The chemical reaction near the crucible wall during directional solidification of Si crystals for solar cells has been investigated. Fragments of the crucible that were used for the crystal growth of a Si ingot were examined. As results, we found that a chemical reaction took place at the coating/crucible interface and that silicon oxynitride particles precipitated near the crucible wall. The oxynitride precipitates were determined as stoichiometric Si2N2O and were revealed not to be amorphous but of orthorhombic crystal symmetry. We show crucial evidence of the formation of stoichiometric Si2N2O microcrystalline precipitates inside the Si crystal.

We investigated oxygen precipitates and grain boundaries (GBs) in multicrystalline silicon for solar cells. We observed the distribution of GBs on both sides of the specimen by using an electron backscattering pattern to be able to compare it with the distribution of oxygen precipitates revealed by infrared absorption spectroscopy. We precisely examined the relationship between oxygen precipitates and various GBs, which are coincidence site lattice GBs ( ) and random GBs, and found that the distribution of oxygen precipitates coincided with that of random GBs. Furthermore, we annealed the specimen in which the oxygen precipitates already existed around the random GBs, and found that the dispersion and coalescence of the precipitates took place.

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Electrically active light-element complexes called thermal donors and shallow thermal donors in silicon crystals grown by the cast method were studied by low-temperature far-infrared absorption spectroscopy. The relationship between these complexes and either crystal defects or light-element impurities was investigated by comparing different types of silicon crystals, that is, conventional cast-grown multicrystalline Si, seed-cast monolike-Si, and Czochralski-grown Si. The dependence of thermal and the shallow thermal donors on the light-element impurity concentration and their annealing behaviors were examined to compare the crystals. It was found that crystal defects such as dislocations and grain boundaries did not affect the formation of thermal or shallow thermal donors. The formation of these complexes was dominantly affected by the concentration of light-element impurities, O and C, independent of the existence of crystal defects.