Defects and gettering of impurities in silicon

Processes of formation of extended defects in silicon and the role of impurities in them, as well as the gettering of impurities from precipitated state, the electrical activity of impurity precipitates and their impact on performance of solar cells are considered in the thesis.; The nucleation and growth of voids and vacancy-type dislocation loops during Si crystal growth under Si vacancy supersaturation conditions have been numerically modeled. The two processes are treated in conjunction with each other. Based on the competition between them, the Si vacancy formation enthalpy range and the void nucleation temperature are determined.; The role of oxygen in the formation of voids in Si has been considered, and the mathematical description of the process has been formulated. It is shown that experimentally observed composite void-oxide defects are likely to nucleate first as simple oxide precipitates and later to develop into voids with their surfaces covered by the oxide layer.; Physical and numerical modeling of impurity gettering from multicrystalline Si for solar cell fabrication has been carried out using Fe as a model impurity. A variable temperature gettering process is modeled and predicted to provide high gettering efficiency and short gettering times.; A quantitative model of the electrical activity of metallic precipitates in Si has been developed. An emphasis is made on the properties of the Schottky junction at the precipitate-Si interface as well as the carrier diffusion and drift in the Si space charge region. Carrier recombination rate is found to be primarily determined by the thermionic emission charge transport process across the Schottky junction rather than the surface recombination process. It is shown that the precipitates can have a very large minority carrier capture cross-section.; The above-mentioned model of the process of impurity gettering from Si by an Al layer has been combined with a solar cell device model. This provides a way of monitoring the solar cell efficiency evolution during the gettering process. Al indiffusion and the back surface field formation are also considered. Impact of the two processes on the thick solar cell efficiency is determined. The model provides a means to optimize the temperature regime of the Al gettering and indiffusion processes.