Permanent annihilation of thermally activated defects which limit the lifetime of float-zone silicon

We have observed very large changes in the minority carrier lifetime when high purity float-zone (FZ) silicon wafers are subject to heat-treatments in the range of 200-1100 degrees C. Recombination centres were found to become activated upon annealing at 450-700 degrees C, causing significant reductions in the bulk lifetime, detrimental for high efficiency solar cells and stable high powered devices. Photoluminescence imaging of wafers annealed at 500 degrees C revealed concentric circular patterns, with lower lifetimes occurring in the centre, and higher lifetimes around the periphery. Deep level transient spectroscopy measurements on samples extracted from the centre of an n-type FZ silicon wafer annealed at 500 degrees C revealed a large variety of defects with activation energies ranging between 0.16-0.36 eV. Our measurements indicate that vacancy related defects are causing the severe degradation in lifetime when FZ wafers are annealed at 450-700 degrees C. Upon annealing FZ silicon at temperatures >800 degrees C, the lifetime is completely recovered, whereby the defect-rich regions vanish and do not reappear (permanently annihilated). Our results indicate that, in general, as-grown FZ silicon should not be assumed to be defect lean, nor can it be assumed that the bulk lifetime will remain stable during thermal processing, unless annealed at temperatures >1000 degrees C. (C) 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim