PHOTOCHEMICAL EFFECTS IN CDS CRYSTALS

At temperatures near and above room temperature, thermally-stimulated- current measurements on CdS crystals, which had been annealed in sulphur vapour at pressures up to 27 atmospheres, showed two major electron traps with levels 0·63 and 0·85 ev below the conduction band. These traps could be detected only if the crystal had previously been illuminated at temperatures in excess of 185°K. If the illumination was carried out between 185 and 245°K, the thermally-stimulated-current curve corresponding to the 0·85 ev trap became progressively larger as the temperature of illumination increased. At still higher temperatures, the thermally-stimulated- current curve corresponding to the 0·85 ev trap decreased, whereas that corresponding to the 0·63 ev trap appeared and increased in magnitude. The measurements show that for every two 0·85 ev traps which disappeared, one 0·63 ev trap appeared. It is suggested that the 0·63 ev trap is an aggregate of cadmium vacancies, created from the less complex 0·85 ev traps.