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, N 2 , and H 2 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:It is shown that the lower equilibrium partial pressure of HBO gas at higher temperatures causes slower B removal rate.

Mechanisms and Kinetics of Boron Removal from Silicon by Humidified Hydrogen

“…Moreover, the rate of B removal is higher using the binary H 2 -H 2 O gas mixtures than other types of gas mixtures, such as humidified argon and nitrogen [12] , as well as the Ar-H 2 -H 2 O gas mixtures [4] . Moreover, the rate of B removal is increased with increasing H 2 O content of the gas mixture [16] , as long as a stable passive oxide layer over the melt surface does not formed through silicon oxidation to SiO 2 [12] .…”

“…The removal of boron by different reactive gases, such as O 2 [2,3] , H 2 [4,5] , CO 2 [3] , and H 2 O [6,7,8,9,10,11,12] , has been studied through several research groups. It has been observed that the application of the H 2 -O 2 gas mixtures in plasma refining is more effective in terms of the kinetics of B removal compared to the single H 2 , O 2 , CO 2 gases [13,14] .…”

“…It has been observed that the application of the H 2 -O 2 gas mixtures in plasma refining is more effective in terms of the kinetics of B removal compared to the single H 2 , O 2 , CO 2 gases [13,14] . Recent experimental studies on B removal by H 2 O-containing gases in top gas blowing technique has shown the possibility of B removal by this technique to extreme low levels, below 1 ppmw [4,12,15] . The presence of hydrogen in the refining gas causes the formation of H-B-O volatile species such as the HBO gaseous species on the melt surface, which are more volatile than the H-B and B-O species [12] .…”

“…Recent experimental studies on B removal by H 2 O-containing gases in top gas blowing technique has shown the possibility of B removal by this technique to extreme low levels, below 1 ppmw [4,12,15] . The presence of hydrogen in the refining gas causes the formation of H-B-O volatile species such as the HBO gaseous species on the melt surface, which are more volatile than the H-B and B-O species [12] . The effects of process parameters such as gas composition, gas flow rate, lance parameters and temperature on the B removal kinetics has been studied recently.…”

“…The effects of process parameters such as gas composition, gas flow rate, lance parameters and temperature on the B removal kinetics has been studied recently. It has been observed that the rate of B removal is decreased with increasing temperature [4,12,15] .…”