Electronic density of states in amorphous selenium

Abstract

Steady-state and transient photoconductivity methods are used to investigate the electronicdensity of states in evaporated layers of amorphous selenium. From the temperaturedependence of the steady-state photocurrent and, independently, from an analysis ofthe post-transit currents of time-of-flight transients, energy levels in the gap at1.43 ± 0.02 eVand 0.40 ± 0.02 eV above the valence band have been determined for the occupied state of thenegative-U centres. An absorption band around 1.50 eV is seen in the spectraldistribution of the photocurrent. The distribution of tail states may—to firstapproximation—be described by a steep exponential with a characteristic width of meV at the valence band and a more steeply declining functional of similar width at theconduction band.