Purification of Silicon by Directional Solidification

Abstract

Purification of silicon by directional solidification under the solidification rate from 5.9×10−6 to 2.2×10−5 m/s was applied for removing aluminium, iron and titanium. Aluminium content was decrease of up to the range of two to three orders from about 100 mass ppm. Iron and titanium contents were decreased under 0.1 mass ppm from about 300 and 20 mass ppm, respectively. Aluminium content profile in solidified silicon could be expressed by the Sceil’s equation and Burton’s equation. Iron and titanium content profiles could not be confirmed by those equations, because detective values limit for iron and titanium were higher than the calculated content.High initial impurity contents lowered efficiency for removing impurity contents in silicon. This was assumed that the stability of planar solid/liquid interface was broken by a low temperature gradient of the liquid phase according to the constitutional supercooling theory.SOG-Si which was experimentally produced through the direct reduction process and the molten purification process showed low impurities contents of less than 0.1 mass ppm of aluminium, iron and titanium. Conversion efficiency of the multicrystalline cell made by those silicon had almost the same performance as made from electrical-grade silicon.