Defects in Cadmium Sulfide Crystals

Abstract

At low temperatures certain crystals of CdS emit green light when struck a blow with a hard object. Many of these crystals show unusual electrical properties. When cooled in the dark, the conductivity goes to very low values, 10−12 (Ω-cm)−1 or less; then, upon exposure to light of wavelength less than 6900 Å, the conductivity increases irreversibly to values as high as 10−2 (Ω-cm)−1. The defect structure of these crystals has been studied by the technique of thermally stimulated currents in the temperature range 80° to 300°K. Five current peaks are observed with different intensities in different crystals. When the crystal is illuminated with light of wavelength longer than 6900 Å, the traps noted above can be filled without changing the conductivity of the crystal. The energy of the traps can be determined by the threshold wavelength of light required to fill the various levels with electrons from the level located 6900 Å (1.79 eV) below the conduction band as well as by the position of the Fermi level when the traps are being emptied.