Precipitation and hydrogenation of iron in multicrystalline silicon

Abstract

This thesis studies the precipitation and the hydrogenation of dissolved iron in multicrystalline silicon (mc-Si). Photoluminescence imaging is used to obtain both the concentration and the spatial distribution of interstitial iron across mc-Si wafers, with an image resolution of 25 micrometres. In the first part, as-cut mc-Si wafers were examined in order to better understand the internal gettering of iron that occurs during the mc-Si ingot directional solidification process. A simple one-dimensional diffusion-capture model was developed to quantify the gettering process. Relaxation gettering by the grain boundaries was found to be present during ingot cooling, and results indicate that iron must have been super-saturated before the onset of precipitation. A systematic study of the precipitation kinetics of iron in mc-Si was then performed, with respect to the annealing temperature, time, iron super-saturation level, and different types and densities of precipitation sites in mc-Si. Annealing temperatures in the range of 400 - 700C (Celsius degree) were examined in detail. The precipitation of iron approximately follows an exponential decay, which is consistent with the classical Ham's precipitation model. However, the precipitation kinetics were found to demonstrate an increasing dependence on the level of the initial iron super-saturation as the super-saturation level decreases. A higher level of initial iron super-saturation was shown to result in a faster precipitation process. The degree of super-saturation only becomes irrelevant for a high level well above 1000. The dependence of the precipitation kinetics on the initial super-saturation level is likely related to the chemical energy required to initiate the precipitation process. Precipitation of iron was found to occur both at some of the grain boundaries and also at the intra-grain dislocations. The precipitation of iron, at temperatures where the dissolved iron is super-saturated, results in a significant reduction of the interstitial iron concentration by 1 - 2 orders of magnitude. However, the process is inefficient compared to the external gettering of iron via phosphorous diffusion. More importantly, annealing at elevated temperatures leads to a degradation of the mc-Si material quality, which offsets the benefit of the reduced dissolved iron concentration. The last section investigates the impact of hydrogen incorporation on the changes in the interstitial iron concentration and distribution in mc-Si wafers. Hydrogen was introduced into the silicon bulk by annealing wafers with plasma-enhanced chemical vapour deposited silicon nitride films, at 400 - 900C and for minutes to hours. Effective hydrogenation of the interstitial iron was observed. The concentration of interstitial iron was shown to reduce by more than 90% after a 30-minute anneal at 600 - 900C with silicon nitride films. The most effective hydrogenation of iron was found to take place at 700C, where more than 99% of iron was hydrogenated after 30 minutes. Results indicate that the observed reduction in interstitial iron concentration is not caused by an enhanced internal gettering of iron, as some authors suggested. The hydrogenation process is conjectured to be the pairing of positively charged iron with negatively charged hydrogen, forming less recombination active Fe-H complexes in silicon. Show more