Space charge generation in ZnS:Mn alternating-current thin-film electroluminescent devices

The electrical characteristics of ZnS:Mn alternating-current thin-film electroluminescent (ACTFEL) devices grown by atomic layer epitaxy are assessed as a function of the thickness of the phosphor layer using capacitance-voltage (C-V) and internal charge-phosphor field (Q-Fp) analysis. Deviations from the ideal in the measured C-V and Q-Fp characteristics are ascribed to the generation of space charge in the phosphor layer during ACTFEL device operation. C-V overshoot deviations are correlated to space charge generation via a simulation employing a single sheet of charge model which assumes that the centroid of the generated space charge is located at a discrete sheet within the phosphor. Space charge generation in these atomic layer epitaxy (ALE) ZnS:Mn ACTFEL devices is ascribed to impact ionization of the zinc vacancy portion of chlorine-zinc vacancy self-activated defect complexes. A thermodynamic argument is provided which suggests that zinc vacancies are created via self-compensation of ZnS when the ZnS is unintentionally doped with chlorine. It is contended that space charge generation could even be desirable in ALE ZnS:Mn ACTFEL devices because it leads to better aging stability and improved performance.