Aging studies of evaporated ZnS:Mn alternating-current thin-film electroluminescent devices

A study of the aging characteristics of evaporated ZnS:Mn alternating-current thin-film electroluminescent (ACTFEL) devices is undertaken by monitoring the capacitance-voltage (C-v) characteristics at various temperatures as a function of aging time. Short-term ACTFEL aging is characterized by a rigid shift in the C-V curve to higher turn-on voltage with aging time. Additionally, the insulator and phosphor capacitances are found to be independent of aging time, the internal phosphor threshold voltage increases slightly with aging time, and the conduction and polarization charges are observed to decrease with aging time. The activation energy for ACTFEL aging is estimated to be about 0.2 eV. These experimental observations lead to a picture for ACTFEL aging in which atomic rearrangement at insulator/phosphor interfaces gives rise to the formation of deep level, fixed charge states. Transported electrons subsequently trapped in these deep levels reduce the amount of charge available for conduction with a concomitant reduction in the polarization charge. The reduction in the polarization charge is responsible for the observed increase in the turn-on voltage with aging.