Interband Absorption Spectra of Disordered Semiconductors in the Coherent Potential Approximation

Abstract

The interband absorption spectra of a system with Gaussian site diagonal disorder are calculated in the coherent potential approximation. The electron-hole vertex part due to interband stochastic correlation is taken into account. The relaxation of the k -selection rule and the effect of localization are studied through the calculation of the optical transition matrix elements. It is clarified that the band tail states contribute to an exponential absorption tail (the so-called Urbach tail) as a result of interplay between transfer energy and Gaussian-distributed site energies. Introduction of a negative interband correlation leads to a red shift of the absorption edge. This effect is discussed in connection with the photo-structural change observed in chalcogenide glasses.