Dark Discharge in Amorphous As2Se3 Films

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Abstract
The dark‐discharge rate of corona‐charged amorphous films of As2Se3 is dependent upon the electric field, temperature, and the duration of discharge. The dark‐discharge current was found to result from both bulk generation of holes and from surface injection of holes. Negative charges are immobile in the material. The surface contribution gradually diminishes over a period of weeks as the film ages, or it can be eliminated by applying a thin organic insulating layer to the photoconductor surface. The bulk generation contribution, identified by its linear dependence upon film thickness, is the subject of more detailed analysis. The discharge rate increases slightly at higher electric fields. During the period of a single discharge measurement, the rate drops faster than can be explained by the field dependence. In addition, the discharge rate increases rapidly with temperature. There is no unique activation energy characterizing the discharge process. To account for the discharge time and temperature dependencies, two sets of generation centers are postulated to lie within the band gap of the material. The first set lies 0.6–0.78 eV above the hole conduction states and the density of centers at each energy increases slowly with increasing energy. The second set is located at 0.8–0.86 eV above the hole conduction states, whose density is more than 2 orders of magnitude greater than that of the shallower centers. A first‐order kinetic mechanism is assumed for the hole generation from each energy level. The density of the first set of centers has been calculated to be 1014 cm−3 and the second set is in the range 1016−1017 cm−3. A Poole‐Frenkel type of relationship is used to correlate the electric field dependency of the dark‐discharge rate. Using this model, the surface potential can be predicted as a function of the discharge time, the temperature, and the initial field, provided no severe internal field distortion exists in the bulk of the photoconductor.