Electronic density of states in amorphous selenium

Steady-state and transient photoconductivity methods are used to investigate the electronic density of states in evaporated layers of amorphous selenium. From the temperature dependence of the steady-state photocurrent and, independently, from an analysis of the post-transit currents of time-of-flight transients, energy levels in the gap at 1.43 +/- 0.02 eV and 0.40 +/- 0.02 eV above the valence band have been determined for the occupied state of the negative-U centres. An absorption band around 1.50eV is seen in the spectral distribution of the photocurrent. The distribution of tall states may-to first approximation-be described by a steep exponential with a characteristic width of similar to24 meV at the valence. band and a more steeply declining functional of similar width at the conduction band.