R-Al2O3基板上EuBa2Cu3O7-δ薄膜の超伝導特性及び結晶成長に対する厚膜CeO2バッファ層の表面性状の影響

Abstract

A buffer layer is indispensable for preventing chemical reactions between high-temperature superconducting thin films and R-Al2O3. CeO2 is a promising buffer layer. However, when a CeO2 buffer layer with a thickness of more than 50 nm is sputter-deposited onto a R-Al2O3 substrate, grains with facets grow and a high-quality EuBa2Cu3O7-δ (EBCO) thin film do not grow on the buffer layer. In order to fabricate a flat and facet-free CeO2 buffer layer and a high-quality EuBa2Cu3O7-δ (EBCO) thin film, we examined the effects of off-center distance (Doff) and substrate temperature (Ts) of the CeO2 buffer layer on the properties of CeO2 buffer layers and EBCO thin films. Doff was defined by the distance from the on-center position to the off-center position. The deposition rate (Rd) was controlled by Doff. A 300-nm-thick CeO2 buffer layer and a 150-nm-thick EBCO thin film were prepared by RF and DC magnetron sputtering, respectively. The surface morphology of CeO2 buffer layer was dependent on Ts and Doff. At Ts = 650°C and Doff = 30 mm, minute grains grew. When Doff was increased to 50 mm, grains with (111) facet planes grew. The surface roughness (Rz) of the CeO2 buffer layer rapidly increased as Doff was increased. The orientation of the EBCO thin film was dependent on the Doff of the buffer layer. At Doff = 0 ∼ 30 mm, only (00l) peaks of an EBCO thin film were observed in X-ray diffraction patterns. Typical rectangular grains were observed on the surface of the thin film. At Doff values over 40 mm, (110) or (103) peaks, in addition to (00l) peaks, were observed. At Doff = 30 mm, the EBCO thin film exhibited a critical temperature (Tce) of approximately 89 K and a critical current density (Jc) of approximately 3.6 MA/cm2.