Light Element Impurities and Related Defects in Polycrystalline Silicon for Photovoltaic Application

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Title: Light Element Impurities and Related Defects in Polycrystalline Silicon for Photovoltaic Application
Author: Lu, Jinggang
Advisors: David E. Aspnes, Committee Member
George A. Rozgonyi, Committee Chair
Carlton Osburn, Committee Member
Gerd Duscher, Committee Member
Abstract: 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 investigated. It is found that a high concentration of interstitial oxygen (Oi) will precipitate readily in polycrystalline sheet and ribbon silicon, and the precipitation of substitutional carbon (Cs) is mainly controlled by oxygen precipitation. By monitoring the 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. It is concluded that a low oxygen content is vital important to prevent extensive oxygen precipitation. On the contrary, a high carbon content (~ 1x 10¹⁸ cm⁻³) can be tolerated as long as the initial Oi concentration is low. 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. The gettering of nitrogen at GBs is likely to be responsible for the denuded zone formation, considering the enhancement of nitrogen impurities on oxygen precipitation. The impact of GBs on carbon precipitation in RGS ribbons has also been studied. Infrared microspectroscopy indicates a higher remaining Cs concentration in the intra-grain region and preferential etching reveals a 20 to 30 μm wide precipitation band near GBs. Assuming that the tensile strain associated with carbon precipitates must be relaxed in order for the precipitation to proceed, it is shown that the precipitation band formation is mainly controlled by diffusion of vacancies from the intra-grain region to GBs.
Date: 2004-11-04
Degree: PhD
Discipline: Materials Science and Engineering

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