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Abstract
Thin single-crystal Nb films have been grown on (0001)S, (0110)S, (2110)S and (0112)S α-Al2O3 (sapphire) surfaces by molecular-beam expitaxy. The same orientation relationship between the bulk of the Nb films and the sapphire substrates is observed for all four systems. The atomic structure of the interface between the Nb films and the sapphire substrates has been determined by high-resolution transmission electron microscopy (HRTEM). The HRTEM studies revealed a unique building principle for the atomic structure of all investigated Nb-Al2O3 interfaces. Two rules characterize the building principle. The first rule states that Nb atoms or ions occupy Al lattice sites at the interface. This rule determines the position of the Nb atoms or ions of the first plane at the interface and thus the growth direction. The second rule determines the orientation of the Nb crystal with respect to the first Nb layer. According to the second rule the Nb atoms of the second layer adjacent to the interface are positioned as close as possible to the Al lattice sites of a continued Al lattice of the sapphire.
It is shown that the unique building principle is established because the bcc unit cell of the Nb corresponds closely to the morphological unit cell of sapphire. The morphological unit cell connects the Al lattice sites in the sapphire lattice. The correspondence between the two unit cells, the unique building principle and in the case of the (0112)S system the dislocation network explain the formation of the same orientation relationship for all systems investigated.
