Thickness effects of SrTiO₃ buffer layers on superconducting properties of YBa₂Cu₃O_7-δ coated conductors

A thin layer of SrTiO₃ has been successfully used as a buffer layer to grow high quality superconducting YBa₂Cu₃O _7-δ (YBCO) thick films on polycrystalline metal substrates with a biaxially oriented MgO template produced by ion-beam-assisted deposition (IBAD). Using this architecture, 1.5 μm-thick-YBCO films with an in-plane mosaic spread in the range of 2-3° in full width at half maximum, and a critical current density over 2 MA/cm² in self-field at 75 K have been achieved routinely. More interestingly, we found the thickness of SrTiO₃ buffer layers strongly affects the properties of YBCO films. The critical current density of YBCO films increases dramatically when the thickness of SrTiO₃ buffer layer reaches an optimum range of 40-120 nm. Microstructure studies, including transmission electron microscopy (TEM), X-ray diffraction (XRD) and scanning electron microscopy (SEM), and ion-milling experiments suggest that the YBCO texture evolution, STO outgrowths and STO surface dents are strongly related to STO thickness. They are the key factors for YBCO self-field J_c variation.