Evidence for band-to-band impact ionization in evaporated ZnS:Mn alternating-current thin-film electroluminescent devices

Abstract

Evidence is presented that the normal operation of evaporated ZnS:Mn alternating-current thin-film electroluminescent (ACTFEL) devices involves electron-hole pair generation by band-to-band impact ionization. Four observations are offered to support this assertion. These observations involve: (i) empirical field-clamping trends, (ii) experimental and simulated trends in charge transfer characteristics, (iii) experimental attempts to assess the interface distribution using a field-control circuit, and (iv) Monte Carlo simulation trends. Furthermore, the absence of overshoot in measured capacitance-voltage and internal charge-phosphor field curves indicates that a majority of the holes created by impact ionization are trapped at or near the phosphor/insulator interface. The multiplication factor (i.e., the total number of electrons transferred across the phosphor divided by the number of electrons injected from the phosphor/insulator cathode interface) is estimated, from device physics simulation of experimental trends, to be of the order 4–8 for evaporated ZnS:Mn ACTFEL devices operating under normal conditions.