Formation of secondary phases in YBa2Cu3O7-δ/SrTiO3 multilayers

The YBa2Cu3O7-delta (YBCO)/SrTiO3 (STO) material system has a potential for device applications such as superconducting field effect transistors, filters and delay lines, due to the possibility to grow heteroepitaxial structures with the necessary orientation of the highly anisotropic superconductor YBCO. To investigate microstructure and formation of secondary phases in this material system, YBCO/STO multilayers were grown by DC and rf magnetron sputtering, respectively. The formation of a previously unidentified cubic (or pseudo-cubic) phase with a lattice parameter of 0.414 nm, containing Ba, Ti, O, and Y, is reported. It is suggested that this phase is Y-substituted BaTiO3 (BTO), with Y in both the Ba and Ti positions of the perovskite unit cell. This new phase was observed to predominantly nucleate on the substrate surface and to be embedded in the layered structure. The precipitates were found to have a cube-on-cube orientational relationship to the LaAlO3 (LAO) substrate, as well as to the STO and YBCO layers. Y2O3 inclusions were also found in the multilayers. (C) 1998 Elsevier Science B.V. All rights reserved.