凝聚态光物理学5.pptVIP

* * * 5 Luminescence 5.1 Light emission in solids 5.2 Interband luminescence 5.3 Photoluminescence 5.4 Electroluminescence 5.1 Light emission in solids The reverse process of absorption – emission Emission in solids is called luminescence. Luminescence mechanisms: Photoluminescence (PL) Electroluminescence (EL) 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 for 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 bottom 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 solids Total rate: The luminescent efficiency ?R: If ?R ?NR, ?R ? 1, maximum possible amount of light is emitted. 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 luminescence The interband luminescence corresponds to annihilation of an electron-hole pair (electron-hole recombination) 5.2.1 Direct gap materials The 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 – 10-9 s. The tra