Study on the deposition of $YBa_2Cu_3O_{7-δ}}$ bi-crystalline thin films using the $BaF_2$ -process and their properties

Abstract

$YBa_2Cu_3O_{7-δ}(YBCO)$ thin films were formed using the $BaF_2$-process. This process comprises two steps. First, a precursor film which is a fine mixture of $BaF_2$ and metallic Y and Cu was deposited using thermal co-evaporation. Then it was heated in a flowing atmosphere containing $N_2$ and $O_2$ gases and water vapor to obtain crystallized YBCO films. To find optimal annealing conditions, eight $SrTiO_3(STO)$ single crystals were used as substrates. As a result, annealing temperature of about 720℃, oxygen partial pressure of 100ppm, and water temperature of 50℃ were found to be the most appropriate annealing conditions. Using these conditions, YBCO bi-crystalline thin films were formed on 30℃ and 40℃ [001]-tilt STO bi-crystals. The pole figures and the X-ray diffraction(XRD) data showed good epitaxial growth of the bi-crystalline samples, but their Scanning electron microscope(SEM) images not so good. The critical current densities were 263 and 51MA/㎠ at 77K and zero field for 30℃ and 45℃ bicrystalline samples, respectively. These poor characteristics may be partially due to the considerable amount of fluoride remaining in the annealed films. Resistive transitions at various magnetic fields were measured and the critical temperatures were found to be 89.5K and 88.5K for 30℃ and 45℃ bi-crystalline samples, respectively. Next, it was attempted the preparation of precursor films using ``$YF_3+BaF_2$`` bulk as one of the two evaporation sources. From this, YBCO superconducting thin films were also obtained, the properties of which were comparable to those of $BaF_2$-processed samples. It was also attempted to use Ba metal instead of $BaF_2$, and $YF_3$ instead of Y metal in this process and obtained similar results. Though the use of CuF was not attempted, similar results are likely to be shown. Furthermore, the preparation of precursor film using one-source by thermal co-evaporation or $CO_2$ laser evaporation is expected to be come true someday.