Weak Absorption Tails in Amorphous Semiconductors

Optical absorption measurements near the absorption edge are presented for three bulk semiconductor glasses: ${\mathrm{As}}_2$ ${\mathrm{S}}_3$, ${\mathrm{Ge}}_{33}$ ${\mathrm{As}}_{12}$ ${\mathrm{Se}}_{55}$, and ${\mathrm{Ge}}_{28}$ ${\mathrm{Sb}}_{12}$ ${\mathrm{Se}}_{60}$. The weak absorption tails observed below the exponential part of the edge also follow an exponential law, and they are not due to a light-scattering artifact. We associate them with localized states in the band gap. The results are interpreted in terms of a model in which optical transitions occur between localized states below the mobility edge and extended states of the opposite band.