Optical switching in thin film electroluminescent devices with inherent memory characteristics

Quantitative data is presented concerning the optical process of writing and erasing of memory electroluminescent (EL) devices. The switching process is dependent on the energy of the exciting photons and the energy required varies from approximately 10 µJ/cm²at 325 nm (associated with bandgap photoconductivity)to about 75 mJ/cm²at 515 nm (associated with impurity photoconductivity). Experimental evidence is presented to show that necessary and sufficient conditions for EL cell erasure are the photoconductive discharging of the insulator-ZnS charge and the decay of the bulk excitation of the ZnS:Mn film. The technique of photoinduced detrapping of the stored space charge is shown to lead to the determination of midgap as the approximate location in energy of the deep traps in one of our sample films.