Edge emission and exciton recombination in cadmium sulphide

Abstract

The Stokes (U.V.) and anti-Stokes (A.S.) photo-excited emissions of large, doped and undoped CdS single crystals, grown under controlled partial pressures of cadmium and sulphur, at liquid helium temperatures were examined to establish a correlation between the crystal growth conditions and the spectral distribution of the green edge and exciton emissions. Anti-Stokes excitation spectra were also obtained. Two longitudinal optical phonon assisted series constituted the green emission. The “high energy series" (H.E.S.) was attributed to the recombination of free electrons with holes bound to acceptors some 0.17eV above the valence band, the "low energy series" (L.E.S.) to a distant-pair recombination process involving electrons bound to donors some 0.03eV below the conduction band and holes bound to the same acceptor. The mean separation between the donors and acceptors was about 100 Å. Only the L.E.S. was observed in A.S. excited green emission. The I(_1)and I(_2) bound excitons which dominated the blue emissions are associated with exciton recombination at neutral acceptors and neutral donors respectively. I*(_2) emission, associated with excitons bound to neutral donors loosing some of their recombination energy in raising the donor electron to an excited state of the donor, was observed and used to evaluate a donor ionisation energy of 0.026eV. Blue emission was excited by A.S. radiation in several crystals and ascribed tentatively to I(_1) emission. A model is developed to explain the variation of the emission characteristics with crystal growth conditions. A cadmium vacancy- donor impurity complex is suggested as the acceptor involved in the green and exciton emissions, with the hole in an excited state of the complex, and as the centre through which the two-step A.S. excitation process proceeds. Sulphur vacancy-acceptor impurity complexes and donor impurities are suggested to explain the donors associated with the L.E.S. and 1(_2) emissions.