Experimental results on the stability of ac thin-film electroluminescent devices

Experimental results on the stability of ac-coupled thin-film electroluminescent devices are presented. The luminance-voltage characteristics of ZnS devices, including both memory and nonmemory types, were measured throughout constant luminance aging experiments. Deposition conditions were varied, and post-deposition treatments by ion implantation and thermal annealings were investigated. We find a pervasive shift in the threshold voltage, and a corresponding loss of hysteresis in memory devices. We postulate that memory loss results from the creation of shallow interface states. The responsible physical mechanism is very sensitive to both hydrogen and oxygen incorporation in the ZnS film; in similar fashions, H or O increases the rate of memory loss. By comparison, the devices were insensitive to common contaminants such as Na, Cl, and F. We have found that hydrogen is generated during the deposition process. Even so, some of our devices exhibited significant memory for several thousand hours.