Boron removal by titanium addition in solidification refining of silicon with Si-Al melt
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I. INTRODUCTION
THE production of “clean and reproducible” energy for power generation using solar cells, particularly by polycrystalline Si solar cells, has increased significantly with the steady aggravation of the problem of environmental pollution. Since the material for the Si solar cell is derived from expensive off-spec semiconductor grade Si (SEG-Si), it is expected that the solar grade Si (SOG-Si) will be in short supply or its cost will be elevated with the increasing demand for solar cells. A metallurgical refining process for SOG-Si with metallurgical grade Si (MG-Si) as the starting material has been developed in Japan;[1] however, a more economical refining process is required for achieving widespread use of solar cells. Among the typical harmful impurities for the Si solar cell, such as Fe, Ti, Al, P, and B, it is difficult to remove B during solidification refining and the vacuum treatment due to its large segregation coefficient in Si, 0.8,[2] and its comparatively low vapor pressure. In recent years, processes such as H2O-added Ar plasma treatment have been developed[3] for oxidation removal of B; however, further improvement for cost reduction is required. The authors have investigated the Si solidification refining process using a Si-Al melt at low temperature[4–9] by using the thermodynamic instability of impurity elements in solid Si at a lower temperature, which is expected from the retrograde solidus lines of impurities in Si.[10] This process consists of the following steps: (1) alloying MG-Si with Al to form the Si-Al melt, (2) solidification of Si from this melt, and (3) collection of refined Si by acid leaching. In our previous study, the segregation of B between solid Si and the Si-Al melt was investigated[9] for B removal in this refining process. The result indicated that the removal fraction of B from Si by solidification refining would be larger than TAKESHI YOSHIKAWA, formerly Graduate Student, is Research Associate, Department of Materials Engineering, The University of Tokyo, Tokyo 113-8656, Japan. Contact e-mail: [email protected] KENTARO ARIMURA, formerly Undergraduate Student, Department of Materials Engineering, The University of Tokyo, is with Daiwa Securities Co. Ltd., Tokyo 100-8101, Japan. KAZUKI MORITA, Associate Professor, is with the Department of Materials Engineering, The University of Tokyo. Manuscript submitted April 11, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS B
90 pct. However, further treatment for B removal is necessary to achieve a low-cost Si refining process. In the present study, an additional B removal technique was investigated. Since B forms a thermodynamically stable compound, TiB2, with Ti, it can be removed from the Si-Al melt in the form of TiB2 by adding Ti before beginning the solidification of Si from the melt. In our previous work for determining the thermodynamic property of B in molten Al,[11] TiB2 solubility in molten Al was found to be rather small with a large temperature dependence. Furthermore, no significant differ
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