A Study of the Optical Properties of Semiconductor Crystallites

The topic of this thesis is the fundamental optical properties of semiconductor crystallites, in particular crystallites of CdSexS1-x embedded within a silica glass host. After a brief introduction, a review will be presented of the considerable volume of published work relating to this material together with some relevant theory which can be utilised in the interpretation and understanding of the properties of such crystallites. This will be followed by a chapter describing the principles underlying the preparation of crystallites within a glass matrix, which is in many ways analogous to the crystallisation which occurs in highly saturated aqueous solutions. The equipment used to characterise and probe the properties of this material will then be discussed. Results will then be presented of experimental measurements on microcrystallites. The use of the technique of Raman scattering from lattice vibrations to determine crystallite stoichiometry will be described. The absorption and photoluminescence of crystallites will then be considered followed by a discussion of the origins of the spectral features observed. The light emitting properties of crystallites and their dependence on crystallite size, stoichiometry, sample temperature and the intensity of the excitation source will then be considered. Results of a time resolved pump probe experiment will then be presented and discussed. This will be followed by a description of an experimental study of sample preparation. The effects of glass doping levels and crystallite growth temperature will be described. This will be followed by a description of an experimental study of the effects of phonon confinement within crystallites. The experimental results relating to phonon confinement will then be compared with the predictions of a phonon confinement model discussed in chapter two. There will then follow a brief discussion of preliminary work performed on other systems of crystallites. Finally, consideration will be given to areas in which the work of this project could be continued and extended. Of the work presented in this thesis the phonon confinement results represent the first reported quantified observation of quasi-zero dimensional phonons in the microcrystallite doped glass system. The effect of phonon confinement upon the values for crystallite stoichiometry obtained from Raman scattering measurements is clarified and quantified. The thesis also presents a novel method of controlling the sizes of crystallites within a doped glass by altering the initial doping concentration. In addition, results of a preliminary investigation of InAs crystallites and coloured glasses prepared using a silicon dopant will be presented which represent new areas of research