Boron Removal from Silicon by Moisturized Gases
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The photovoltaic (PV) industry is in rapid growth and a large supply of PV feedstock materials must be provided to maintain this growth. Since silicon is still the dominant material for the fabrication of solar cells, low-cost solar-grade silicon (SoG-Si) feedstock is demanded by the PV industry and a continuous growth for it is expected. The most cost-effective and direct approach for producing SoG-Si is to purify and upgrade metallurgical-grade silicon produced from raw materials. The majority of impurities existing in MG-Si are effectively removed through directional solidification of molten silicon, which is a key process in silicon solar cells production. However, the removal of boron and phosphorus by this method is difficult and expensive due to the relatively large distribution coefficients of these elements between solid and liquid silicon during solidification. Therefore, the elimination of these elements to the levels required for SoG-Si feedstock requires other processes. In the present work, the boron removal by top-gas blowing of H2, H2-H2O, Ar-H2, Ar-H2-H2O, He-H2O and He-H2-H2O over silicon melt is studied. It is shown that the type of gas mixture and the concentration of water vapor in the gas affect the rate of boron removal. Moreover, it is indicated that the kinetics of boron removal from silicon is depending on the mass transport of the gaseous species in the system and also the chemical reactions at the melt surface.