Boron Removal from Silicon by Humidified Gases

Abstract

Boron (B) is one of the most problematic impurities to remove from metallurgical grade silicon in the production of more pure solar grade silicon (SoG-Si). In the present work, recent progresses in the application of reactive gases for B removal from molten silicon is reviewed. Moreover, in order to clarify the mechanisms and kinetics of gas-based B-refining, an experimental procedure using humidified Ar, N2, and H2 gases applied to boron-doped silicon melt is described. It is shown that the kinetics and extent of B removal is depending on the type of humidified gas. The thermodynamics and kinetics of B removal from molten silicon are studied to explain experimental observations. The mass transfer coefficients of B are calculated and possible mechanisms for B removal by the reactive gases are proposed: $$ \begin{aligned} 1/2{\text{H}}_{2} ({\text{g}}) &= \underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle-}$}}{{\text{H} ,}} \hfill \\ \underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle-}$}}{\text{B}} + \underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle-}$}}{\text{H}} + {\text{H}}_{2} {\text{O(g)}} &= {\text{HBO(g)}} + {\text{H}}_{2} . \hfill \\ \end{aligned} $$ It is shown that the lower equilibrium partial pressure of HBO gas at higher temperatures causes slower B removal rate.