An investigation was made of the kinetics of in-diffusion and the formation of the interstitial fraction (Fei) of iron. The p-type Cz-Si with a resistivity of 10Ωcm was . intentionally contaminated with Fe by in-diffusion from a surface layer of FeCl3 at 700C followed by cooling at a rate of ∼3.3K/s. The concentration of Fei was measured both by microwave photo conductance decay and deep level transient spectroscopy. In the microwave photo conductance decay measurements, the Fei concentration was determined using the effect of light-induced splitting of the iron-boron pairs (FeB), while in the deep level transient spectroscopy measurements Fei was monitored by the donor electronic state at 0.43eV above the valence band. A linear dependence was observed between the minority carrier lifetime (τ) and the inverse Fei concentration. This confirmed that Fei was the predominant recombination centre. In the present investigations a material relevant for solar cells was used, with a resistivity of 10Ωcm. It was found that the concentration of interstitial iron decreased with increasing time for in-diffusion of Fe, given identical cooling condition. This decreasing concentration of Fei was believed to be due to the formation of more iron precipitates that served as sinks for fast diffusing Fei. A high-temperature anneal at 1000C for 60s, followed by fast cooling (∼33K/s), resulted in dissolution of the precipitates and the freezing of Fe into interstitial positions, where the concentration of Fei increased with increasing in-diffusion time.

Lifetime and DLTS Studies of Interstitial Fe in p-type Si. M.Syre, E.Monakov, A.Holt, B.G.Svensson: Physica Status Solidi C, 2011, 8[3], 721–4