R-Al₂O₃基板上EuBa₂Cu₃O_7-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

抄録

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 EuBa₂Cu₃O_7-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 EuBa₂Cu₃O_7-δ (EBCO) thin film, we examined the effects of off-center distance (D_off) and substrate temperature (T_s) of the CeO₂ buffer layer on the properties of CeO₂ buffer layers and EBCO thin films. D_off was defined by the distance from the on-center position to the off-center position. The deposition rate (R_d) was controlled by D_off. 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 T_s and D_off. At T_s = 650°C and D_off = 30 mm, minute grains grew. When D_off was increased to 50 mm, grains with (111) facet planes grew. The surface roughness (R_z) of the CeO₂ buffer layer rapidly increased as D_off was increased. The orientation of the EBCO thin film was dependent on the D_off of the buffer layer. At D_off = 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 D_off values over 40 mm, (110) or (103) peaks, in addition to (00l) peaks, were observed. At D_off = 30 mm, the EBCO thin film exhibited a critical temperature (T_ce) of approximately 89 K and a critical current density (J_c) of approximately 3.6 MA/cm².