Lifetime and DLTS studies of interstitial Fe in p-type Si

Fe is one of the most prominent metallic impurities in solar-grade Si. In this work we have investigated the kinetics of in-diffusion and formation of the interstitial fraction (Fe-i). P-type Cz-Si with a resistivity of 10 Omega-cm has been intentionally contaminated with Fe by in-diffusion from a surface layer of FeCl3 at 700 degrees C followed by cooling with a rate of similar to 3.3 K/s. The concentration of Fe-i has been measured both by microwave photo conductance decay (mu-PCD) and deep level transient spectroscopy (DLTS). In the mu-PCD measurements, the Fe-i concentration has been determined using the effect of light-induced splitting of the iron-boron pairs (FeB), while in the DLTS measurements Fe-i has been monitored by the donor electronic state at 0.43 eV above the valence band. We have observed a linear dependence between the minority carrier lifetime (tau) and the inverse Fe-i concentration. This confirms Fe-i as the dominating recombination centre. In the present investigations we use a material relevant for solar cells with a resistivity of 10 Omega-cm. We have found that the concentration of interstitial iron decreases with increasing time for in-diffusion of Fe, provided identical cooling condition. This decreasing concentration of Fe-i is believed to be due to formation of more iron precipitates that serve as sinks for fast diffusing Fe-i. A high temperature anneal at 1000 degrees C for 1 minute followed by fast cooling (similar to 33 K/s) results in dissolution of the precipitates and freezing Fe into interstitial positions, where the concentration of Fe-i increases with increasing in-diffusion time. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim