Removal of B from Si by solidification refining with Si-Al melts
- PDF / 346,790 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 35 Downloads / 218 Views
I. INTRODUCTION
IN this decade, the amount of solar cell production, especially that of polycrystalline Si, has increased significantly with the growing demand for clean energy. As the material resource for polycrystalline Si solar cells is dependent on expensive off-grade Si for semiconductors (SEG-Si), solargrade Si (SOG-Si) would become in short supply with an increasing demand for solar cells in the near future. In order to provide a cost reduction and productivity increase, a new metallurgical refining process of SOG-Si using metallurgicalgrade Si (MG-Si) as a starting material has been developed in Japan.[1] However, the development of a much-lower-cost refining process is required for spreading solar cells widely. In the metallurgical refining of Si, the effective removal of B a crucial matter. The difficulty of B removal is due to its large segregation coefficient in Si (0.8),[2] and its low vapor pressure in molten Si. Currently, the oxidation treatment for B removal with H2O-added Ar plasma is performed in the aforementioned process, but it involves a long processing time and electrical costs. Also, the Ca-added acid leaching treatment, which is useful for removal of metallic impurities, is found to not be effective for B removal.[3] On the other hand, the present authors are trying to develop a solidification refining process for Si with an Si-Al melt at low temperatures[4–8] from the retrograde solidus line of impurities in Si,[9] which indicates the thermodynamic instability of impurities at lower temperatures. However, B removal in this refining has not been clarified thermodynamically. In order to discuss B removal by the solidification refining of Si with Si-Al melts, the segregation ratio of B between solid Si and Si-Al melt at 1273 to 1473 K was investigated in the present work. As both Al and B are low-diffusive elements TAKESHI YOSHIKAWA, formerly Graduate Student, is Research Fellow, Department of Materials Engineering, The University of Tokyo, Tokyo 1138656, Japan. Contact e-mail: [email protected] 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
in solid Si (e.g., B diffusion is 1.1 1014 cm2/s at 1273 K[10] and Al diffusion is 4.0 1013 cm2/s at 1273 K[11]), solidification of Si from an Si-Al melt is considered to be adequate for the solute equilibrium between solid Si and the Si-Al melt, rather than a simple solid-liquid equilibration technique. Hence, the temperature-gradient-zone melting (TGZM) method[12] was employed as in the previous studies.[4–6] The segregation ratio of B between solid Si and the Si-Al melt, as well as the thermodynamic property of B in solid Si, was determined by measuring the Al distribution of solidified Si from the Si-Al melt zone. II. EXPERIMENTAL Figure 1 shows an experimental apparatus. The singlecrystalline Si (11N purity) was cut into (111)-oriented plates of 15 15 10 mm and 15 15 4 mm. Aft
Data Loading...
