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1、认识到了贫困户贫困的根本原因，才能开始对症下药，然后药到病除。近年来国家对扶贫工作高度重视，已经展开了“精准扶贫”项目课程内容l绪论l经典传播l带间吸收l激子l发光l半导体量子阱、自由电子、分子材料l发光中心1认识到了贫困户贫困的根本原因，才能开始对症下药，然后药到病除。近年来国家对扶贫工作高度重视，已经展开了“精准扶贫”项目2认识到了贫困户贫困的根本原因，才能开始对症下药，然后药到病除。近年来国家对扶贫工作高度重视，已经展开了“精准扶贫”项目第四讲第四讲 ExcitonsThe concept of excitonsFree excitonsFree excitons at high den
2、sityFrenkel excitons3认识到了贫困户贫困的根本原因，才能开始对症下药，然后药到病除。近年来国家对扶贫工作高度重视，已经展开了“精准扶贫”项目The concept of excitonsThe concept of excitonsIn semiconductors and insulators:photon absorption electrons in the conduction band and holes in the valence band.Exciton:bound electron hole pair by Coulomb interactionObser
3、ved in semiconductors;large radius;delocalized states;move freely;binding energy 0.01 eV Observed in insulators and molecular crystals;smaller radius;localized states;less mobile and hoping;binding energy 0.1-1 eV.Stable excitons will only be formed if the attractive energy kBT(0.026eV at room tempe
4、rature)Free excitons are stable at cryogenic temperature.Tight bound excitons are stable at room temperature.Two types of excitons:Wannier激子（自由激子）激子（自由激子）Frenkel激子激子（束缚激子）（束缚激子）:4认识到了贫困户贫困的根本原因，才能开始对症下药，然后药到病除。近年来国家对扶贫工作高度重视，已经展开了“精准扶贫”项目The concept of excitonsFree excitonsFree excitons at high dens
5、ityFrenkel excitons第四讲第四讲 Excitons5认识到了贫困户贫困的根本原因，才能开始对症下药，然后药到病除。近年来国家对扶贫工作高度重视，已经展开了“精准扶贫”项目Binding energy and radius of free excitonsBinding energy and radius of free excitonsFree excitons:weakly bound electron-hole pair;a hydrogenic system Applying the Bohr model to the exciton,considering diele
6、ctric constant r of the medium and the reduced mass of electron and hole.The energy of the nth level relative to the ionization limit RH is the Rydberg constant of the hydrogen atom(13.6 eV).RX:exciton Rydberg constant.The radius of the electron-hole orbit:aH is the Bohr radius of the hydrogen atom(
7、5.29 10-11m)and ax is the exciton Bohr radius.ground state with n=1 has the largest binding energy and smallest radius.n1:less strong binding energy and larger radius.Biding energy tends to decrease and ax to increase as r increase.6认识到了贫困户贫困的根本原因，才能开始对症下药，然后药到病除。近年来国家对扶贫工作高度重视，已经展开了“精准扶贫”项目Rx tends
8、 to increase and ax to decrease as Eg increases.Causes:r tends to decrease and to increase as the band gap increases.In insulators with band gaps greater than about 5 eV,ax becomes comparable to the unit cell size,and the Wannier model is no longer valid.narrow gap semiconductors:RH is so small that
9、 exciton effects is hard to observe.(Eg=1-3eV,free excitons behavior is best observed)7认识到了贫困户贫困的根本原因，才能开始对症下药，然后药到病除。近年来国家对扶贫工作高度重视，已经展开了“精准扶贫”项目Exciton absorptionExciton absorptionFree excitons are typically observed in direct gap semiconductors.(hard to observe in the absorption spectra of indire
10、ct semiconductor)The group velocity of an electron or hole in a band is given by:At the Brillouim zone centre of direct semiconductor:k=0 and zero gradient.Eectron-hole pairs created by direct transition and have the same velocities.Therefore,strong excitons occur in the spectral region close to the
