Self-activated emission in ZnS and ZnSe

Abstract

In an earlier investigation by the authors concerned with the production of semiconducting dice of ZnS, it was shown that all their samples were contaminated with trace concentrations of copper, and the lowest resistivity ( approximately 102 ohm cm) could be obtained by heating the crystalline material in molten zinc containing 1% Ga and 1/2% Al. The broadband photoluminescence of all the samples used in these conductivity studies has now been measured, and identified as a self-activated emission. At low copper concentrations the emission shifts to higher photon energies on cooling to 80K, but with copper concentrations exceeding 0.85 ppm the emission shifts to lower energies on cooling. A model is suggested to explain this effect. It involves recombination from distant donors to complex centres consisting of (GZn+-AlZn3+) A-centres associated with CuZn+ copper green centres at low copper concentrations, and with (CuZn+-CuI+) copper blue centres at higher concentrations. The model can also explain why heating in Ga+Al is particularly effective in reducing the resistivity. Similar effects occur in ZnSe, where the model can explain why excitonic emission is observed in dice heated in zinc containing 1% gallium.