Exactly exponential band tail in a glassy semiconductor

Transient-photocurrent measurements in glassy ${\mathrm{As}}_2$ ${\mathrm{Se}}_3$ reveal an accurate power-law behavior spanning more than eight decades of time. In the multiple-trapping model this requires a density of valence-band-tail states varying exponentially with energy over almost five decades. The accuracy of this exponential and its continuation deep into the gap are inconsistent with existing models for band tails based on short-range potential fluctuations, but show a remarkable degree of perfection in the electronic structure of this ostensibly random material.