Effects of Solidification Rate and Settling Time of SiC Particles on the Macrosegregation of Carbon in Silicon Ingots

The macrosegregation of carbon in silicon was studied aiming for silicon refining for photovoltaic applications. Experiments of directional solidification upward and of settling of SiC particles in the melt were carried out in a Bridgman furnace using electronic grade silicon intentionally contaminated with carbon. Mold pulling velocities in the range between 5 and 80 μm/s and settling time periods of 1 and 6 hours were adopted in the experiments. In the resulting cylindrical ingots, the macrosegregation of carbon along the axial direction was measured and their macro and microstructures revealed and examined. In all directional solidification experiments, the carbon concentration increases to the ingot top as result of an accumulation of SiC particles in this region. For the lower velocities (<40 μm/s), a larger particle accumulation and columnar grains aligned axially and pointing to the ingot top are seen, which is consistent with the pushing of SiC particles by a planar solid–liquid interface. For the settling experiments, the carbon concentration increases from the ingot top to the bottom as a result of the accumulation of SiC particles at the bottom. This accumulation is probably caused by the settling of SiC particles, but an increase in the settling time from 1 to 6 hours have no significant effect in the macrosegregation profile. Finally, in the conditions of the present experiments, the directional solidification of silicon at mold pulling velocities lower than 40 μm/s is more effective to remove carbon from silicon than the settling of SiC particles during times up to 6 hours.