Possible Candidates for Impurities in mc-Si Wafers Responsible for Light-Induced Lifetime Degradation and Regeneration

We examine the light-induced carrier lifetime degradation and regeneration at elevated temperature in multicrystalline silicon (mc-Si) wafers of different thicknesses. The experimental results show that the thinner the wafer the less pronounced the degradation is and the faster the regeneration takes place. We interpret this result in the framework of a recently proposed defect model, where the lifetime regeneration is attributed to the diffusion of the recombination-active impurity to the wafer surfaces, where it is permanently trapped. Modeling the measured thickness-dependent lifetime evolutions enables us to determine the diffusion coefficient of the impurity to be in the range (5 +/- 2) x 10(-11) cm(2) s(-1) at a temperature of 75 degrees C. Comparing the diffusion coefficient extracted from our measurements with data published in the literature allows us to exclude most impurities. Despite the large uncertainties in the diffusion coefficient data reported in the literature, reasonable agreement is only obtained for nickel, cobalt, and hydrogen. One important practical implication of our study is that mc-Si wafers thinner than 120 mu m do not suffer from pronounced light-induced lifetime degradation.