Getting and poisoning of silicon wafers by phosphorus diffused layers

The effectiveness of phosphorus gettering, and the possible re-injection of impurities from the gettering layer during subsequent annealing, has been studied through the use of float-zone samples deliberately contaminated with iron. Lifetime measurements reveal that phosphorus gettering, in this case at 880°C, initially removes more than 99% of the iron from the wafer bulk, to levels below 1×1011cm-3. However, upon further annealing at temperatures greater than the gettering temperature, some of the iron is injected back into the wafer. Annealing at 900°C caused a significant amount of this ‘poisoning’, with 7% of the pregettered iron returning to the bulk, resulting in a final Fe concentration around 5×1011cm-3. At 800°C there was no detectable re-injection of Fe within uncertainty. The results may have implications for optimising industrial metallisation fire-through processes for multicrystalline solar cells, which contain relatively high levels of iron and other metal impurities