Atomic structure of epitaxial Nb-Al2O3 interfaces .2. Misfit dislocations

Four Nb-Al2O3 interfaces were generated by depositing Nb on (0001)(S), (<01(1)over bar 0>)(S), (<2(1)over bar><(1)over bar 0>)(S) and (<01(1)over bar 2>)(S) alpha-Al2O3 (sapphire) surfaces by molecular-beam epitaxy. High-resolution transmission electron microscopy (HRTEM) studies showed that the interfaces are semicoherent for the film thicknesses investigated, that is coherent regions alternate with misfit dislocations at the interfaces. The visible displacement field around the core of the dislocations is restricted to the Nb lattice. The Burgers vector of the misfit dislocations is therefore assigned to the lattice of the Nb. The Burgers vector is determined by HRTEM using the concept of a Burgers circuit around the core of the misfit dislocations. Since this core is in general at the interface, the Burgers circuit has to cross the interface. The dislocation networks, which accommodate the mismatch between the lattice of the Nb and the sapphire, are built up by misfit dislocations with a Burgers vector of 1/2 (111). This Burgers vector corresponds to the Burgers vector of bulk dislocations in Nb. The Burgers vector of the misfit dislocations is not always parallel to the interface nor are all misfit dislocations pure edge dislocations. Only the edge component b(MD) of the Burgers vector, which is parallel to the interface, accommodates the lattice mismatch at these interfaces. Screw components b(screw) and/or edge components b(perpendicular to), which are perpendicular to the interface, are in general compensated in the networks. Only in the system where the interface is parallel to (<01(1)over bar 2>)(S) does an array of misfit dislocations with an uncompensated Burgers vector compoennt b(perpendicular to) lead to a tilt of the Nb lattice with respect to the sapphire lattice.