Thickness effects of SrTiO3 buffer layers on superconducting properties of YBa2Cu3O7−δ coated conductors

A thin layer of SrTiO3 has been successfully used as a buffer layer to grow high quality superconducting YBa2Cu3O7−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 Jc variation.