Interband optical transitions in disordered semiconductors

The interband absorption coefficient is calculated for the frequency range, ω, below the threshold value (“optical tail”). The tail is attributed to the influence of the random force field present in a disordered system. Two cases are considered: that of the “smooth” random field possessing a correlation function which is continuous and differentiable as many times as needed and that of the Coulomb field produced, for example, by randomly distributed charged centres. In both cases an exponential optical tail is obtained for a sample which is in a certain sense nearly intrinsic: In α ∼ – (Δ – ħ ω)/ ω , where Δ is the band gap that would have been observed without fluctuations of the force field and ω is some characteristic energy. The tail is correlated with the density of states tail but does not reproduce it, the latter being Gaussian in the same energy range. In the most interesting case the formation of an optical tail is due to quantum effects in contrast to the density of states tail which may be obtained in a purely classical approximation as well. In the Coulombic case an explicit form of the dependence of ω on impurity concentration n is obtained ω ∼ n 2/5 . This seems to be in agreement with the experimental data available.