Lattice mismatch effects on the hetero interface structure of YBCO films grown by liquid phase epitaxy

Electronic and magnetic properties of oxide superconducting films grown on a substrate strongly depend on the film/substrate interface. In this study, the lattice mismatch effects on the interface structure of YBCO films grown by liquid phase epitaxy were investigated by characterizing two model systems. These two systems, YBCO/ NdGaO₃(f: 0.84∼0.90%) and YBCO/MgO(f: 9.1∼25%), differ by their small and large lattice mismatches, respectively. In the case of the YBCO/NdGaO₃ interface, misfit dislocation network is introduced to accomodate lattice strain along the interface. The misfit dislocations are composed of [110]_NdGaO3, [002]_NdGaO3, and [100]_YBCO edge type dislocations depending on the crystal orientation, and hence extra half planes exist both in NdGaO₃ and YBCO crystals. The orthogonal sets of misfit dislocations are dissociated to form nodes which also have an edge component. The critical current density J_ci in the YBCO film is as low as 9.0 × 10² A/cm² at 77 K in 1 T. In the case of the YBCO/MgO interface, geometrical misfit dislocations are formed at the interface. The geometrical misfit dislocations accompany local lattice bending in the vicinity of the interface. In the YBCO film, a high density of edge dislocations with their Burgers vector perpendicular to the interface exist, which result in the formation of stacking faults on the ab plane in the YBCO crystals. Such defects may be introduced during crystal growth by following the local lattice bending due to the geometrical misfit dislocation. The J_ci in the YBCO film is as high as 5.4 × 10⁵ A/cm² at 77 K in 1 T. The difference in critical current densities between two specimens presumably originates from the density of pinning centers for magnetic flux in YBCO crystals.