Removal of Boron from Silicon by Moist Hydrogen Gas

Abstract

New and cheaper refining methods for production of metallurgical silicon are needed to meet the increasing demands for low-cost, high-quality silicon for the solar cell industry. One promising refining method for boron is moist hydrogen treatment. In this work, an evaporation unit has been used to produce wet hydrogen gas, which subsequently has been sparged on top of silicon melts. The effect of temperature and gas composition on boron removal has been studied. The main results show that boron is removed from liquid silicon and the removal rate is controlled by chemical reaction depending on p H 2 O and p H 2 . Water vapor treatment of molten silicon can alone remove boron. However, in combination with hydrogen gas, the removal rate is significantly increased. In addition, the rate of boron removal in silicon has been found to decease with increasing temperature.

Boron Removal from Silicon by Humidified Gases

“…Hence, increasing the refining temperature has to increase the kinetics of B removal, if Reaction [2] is the controlling step of B removal. However, the results of this work and the previous works [8,15] show that the rate of B removal is decreased with increasing temperature and Reaction [2] cannot explain the effect of temperature. If we assume that Reaction [6] is the main reaction for B removal, and that H 2 does not play a role in the removal kinetics, it is difficult to explain the difference in B removal rate with gas composition change in experiments 1, 2, and 3 where the steam concentration in the reactive gas phase is the same, while the carrier gas different (Ar, N 2 , H 2 , respectively).…”

“…If we assume that Reaction [6] is the main reaction for B removal, and that H 2 does not play a role in the removal kinetics, it is difficult to explain the difference in B removal rate with gas composition change in experiments 1, 2, and 3 where the steam concentration in the reactive gas phase is the same, while the carrier gas different (Ar, N 2 , H 2 , respectively). According to the present results and the previous work at NTNU, [8] the B removal rate is higher than any other type of gas when H 2 -H 2 O mixtures are used. Therefore, we may conclude that hydrogen plays a key role in the reaction mechanism as observed in literature as well.…”

“…Lower rate constants have been reported from the non-plasma-aided blowing of H 2 -H 2 O gas mixtures (1.0 9 10 À6 to 1.3 9 10 À5 m/s). [7,8] The highest rate constant for such conditions has been reported for H2-3.2 pctH 2 O at 1723 K (1450°C). [8] In the present study, the gas-based removal of B from molten silicon is studied experimentally.…”

“…The removal of B from silicon by gas treatment has been studied by several researchers using plasma technique or gas blowing. The application of H 2 O gas in the reactive gas in plasma refining [1][2][3][4][5][6] and H 2 -H 2 O gas mixtures through non-plasma-aided gas blowing [7,8] has shown faster B removal compared to the application of pure H 2 , [8][9][10] O 2 , [9,11] and CO 2 [11] as reactive gases. The plasma refining experiments with H 2 -O 2 mixtures [12,13] as the reactive gas have also indicated faster B removal from silicon than using H 2 , O 2 , CO 2 gases.…”

“…[7,8] The highest rate constant for such conditions has been reported for H2-3.2 pctH 2 O at 1723 K (1450°C). [8] In the present study, the gas-based removal of B from molten silicon is studied experimentally. The obtained results are discussed based on thermodynamic and kinetic considerations of the refining system.…”