11、 fundamental band gap.Band edge absorption spectrum for a direct gap semiconductor with excitonic effects included.The dashed line shows the expected absorption when the excitonic effects are ignored.Free excitons can only be observed in very pure samples.Impurities:screening the Coulomb interaction
12、 in the exciton and thereby strongly reduce the binding forces;generating electric field and tend to ionize excitons.Exciton creation:electron-hole pairs and same velocities.The energy of exciton absorption is:8认识到了贫困户贫困的根本原因，才能开始对症下药，然后药到病除。近年来国家对扶贫工作高度重视，已经展开了“精准扶贫”项目Experimental data for free exc
13、itons in GaAs Exciton absorption of ultra pure GaAs at 1.2 K.hydrogen-like energy spectrum of the exciton in the vicinity of the band gap.E1=1.5149 eV,E2=1.5180 eV,E3=1.5187 eV The experimental Rx=4.2 meV is in good agreement with the calculated value.Eg=l.5191 eV,agree with other measurements.9认识到了
14、贫困户贫困的根本原因，才能开始对症下药，然后药到病除。近年来国家对扶贫工作高度重视，已经展开了“精准扶贫”项目The concept of excitonsFree excitonsFree excitons at high densityFrenkel excitons第四讲第四讲 Excitons10认识到了贫困户贫困的根本原因，才能开始对症下药，然后药到病除。近年来国家对扶贫工作高度重视，已经展开了“精准扶贫”项目Mort density Nmott:the density at which the exciton-exciton distance is equal to the exc
15、iton diameter:High density is achievable with a focussed laser beam.uThe exciton density can be controlled by tuning the laser powerLow density,the exciton-exciton interactions are negligible;the exciton wave functions begin to overlap at high density and the interaction will become significant.11认识
16、到了贫困户贫困的根本原因，才能开始对症下药，然后药到病除。近年来国家对扶贫工作高度重视，已经展开了“精准扶贫”项目Density Effects:1.electron-hole plasmaweakening and broadening of the exciton absorption line is observed(absorption saturation,nonlinear effects).2.Biexcitons(exciton molecules)equivalent process to the formation of an H2 molecule;new feature
17、 line can be found.3.electron-hole droplets4.Bose-Einstein condensation(Stotal=0 or 1)Broad feature line at lower energy than the free exciton12认识到了贫困户贫困的根本原因，才能开始对症下药，然后药到病除。近年来国家对扶贫工作高度重视，已经展开了“精准扶贫”项目The concept of excitonsFree excitonsFree excitons at high densityFrenkel excitons第四讲第四讲 Excitons1
18、3认识到了贫困户贫困的根本原因，才能开始对症下药，然后药到病除。近年来国家对扶贫工作高度重视，已经展开了“精准扶贫”项目Frenkel excitons occurring in large band gap materials with small dielectric constants and large effective masses.small radii and large binding energies,0.1 eV to several eV,stable at room temperature.propagating through the crystal by hopp
19、ing.Localized on the atom site,may therefore be considered as excited states of the individual atoms or molecules,especially for n=1exciton energy.Theoretical treatment of Frenkel excitons is more complicated.14认识到了贫困户贫困的根本原因，才能开始对症下药，然后药到病除。近年来国家对扶贫工作高度重视，已经展开了“精准扶贫”项目Rare gas crystals Crystallize
20、at cryogenic temperatures.Large band gap,Neon has the largest band gap in nature.Exciton transitions all occur in the vacuum ultraviolet spectral range Binding energies are very large.A close correspondence between the n=1 exciton energies in the crystals and the optical transitions of the isolated
