Defects in Solids: AN AB Initio Study.

Available from UMI in association with The British Library. This thesis presents a method by which properties of defects in solids can be obtained from first principles, that is, they can be derived from a knowledge of only the atomic numbers of the constituent atoms. The method calculates the total energy of a hydrogen-terminated cluster of up to 100 atoms with the defect placed in the centre. From these calculations structural, vibrational and to some extent electronic properties can be obtained. The method is first used in a study of the properties of topological defects in amorphous phosphorus. The stability and vibrational properties of several defects previously used to explain photoluminescence data are determined and some shown to be unstable. The properties of interstitial hydrogen in semiconductors are then investigated and models for the normal and anomalous muonium centres presented. The interaction of hydrogen with dopant atoms is then considered in gallium arsenide and it is shown that the stable equilibrium sites are very different from the more well-known behaviour in silicon. It is shown that the hydrogen bonds directly to the donor atom in an antibonding direction, leading to a broken bond in the host lattice, explaining the absence of one expected infrared absorption line. In the p-type material the hydrogen moves into a bond-centred site giving rise to an anomalously low bend frequency. Finally the properties of nitrogen in diamond are investigated. All the experimental information about a substitutional nitrogen atom is reproduced and an explanation given for an "anomaly" in the infrared absorption. Two models for the structure of platelets are investigated, with the model due to Lang being found incapable of giving the high frequency absorption seen by experiment. In contrast it is shown that the Humble CN model can give rise to all the principle absorptions.