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 exhibiti ng memory. The phenomenon is seen in Zn-rich, but not in S-rich ZnS:Mn films. Low-field electrical characterization has previously shown tha t 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 i s believed to sustain electroluminescence at voltages below threshold. The trap is thought to be created by an excess of S vacancies (donor sites) over other trapping or recombination centers.