Submitted by Paul William Leary to the University of Exeter as a thesis for the degree of Doctor of Philosophy in Physics in the Faculty of Science, September 1997.


Presented in this thesis are theoretical investigations into light impurity defects (H and C) in both silicon and germanium. The calculations are performed using the AIMPRO code, an ab initio local density method applied to large hydrogen terminated clusters. The silicon self-interstitial (Sii) is a very important defect in Si, since it mediates dopant diffusion, and we first focus on this defect. Several low energy configurations are found, and the defect is bistable, switching configuration with charge state. The migration of the defect is discussed both in terms of the metastable configurations, and effects of varying charge state. The effects of hydrogen interaction with Sii are investigated, and the calculations on the IH2 defect agree well with experimental observations. Preliminary calculations on Sii aggregates are also discussed. Carbon defects in silicon are then investigated, and here, several unexplained properties of various centres are addressed. Calculations are performed on the Cs, Ci, and Ci-Cs defects, and our results support the proposed structural models for these centres. The calculated local modes of all of these defects are in good agreement with experiment, and the energies of the later defect in various charge states are consistent with the bistable properties of the centre. The effects of hydrogen interactions with carbon defects (both Cs and Ci) are then investigated, and a number of new results are found. The results on the Cs-H defect are consistent with experiment, and again, there is evidence for bi-stability of the centre. The Ci-H defect has a low migration energy, and therefore we expect it to be a very reactive species. The interaction of this centre with other impurities, in particular Cs is discussed. The results lead to an identification of the structure of the T-centre defect as Cs-CiH; comments are also made on several other CH related centres observed by photoluminescence. Finally, the interaction of intrinsic defects with H, and the Cs defect are investigated in germanium. Parallels are drawn between these results and those in Si, and the thesis ends with preliminary results on the effects of Cs in Si/Ge alloys.
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Last modified: Thur Jan 22 12:21:20 GMT 1998 by Paul Leary