Purification of metallurgical silicon by horizontal zone melting

In this study, we aimed at systematically determining the potential of the zone melting (ZM) technique to remove impurities from Metallurgical Grade Silicon (MG- Si) in an Electron Beam Furnace (EBF), using a water-cooled copper crucible. Our focus was on obtaining solar grade silicon, with the purity between Electronic Grade Silicon (EG-Si) and MG-Si, at lower cost than the silicon obtained by the Siemens process. The MG- Si (99.855% purity in mass, or 1,450 ppm of impurities) was processed by 1 and 2 passes of ZM at speed of 1 mm/min and 10 mm/min. The ZM process reduced in 98% the total amount of impurities present in the MG-Si, increasing the purity from 99% to 99.999%, an intermediate stage to achieve the electronic grade (> 99.9999%). Boron remained near the same after the ZM due to its vapor pressure be lower than the pressure of the furnace chamber and due its distribution coefficient in silicon be near the unit. Carbon and oxygen in the MG-Si were reduced from 106 to 35 ppm and from 30 ppm to 5 ppm, respectively, after ZM, and these values are very close to the levels in the electronic grade silicon. The electrical resistivity showed to be dependent on the boron concentration, but not on the phosphorus or the total amount of impurities. All ingots processed by ZM exhibited p-type characteristics, and it means that boron was really the dominant dopant.