Thin monocrystalline Nb films were grown onto (00•1), (01•0), (¯21•0) and (01•2) -phase (sapphire) surfaces by means of molecular beam epitaxy. The same orientation relationship, between the bulk of the Nb films and the sapphire substrates, was observed for all 4 systems. The atomic structure of the interface between the Nb films and the sapphire substrates was determined by means of high-resolution transmission electron microscopy. The results revealed the existence of an unique build-up principle for the atomic structure of all of the investigated interfaces. It was found that 2 rules characterized the build-up principle. The first rule required that Nb atoms or ions should occupy Al lattice sites at the interface. This rule determined the position of the Nb atoms or ions of the first plane at the interface, and therefore controlled the growth direction. The second rule governed the orientation of the Nb crystal with respect to the first Nb layer. According to this rule, the Nb atoms of the second layer adjacent to the interface were situated as close as possible to the Al lattice sites of an extrapolated Al lattice of the sapphire. It was shown that this unique build-up principle applied because the body-centered cubic unit cell of the Nb corresponded closely to the morphological unit cell of sapphire. The latter cell connected the Al lattice sites in the sapphire lattice.

G.Gutekunst, J.Mayer, M.Rühle: Philosophical Magazine A, 1997, 75[5], 1329-55