The effect of grain boundaries on instability at the crystal/melt interface during the unidirectional growth of Si

The instability of crystal/melt interfaces including numerous Σ3, small-angle and large-angle grain boundaries during the unidirectional growth of Si was investigated, using an in situ observation system. Planar crystal/melt interfaces including Σ3 or small-angle grain boundaries were found to undergo local morphological transformation at the grain-boundary positions during high-velocity growth. The data also confirmed that the critical growth velocity for the appearance of instability at a crystal/melt interface including grain boundaries is significantly lower than that for a single crystal Si/melt interface. The ambient temperature field at the crystal/melt interface was calculated, taking into account the effective thermal conductivities of grain boundaries and the results show that a negative temperature gradient readily forms in the case that grain boundaries are present.