Photochemical effects in cadmium sulphide crystals

Abstract

Conductivity glow curve experiments have shown that six major sets of electron traps can be found in cadmium sulphide crystals prepared in this laboratory. Traps which lie 0.25 eV below the conduction band are associated with excess cadmium. These centres disappear when a crystal is irradiated with white light at or near room temperature, while two new traps appear with depths of 0.41 and 0.83 eV. The process is reversible, and the initial 0.25 eV levels can be restored by heating at 100°C in the dark. The 0.25 eV trap is not found in sulphur-rich crystals. Experiments on sulphur-rich samples as grown, indicate the presence of traps lying 0.41, 0.83 and 0.63 eV below the conduction band. Heat treatment at 100°C removes all trace of these three centres from a subsequent glow curve. The 0.41 and 0.83 eV centres can be produced by illuminating a crystal which is maintained at a temperature between -50 and +20°C. The 0.63 eV trap only is created when the crystal is held at +80°C during the irradiation. A tentative explanation of these effects is given in terms of associated defects.