Subboundary-free zone-melt recrystallization of thin-film silicon

Scanned zone-melt recrystallization (ZMR) of amorphous Si 1-μm films on SiO₂ results in subboundary-free material provided the thermal gradient along the scan is reduced to 4 K/mm or less. Below this value the usual ZMR subboundaries consisting of networks of in-plane edge dislocations are replaced by rows of threading dislocations. We account for the two kinds of crystallization by extending the faceted solidification model that we developed previously. We consider profiles of the solidification front at the intersections of {111} facet pairs where subboundaries are known to form, and postulate that the profiles are aligned approximately normal to the scan in the high gradient case, but become tilted toward the plane of the SiO₂ cap layer for the low gradient case. The tilting accounts in a natural way for subboundary removal and the transition from in-plane to threading dislocations.