Uniform and filamentary transport in d.c. thin-film ZnS: Mn electroluminescent devices

Abstract

Thin-film ZnS is highly insulating until the applied field rises above approximately 1 MVcm⁻¹. The current then rises almost exponentially with increasing voltage, and current densities in excess of 1 A cm⁻² 2 can be maintained. If the ZnS is doped with Mn, the resulting efficient electroluminescence provides a means of spatially and temporally mapping the local current density. Device behaviour under both steady-state and non-equilibrium conditions has been investigated including laterally uniform current, filamentary current flow, kinetic electroluminescence and negative-capacitance behaviour. This behaviour is inconsistent with previously reported transport models but can be understood, at least qualitatively, within the framework of the novel high-field transport mechanism proposed. This is based on tunnelling from states within the bandgap and the dynamics of interacting regions of space charge within the ZnS.