HRTEM STUDY OF ATOMIC FACETING OF ASYMMETRICAL TWIN AND ASYMMETRICAL HETERO-TWIN BOUNDARIES IN NISI2 SI/

Based on computer calculations, the structures of coincidence grain bo undaries can be described in terms of a limited number of repeating '' structural units''. A structural unit is defined as a small group of a toms arranged in a characteristic configuration. In symmetrical tilt b oundaries, where the interface lies parallel to a high-density plane o f the coincidence site lattice (CSL), these periodically repeating uni ts interconnect so that the boundary plane is essentially flat. Howeve r, in asymmetrical tilt boundaries, which are generally of longer peri ods, the interface does not lie parallel to a high-density plane of th e CSL. Several authors have suggested that a long-period asymmetrical boundary may facet on an atomic scale into short-period symmetrical bo undaries. In this research, we investigated the atomic structures of t wo different boundaries (111BAR)1/(115BAR)2 and (221BAR)1/(001)2 for b oth twins in NiSi2 thin films and '' hetero-twin'' in NiSi2/Si by mean s of a high-resolution imaging method. Our research shows that, just l ike the case of asymmetrical boundaries in metals, long-period SIGMA = 3 (CSL and CCSL) (111BAR)1/(115BAR)2 and (221BAR)1/(001)2 asymmetrica l boundaries in NiSi2 /Si dissociate into (111BAR)1/(111BAR)2 and (112 BAR)1/(112BAR)2 symmetrical boundaries on an atomic scale, which is in good agreement with the prediction of the structural unit model.