Electron spin resonance in cadmium sulphide

Abstract

For some time past there has been a need for a more positive identification of the nature of the defect centres which give rise to energy levels in the forbidden gap in cadmium sulphide. Electron spin resonance (e.s.r.) techniques have proved very useful in this type of investigation in other materials. Since little similar work on CdS has previously been reported, the purpose of the research described in this thesis, has been to examine the usefulness of the technique in studying CdS. Initially it was necessary to construct a sensitive x-band microwave spectrometer, which operates at temperatures down to 1.5ºK and has provision for continual. illumination of the samples, Electron spin resonance absorption signals have been detected in undoped single crystals of GdS which can be attributed to four different defect centres. The occurrence of the resonance signals can be correlated with the resistivities and edge luminescence spectra of the samples. This indicates that the centres responsible for the resonance absorption are important in determining the electrical and optical properties of CdS and are those which this work was initiated to study. Tentative models for the various defects have been proposed. The most important feature of the work reported in this thesis is the isolation of an e.s.r. signal which is thought to be associated with the class 2 centres which provide the photo- conductive sensitisation and possibly act as I.R. emission centres in CdS. The model proposed for such centres is of compensated acceptor complexes with levels approximately 0.7eV above the valence band. A centre consists of four cadmium vacancies in nearest neighbour association. The models for the other three centres have not been discussed as fully as that mentioned above because of the lack of experimental data. However it is evident that e.s.r. techniques are a very valuable tool for investigating the nature of defect centres in CdS and that continuation of the work should prove invaluable in providing an unabiguous identification of the atomic structure of the defect complexes.