Abstract
We introduce a two-band tight-binding Hamiltonian which describes in a unified manner the behaviour of electrons in a three-dimensional amorphous semiconductor as well as in an amorphous semiconductor superlattice. On the basis of this Hamiltonian, we calculate the density of states D(E) and the optical absorption spectra I(E). Our assertion is that the exponential of Urbach tail of I(E) corresponds to the transition region from the essential band-to-band absorption to the deep tail. As a consequence, the exponential tail is by no means an exclusive feature of the Gaussian distribution as argued before. We also evaluate the slopes of the exponential tails E O and E U of D(E) and I(E) respectively for amorphous semiconductor superlattices and discuss their dependence on the width of the well layer.