Interaction of Iron with Extended Defects in Multicrystalline Silicon Studied by Local Gettering

In order to adjust temperature treatments during solar cell processing to multicrystalline silicon, the interactions of metal impurities in multicrystalline silicon with extended defects like grain boundaries have to be understood. For this purpose, we investigate neighboring wafers of block-cast multicrystalline silicon intentionally doped with iron during crystal growth. Samples are investigated with LBIC after aluminum gettering (AlG) experiments with an Al layer only on parts of the sample at different temperatures. LBIC data are quantitatively analyzed using two- dimensional gettering simulations and pattern recognition techniques combined with multivariate statistics. LBIC- images and simulations show pronounced regions of reduced excess carrier recombination around grain boundaries (denuded zone) in uncovered regions. This can be attributed to impurity accumulation at grain boundaries and their related depletion in the adjacent bulk. In covered regions, no denuded zones are observed. Temperature variation provides evidence that our experiments are mainly in the regime where gettering efficiency is limited by impurity precipitate dissolution, unless the temperature is chosen well above the solubility temperature corresponding to the average iron concentration.