Thermal activation and deactivation of grown-in defects limiting the lifetime of float-zone silicon

By studying the minority carrier lifetime in recently manufactured commercially available n- and p-type float-zone (FZ) silicon from five leading suppliers, we observe a very large reduction in the bulk lifetime when FZ silicon is heat-treated in the range 450-700 degrees C. Photoluminescence imaging of these samples at the wafer scale revealed concentric circular patterns, with higher recombination occurring in the centre, and far less around the periphery. Deep level transient spectroscopy measurements indicate the presence of recombination active defects, including a dominant center with an energy level at similar to E-v + 0.5 eV. Upon annealing FZ silicon at temperatures > 1000 degrees C in oxygen, the lifetime is completely recovered, whereby the defects vanish and do not reappear upon subsequent annealing at 500 degrees C. We conclude that the heat-treatments at >1000 degrees C result in total annihilation of the recombination active defects. Without such high temperature treatments, the minority carrier lifetime in FZ silicon is unstable and will affect the development of high efficiency (>24%) solar cells and surface passivation studies.