Investigation of subgrains in directionally solidified cast mono-seeded silicon and their interactions with twin boundaries
Résumé
Directional solidification of a cast mono silicon seed and of a float-zone (FZ) silicon seed was performed and the
grain and defect structures of the seeds as well as of the regrown parts are analyzed. In situ X-ray diffraction
imaging enabled the observation of the dislocation arrangements. During the heating process, in the FZ seed,
mobile dislocations glide on {111} planes, whereas in the cast mono seed dislocations are arranged in a mainly
immobile cellular structure. Ex situ grain orientation mappings reveal the presence of subgrains with misorientations
up to 3◦ in the regrown part of the cast mono-seeded sample, which are not observed in the regrown
part of the FZ-seeded sample. Subgrain boundaries characterized by misorientations around the [001] growth
axis propagate roughly along the growth axis and increase their misorientation by merging with new subgrain
boundaries appearing in their vicinity. Although the first inception of subgrain formation cannot be revealed, the
comparison of the dislocation arrangements in the two seeds strongly suggests an influence of the latter on
subgrain formation. In the regrown part, interactions between subgrain boundaries and twin boundaries show
that they can follow Σ3{111} and Σ9{221} grain boundaries or cross Σ3{111} grain boundaries. Whether Σ3
{111} GBs are crossed or not depends among other things on the orientation of the grains on either side of the
twin. It demonstrates that the grain orientation relationship and not only the grain boundary character play an
important role in the subgrain structure evolution and redistribution in a multicrystalline silicon ingot.
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