Charge generation from band-gap states in amorphous selenium films

Band-gap states have been detected in amorphous selenium films by the observation of dark currents. These currents are a function of the selenium thickness, time, electric field $E$, and temperature. By studying the dependence on film thickness, the dark current due to bulk generation of charge is separated from the surface contribution. Subtracting the surface current from the measured dark currents permits the study of the remaining bulk contribution. The bulk dark current depends exponentially on $E^{\frac{1}{2}}$, consistent with the Poole-Frenkel effect. The time and temperature dependence reveal that the states involved are located 0.7-1.0 eV from the valence-band edge with a density of states of approximately ${10}^{14}$ ${\mathrm{cm}}^{-3\mathrm{}}$.