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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.

International audience; To control the final grain structure and the density of structural crystalline defects in silicon (Si) ingots is still a main issue for Si used in photovoltaic solar cells. It concerns both innovative and conventional fabrication processes. Due to the dynamic essence of the phenomena and to the coupling of mechanisms at different scales, the post-mortem study of the solidified ingots gives limited results. In the past years, we developed an original system named GaTSBI for Growth at high Temperature observed by Synchrotron Beam Imaging, to investigate in situ the mechanisms involved during solidification. X-ray radiography and X-ray Bragg diffraction imaging (topography) are combined and implemented together with the running of a high temperature (up to 2073 K) solidification furnace. The experiments are conducted at the European Synchrotron Radiation Facility (ESRF). Both imaging techniques provide in situ and real time information during growth on the morphology and kinetics of the solid/liquid (S/L) interface, as well as on the deformation of the crystal structure and on the dynamics of structural defects including dislocations. Essential features of twinning, grain nucleation, competition, strain building, and dislocations during Si solidification are characterized and allow a deeper understanding of the fundamental mechanisms of its growth.