Thin film AC ZnSxSe1-x:Mn EL
Conventional ZnS:Mn electroluminescence (EL) device has a good level of brightness but needs a high operating voltage. The best way to lower the operating voltage seemed to be by reducing the band gap of the host material and to achieve this, ZnSe had been proposed before. However, our experiments showed that when the ZnSe:Mn EL was operated at low voltages the brightness level and contrast ratio decreased, due to the yellowish coloration of the ZnSe film. The authors have developed a host material which combines both advantages; the high brightness level of ZnS, and the low operating voltage of ZnSe. A material with a band gap between those of ZnS (3.7 eV) and ZnSe (2.7 eV) was tested. By coevaporation of ZnS and ZnSe, a solid solution ZnSxSe1-xwas found in an evaporated film. By measuring the absorption edge and using X-ray techniques, we found that the band gap varied in proportion to the concentration of selenium. A thin film of ZnS0.4Se0.6EL sandwiched between Y2O3insulating layers can operate at a low voltage (120 V) with a high level of brightness (>100 fL at 1 KHz), with a very small upward voltage shift (10 V) during aging.
ac electroluminescence in thin‐film ZnSe : Mn
ac electroluminescence from thin films of Mn‐doped ZnSe, sandwiched between two insulating films, shows a hysteresis in light output versus applied voltage, similar to that observed in ZnS : Mn. We also find that the threshold voltage for light emission is approximately a factor of 2 smaller for ZnSe structures than for similarly prepared ZnS structures of identical dimensions.
The memory effect of ZnS : Mn ac thin‐film electroluminescence
The electroluminescence of ZnS : Mn thin‐film memory devices is observed to occur in two components, one which is spatially homogeneous and one from localized bright regions less than ∼1 μm in diameter. The brightness‐voltage hysteresis, or memory effect, is observed to reside exclusively in the luminescence of the localized bright regions in our devices. It is concluded that the memory effect arises from a negative resistance of a filamentary ac conduction mechanism in these regions.