We investigated twin boundary formation during directional solidification of the compound semiconductor InSb. We directly observed a groove formed at the junction of a solid–liquid interface and a Σ9 grain-boundary by optical microscopy. When the depth of the grain-boundary groove reached about 100 μm, rapid growth occurred to fill the groove. We found that two facet planes of the grain-boundary groove became twin boundary planes. Calculations of the change in free energy associated with nucleation at the grain-boundary groove indicates that the preferential twin nucleation occurs at the bottom of the groove, consistent with the direct observations in this study.
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