The photoconductive and luminescent properties of cadmium sulphide

Abstract

An evaluation is presented of some of the photoconductive and luminescent properties of CdS crystals grown by sublimation in sealed tubes under excess pressures of cadmium and sulphur. It is demonstrated that the use of impure starting material, for crystal growth, can lead to anomalous results. Eight discrete sets of electron traps were found in the present samples using the thermally stimulated current technique. The trapping parameters and the variation of trap density with growth conditions are given in the text. It is suggested that one of these traps, which had a thermal activation energy of 0.15 eV, is associated with cadmium interstitials. The densities of two traps, with activation energies of 0.53 eV and 0.60 eV, changed in magnitude when differing conditions of illumination were used prior to measurement. Such changes are shown to be commensurate with traps of a -photochemical nature. Three and four probe measurements are also described, which were made to determine how the crystal surface and the indium contacts modify the thermally stimulated current. Considerable differences were found in the spectral response and infrared quenching of photocurrent measured on cadmium and sulphur rich samples. A disparity was found between the thermal and optical separations of the sensitising Class 11 centres from the valence band, which indicates that these centres are singly negatively charged when occupied by electrons. The structured luminescence, in the region 1.6 µm to 2.0 µm, has been attributed to radiative recombination of electrons from the sensitising Class 11 centres with holes in the upper branches of the valence band.