Dissociation of screw dislocations in (001) low-angle twist boundaries:: a source of the 30° partial dislocations in silicon

The first experimental evidence for dissociation of grain boundary screw dislocations is presented for (001) low-angle twist boundaries in silicon. Using a combination of high-resolution electron microscopy and the weak-beam technique of transmission electron microscopy, it is found that the grain-boundary screw dislocations (b =1/2[110]) can dissociate in the (111) plane into 30" partials, forming an intrinsic stacking fault, as do lattice screw dislocations of the glide set. On dissociation one partial dislocation stands off the grain-boundary plane. Some segments of the grain-boundary screw dislocations, however, may remain undissociated. An atomic model for the undissociated screw dislocation core, as well as a mechanism of its transformation into cores of individual 30 degrees partials upon dissociation, are proposed on the basis of classical molecular dynamics simulations with an empirical interatomic potential. The model enables an understanding of the results of electron microscopy investigations.