ATOMIC-STRUCTURE OF EPITAXIAL NB-AL2O3 INTERFACES .1. COHERENT REGIONS

Thin single-crystal Nb films have been grown on (0001)(S), (<01(1)over bar 0>)(S), (<2(11)over bar0>)(S) and (<01(1)over bar 2>)(S) alpha-Al 2O3 (sapphire) surfaces by molecular-beam expitaxy. The same orientati on relationship between the bulk of the Nb films and the sapphire subs trates is observed for all four systems. The atomic structure of the i nterface between the Nb films and the sapphire substrates has been det ermined by high-resolution transmission electron microscopy (HRTEM). T he HRTEM studies revealed a unique building principle for the atomic:s tructure of all investigated Nb-Al2O3 interfaces. Two rules characteri ze the building principle. The first rule states that Nb atoms or ions occupy Al lattice sites at the interface. This rule determines the po sition of the Nb atoms or ions or me 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 interfac e are positioned as close as possible to the Al lattice sites of a con tinued Al lattice of the sapphire. It is shown that the unique buildin g principle is established because the bcc unit cell of the Nb corresp onds closely to the morphological unit cell of sapphire. The morpholog ical unit cell connects the Al lattice sites in the sapphire lattice. The correspondence between the two unit cells, the unique building pri nciple and in the case of the (<01(1)over bar 2>)(S) system the disloc ation network explain the formation of the same orientation relationsh ip for all systems investigated.