Nature of impurity states in doped amorphous silicon

We calculate the electronic structure of doped amorphous silicon in a tight-binding framework. Substitutional impurities are randomly introduced at fourfold-coordinated sites of a previously published model of a-Si:H. Our most important result is that single impurities have much larger binding energies than those in the bulk crystal so that their spectral distribution gives an important contribution to the Urbach tails. We analyze the origin of this situation and present an original physical interpretation in terms of an impurity-induced shift of the localized states characterizing the undoped material. Finally we compare our predictions to experimental data and discuss the effect of long-range potential fluctuations.