Zinc Sulfide Phosphor Constitution and Its Effect on Electron Traps

Abstract

The phosphorescence and thermoluminescence of zinc sulfide phosphors are governed by the mechanism of electron trapping in the phosphor crystal lattice. Experiments have been carried out to find the effect of phosphor constitution and method of preparation on the numbers and the energy distribution of electron traps. In order to investigate the traps, use has been made of the thermoluminescence vs. temperature characteristics of the phosphors. These experiments show that pure zinc sulfide and zinc sulfide activated by silver and manganese possess similar ``shallow'' electron trap configurations after preparation and that these configurations are not markedly dependent on the particular activator. However, the introduction of copper impurity to zinc sulfide causes a marked change in the number and distribution of electron traps and is responsible for the long afterglow of these phosphors. Investigations of the effect of firing temperature and crystal form on the electron traps show that below the blende‐wurtzite transition temperature (1020°C) the proportion of deep traps increases with firing temperature but that above this temperature the trap distribution is independent of the firing temperature or of fluxes and excess of sulfur. Experiments attempting to show that these traps were due to stoichiometric excess of zinc gave inconclusive results, but it is clear that the deep electron traps which produce the long afterglow characteristics of zinc sulfide are due both to the presence of copper impurity and to some change produced in the zinc sulfide lattice at high temperature without copper being present. Other studies have been made of the effect of the mixed crystal proportions in the ZnS–CdS–Cu phosphor system on the electron traps. At high CdS contents some relatively deep electron traps are produced. Measurements of the fluorescence vs. temperature characteristics of the above phosphors show how the phosphor constitution and firing temperature affect the temperature at which the thermal quenching of fluorescence becomes appreciable.