第四讲-激子与发光ppt课件.ppt

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1、课程内容绪论经典传播带间吸收激子发光半导体量子阱、自由电子、分子材料发光中心12第四讲ExcitonsTheconceptofexcitonsFreeexcitonsFreeexcitonsathighdensityFrenkelexcitons3TheconceptofexcitonsInsemiconductorsandinsulators:photonabsorptionelectronsintheconductionbandandholesinthevalenceband.Exciton:boundelectron–holepairbyCoulombin

2、teractionObservedinsemiconductors;largeradius;delocalizedstates;movefreely;bindingenergy~0.01eVObservedininsulatorsandmolecularcrystals;smallerradius;localizedstates;lessmobileandhoping;bindingenergy~0.1-1eV.Stableexcitonswillonlybeformediftheattractiveenergy>>kBT(0.026eVatroomtempera

3、ture)Freeexcitonsarestableatcryogenictemperature.Tightboundexcitonsarestableatroomtemperature.Twotypesofexcitons:Wannier激子（自由激子）Frenkel激子（束缚激子）:4TheconceptofexcitonsFreeexcitonsFreeexcitonsathighdensityFrenkelexcitons第四讲Excitons5BindingenergyandradiusoffreeexcitonsFreeexcitons:weaklyb

4、oundelectron-holepair;ahydrogenicsystemApplyingtheBohrmodeltotheexciton,consideringdielectricconstantrofthemediumandthereducedmassofelectronandhole.TheenergyofthenthlevelrelativetotheionizationlimitRHistheRydbergconstantofthehydrogenatom(13.6eV).RX:excitonRydbergconstant.Theradiusof

5、theelectron-holeorbit:aHistheBohrradiusofthehydrogenatom(5.2910-11m)andaxistheexcitonBohrradius.groundstatewithn=1hasthelargestbindingenergyandsmallestradius.n>1:lessstrongbindingenergyandlargerradius.Bidingenergytendstodecreaseandaxtoincreaseasrincrease.6Rxtendstoincreaseandaxtodec

6、reaseasEgincreases.Causes:rtendstodecreaseandtoincreaseasthebandgapincreases.Ininsulatorswithbandgapsgreaterthanabout5eV,axbecomescomparabletotheunitcellsize,andtheWanniermodelisnolongervalid.narrowgapsemiconductors:RHissosmallthatexcitoneffectsishardtoobserve.(Eg=1-3eV,freeexcitons

7、behaviorisbestobserved)7ExcitonabsorptionFreeexcitonsaretypicallyobservedindirectgapsemiconductors.(hardtoobserveintheabsorptionspectraofindirectsemiconductor)Thegroupvelocityofanelectronorholeinabandisgivenby:AttheBrillouimzonecentreofdirectsemiconductor:k=0andzerogradient.Eectron-ho

8、lepai