Investigation of subgrains in directionally solidified cast mono-seeded silicon and their interactions with twin boundaries

International audienceDirectional solidification of a cast mono silicon seed and of a float-zone (FZ) silicon seed was performed and thegrain and defect structures of the seeds as well as of the regrown parts are analyzed. In situ X-ray diffractionimaging 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 mainlyimmobile cellular structure. Ex situ grain orientation mappings reveal the presence of subgrains with misorientationsup to 3◦ in the regrown part of the cast mono-seeded sample, which are not observed in the regrownpart of the FZ-seeded sample. Subgrain boundaries characterized by misorientations around the [001] growthaxis propagate roughly along the growth axis and increase their misorientation by merging with new subgrainboundaries appearing in their vicinity. Although the first inception of subgrain formation cannot be revealed, thecomparison of the dislocation arrangements in the two seeds strongly suggests an influence of the latter onsubgrain formation. In the regrown part, interactions between subgrain boundaries and twin boundaries showthat 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 thetwin. It demonstrates that the grain orientation relationship and not only the grain boundary character play animportant role in the subgrain structure evolution and redistribution in a multicrystalline silicon ingot