Microstructure of SrTiO₃ buffer layers and its effects on superconducting properties of YBa₂Cu₃O_7-δ coated conductors

A thin layer of SrTiO₃ (STO) has successfully been 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. Using this architecture, 1.5-μm-thick YBCO films with an in-plane mosaic spread in the range of 2.5° to 3.5° in full width at half-maximum and critical current density over 2 × 10⁶ A/cm² in self-field at 75 K have routinely been achieved. It is interesting to note that the pulsed laser deposition growth conditions of SrTiO₃ buffer layers, such as growth temperature and oxygen pressure, have strong effects on the superconducting properties of YBCO. Detailed studies using transmission electron microscopy, scanning electron microscopy, and atomic force microscopy were used to explore the microstructures of STO deposited at different conditions and to understand further their effects on the growth and properties of YBCO films.