Growth of High-Temperature Oxide Superconductor Thin Films on Ultrasmooth Substrates and Elucidation of Their Growth Mechanism.

了解氧化物超导体的生长机制对于获得具有原子表面平坦度的高质量薄膜非常重要。在这项研究中，我们通过溅射在 MgO(100) 超光滑台阶基底上沉积 YBCO 薄膜，并通过原子力显微镜 (AFM) 研究了生长的早期阶段。并讨论了衬底表面结构对YBCO薄膜生长机制和表面结构的影响。通过HiD22刻蚀和高温退火获得了不同台阶宽度和台阶高度的MgO(100)台阶衬底的原子平坦表面，在该衬底上采用溅射方法生长了c轴取向的YBCO薄膜，并用AFM观察了薄膜生长的早期阶段。当在宽平台宽度和高台阶高度上生长时，3 晶胞 (u.c.) 厚的 YBCO 薄膜沿台阶边缘观察到 3D 岛结构。认为当平台宽度大于沉积物质的扩散长度并且台阶高度远高于YBCO的晶胞长度时，YBCO的沉积物质优先在台阶边缘处被捕获并在那里形成岛结构。另一方面，当薄膜厚度为3u.c时，在窄平台宽度和低台阶高度的衬底上生长的薄膜上观察到二维晶粒并且没有观察到岛。此类基板上的生长模式从 2D 逐层生长变为 3D 凸缘生长，膜厚约为 7u.c。当膜厚度为10u.c时观察到螺旋结构的起始。结果表明，当膜厚大于10u.c时，YBCO薄膜的生长机制由3D凸缘生长转变为螺旋生长。同时采用硫酸和磷酸化学刻蚀法获得超光滑蓝宝石台阶衬底，并在其上溅射生长ZnO和GaN薄膜。在台阶基底上生长的薄膜的生长性能优于未处理基底上的薄膜。

It is very important to know the growth mechanism of oxide superconductors to obtain high-quality films with atomic surface flatness. In this study, we deposited YBCO thin films on MgO(100) ultra smooth step substrates by sputtering and investigated the early stages of the growth by atomic force microscopy (AFM). And discussed the influence of the surface structure of the substrates on the growth mechanism and surface structure of YBCO films.Atomically flat surface of MgO(100) step substrates with different terrace width and step height have been obtained by HィイD22ィエD20 etching and high-temperature annealing, c-axis oriented YBCO thin films were grown on such substrates by sputtering and observed the early stages of the films growth by AFM. 3D island structures were observed along the step edges for 3 unit cell(u.c.) thick YBCO films when grown on wide terrace width and high step height. It is considered that the deposited species of YBCO are preferentially captured at the step edges and form island structures there when the terrace width is wider than diffusion length of deposition species and the step height is much higher than the unit cell length of YBCO. On the other hand, 2D grains and no island were observed on the films grown on the substrates with narrow terrace width and low step height when the film thickness was 3u.c. Growth mode on such substrates changed from 2D layer-by-layer growth to 3D ledge growth at the film thickness around 7u.c. The initiation of spiral structures was observed when the film thickness is lOu.c. It is concluded that the growth mechanisms of YBCO film changed from 3D ledge growth to spiral growth when the film thickness is thicker than lOu.c.Ultra smooth sapphire step substrates were also obtained by chemical etching using sulfuric and phosphoric acids and ZnO and GaN thin films were grown on them by sputtering. The crystal quality of thin films grown on step substrate was better than that on non-treated substrates.