抄録
With a view to studying the properties of the optical-absorption edge in a–Si, we calculate in the CPA the density of states (DOS), the absorption spectra and the spectral density on the basis of the site-disordered Abe-Toyozawa model. As the probability distribution for site disorder, we use the bilateral exponential distribution and the distribution defined by an exponential factor with the fourth-order power in its argument as well as the common Gaussian distribution. Our assertion is that, to make the model realistic, we must choose the degree of disorder small enough relative to the band width such that we can identify the three regions of absorption with high, intermediate and low energies. The intermediate region in this case universally shows an exponential behaviour, irrespective of the type of the probability distribution of “static” disorder. We compare the absorption spectra I(E) calculated from the Kubo formula with the convolution Iconv(E) in which the k-selection rule is completely relaxed. We clarify the conditions under which Iconv(E) serves as a good approximation of I(E).
