Macrosegregation of Impurities in Directionally Solidified Silicon

Directional solidification of molten metallurgical-grade Si was carried out in a vertical Bridgman furnace. The effects of changing the mold velocity from 5 to 110 μm seconds^–1 on the macrosegregation of impurities during solidification were investigated. The macrostructures of the cylindrical Si ingots obtained in the experiments consist mostly of columnar grains parallel to the ingot axis. Because neither cells nor dendrites can be observed on ingot samples, the absence of precipitated particles and the fulfillment of the constitutional supercooling criterion suggest a planar solid–liquid interface for mold velocities ≤10 μm seconds^–1. Concentration profiles of several impurities were measured along the ingots, showing that their bottom and middle are purer than the metallurgical Si from which they solidified. At the ingot top, however, impurities accumulated, indicating the typical normal macrosegregation. When the mold velocity decreases, the macrosegregation and ingot purity increase, changing abruptly for a velocity variation from 20 to 10 μm seconds^–1. A mathematical model of solute transport during solidification shows that, for mold velocities ≥20 μm seconds^–1, macrosegregation is caused mainly by diffusion in a stagnant liquid layer assumed at the solid–liquid interface, whereas for lower velocities, macrosegregation increases as a result of more intense convective solute transport.