Recombination kinetics in CdS.

In high-purity cadmium sulfide crystals,, at low temperatures and high excitation intensities, emission lines attributed to free and bound exciton recombination are observed in the spectral range 4860 to 5090 A. In addition, the main peaks of two broad emission bands, which are repeated at lower energies with the simultaneous emission of one or more longitudinal optical phonons, are observed at about 51^-0 and 5180 A.. The high energy band, which is d.ominant at liquid, nitrogen temperatures, is d.ue to free electrons recombining with holes bound at cadmium vacancy acceptors. The low energy band, which is dominant at liquid helium temperatures, is due to electrons bound to shallow donors recombining with the bound holes. The photoluminescence efficiency and photoconductivity response of cadmium sulfide crystals were measured and the data interpreted in terms of an energy band model involving the donor and acceptor levels previously established as being involved in the radiative transitions. In addition, an effective recombination center (consisting of deep acceptor-like recombination centers) and non-radiative surface recombination centers are required to account for the non-radiative transitions. The results of the thesis are divided into four topics and are summarized below. The first topic deals with the controversy in the literature regarding the origin of the high energy emission band at about 51^-0 A. Two recent papers, which identify this band, as being due to bound electron-to-bound hole transitions, are analyzed and. it is shown that their conclusions are incorrect. Further ii, analysis and experiments show that their data support the free electron-to-bound electron interpretation of other authors. The second topic was the effect of surface recombination centers on the luminescence efficiency. These states are believed to be mainly chemisorbed oxygen ions. Non-radiative surface recombination is reduced by applying an electric field to counteract the electric field in the charge depletion layer next to the surface, or by phot o-d.es orb ing the oxygen ions. This electric field d.raws minority carriers to the surface where they recombine non-radiatively. The luminescence efficiency is found to be lowest when the electron-hole pairs are generated.,, closest to the surface. This is interpreted as meaning that a greater fraction of the carriers can reach the surface to recombine and that ambipolar diffusion of carriers into the interior of the crystal does not take place. It was also found, that heating CdS briefly in a nitrogen ambient produces free-to-bound and bound-to-bound transitions associated with nitrogen acceptors 130 meV above the valence band. The nitrogen impurities are near the surface since these band.s are removed by a short etch in concentrated hyd.rochloric acid. The third topic was the recombination kinetics of excitons and the bound, electron-to-bound hole luminescence. In all cases, the exciton efficiency increases with increasing excitation intensity as expected since the formation of excitons depends on the prod.uct of the free carrier densities. The bound-to-bound emission efficiency is high and varies slowly with excitation intensity. The efficiency falls slowly both at high and at ill. low excitation intensity. The fall-off in efficiency at high excitation intensity is accompanied by an increase in efficiency of the free-to-bound emission band. The decrease in efficiency at low excitation intensities may be due to non-radiative surface recombination. The last topic was the recombination kinetics of the free electron-to-bound hole luminescence. Using the energy band model mentioned earlier, the data was analyzed to obtain the electron and hole lifetimes, the luminescence efficiency, and the electron and hole capture cross-sections of the cadmium vacancy acceptor and the other d.eep recombination center. The internal luminescence efficiency is near unity as long as the minority carriers (holes) are quickly captured by the radiative recombination centers (cadmium vacancies). At high temperature the luminescence efficiency is low because the cadmium vacancy centers act as traps rather than recombination centers, while at high excitation intensities the efficiency drops because the radiative transitions saturate.