Control of Variants in Heteroepitaxy by Substrate Miscut

This paper discusses variants that occur in heteroepitaxy when the substrate possesses symmetries that are absent from the epilayer. Variants are enumerated in terms of 2-D symmetries of the substrate and epilayer, as they exist terminated at their interface. In this formulation, all possible cases can be presented in a single table. The role of miscut in controlling otherwise equal variants proportions is discussed with a view to eliminating unwanted variants and thereby improving the translational invariance of the materials. We summarize an experimental study of a (3m) symmetry epilayer growing on a (2mm) substrate, specifically Cu3Au (111) grown by molecular beam epitaxy on Nb (110), for which the two variants predicted from symmetry are stacking twins. Miscuts of about 1° along the indicated azimuth are sufficient to eliminate all except ~ 0.1% of the less favored variant. This has interesting consequences in the context of the theory. A detailed understanding of the mechanisms for this example requires information about nanostructures that develop on the miscut Nb (011) surface. State-of-the-art information about novel substrate nanostructures, derived from scanning probe and low energy electron microscopies, is presented, and the prospects for a predictive science of variant control in heteroepitaxial growth are assessed.