Functional Interfaces in Epitaxial YBa2Cu3O7-δ with Artificial Grain Boundary Junctions
Doctoral thesis, 2010
The properties of superconducting devices based on grain boundaries in YBa2Cu3O7-δ (YBCO) thin films are determined by the atomic structure of the grain boundary plane. To obtain these devices based on single grain boundaries, biepitaxial fabrication techniques has been used. The biepitaxial system was chosen due to its flexible possibilities to manufacture complex grain boundary structures. The majority of all YBCO films that are produced are c-axis oriented. So, beside 90° a-axis tilt boundaries not many systems, other than c-axis tilt boundaries have been examined. By the biepitaxial technique on (110) SrTiO3 (STO) and (110) MgO substrates 45° tilt and twist boundaries can be obtained. They are used in the research of d-wave symmetry and qubits for future applications in i.e. quantum computers. The development of a biepitaxial method for arbitrary misorientation angles is not a trivial task. Hence, further information about the control of epitaxial orientation relationships will contribute to the improvement of the flexibility of the biepitaxial method. The purpose of this work is to investigate nucleation and evolution of grain boundaries in YBCO epitaxial thin films grown on SrTiO3 (STO) bicrystal substrates, CeO2/STO bicrystal substrates, biepitaxial (110) STO with () 110 CeO2 (CEO) template layer and biepitaxial (110) MgO with (110) STO template layer. Both misorientation between adjacent YBCO grains and the detailed geometry of the interfaces are of importance. The aim of this work is to gain understanding of how the geometry of the YBCO grain boundary can be controlled. This is crucial for the understanding and optimization of the novel electrical devices based on high-Tc superconductors. Particular attention has been paid to the nucleation and the subsequent evolution of the YBCO film and the grain boundary structures present. The development of a biepitaxial method for arbitrary misorientation angles is not a trivial task. Hence, further information about the control of epitaxial orientation relationships will contribute to the improvement of the flexibility and reproducibility of the biepitaxial method. The microstructural investigation has been performed using scanning electron microscopy and transmission electron microscopy. General characteristics of the structure and their correlation to transport properties of a-axis tilt TBCO grain boundaries have been deduced based on the large number of grain boundary junctions investigated.