Chemical Transport Deposition of Purified Poly-Si Films from Metallurgical-grade Si Using Subatmospheric-pressure H 2 Pl
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1245-A10-01
Chemical Transport Deposition of Purified Poly-Si Films from Metallurgical-Grade Si Using Subatmospheric-Pressure H2 Plasma
Kiyoshi Yasutake1,2,3, Hiromasa Ohmi1,2,3 and Hiroaki Kakiuchi1,3 1
Department of Precision Science and Technology, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan
2
Research Center for Ultra-Precision Science and Technology, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan 3
Japan Science and Technology Agency, CREST, 5 Sanbancho, Chiyoda-ku, Tokyo, 102-0075, Japan
ABSTRACT Purified Si film is prepared directly from metallurgical-grade Si (MG-Si) by chemical transport using subatmospheric-pressure H2 plasma. The purification mechanism is based on the selective etching of Si by atomic H. Since most metals are not etched by H, this process is efficient to reduce metal impurities in Si films. It is demonstrated that the concentrations of most metal impurities (Fe, Mn, Ti, Co, Cr, Ni, etc.) in the prepared Si film are in the acceptable range for applying it to solar-grade Si (SOG-Si) material, or below the determination limit of the present measurements. On the other hand B and P atoms, which make volatile hydrogen compounds such as B2H6 and PH3, are difficult to eliminate by the present principle. From the infrared absorption measurements of the etching product produced by the reaction between H2 plasma and MG-Si, it is found that the main etching product is SiH4. Therefore, a remote-type chemical transport process is possible to produce SiH4 gas directly from MG-Si. Combining other purifying principle (such as a pyrolysis filter), this process may have an advantage to eliminate B2H6 and PH3 from the produced SiH4 gas. INTRODUCTION The recent explosive increase in photovoltaic (PV) market causes the shortage of high-purity Si material for solar cell production [1,2]. To solve this problem and make Si solar cells less expensive, reducing the cost of polycrystalline Si for solar cells continues to be a high priority issue in PV industries. Hence, there have been many efforts to produce SOG-Si feedstock from MG-Si at a high efficiency by substituting metallurgical processes such as directional solidification for conventional chemical purification processes such as Siemens process [3–5]. In the metallurgical process [6,7], not only long solidification time but also high temperature to melt Si is necessary, and the elimination rates for metal impurities typically range between 10−2 and 10−3 in a single solidification process.
On the other hand, in terms of reducing the consumption of high-purity Si material, thin-film Si solar cells are considered to be preferable [8]. Generally, Si thin films are fabricated by various chemical vapor deposition (CVD) techniques [9,10]. A high purity feedstock gas, such as SiH4, is needed in the CVD process. SiH4 gas is also produced from MG-Si through the classical chemical purification process. SiH4 gas is not only toxic, but also a precious compound. There
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