Xerographic spectroscopy of gap states in amorphous semiconductors

Analysis of the temperature-dependent decay of surface voltage on an amorphous film after charging but prior to exposure (xerographic dark decay) and of residual decay after exposure can, in combination, be used to map the density of states and position the Fermi level. The procedure is illustrated for $a$-Se where both electron and hole residuals can be measured. $a$-Se is found to be characterized by a relatively discrete gap-state structure. These measurements readily discern thermostructural and photostructural effects on gap-state populations. Thus, during structural relaxation of glassy films in the As:Se alloy system, systematic variation in a number of localized states distributed throughout the mobility gap is observed. This observation is consistent with the view that native defects play a key role in photoelectronic behavior of amorphous chalcogenides.