Mass Transfer of Phosphorus in Silicon Melts Under Vacuum Induction Refining
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SOLAR energy is in a great demand because it is inexhaustible and cleaner than any conventional energy resource. Creating stable, high-efficiency, low-cost solar cells has sparked wide interest in photovoltaic energy conversion. To make solar cells less expensive, it is necessary to reduce the cost of the raw material. Using relatively inexpensive metallurgical-grade silicon (MGSi) as a starting material is one way to make solar-grade silicon (SOG-Si) by refining. Phosphorus is a typical impurity in MG-Si. As a Group V element, phosphorus acts as an electron ‘‘donor’’ in silicon (a Group IV element), in which case electrical conduction is mainly caused by free electrons, and the silicon is doped n-type to signify the dominance of negatively charged carriers in the conduction of current. For technical reasons, most silicon wafers or sheets used for solar cells are p-type and doped with boron.[1] As is shown in Figure 1, the limits on impurity concentrations in p-type silicon for impurities determining the degradation threshold of solar cells are reported by Bathey and Cretella,[2] Gribov and zinov’ev,[3] and Dietl.[4] The required maximum limit for phosphorus content in SOG-Si should be less than 0.1 ppma (0.11 ppmw). Phosphorus is unavoidably present in the silica and carbon raw materials used for the arc-furnace reduction SONG-SHENG ZHENG and JING CAI, Ph.D. Candidates, WEN-HUI CHEN, Master’s Student, JIN-TANG LI, Assistant Professor, and XUE-TAO LUO, Professor, are with the Department of Materials Science and Engineering, Xiamen University, Xiamen 361005, P.R. China. Contact e-mail: [email protected] CHAO CHEN, Professor, is with the Department of Physics, Xiamen University. Manuscript submitted June 28, 2009. Article published online August 19, 2010. 1268—VOLUME 41B, DECEMBER 2010
process. MG-Si invariably exhibits a phosphorus concentration level in the 10 ppma to 50 ppma (11 ppmw to 55 ppmw) range. With segregation coefficients equal to 0.35, limited phosphorus can be removed by solidification. Vacuum refining is a comparatively effective process to eliminate phosphorus from silicon melts. Experiments to remove phosphorus under vacuum conditions have been reported by Zheng et al.,[5] Miyake et al.,[6] Yuge et al.,[7] Suzuki et al.,[8] Pires et al.,[9] and Ikeda and Maeda,[10] and the thermodynamics of phosphorus in molten silicon also has been presented by Miki et al.[11] and Zaitsev et al.[12] Among various different types of vacuum-refining processes, vacuum refining with induction heating is one economic way to refine molten silicon. Vacuum refining in induction melting units is achieved through evaporation of volatile impurities into the gas phase above the melt. Vacuum induction melting can provide a close control of the melt composition and temperature while simultaneously preventing undesired contamination by reactive gases such as oxygen. Similarly, through the use of low pressures and inductive stirring, the rate of refining reactions can be improved, and purification processes that would not take place at atmosp
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