Interstitial iron concentrations across multicrystalline silicon wafers via photoluminescence imaging

We present high-resolution images of the lateral distribution of interstitial iron across wafers from various positions along the length of a directionally solidified multicrystalline silicon ingot. Iron images were taken on wafers in the as-cut state and also after two different phosphorus gettering steps performed at 845°C for 30 min, one with an additional anneal at 600°C for 5 h (referred to as extended gettering). The iron images were obtained by taking calibrated photoluminescence (PL) images of the low injection carrier lifetimes, before and after dissociation of iron-boron pairs via strong illumination. The iron images clearly reveal the internal gettering of iron during ingot cooling to grain boundaries and dislocation clusters, resulting in much lower dissolved iron concentrations at those features. In contrast, the PL images of gettered wafers exhibit a reversed distribution of dissolved iron compared to the as-cut wafers, in other words, with higher interstitial iron concentrations at the grain boundaries than within the grains, most probably owing to the precipitated iron at the grain boundaries partly dissolving during the high-temperature gettering process. Phosphorus gettering was found to result in a significant reduction of interstitial iron both inside the grains and at grain boundaries. The extended gettering resulted in a further significant reduction in all parts of the wafers and along all sections of the ingot.