Light element impurities and related defects in polycrystalline silicon for photovoltaic application

This thesis examines light element impurities and related defects in polycrystalline sheet and RGS (Ribbon Growth on Substrate) ribbon silicon. The interaction dynamics between oxygen, carbon, nitrogen, and intrinsic point defects, as well as the role of grain boundaries (GBs) on oxygen and carbon precipitation have been studied.; Sheet silicon wafers subject to various heat treatments have been studied by infrared microspectroscopy. An interstitial oxygen (Oi) denuded-zone near the top surface is observed in the virgin wafer grown with a more aggressive thermal profile. The Oi denuded zone is sustained as a 250 mum wide precipitate-denuded-zone during annealing, mainly due to the critical role of the initial Oi concentration and quenched-in vacancies on precipitate nucleation. It is found that carbon precipitation is well correlated with that of oxygen. By monitoring the substitutional carbon (Cs) reduction by infrared absorption and the precipitate density by preferential etching, it is concluded that formation of interstitial carbon by trapping silicon self-interstitials is an indispensable step for the observed fast Cs precipitation.; Carbon and oxygen precipitation in four sets of RGS wafers with similar carbon contents and very different oxygen concentrations has been studied. It is found that carbon precipitation in an oxygen containing wafer consists of two distinct steps; namely, an initial rapid oxygen-carbon co-precipitation in the very first hour annealing, followed by slow precipitation during subsequent prolonged annealing. A high oxygen content enhances carbon precipitation throughout the two steps. It is shown that the formation of interstitial carbon in the presence of excess silicon self-interstitials generated during oxygen precipitation plays an important role in increasing the carbon precipitation rate in the first hour annealing. Because of the absence of interstitial injection during the following slow precipitation process, the enhancement effect of oxygen can only arise from an increase in precipitation sites. It is proposed that the oxygen-carbon co-precipitates formed in the very first hour annealing provide sites for continuous carbon precipitation. This explains why carbon impurities precipitate faster in a high oxygen containing wafer, even after removal of all the interstitial oxygen from the silicon matrix.; The impact of GBs on oxygen precipitation in sheet silicon has been investigated. Infrared microspectroscopy shows nitrogen gettering at GBs, and preferential etching reveals a precipitate-denuded-zone near GBs. (Abstract shortened by UMI.)...