Structure of high-tilt YBa₂Cu₃O_x thin films by pulsed laser deposition

The graphoepitaxial seeding provides a possibility of fabrication of epitaxial thin films of anisotropic materials with unusual orientation. This technique allows preparation of YBa₂Cu₃O_x (YBCO) thin films with c axis position close to the substrate plane - the high-tilt YBCO films - that may be applied for fabrication of scalable intrinsic Josephson structures. The structure of YBCO films deposited on 14–23º tilted-axes NdGaO₃ (NGO) substrates with pulsed laser deposition was studied with the X-ray diffraction techniques. Two orientations of YBCO grains, pseudo-c (001)_YBCO||(110)_NGO and pseudo-a (100)_YBCO||(110)_NGO (the high-tilt orientation), were observed. The dominant orientation of the film is set in the very beginning of deposition with overgrowth of the pseudo-c seeds with the pseudo-a grains or vice versa. The disorder in the top layer of the film rapidly increases with the volume part of pseudo-a grains, while the crystallinity and the mosaicity of the main layer are not affected by the film orientation. Symmetric (103) grain boundaries between the pseudo-c and pseudo-a grains dominate in the top layer. The pseudo-c grains experience a significant effect of the substrate-induced strain; the introduction of pseudo-a grains results in a relaxed structure. Secondary seeding of the pseudo-c grains is observed in thick films, grown with the vapor-liquid-solid mechanism. The lattice of such grains is completely strained by the bottom pseudo-a oriented layer.