Recombination lifetime and trap density variations in multicrystalline silicon wafers through the block

We have studied the variations in recombination lifetime and density of non-recombinative traps between wafers through the entire height of a multicrystalline silicon block. We find that the low lifetime regions in the very bottom and top part of an ingot are described by a relatively high density of non-recombinative traps. It is argued that in the bottom region in-diffused impurities from the crucible, as well as grain boundaries and dislocations, could be the major trap contributors. In the upper region, trapping is believed to be due to high concentrations of metallic impurities, caused by segregation during growth and solid-sate diffusion during cooling of the ingot. Moreover, we have investigated the gettering response of wafers from different positions within the block from a 50 Omega/sq phosphorus emitter diffusion. The results show that the most notable lifetime enhancements occur in regions of the block where the trap density is high, indicating the relevance of mobile impurities as non-recombinative trapping centers.