The behaviors of carbon, nitrogen, and oxygen in multicrystalline silicon grown by the cast method were investigated using infrared absorption spectroscopy. A microscopic distribution map of the impurities and their precipitates was obtained with a spatial resolution of a few tens of µm. The distribution and bonding states depended on the impurity: nitrogen and oxygen precipitated as crystalline nitride (α-Si3N4) and amorphous oxide (SiO2), respectively, while carbon distributed homogeneously as isolated substitutional atoms. The nitrogen and oxygen precipitates remained independent, and thus, they might not have formed a compound oxynitride like Si2N2O.

Multicrystalline silicon grown by the cast method and used in photovoltaic cells includes nitrogen from the Si3N4 coating of the SiO2 crucible. We investigated the distribution of nitrogen, existing as NN or NNO complexes in silicon, by Fourier transform infrared spectroscopy. We found that the NN peak intensity was higher at the upper part of the silicon ingot, while the NNO peak intensity was higher at the lower part. There was a complementary relationship between the distribution of NN and that of NNO; this implied that NNO complexes were formed by NN complexes that had combined with interstitial oxygen during crystal growth.