21、atoms(For Xenon 5p65p56s).The radius increases with n,delocalized,correspondence gets weaker.15认识到了贫困户贫困的根本原因，才能开始对症下药，然后药到病除。近年来国家对扶贫工作高度重视，已经展开了“精准扶贫”项目Alkali halides Large direct band gaps(5.9 eV 13.7 eV)LiF has the widest band gap of any practical optical material.Eg and exciton binding energy t
22、ends to increase with decreasing anion and cation size.The excitons are localized at the negative(halogen)ions.Strong exciton effects at RT because of large binding energy(0.8eV and 1.9eV)16认识到了贫困户贫困的根本原因，才能开始对症下药，然后药到病除。近年来国家对扶贫工作高度重视，已经展开了“精准扶贫”项目Principles of luminescenceInterband luminescence Ph
23、otoluminescenceElectroluminescent第五讲第五讲 Interband Luminescence17认识到了贫困户贫困的根本原因，才能开始对症下药，然后药到病除。近年来国家对扶贫工作高度重视，已经展开了“精准扶贫”项目发光的定义发光的定义u固体中的电子受到外界能量的激发，从基态跃迁到激发态，这是一种非平衡态。处于激发态的电子具有一定的寿命，以一定几率回落到基态，并把多余的能量以各种形式释放出来。如果以光能的形式释放，称为发光过程。u任何物体在一定温度下均有热辐射（热发光）。为了区分其它发光形式和热发光，严格的固体发光概念不包含热发光。发光现象有两个主要特征：发发光光
24、为为固固体体吸吸收收外外界界能能量量后后，所所发发出出总总辐辐射射超超出出热热发发射射的的部分。（发光的定义，指出了与热辐射的区别）部分。（发光的定义，指出了与热辐射的区别）外外界界激激发发源源对对物物体体的的作作用用停停止止后后，发发光光现现象象会会持持续续一一段段时时间。（发光与散射、反射等现象的区别）间。（发光与散射、反射等现象的区别）18认识到了贫困户贫困的根本原因，才能开始对症下药，然后药到病除。近年来国家对扶贫工作高度重视，已经展开了“精准扶贫”项目发光的分类发光的分类依据激发方式不同，固体发光可分为以下几种形式：u光致发光：如荧光灯，PDP。u电致发光：如LED。u阴极射线发光：
25、CRT。u高能射线或粒子（X射线，射线，粒子等）发光：如医学胸透。u化学发光：如荧光棒。u生物发光：如萤火虫。u机械发光：摩擦发光。19认识到了贫困户贫困的根本原因，才能开始对症下药，然后药到病除。近年来国家对扶贫工作高度重视，已经展开了“精准扶贫”项目Light emission in solidsR=1/A，radiative lifetime of transitionInjected electron or holeRelax to the minimum energy stateThe photon is emitted when an electron in an excited
26、state drops down into an empty state in the ground state band-Luminescence.If the upper level has a population N at time t,the radiative emission rate is given by:A:Einstein coefficient.20认识到了贫困户贫困的根本原因，才能开始对症下药，然后药到病除。近年来国家对扶贫工作高度重视，已经展开了“精准扶贫”项目Photon absorptionPhoton emissionutransitions which ha
27、ve large absorption coefficients also have high emission probabilities and short radiative lifetimes.uphotons can be absorbed to any state within the excited state band,no matter how far it is above the bottom of the band.uElectrons and holes relax rapidly to the lowest levels of excited state,and t
28、he light will therefore only be emitted within a narrow energy range from the lowest levels in the excited state band.uNormally,the absorption and emission spectra are not same.21认识到了贫困户贫困的根本原因，才能开始对症下药，然后药到病除。近年来国家对扶贫工作高度重视，已经展开了“精准扶贫”项目uThe luminescent intensity at frequency :The matrix element M:
29、Fermis golden ruleThe joint density of state g(h)the occupancy factors give the probabilities that the relevant upper level is occupied and the lower level is empty.22认识到了贫困户贫困的根本原因，才能开始对症下药，然后药到病除。近年来国家对扶贫工作高度重视，已经展开了“精准扶贫”项目光致发光的效率光致发光的效率辐射跃迁并不是激发态电子回到基态的唯一途径。另一途径：无辐射跃迁，发射声子（吸收光能转变为热）。消弱发光。设无辐射跃迁寿
30、命为NR，同时考虑辐射跃迁和无辐射跃迁，激发态电子数变化速率：发光效率R定义为辐射跃迁速率/总跃迁速率：u高效发光材料要求辐射跃迁寿命R远小于无辐射跃迁寿命NR。23认识到了贫困户贫困的根本原因，才能开始对症下药，然后药到病除。近年来国家对扶贫工作高度重视，已经展开了“精准扶贫”项目Principles of luminescenceInterband luminescence PhotoluminescenceElectroluminescent第五讲第五讲 Interband Luminescence24认识到了贫困户贫困的根本原因，才能开始对症下药，然后药到病除。近年来国家对扶贫工作高度
