TIME-RESOLVED DARK INJECTION FROM VARIOUS METALS INTO AN AMORPHOUS-SEMICONDUCTOR

The evolution from transient conditions to dark-current steady state following the application of a step field to a-As2Se3 sandwich cells fitted with Bi, Ni, Ag, Au, Sn, C, Ga, Al, and Cu has been measured. J-vs-E characteristics are in almost all cases found to be similar to those already published on the Au : a-As2Se3 system. These J-vs-E characteristics show both an extensive region of linear field dependence and a superlinear region in which the current is not space-charge limited. Dark currents in a-As2Se3 and related chalcogenides have been described in terms of a single bulk-controlled mechanism operative at all fields, meaning that contact invisibility (Ohmicity) has been tacitly assumed. At fields exceeding 104 V/cm, the hole transit time in a-As 2Se3 becomes the dielectric relaxation time and under these circumstances an Ohmic contact will, under step-field excitation, exhibit a well-characterized space-charge-limited transient response. None of these contacts exhibit this response; instead they behave as finite injectors supplying holes with a diminishing efficiency in the time after application of a voltage step and at a rate which is always much less than CV per transit time. The resulting small-signal transient response exhibits a local maximum at a time following excitation which is significantly affected by the contact-reservoir decay time. The steady-state dark-current density above 104 V/cm must therefore reflect an injection efficiency relatively insensitive to the choice of contact.