DECARBURIZATION OF SILICON MELT FOR SOLAR-CELLS BY FILTRATION AND OXIDATION

In studies on the decarburization of liquid silicon, two types of carbon were identified in a silicon melt containing more carbon than the solubility. One was in the form of silicon carbide particles and the other was in the form of dissolved carbon. The silicon was filtered to remove the carbide particles. Using silica or silicon carbide as the material for the filter, the carbon content was reduced from 100 to 30 to 60 ppmw. Oxidation was done under reduced pressure to eliminate the rest of the carbon as carbon monoxide, and the final carbon content was as low as 10 ppmw. Silica was chosen as oxidizing reagent. To avoid a reverse reaction, Ar was introduced on the surface to expel extraneous CO or CO2 gas evolved. The thermodynamics of the oxidation of carbon in silicon melts is also discussed.