The importance of CeO2 growth temperature and its post-annealing for the improvement of the microwave surface resistance of DyBa2Cu3Oz films

We report here the influence of growth temperature of CeO2 buffer layer T(CeO2) as well as the post-annealing of the buffer layer on the crystallinity and the microwave surface resistance Rs of DyBa2Cu3Oz (DBCO) films grown by pulsed laser deposition (PLD). It is found that (i) an increase in the T(CeO2) facilitates the epitaxial growth of the CeO2 films, which is a prerequisite to obtain the high quality superconducting films and (ii) the post-annealing of buffer layer at 1050 °C in flowing O2 for 1 h leads to a profound improvement in the morphology and in the crystallinity of CeO2 films. Apparently, there exists a critical growth temperature (T(CeO2)=800-820∘C, as found in this and previous study [J.C. Nie, H. Yamasaki, Y. Nakagawa, K.D. Bagarinao, M. Murugesan, H. Obara, Y. Mawatari, J. Crystal Growth 284 (2005) 417]) for CeO2, below which the crystalline quality of CeO2 films might not be improved merely by the post-annealing. It is explained that for T(CeO2)<800∘C, the as-grown CeO2 grains are longitudinal in shape, and it forms corrugated structure upon annealing. This poor morphology yields a deteriorated crystallinity (i.e., a large value of Δω and Δφ, and the formation of secondary phase) for the CeO2 as well as the overlying DBCO films, and hence a poor microwave performance of DBCO films for T(CeO2)<800∘C. We also observed that the Rs in DBCO films monotonously decreased with increase in the growth temperature of CeO2. Further, the post-annealing of the CeO2 buffer layer prior to DBCO deposition greatly helps to reduce the Rs at the liquid N2 temperature region, which is immensely required for the use of superconducting films in the passive microwave device components. Thus, the 800-820 °C of T(CeO2) and the post-annealing of CeO2 at 1050 °C in flowing O2 for 1 h may be readily exploited to grow RBCO (R=Y or rare-earth elements) films for microwave applications on the technologically viable r-Al2O3 substrates.