Structure of the band tails in amorphous selenium

Transient photocurrent measurements on evaporated a-Se layers indicate the presence of two sets of discrete traps in the band tail region. The shallower traps, at E-V + 0.20 eV and E-C - 0.28 eV, are found to be electrically neutral, while the deeper ones at E-V + 0.38 eV and E-C - 0.53 eV are related to the charged negative-U centres of a-Se. The density of the discrete traps is of the same order of magnitude as the disorder-induced background density in the valence and conduction band tails, preventing the characterization of the a-Se tail-state densities by a simple functional form.