The shape of the self-activated cathodoluminescence band in ZnS: Cl crystals

The shape of the self-activated (SA) cathodoluminescence (CL) band of ZnS: Cl crystals was measured at temperatures in the range from liquid nitrogen to room temperature. Around the SA CL band maximum the shape is Gaussian. For the low and high energy regions of the band the shape becomes exponential. These results are discussed in terms of the Dow and Redfield [1] internal electric microfield model for exponential edges. The shape parameters for the ZnS self-activated cathodoluminescence band are given in table 1. A spectral correction procedure was employed to obtain the spectra in absolute terms (photons emitted per second in a constant bandwidth resolution interval) from the raw count-rate spectra and the vital importance of this correction is emphasised. When the temperature is decreased, as usually reported, the peak of the uncorrected self-activated band shifts to lower energies. The peak of the corrected band however shifts in the opposite direction, i.e., to higher energies. This corrected change is in agreement with that of the edge emission band, that it is in agreement with the increase in the width of the forbidden energy gap with decreasing temperature in ZnS (table 2). The temperature dependence of the blue-silver (B-Ag_I) and the red-tin (R-Sn) impurity bands in cubic structure ZnS were also investigated. It was found that the uncorrected and corrected B-Ag_I peak energy decreases whilst that of the R-Sn band increases as the temperature is reduced.