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 50Ω/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.
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