Electron and Hole Trapping in ZnS Phosphors Investigated by Electron-Paramagnetic-Resonance and Luminescence Methods

Using electron-paramagnetic-resonance (EPR) and luminescence measurements, new information regarding trap distributions and electron transitions has been obtained for a series of ZnS: Cu, Cl, Cr phosphors. Electrons can be trapped at ${\mathrm{Cr}}^{++}$ sites, giving rise to an EPR signal upon irradiation; the number of spins depends on the Cr concentration (for a given excitation intensity). These traps are relatively deep and show up in a glow curve only at temperatures of about 100°C. Infrared irradiation (IR) can release electrons from these traps and shows a maximum response at about 1.1 μ; this is quite different from the usual ir dependence shown by ZnS phosphors, which show a minimum at this wavelength. Electrons released by IR from the Cr traps at liquid-nitrogen temperature are mostly retrapped in shallower levels (when these are empty) but show no EPR signal in such levels. The electrons can be returned to the deep traps by heating up to -30°C. Electrons can also be removed from the Cr traps by the usual hole quenching mechanism which follows the normal ir wavelength response.