HRTEM study of atomic faceting of asymmetrical twin and asymmetrical hetero-twin boundaries in NiSi 2/Si

Abstract

Based on computer calculations, the structures of coincidence grain boundaries can be described in terms of a limited number of repeating “structural units”. A structural unit is defined as a small group of atoms arranged in a characteristic configuration. In symmetrical tilt boundaries, where the interface lies parallel to a high-density plane of the coincidence site lattice (CSL), these periodically repeating units interconnect so that the boundary plane is essentially flat. However, in asymmetrical tilt boundaries, which are generally of longer periods, the interface does not lie parallel to a high-density plane of the CSL. Several authors have suggested that a long-period asymmetrical boundary may facet on an atomic scale into short-period symmetrical boundaries. In this research, we investigated the atomic structures of two different boundaries ( 1 11) 1/(1 1 5) 2 and (2 2 1) 1/(001) for both twins in NiSi 2 thin films and “hetero-twin” in NiSi 2/Si by means of a high-resolution imaging method. Our research shows that, just like the case of asymmetrical boundaries in metals, long-period Σ = 3 (CSL and CCSL) ( 1 11) 1/(1 1 5) 2 and (2 2 1) 1/(001) 2 asymmetrical boundaries in NiSi 2/Si dissociate into ( 1 1 1 ) 1/( 1 11) 2 and ( 1 12) 1/(1 1 2) 2 symmetrical boundaries on an atomic scale, which is in good agreement with the prediction of the structural unit model.