheexcitedstateband.Normally,theabsorptionandemissionspectraarenotsame.21PhotonabsorptionPhotonemisTheluminescentintensityatfrequency:ThematrixelementM:Fermi’sgoldenruleThejointdensityofstateg(h)theoccupancyfactorsgivetheprobabilitiesthattherelevantupperlevelisoccupiedandthelowerlevelisempty.62Theluminescentintensityatf光致發(fā)光的效率輻射躍遷并不是激發(fā)態(tài)電子回到基態(tài)的唯一途徑。另一途徑：無輻射躍遷，發(fā)射聲子（吸收光能轉(zhuǎn)變?yōu)闊幔?。消弱發(fā)光。設(shè)無輻射躍遷壽命為NR，同時(shí)考慮輻射躍遷和無輻射躍遷，激發(fā)態(tài)電子數(shù)變化速率：發(fā)光效率R定義為輻射躍遷速率/總躍遷速率：高效發(fā)光材料要求輻射躍遷壽命R遠(yuǎn)小于無輻射躍遷壽命NR

。63光致發(fā)光的效率輻射躍遷并不是激發(fā)態(tài)電子回到基態(tài)的唯一途徑。另PrinciplesofluminescenceInterbandluminescencePhotoluminescenceElectroluminescent64第五講InterbandLuminescencePrinciplesofluminescence24第五InterbandluminescenceInterbandluminescence:inasemiconductor,anelectronthathasbeenexcitedintotheconductionbanddropsbacktothevalencebandbytheemissionofaphoton.Correspondingtotheannihilationofanelectron-holepair(electron-holerecombination).1.DirectgapmaterialsTheopticaltransitionsaredipole-allowedandhavelargematrixelements.radiativelifetime:10-8-10-9s;luminescentefficiencyishigh.injectedelectronsandholesrelaxveryrapidlytothelowestenergystates.electronandholethatrecombinemusthavethesamekvector,downwardverticalarrow.Nomatterhowweexcitetheelectronsandholesinthefirstplace,luminescenceatenergiesclosetothebandgapisalwaysobtained.65InterbandluminescenceInterbanLuminescencespectrumandabsorptionofaGaNepilayerat4K.Thephotoluminescence(PL)wasexcitedbyabsorptionof4.9eVphotons.Theemissionspectrumconsistofanarrowemissionlineat3.5eVclosetothebandgapenergy,whiletheabsorptionshowstheusualthresholdatEgwithcontinuousabsorptionfor?>Eg.Theemissionandabsorptionspectraarenotthesame,eventhoughtheyaredeterminedbythesamematrixelement.Thebandgapcorrespondstothethresholdforopticalabsorption,buttotheenergyoftheopticalemission.66Luminescencespectrumandabso2.IndirectgapmaterialsRequiringemittingbothaphononandaphotonduringthetransition.asecond-orderprocess,withasmalltransitionprobability.longerradiativelifetime,smallerluminescentefficiency.Theindirectgapmaterialssuchassiliconandgermaniumaregenerallybadlightemitters.672.IndirectgapmaterialsRequiPrinciplesofluminescenceInterbandluminescencePhotoluminescenceElectroluminescent68第五講InterbandLuminescencePrinciplesofluminescence28第五Photoluminescenceinadirectgapsemiconductor:interbandluminescenceexcitedbyaphotonwithenergygreaterthanEg.ExcitationandrelaxationPhotonsabsorptionfromanexcitationsource(laserorlamp),electrons(inconductionband)andholes(invalenceband)arecreated.hvL>EgTheelectronsandholesrapidlyrelaxtothebottomoftheirbandsbyphononemission(~10-13s)beforerecombiningbyemittingaphoton(~10-9s).occupancyfactorsshownbytheshadingcanbecalculatedbyapplyingstatisticalphysicstotheelectronandholedistributions.(a)SchematicdiagramoftheprocessesoccurringduringPLinadirectgapsemiconductorafterexcitationatfrequencyL.(b)Densityofstatesandleveloccupanciesfortheelect