31、重视，已经展开了“精准扶贫”项目Interband luminescenceInterband luminescence:in a semiconductor,an electron that has been excited into the conduction band drops back to the valence band by the emission of a photon.Corresponding to the annihilation of an electron-hole pair(electron-hole recombination).1.Direct gap m
32、aterialsThe optical transitions are dipole-allowed and have large matrix elements.radiative lifetime:10-8-10-9s;luminescent efficiency is high.injected electrons and holes relax very rapidly to the lowest energy states.electron and hole that recombine must have the same k vector,downward vertical ar
33、row.No matter how we excite the electrons and holes in the first place,luminescence at energies close to the band gap is always obtained.25认识到了贫困户贫困的根本原因，才能开始对症下药，然后药到病除。近年来国家对扶贫工作高度重视，已经展开了“精准扶贫”项目Luminescence spectrum and absorption of a GaN epilayer at 4 K.The photoluminescence(PL)was excited by
34、absorption of 4.9 eV photons.The emission spectrum consist of a narrow emission line at 3.5 eV close to the band gap energy,while the absorption shows the usual threshold at Eg with continuous absorption for Eg.The emission and absorption spectra are not the same,even though they are determined by t
35、he same matrix element.The band gap corresponds to the threshold for optical absorption,but to the energy of the optical emission.26认识到了贫困户贫困的根本原因，才能开始对症下药，然后药到病除。近年来国家对扶贫工作高度重视，已经展开了“精准扶贫”项目2.Indirect gap materialsRequiring emitting both a phonon and a photon during the transition.a second-order pr
36、ocess,with a small transition probability.longer radiative lifetime,smaller luminescent efficiency.The indirect gap materials such as silicon and germanium are generally bad light emitters.27认识到了贫困户贫困的根本原因，才能开始对症下药，然后药到病除。近年来国家对扶贫工作高度重视，已经展开了“精准扶贫”项目Principles of luminescenceInterband luminescence P
37、hotoluminescenceElectroluminescent第五讲第五讲 Interband Luminescence28认识到了贫困户贫困的根本原因，才能开始对症下药，然后药到病除。近年来国家对扶贫工作高度重视，已经展开了“精准扶贫”项目uPhotoluminescence in a direct gap semiconductor:interband luminescence excited by a photon with energy greater than Eg.Excitation and relaxationPhotons absorption from an exci
38、tation source(laser or lamp),electrons(in conduction band)and holes(in valence band)are created.hvLEgThe electrons and holes rapidly relax to the bottom of their bands by phonon emission(10-13s)before recombining by emitting a photon(10-9s).occupancy factors shown by the shading can be calculated by
39、 applying statistical physics to the electron and hole distributions.(a)Schematic diagram of the processes occurring during PL in a direct gap semiconductor after excitation at frequency L.(b)Density of states and level occupancies for the electrons and holes after optical excitation.29认识到了贫困户贫困的根本原
40、因，才能开始对症下药，然后药到病除。近年来国家对扶贫工作高度重视，已经展开了“精准扶贫”项目Low carrier densities At low carrier densities,the occupancy of the levels is small and+1 factor in fe(E)can be ignored.The electron and hole distribution will be described by classical statistics.Fermi Boltzmann distribution:The luminescent intensity at
