Structure, chemistry and electrical properties of extended defects in crystalline silicon for photovoltaics

The electronic properties of present-day multicrystalline silicon (mc-Si) materials for photovoltaic applications are strongly influenced by point defects, their mutual interaction and their interaction with dislocations and grain boundaries. This paper presents results from fundamental investigations of metal impurity interaction with extended defects, namely a small-angle grain boundary and bulk microdefects. It is shown that the distribution of copper suicide precipitates closely follows the density of bulk microdefects indicating the underlying physics of 'good' and 'bad' grains frequently observed in mc-Si. Co-precipitation of copper and nickel in the same samples leads to virtually the same distribution of multi-metal silicide precipitates which according to light-beam induced current measurements show the same recombination activity as single-metal suicide particles. Transmission electron microscopy is used to show that for copper-rich and nickel-rich conditions two types of silicides co-exist, i.e. Cu3Si precipitates containing a small amount of nickel and NiSi2 precipitates containing some copper. Finally, phosphorus-diffusion gettering (PDG) is discussed as the main gettering process used in present-day silicon photovoltaics. Special emphasis is put on the effect of extended defects and their interaction with metal impurities on PDG kinetics. It is shown that different limiting processes will be simultaneously operative in mc-Si as a result of inhomogeneous bulk defect distributions. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim