Monocrystalline molecular beam epitaxial Nb films were deposited onto 4 different sapphire surfaces, and the atomic structures of the interfaces between Nb and sapphire were revealed by high-resolution transmission electron microscopy. The interfaces consisted of coherent regions and misfit dislocations. Electron micrographs showed that the atomic structures in the coherent regions of all of the interfaces could be described by the same building principle. The latter was characterized by 2 simple rules which determined the positions of the Nb atoms in the first 2 layers next to the interface. This building principle, and a coincidence between the body-centered cubic unit cell of Nb and the morphological unit cell of sapphire, explained the unique orientation relationship which was found for all 4 systems. Mismatch between the Nb lattice and the sapphire lattice was accommodated by misfit dislocations. The Burgers vectors of the misfit dislocations, and the geometry of the dislocation networks, were determined by means of high-resolution transmission electron microscopy. The Burgers vector of each of the misfit dislocations was of ½<111> type. This corresponded to the Burgers vector of a bulk dislocation in Nb. The edge component of the Burgers vector, which was parallel to the interface, accommodated the lattice mismatch in these networks. The Burgers vector components, which did not accommodate any lattice mismatch, were generally compensated in the networks.

G.Gutekunst, J.Mayer, M.Rühle: Materials Science Forum, 1996, 207-209, 241-4