Assessing the role of iron-acceptor pairs in solar grade multicrystalline silicon wafers from the metallurgical route

The recombination parameters of iron-boron (FeB), iron-aluminium (FeAl) and iron-gallium (FeGa) pairs in Fe implanted FZ monocrystalline silicon wafers and Fe contaminated multicrystalline silicon (mc-Si) wafers of solar grade feedstock from the metallurgical route have been studied by combining the Deep Level Transient Spectroscopy (DLTS) and Microwave Photoconductive Decay (µ-PCD) techniques. Energy levels associated with FeB, FeAl and FeGa pairs were detected in the monocrystalline samples. The activation energy and capture cross section of these levels were determined. FeGa gave the strongest recombination effect, reducing the minority carrier lifetime of the sample from about 39 µs to 0.7 µs at a concentration of 4x1013 cm-3. No electrically active iron-acceptor pairs could be detected in the mc-Si wafer. However, it was demonstrated that micrometer-sized clusters, most likely composed of metallic oxides, collect iron from the bulk. This iron collection may reduce the available amount of iron for creating electrically active iron-acceptor pairs below the detection limit of DLTS. The contamination did, however, degrade the lifetime from 40 µs to less than 1 µs in the wafer. This is likely a result of at least three overlapping energy levels believed to be related to iron (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)