Sequential Purification and Crystal Growth for the Production of Low Cost Silicon Substrates: Quarterly Technical Progress Report No. 5 for the Period January 1 - March 31, 1981

The need for low cost silicon for solar cell applications has stimulated the study of direct purification of metallurgical-grade silicon (MG-Si). The technique of sequential purification of molten silicon was investigated. The sequential purification steps included (1) physical separation of insoluble impurities,(2) reactive gas treatment,(3) slagging and (4) impurity redistribution using ingotpulling. The results have shown that the ingot pulling is the most effective step of the purification process. This has been predicted since the majority of metal impurities in silicon have very low segregation coefficients (from 10 -2 to 10 -6). In theory the ingot pulled from the molten silicon will be two to six orders of magnitude lower in the metal concentration than that in the originalsilicon used for melting. By the same token the pulled ingots can be remelted and repulled. The purity of the silicon can be improved by another 2 to 6 orders of magnitude. The process can be repeated for further purification. The purpose of this paper is to report the purity, structural and electrical characteristics of silicon ingots obtained by various number of crystal pulling.