Improving the material quality of silicon ingots by aluminum gettering during crystal growth

We present a method for the purification of silicon ingots during the crystallization process that reduces significantly the width of the low charge carrier lifetime region at the ingot top. The back-diffusion of impurities from the ingot top is suppressed by adding a small amount of pure aluminum into the silicon melt right at the end of the solidification. We study the aluminum gettering effect by instrumental neutron activation analysis (INAA) and Fei imaging. Furthermore, we present a model for aluminum gettering of Fe in the silicon ingot that is in agreement with literature data for aluminum gettering at lower temperature. The distribution of iron in the ingots with and without aluminum is fairly well predicted by a combination of this model with a model for Fe contamination from the crucible system. A simulation with varying Al content exhibits further potential for an increased yield of silicon wafers with high charge carrier lifetime.