The effects of preferential pairing and its detection using electron radiation damage

A study of the electron radiation damage of the donor-acceptor pair green edge emission of cadmium sulphide at 4.2 K has led to possibly the first reported observation of preferential pairing. Irradiations of cadmium sulphide at 4.2 K with electrons of 260 keV energy, which were able to produce an increase in the donor concentration through the displacement of sulphur ions, were found to produce a wavelength shift in the peak positions of the donoracceptor pair cathodoluminescence emission band. These shifts were to shorter wavelengths in as-grown or excess cadmium-fired samples and to longer wavelengths in excess sulphur-fired samples. Theoretical considerations show that the latter shift may be reasonably interpreted to occur as the result of preferential pairing of the donor and acceptor centres, although a possible ambiguity arises due to impurity banding effects.