Edge emission in CdS crystals.

The aim of this work was to investigate the luminescent mechanism in pure CdS single crystals and to attempt to obtain unambiguous evidence in favour of one of the two models proposed in the literature (the Schon-Klasens or the Lambe-Klick model). Experiments have been carried out on undoped specimens obtained from three different sources and an attempt has been made to correlate transport, electrical, optical and luminescent properties measured on the same specimens. The results of electron transport measurements, based on drift mobility techniques, on a number of specimens indicated that the drifting electrons interact predominately with either a shallow centre lying at about 0.03ev or with a deeper level at 0.l6ev below the conduction band. Moderate heat treatment (360C) led to a progressive reduction in the density of the 0.16ev centres and also quenched the emission. The other properties studied were (a) the free electron density as a function of temperature and excitation intensity, (b) the temperature and excitation intensity dependence of the luminescence (c) its spectral distribution, and (d) the optical quenching spectrum. From the results of these experiments it is concluded that: (i) the 0,16ev centres are not directly connected with the luminescent transition and a model based on the Lambe-Klick transition appears unlikely, and (ii) a class II centre (Sp > Sn) lies between 0.13 and 0.15ev above the valence band, which accounts for the supralinearity observed in the electron density as a function of excitation intensity, and also explains the temperature dependence of n. The results of the electrical measurements have been fitted to a three centre model of a photoconductor. The free carrier densities and occupation of the centres have been calculated by computer as a function of temperature and excitation intensity for various sets of parameters characterising the centres. On the basis of this model the dependence of luminescence on the temperature and excitation intensity has been predicted. A comparison between the computed and experimental curves indicates that only a model in which the radiative recombination is associated with a trapped hole (in a class II centre) and a free electron (Schon-Klasens model) can predict the experimental results correctly. It appears likely that the luminescent centre is a class II centre. The reduction in the density of 0.16ev centres below the conduction band and the loss of emission appear to be two effects which are not directly associated. It is tentatively suggested that the centre seen in transport measurements is a cadmium interstitial and that the loss of edge emission by heat treatment in a vacuum, is probably due to the creation of cadmium vacancies at the surface. A reduction in the density of luminescent sites is not essential to explain the effect which is observed