Thermal and Electrolytic Activation of Photoconductivity in CdS Crystals

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Abstract
The photoconductivity and dark conductivity of cadmium sulphide crystals grown by the Frerichs technique can be increased by heat treatment in vacuum or in air. Free cadmium is present on the surface and at internal cracks of a crystal as grown. Heating at 100–300°c in the presence of the free cadmium leads to a non-stoichiometric excess of cadmium in the lattice, and considerable activation is achieved. At temperatures in excess of 300°c thermal dissociation occurs, sulphur is lost and a further non-stoichiometric excess of cadmium is formed. The application of an electric field assists dissociation by electrolytic processes. The activation of cadmium sulphide crystals resulting from dielectric breakdown previously reported by the author (Woods 1056) can now be understood. Heat treatment in vacuum produces a high surface conductivity accompanied by a high photosensitivity to blue light. Both are reduced by exposure to oxygen at room temperature. Heat treatment in air produces electron trapping levels which are not created by corresponding treatment in vacuum.