Mechanisms and Kinetics of Boron Removal from Silicon by Humidified Hydrogen
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In alumina crucible : Al þ B þ O ¼ AlBO2 ðgÞ Based on the obtained results, it has been proposed that boron removal from silicon melt by humidified hydrogen is controlled both by the chemical reaction for boron gasification and mass transport in the adjacent gas phase. DOI: 10.1007/s11663-015-0566-9 Ó The Minerals, Metals & Materials Society and ASM International 2015
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INTRODUCTION
SILICON is still the dominant semiconductor material used in the photovoltaic (PV) industry. Solar grade silicon (SoG-Si) is a high purity material with above 99.9999 pct Si, which can be produced through chemical and metallurgical routes. The purity of the silicon produced by the chemical processes is usually higher than that produced through metallurgical processes; and JAFAR SAFARIAN and STEFAN ANDERSSON, Researchers (PhD), and KAI TANG and JAN ERIK OLSEN, Senior Researchers (PhD), are with SINTEF Materials and Chemistry, Alfred Getz Vei 2, 7465 Trondheim, Norway. Contact emails: [email protected]; [email protected] GABRIELLA TRANELL, Professor, is with the Department of Materials Science and Engineering, Norwegian University of Science and Technology (NTNU), 7491 Trondheim, Norway. KJETIL HILDAL, R&D Engineer (PhD), is with the Process Development Group, Elkem AS-Technology, 4675 Kristiansand, Norway. Manuscript submitted December 4, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS B
often above that required for photovoltaic applications. The most common route to produce SoG-Si from Metallurgical Grade Silicon (MG-Si) is still based around so-called the Siemens technology which includes gasification, distillation and re-deposition of silicon via trichlorosilane, a very energy intensive method which also generates significant amounts of chemical waste.[1] Hence, more cost effective metallurgical processes for making solar grade silicon—with less environmental impact—have been studied and developed in the recent years due to the rapid PV market growth. Boron (B) and phosphorus (P) are the two crucial impurity elements that are most difficult to be removed from silicon, since they cannot be effectively separated by directional solidification, a key process step for removing many impurities such as Fe, Ti, Al, Ca, Mg, etc. The removal of B and P requires the dedicated refining processes and has been the motivation of many purification process studies. In addition to boron slag refining technology, which is currently applied industrially by ELKEM Solar AS, the removal of B can be performed through the
treatment of liquid silicon by oxidative plasma or other gas refining techniques. In these processes, the dissolved B in molten silicon is gasified through the formation of volatile B-containing compounds. The removal of boron by different reactive gases, such as O2,[2,3] H2,[4,5] CO2,[3] and H2O,[6–12] has been studied through several research groups. It has been observed that the application of the H2-O2 gas mixtures in plasma refining is more effective in terms of the kinetics of B removal compared to the single H2, O2, CO2 gases.[1
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