Investigating Internal Gettering of Iron at Grain Boundaries in Multicrystalline Silicon via Photoluminescence Imaging

In this paper, we present measurements and modeling of the reduction in dissolved iron Fe; concentrations near grain boundaries in multicrystalline silicon (mc-Si) wafers. The measurements of the interstitial Fe concentrations are obtained via photoluminescence images taken before and after iron-boron pair dissociation. A simple diffusion-capture model was developed to characterize the removal of interstitial Fe by the gettering sites. The model is based on a numerical solution to the 1-D diffusion equation with two fitting parameters: the diffusion length of dissolved Fe atoms and the effective gettering velocity at the gettering site. By comparing the simulation with a controlled phosphorous gettering process, the model is shown to give good estimation of the diffusion length of Fe atoms. For as-cut multicrystalline silicon wafers from different parts of the ingot, that is, wafers with different average dissolved Fe concentrations [Fe-i], the diffusion lengths of Fe atoms are found to decrease with decreasing average [Fe-i]. This suggests the presence of relaxation precipitation during the internal gettering of dissolved Fe by the grain boundaries in mc-Si during ingot cooling.