Phosphorus removal from upgraded metallurgical-grade silicon by vacuum directional solidification

The removal of phosphorus impurities is among the most important problems in the purification of upgraded metallurgical-grade silicon (UMG-Si) used in the preparation of solar-grade silicon (SOG-Si). Vacuum refining is considered relatively effective for removing phosphorus from molten silicon. Based on the saturated vapor pressure of phosphorus calculated, the removal of phosphorus from UMG-Si at 1823K was performed via an industrial-scale vacuum directional solidification (VDS) furnace in this paper. Amathematical model for phosphorus removal during VDS was proposed to predict the distribution of phosphorus in silicon ingots. An industrial-scale experiment was carried out to confirm the mathematical model. The results demonstrated that phosphorus could be removed effectively main by vacuum evaporation during the VDS process, and thephosphorus concentration was about 4.63-13.41 ppm.The relationship between the phosphorus concentration and the resistivity of the silicon is discussed by experimental and calculated findings. The experimental results show that the resistivity decreases from 0.354 Ω cm to 0.259 Ω cm with increased phosphorus concentration.