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1、5Luminescence5.1 Light emission in solids 5.2 Interband luminescence 5.3 Photoluminescence 5.4 Electroluminescence 5.1 Light emission in solidsThe reverse process of absorption emission Emission in solids is called luminescence. Luminescence mechanisms: Photoluminescence (PL) Electroluminescence (EL
2、) Electrons are injected into the excited state band and relax to the lowest available level. The photon is emitted when an electron in an excited state drops down into an empty state in the ground state band. These empty state are generated by the injection of holes.The spontaneous emission rate fo
3、r a two level: A: Einstein A coefficient; R=A-1: radiative lifetime of the transition. The transition have large absorption coefficients also have high emission probabilities and short radiative lifetime; Upper level is populated.In normal circumstances the electrons relax to within kB T of the bott
4、om of excited state band. The holes follow a similar series of relaxations. Thus light is only emitted within a narrow energy range. Non-radiative relaxation: The excited energy may transfer into heat by emitting phonons or be trapped by defect. 5.1 Light emission in solidsTotal rate: The luminescen
5、t efficiency R:If R NR, R 0, light emission is very inefficient.The efficient luminescence requires that the radiative lifetime should be much shorter than the non-radiative lifetime 5.2 Interband luminescenceThe interband luminescence corresponds to annihilation of an electron-hole pair (electron-
6、hole recombination) 5.2.1 Direct gap materialsThe injected electrons and holes relax very rapidly to lowest energy states. The photons are emitted when electrons at the bottom of the conduction band recombine with holes at the top of the valence band. The typical values of R is in the range 10-8 10-
7、9 s. The transition should be dipole allowed and have large matrix elements and the same k vector (near k=0, thus close to h=Eg). The luminescent intensity at frequency :The PL was excited by absorption of 4.9 eV photons from a frequency doubled copper vapour laser. The spectrum consist of a narrow
8、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 h Eg.5.2.2 Indirect gap materialsIn an indirect materials, conservation of momen- tum requires that a phonon must either be emitted or absorbed when the photon is
9、 emitted.The interband luminesence in an indirect gap material is a second-order process. The R much more longer than for direct transition, therefore this makes the luminescence efficiency small. So the indirect gap materials such as silicon and germanium are generally band light emitters.5.3 Photo
10、luminescence5.3.1 Excitation and relaxation( a) Schematic diagram of the processes occurring during PL in a direct gap semiconductor after excitation at frequency L. The electrons and holes rapidly relax to the bottom of their bands by phonon emission (10-13 s) before recombining by emitting a photo
11、n ( 10-9s). (b) Density of states and level occupancies for the electrons and holes after optical excitation. The distribution functions shown by the shading apply to the classical limit where Boltzmann statistics are valid. The total number density Ne of electrons:The density of state in conduction
12、 band:Fermi-Dirac distribution for the electrons:(The system is in a situation of quasi- equili- brium, thus is no unique Fermi energy. E= 0 corresponds to the bottom of the conduction band or the top of the valence band) The total number density Ne of holes:These two Eqs can be used to calcuulate 5
13、.3.2 Low carrier densities At low carrier densities, the occupancy of the levels is small and +1 factor in fe(E) cab be ignored. The electron and hole distribution will be described by classical situation. Fermi Boltzmann distribution :(valid at low densities and high temperature)PL spectrum of GaAs
14、 at 100 K. The excitation source was a helium 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 and then falls off exponentially due to the Boltzmann factor. The full width at half maximum of the emission line is very close to kBT The i
15、nset give a semi-logarithmic plot of the same data. 5.3.3 DegeneracyAi high carrier densities, the electron and hole distributions are described using Fermi-Dirac statistics. This situation is called degeneracy: In the extreme limit of T = 0Electron-hole recombination can occur between any states in
16、 two bands, therefore there is a broad emission spectrum stating at Eg up to a sharp cut-off at . As finite temperature, the cut off at will be broadened over an energy range kBT.Time-resolved PL spectra of Ga0.47In0.53As at lattice temperature TL=10 K. The sample was excited with laser polse at 610 nm with an energy of 6 nJ and a duration of 8 ps. This generated an initial carrier density of 21024m-3. 5.3.4 Photoluminescen
