Thickness-induced residual stresses in textured YBCO thin films determined by crystalline group method

High quality YBa2Cu3O7−δ (YBCO) superconducting films were fabricated on LaAlO3 substrates in a wide thickness range of 0.2–2 μm. The texture, residual stress and microstructure in the YBCO thin films were systematically investigated as a function of the film thickness. High-resolution X-ray diffraction showed that strictly c-axis epitaxial YBCO films were observed in various film thicknesses. Residual stress in the films was calculated using a modified sin2 ψ method, termed crystalline group method, by assuming a biaxial stress state. The results showed that the compressive stress was obtained for the YBCO films thinner than 1 μm, and it tends to decrease with increasing film thickness. Scanning electron microscopy (SEM) indicated that outgrowths predominate the surface structures of the thinner sample, and pores are major structures for thicker samples. Some microcracks were observed in thick films due to the tensile stress. In addition, the relationship between film thickness and transition temperature (Tc) as well as critical current density (Jc) was also investigated for all YBCO superconducting films. The results exhibited that the values of the Tc and Jc were strongly dependent of the film thickness.