Determination of the gap density of states in amorphous silicon by phase shift analysis of the modulated photocurrent

The density of localized states in the mobility gap of evaporated amorphous silicon films has been measured over a range of 250 meV between the conduction band and the Fermi level. The method employed is based on the analysis of the phase shift between an intensity modulated exciting light and the associated photocurrent induced in the semiconductor. The density of states falls off almost exponentially with energy away from the conduction band. It suggests an overlap of the conduction and valence band tails, a result consistent with the Cohen, Fritzsche, and Ovshinsky model.