Thickness modulation effect of CeO2 layer for YBCO films grown by pulsed laser deposition

Abstract

CeO2 film plays an essential role in nucleation and growth of YBa2Cu3O7−x (YBCO) films. In this work, the dependence of superconducting properties of YBCO on CeO2 films with different thicknesses was investigated, in order to achieve fabrication of high-performance YBCO coated conductors in industrial scale. The crystalline structure and morphology of CeO2 films with thickness ranging from 21 to 563 nm were systematically characterized by means of X-ray diffraction (XRD), atomic force microscope (AFM) and reflection high-energy electron diffraction (RHEED). Additional focus was addressed on evolution of the surface quality of CeO2 films with thickness increasing. The results show that at the optimal thickness of 221 nm, CeO2 film exhibits sharp in-plane and out-of-plane texture with full width of half maximum (FWHM) values of 5.9° and 1.8°, respectively, and smooth surface with a mean root-mean-square (RMS) roughness value as low as 0.6 nm. Combing RHEED and transmission electron microscope (TEM) cross-sectional analysis, it is found that nucleation and growth of CeO2 films at early stage remain in island growth mode with rougher surface, while further increasing the thickness beyond the optimal thickness leads to weak surface quality, consequently resulting in degradation of superconductor layers deposited subsequently. Eventually, a critical current density (Jc) as high as 4.6 × 106 A·cm−2 (77 K, self-field) is achieved on a YBCO film on a thickness-modulated CeO2/MgO/Y2O3/Al2O3/C276 architecture, demonstrating the advantages of CeO2 films as buffer layer in high-throughput manufacture of coated conductors.