Evaporation and removal mechanism of phosphorus from the surface of silicon melt during electron beam melting

An experimental investigation into the removal of phosphorus from molten silicon using electron beam melting has been carried out. The time variation of phosphorus content is obtained at the electron beam power of 9, 15, and 21 kW, respectively. The results show that, at a constant power, the content of phosphorus decreases rapidly within the range of approximately 0-900 s after silicon is melted completely, and then tends to level out with further extension of the melting time. The content of phosphorus is decreased from 33.2 x 10(-4) wt.% to 0.07 x 10(-4) wt.% after 1920 s at a power of 21 kW, which achieves the target for solar-grade silicon of less than 0.1 x 10(-4) wt.%. Moreover, the removal reaction of phosphorus by evaporation from the surface of silicon melt during electron beam melting occurs in accordance with the first order kinetics. The mass transfer coefficients in different removal steps are calculated and discussed, which indicate the removal reaction of phosphorus is controlled by both the transport of phosphorus atom from the bulk to the melt free surface and the vaporization from the free melt surface into the gas phase. (c) 2012 Elsevier B.V. All rights reserved.