41、 frequency :Assuming that the matrix element is independent of frequency.Arising from the joint density of states Arising from the Boltzmann statistics of the electrons and holes.30认识到了贫困户贫困的根本原因，才能开始对症下药，然后药到病除。近年来国家对扶贫工作高度重视，已经展开了“精准扶贫”项目uPL spectrum of GaAs at 100 K.The excitation source was a he
42、lium neon laser operating at 632.8 nm(1.96 eV).The spectrum shows a sharp rise at Eg due to the(h-Eg)1/2 factor.Then falls off exponentially due to the Boltzmann factor.The full width at half maximum of the emission line is very close to kBT 31认识到了贫困户贫困的根本原因，才能开始对症下药，然后药到病除。近年来国家对扶贫工作高度重视，已经展开了“精准扶贫
43、”项目Photoluminescence spectroscopy Photoluminescence(PL)spectra:The sample is excited with a laser or lamp with photon energy greater than the band gap.The spectrum is obtained by recording the emission as a function of wavelength.Photoluminescence excitation spectroscopy(PLE):The luminescence intens
44、ity at the peak of the emission is measured as the excitation wavelength is scanned.32认识到了贫困户贫困的根本原因，才能开始对症下药，然后药到病除。近年来国家对扶贫工作高度重视，已经展开了“精准扶贫”项目GaN:Zn:excitation and emission spectra33认识到了贫困户贫困的根本原因，才能开始对症下药，然后药到病除。近年来国家对扶贫工作高度重视，已经展开了“精准扶贫”项目Principles of luminescenceInterband luminescence Photolu
45、minescenceElectroluminescent第五讲第五讲 Interband Luminescence34认识到了贫困户贫困的根本原因，才能开始对症下药，然后药到病除。近年来国家对扶贫工作高度重视，已经展开了“精准扶贫”项目General principles of electroluminescent devices uElectroluminescence is the process by which luminescence is generated while an electrical current flows through an optoelectronic de
46、vice.uTwo main types of devices:Light Emitting Diodes(LED)and Laser Diode(LD).Structure:epitaxial layer;p-and n-type region;active region.Mechanism:operated in forward bias;electron s and holes injection and recombination in active region.Be same as the photoluminescence and band gap determining the
47、 emission spectra(line emission at Eg with band width of kT).Commercial electroluminescent devices are therefore made from direct gap compounds.Three factors for the choice of materials:1.band gap size;2.lattice matching;3.p-type doping.35认识到了贫困户贫困的根本原因，才能开始对症下药，然后药到病除。近年来国家对扶贫工作高度重视，已经展开了“精准扶贫”项目Ba
48、nd gap of selected III-V semiconductors vs lattice constant“lattice matching”between the epitaxial layers and the substrate:if not,the formed dislocation will degrade the optical quality.Nitride?AlxGa1-xAs:630-870nm,red and infrared LED;perfect lattice matching.GaxInl-xAsyPl-y:0.92 1.65m,as light so
49、urce for optical fiber communication(operated at 1.55 and 1.3 m)GaxIn1-xN:The emission wavelength varied from 360 to 650nm,green and blue LED;Lattice matching and p-doping problems;p-type doping in wide band gap semiconductor will result in deep acceptor levels.And then the low hole density gives th
50、e layers a high resistivity,which causes ohmic heating when the current flows and hence device failure.This a common problem for most of wide gap semiconductors.36认识到了贫困户贫困的根本原因，才能开始对症下药，然后药到病除。近年来国家对扶贫工作高度重视，已经展开了“精准扶贫”项目Light Emitting Diodes(LED)Heavily doping produces degenerate distributions of