Copper Removal from Hypereutectic Cu-Si Alloys by Heavy Liquid Media Separation
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TRODUCTION
IN recent years, silicon gained increased interest predominantly because of the large growth of solar cell industry and increased demand for materials with photovoltaic properties. Mardesich et al.[1] investigated the feasibility to produce Si for solar cells by high-temperature deformation of impure Si at temperatures up to 40 K below its melting point. Recrystallization was incomplete even after 10 hours of anneals where the minority carrier diffusion length was found to be reduced drastically after deformation, perhaps because of contamination or the cooling rate, and it recovered slightly with annealing. These results suggested that high-temperature deformation of untreated Si is not practical. One possible route to achieve higher purity Si is alloying and controlled solidification with elements with significantly lower melting temperatures to produce purified platelets of Si, while the impurities are trapped in a getter alloy. The first attempt has been made by Driole and Bonnier,[2] which alloyed Si with Sb and Sn, and later recovered Si by distillation; however, most impurities were removed by subsequent acid leaching phase rather than alloying. Monner and Giaco[3] achieved a significant improvement in Si purification by separating Si from the Cu-Si alloy using the electrolysis technique. Recently, according to Scott’s patent,[4] solar-grade silicon was achieved by alloying Si with various elements and following with several metallurgical techniques, e.g., directional solidification, gas injection, slagging, and single-crystal ALEKSANDAR M. MITRASˇINOVIC´, formerly Ph.D. Student with the Department of Materials Science and Engineering, University of Toronto, is currently a Postdoctoral Fellow with the Centre for Advanced Coating Technologies (CACT), University of Toronto and with Laboratory for Emerging Energy Research (LEER), University of Waterloo, Waterloo, ON, Canada, and is currently at the University of Toronto, Bahen Centre for Information Technologies, Rm 8268, 40 St. George Street, Toronto, ON M5S 1A4, Canada. Contact e-mail: [email protected] TORSTEIN A. UTIGARD, Professor, is with the Department of Materials Science and Engineering, University of Toronto, 184 College St., Toronto, ON M5S 3E4, Canada. Manuscript submitted February 7, 2011. Article published online December 14, 2011. METALLURGICAL AND MATERIALS TRANSACTIONS B
growth process. Juneja and Mukherjee[5] proposed alloying with Cu followed by electrochemical or acid leaching. Bracht[6] explained the distribution and size of the Cu precipitates depending on the initial Cu concentration and the cooling conditions. Rapid cooling was found to result in platelet-like precipitates distributed homogeneously in the Si crystal,where as slow cooling of samples with Cu concentrations of the order of 1017 cm3 yielded Cu precipitates primarily in the near surface region of the wafer. Presha and Weber[7] recognized passivation and determined the dissociation energy of the Cu-acceptor element pairs such as Ga, In, or B. In the work of Hei
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