Production of Solar-grade Silicon by Halidothermic Reduction of Silicon Tetrachloride
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INTRODUCTION
IN the 21st century, the use of solar cells has increased considerably because solar energy is expected to serve as an alternative source of energy that will also mitigate environmental issues. Worldwide, the amount of electricity produced using photovoltaic (PV) cells has been increasing at the rate of 40 pct per year, reaching a value of 6,941 MW in 2008.[1–3] Currently, more than 92 pct of all solar cells are silicon(Si)-based cells, and the global production volume of polycrystalline Si reached 57,400 tons in 2008; this volume is expected to double in the current decade.[2–4] From 2004 to 2008, this rapid increase in the production of PV cells has resulted in serious shortages in the supply of solar-grade silicon (SOG-Si; solar-grade Si, 6N purity), which has severely hampered the growth of the PV industry, especially in Japan. Before 2004, SOG-Si was supplied commercially only from the off-grade part of semiconductor-grade Si (SEG-Si, 12N purity) manufactured by the Siemens process (Figure 1(a)),[5,6] and to a lesser extent, by the Komatsu process.[7] Nowadays, some polycrystalline Si are manufactured only with the objective of PV applications by the Siemens or similar processes. However, the PV and silicon industries are expected to grow KOUJI YASUDA, formerly Project Research Associate, Institute of Industrial Science, University of Tokyo, Tokyo 153-8505, Japan, is now, Researcher, with the High Performance Materials Department, Osaka Titanium Technologies Co., Ltd, Hyogo 660-8533, Japan. TORU H. OKABE, Professor, is with the Institute of Industrial Science, University of Tokyo. Contact e-mail: [email protected]. KUNIO SAEGUSA, Senior Research Associate, is with the Tsukuba Research Laboratory, Sumitomo Chemical Co., Ltd., Ibaraki 300-3294, Japan. Manuscript submitted April 29, 2010. Article published online October 15, 2010. METALLURGICAL AND MATERIALS TRANSACTIONS B
considerably in the long term, and therefore, the possibility of a shortage in the supply of SOG-Si may reoccur. The main factor responsible for the shortage in SOG-Si supply is the low productivity of the conventional Si-reduction processes. Low productivity results from batch-type processes, slow reactions, and low energy efficiency. The reactions involved in the conventional Si production processes are thermal decomposition and/or hydrogen (H2) reduction of trichlorosilane (SiHCl3) or thermal decomposition of monosilane (SiH4). Hence, to satisfy the growing demand for SOG-Si, it is necessary to develop a new SOG-Si production process that has high productivity. As summarized in Table I and Figure 1, various methods for SOG-Si production/Si purification have been developed and improved by researchers for overcoming the productivity of the Siemens process. These methods are roughly classified into the following three technologies[3,8,9]: (1) thermal decomposition and/or H2 reduction of silane gases by improving the existing Si production methods that are based on the Siemensbased processes[10–13]; (2) metallothermic reduction of
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