Limitations to the Impurity Gettering Effect of Multicrystalline Silicon Wafers

In order to investigate the limitations to the gettering efficiency of multicrystalline solar grade silicon from the metallurgical route, wafers based on different feedstock of this type of materials were selected. The composition of metallic impurities and structural defects varied between the types of feedstock. Several gettering procedures were tested in order to find the maximum gettering potential. Gettering in a POCl3 gas deposition process provided the most promising result, with an increase in the minority carrier lifetime from 12.8 μs to 72.3 μs in wafers containing a low density of dislocations. In wafers with a high density of dislocation clusters the lifetime improvement factor was severely degraded, although the initial lifetime was much higher (~40 μs). Internal gettering of such wafers lead to a decrease in the overall lifetime. The dislocation clusters were most likely the main reason for the degradation of the gettering efficiency. In addition, a higher concentration of grain boundaries that were non-ideal for metallic collection could further reduce the gettering potential.