Radiative lifetime of excitonic photoluminescence in amorphous semiconductors

Abstract

A comprehensive theory for calculating the radiative lifetime of excitons in amorphous semiconductors is presented. Four possibilities for the radiative recombination of an exciton are considered: (i) both excited electron and hole are in their extended states, (ii) electron is in the extended and hole in tail states, (iii) electron is in the tail and hole in extended states and (iv) both in their tail states. Rates of radiative recombination corresponding to each of the four possibilities are derived: (a) within two-level approximation, and at (b) nonequilibrium and (c) equilibrium conditions. It is found that rates derived under the nonequilibrium condition have no finite peak values with respect to the photoluminescence energy. However, considering that the maximum value of a rate derived at equilibrium gives the inverse of the radiative lifetime, the latter is calculated for all the four possibilities in a-Si:H. The radiative lifetime of excitons is found to be in the ns time range for possibilities (i)–(iii) at temperatures 15–20K and in the μs range at 3.7K. When the recombination occurs from the tail states, the radiative lifetime gets prolonged due to the localization of charge carriers. Results agree very well with experiments and are compared with other theories as well.