Radiative Recombination in Cadmium Sulfide

Abstract

Luminescent centers involving Ag impurities were introduced into CdS single crystals through doping with 109Cd radioisotopes. Thus, the Ag concentration increases with time as more 109Cd decays. This enables a study of photoluminescence versus Ag concentration in a given crystal without changing the concentrations of other impurities.A new emission band at 5600 Å results in addition to the 6100 Å band present in Ag-doped crystals using conventional techniques. This new emission is quenched with increasing Ag concentration at high concentrations. Also concentration quenching by the Ag impurities occurs for the green edge, and the bound-exciton emissions I1, and I2. The quenching is explained by assuming a donor–acceptor recombination process.The new emission probably arises from the recombination of a bound electron with a bound hole at a distant donor–acceptor pair, with Ag as the acceptor. The acceptor role of Ag is supported by electrical conductivity measurements on 109Cd-doped crystals. Estimates are obtained for the acceptor binding energy, the donor concentrations, and the separations of pairs responsible for the new 5600 Å emission and the green-edge emission. The 6100 Å emission is attributed to Ag closely associated with other impurities. These conclusions are verified by our temperature annealing and quenching experiments.