Effect of calcium addition on the silicon purification in the presence of low concentration of iron

Abstract

In this research, the efficiency of Si-Ca system for B and P removal from the silicon was initially evaluated during furnace cooling. In the second step, 5 to 25 wt% Ca was added to Si-5 wt.% Fe system and the removal of boron and phosphorus from the solidified silicon were studied. Results showed that the removal of B and P increased by adding more Ca. For 25 wt% Ca, ∼89% of P alongside ∼48% B were removed. These removal rates were about 75% for P and 30% for B better than those that could be obtained by directional solidification of silicon. P removal was not increased to more than 88–90% by increasing the phosphorus concentration of the initial mixture. However, higher boron concentration of the initial mixture caused higher B removal as a result of a new Ca-B compound formation. Presence of about 5 wt% Fe was slightly effective in gathering calcium and decreased its content in the purified silicon. Furthermore, a slight improvement in boron removal just in samples with lower amounts of calcium was observed. Among the samples in this study, the least concentration of iron in the purified silicon was obtained when the overall composition of the initial mixture was nearest the ternary eutectic point of the Si-Fe-Ca system.