Response to Phosphorus Gettering of Different Regions of Cast Multicrystalline Silicon Ingots

Minority carrier lifetimes were measured to determine the effect of phosphorus gettering on cast multicrystalline silicon substrates from central and end regions of two different ingots. One ingot exhibited visibly inferior crystallographic structure, and consistently showed lower lifetimes. For the low quality ingot, wafers from the bottom region did not respond to gettering, whilst those from the top experienced a noticeable lifetime increase. The standard ingot improved markedly at the bottom, but not at all at the top. However, novel crosscontamination experiments showed that all top and bottom wafers contained high concentrations of mobile impurities in comparison to central regions. Defect etching revealed that the top and bottom wafers whose lifetime did not increase with gettering had very high dislocation densities, whereas those with low dislocation densities improved markedly after gettering. For the case of the highly dislocated samples, a sufficient number of recombination centres remain to prevent an increase in the lifetime, even after the mobile impurities have been extracted through gettering. Wafers from central regions had moderate dislocation densities and low concentrations of out-diffusible impurities. Their lifetimes increased after gettering up to a more evenly distributed limit imposed by the crystallography. The value of this crystallographic limit was found to be ingot dependent.