Purification of Multicrystalline Silicon by Cold Crucible Directional Solidification and Impurity Distribution

Abstract

In order to get solar grade silicon, large cold crucible has been used in an induction heat furnace. By controlling the relative location of the crucible and coils, directional solidification was realized. More than 200 kg multi-crystalline silicon ingot was produced in a batch with short work time. The removal rate of most metal impurities was high, typically higher than 99% for transition metals like iron. Non-metallic elements such as boron and phosphorus could not be removed efficiently because of larger equilibrium segregation coefficient. The concentration of phosphorus was one third of the feedstock due to the vaporization in the melting process. The distribution of impurities agreed with the solidification principle. Quartzes and carbon was not used, which ensured silicon prevent from the contamination. Cooperated with other methods, large scale of solar grade silicon was produced.