Memory effect in ZnS:Mn AC thin-film electroluminescent devices with low Mn concentration

Electroluminescent (EL) memory is exhibited in ZnS:Mn AC thin-film EL (ACTFEL) devices with Mn concentrations between 0.2 and 0.7 wt.% (±0.2%). Maximum hysteresis width is observed for Mn concentrations of 0.3 wt.% (±0.2%), compared to 1 wt.% for previous devices exhibiting memory. The phenomenon is seen in Zn-rich, but not in S-rich ZnS:Mn films. Low-field electrical characterization has previously shown the presence of shallow donor sites in Zn-rich ZnS at 0.11 eV below the conduction band. Electron ejection of donors near the interfaces is believed to sustain electroluminescence at voltages below threshold. The trap is thought to be created by an excess of S vacancies (donor sites) over the trapping or recombination centers