Potential fluctuations and density of gap states in amorphous semiconductors

Abstract

The exponential-band-tail (Urbach-tail) problem for structurally disordered material is treated by a simple model in which quantum wells (used to model the effects of local disorder) trap states in the gap. The density of states in the gap is calculated and is shown to have approximately exponential behavior over a range of energy of physical interest. A direct relation is established between the Urbach-tail slope and the glass transition temperature. Agreement with experimentally determined Urbach-tail slopes is obtained for the Ge-Sn-Se glasses for reasonable parameters employed in the present model.