Time-resolved dark injection in dispersive media. Dopeda-As2Se3

Time-resolved hole injection following application of a step field has been studied in a series of doped $a$-${\mathrm{As}}_2$ ${\mathrm{Se}}_3$ films fitted with Au contacts. In either undoped $a$-${\mathrm{As}}_2$ ${\mathrm{Se}}_3$ or in $a$-${\mathrm{As}}_2$ ${\mathrm{Se}}_3$ doped with Ga (which increases hole drift mobility), injection from Au does not occur under space-charge-limited conditions. In Ga-doped samples both the small-signal transient responses and the dark-current steady state, although not space-charge limited, are controlled by an injection efficiency which is found to increase monotonically with hole drift mobility. In $a$-${\mathrm{As}}_2$ ${\mathrm{Se}}_3$ films doped with 4200-ppm Cu, hole drift mobility is reduced by two orders with respect to the undoped film at the same field. Step-field-excited transient response, dark-current steady-state behavior, and drift-mobility data are, in this case, all susceptible to a self-consistent analysis using conventional space-charge-limited theory. In particular, despite the large dispersion in carrier arrival times perceived in time-of-flight experiments, steady-state dark currents, calculated using the experimental transit time, are coincident with measured values. In these highly dispersive media, however, the transit time, defined in the context of a stochastic model is disproportionately weighted by the fastest carriers. It is these faster transit events which control the steady-state current.