Recombination multiphonon

Multiphonon generation is the inverse of multiphonon recombination. Multiphonon generation is a thermally activated process. Electrons are diermally excited from G-R centers into the conduction band and holes are similarly thermally excited from G-R centers into the valence band. The key feature of SRH generation is thermal excitation. A thermally activated process is very dependent on the temperature and on the activation energy. We define a generation lifetime as [60]... 

The excess free carriers (and excitons) do not represent stable excited states of the solids. A fraction of them recombine directly after thermahzation either radiatively or by multiphonon emission. In most materials, nonradiative transitions to defect states in the gap are the dominant mode of decay. The lifetime of free carriers T = 1/avS is determined by the density a of recombination centers, their thermal velocity v, and the capture cross section S, and may span 10-10 s. Electrons, captured by states above the demarcation level, and holes, captured by states below the hole demarcation level, may get trapped. The condition for trapping is given when the occupied electron trap has a very small cross section for recombining with a free hole. The trapping process has, until recently, not been well understood. 

Recombination at and excitation from deep levels are emphasized. Nonradiative transitions at defect levels—Auger, cascade capture, and multiphonon emission processes—are discussed in detail. Factors to be considered in the analysis of optical cross sections which can give information about the parity of the impurity wave function and thus about the symmetry of a particular center are reviewed. 

The recombination of an electron-hole pair, or the capture of a carrier into a deep trap, releases much more energy than can be taken up by a single phonon. Multiphonon recombination is represented by transition A in Fig. 8.3. The probability of the simultaneous emission of n phonons is (Stoneham 1977),... 

Defect recombination is therefore primarily non-radiative. The standard theories of non-radiative transitions are based on the multiphonon processes described in Section 8.1.2. A temperature-independent capture cross-section of about 10 cm" is characteristic of a defect state with a strong electron-phonon coupling (see Fig. 8.6). 

The creation of an excess ehp requires an energy equal to the semiconductor band gap. When excess electron-hole pairs recombine they release this energy by one of several distinct physical mechanisms. When the energy is given to phonons or lattice vibrations, the recombination mechanism is known as multiphonon recombination or Shockley-Read-Hall (SRH) recombination. SRH recombination dominates in the indirect band gap semiconductors Si, Ge and GaP. 

The effect of temperature on deep trap emission is similar to that observed for bandgap emission, with the intensity of the emission decreasing as the temperature increases. This can be explained by the involvement of nonradiative recombination processes dominating at higher temperature. Nonradiative relaxation in CdSe nanoclusters has been assigned to the involvement of a multiphonon relaxation mechanism mediated by a vibrational mode of the surface phenylse-lenolate ligands. ... 

D. V. Lang, C. H. Henry, Nonradiative Recombination at Deep Levels in GaAs and GaP by Lattice-Relaxation Multiphonon Emission, Phys. Rev. Lett. 1975, 35, 1525-1528. 

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