Performance Improvement of Sandwich type High Temperature Superconducting Josephson Junctions by means of Microscopic Control of Engineered Interface Barrier

为了进一步提高夹层型高温超导Josephson结的性能及其可重复性，我们尝试在YBa2Cu3Oy （YBCO）高温超导基电极表面建立超光滑（粗糙度< 2 nm）和热稳定势垒层的制备工艺。本项目采用90度离轴溅射方法制备YBCO电极，因为该方法适合大面积均匀沉积。为了获得c轴取向YBCO薄膜的超平坦表面，首先对生长温度、气体压力、气体流速等生长条件进行了优化。一系列实验表明，在生长表面提供充足的活性氧和增强表面扩散相结合，可以有效地促进二维生长和表面光滑。提高基体表面的生长温度，提高基体表面的温度，降低基体表面的缺陷密度，可以促进合金的扩散。生长温度越高，YBCO的表面越光滑。在770℃下，在SrTiO3和LaSrAlTaO6上生长的c-YBCO薄膜显示出约5 nm的峰谷粗糙度，接近c-YBC溅射薄膜的表面光滑度记录。由于某些成分的再蒸发温度限制了生长温度，因此在机械和/或化学降解的底物表面（如商业底物）上很难实现充分的扩散。在再蒸发温度以下的条件下，使用由原子平面阶梯和单位胞高阶梯组成的阶梯和阶梯底物可以有效地促进YBCO的扩散和抑制非c轴组分的生长。c-YBCO薄膜在750摄氏度的阶梯和阶梯衬底上生长，具有高于82 K的高超导临界温度和优异的表面光滑度，峰谷粗糙度小于2.5 nm，其中观察到一个或两个单位电池高度台阶。这是c-YBCO溅射膜中最光滑的表面之一，满足夹层型结基底电极表面的几何要求。对得到的平面c-YBCO，采用氩离子束辐照表面工程处理。对工程表面的晶体、电子结构及其在后续高温过程中的再结晶机理进行了一系列表征。原位实验结果表明，随着冲击离子动能的增加，非晶结构修饰区的初始厚度逐渐变厚。YBCO的再结晶始于非晶区和下伏YBCO区之间的界面，并向表面扩散。在630℃原位退火1小时后，低能量~ 500 eV离子工程表面很容易转化为YBCO结构。另一方面，对于含有1个keV离子的表面，即使在730摄氏度下退火3小时，其绝缘特性仍然存在。这些结果表明，结合工程高能辐照和缩短上电极沉积过程的时间，将有助于夹层结的制备。对于c-YBCO/界面工程势垒/c-YBCO夹层型结，采用1 keV离子的c-YBCO电极表面工程，成功获得了RSJ型的电流-电压特性，而低能量辐照工程导致超导短路。如上所述，建立了YBCO基电极超光滑表面和c-YBCO/界面工程势垒/c-YBCO夹层型结均匀热稳定的工程势垒的制备工艺。首次阐明了工程区向超导相再结晶的微观机理。最后，通过实现结的优异性能，证明了这些直接和微观控制器件过程的有效性。少

For the purpose to realize further improvements of performances and their reproducibility of sandwich type high temperature superconducting (HTS) Josephson Junctions, we have attempted to establish a fabrication process of ultra smooth (roughness < 2 nm) and thermally stable barrier layer on the surface of YBa2Cu3Oy (YBCO) HTS base electrode. In this project, 90 degree off-axis sputtering has been used for fabrication of the YBCO electrode, because of suitability of this method to large-area and uniform deposition. As the first step, for obtaining the ultra-flat surface of c-axis oriented YBCO films, growth conditions including growth temperature, gas pressure and gas-flow rates were optimized. A series of experiments revealed that a combination of sufficient supply of active oxygen and enhancement of surface diffusion on the growing surface should be effective to promote 2-dimensional growth as well as surface-smoothing. The diffusion could be promoted by an increase of growth tempera … More ture and reduction of defect density of substrate surfaces. The higher growth temperature usually resulted in the smoother surface of YBCO. The c-YBCO films grown on commercial SrTiO3 and LaSrAlTaO6 at 770 degree Celsius showed peak-to-valley roughness about 5 nm, which was close to the record of surface smoothness of c-YBC sputtered films. Since the growth temperature was limited with a re-evaporation temperature of some constituents, sufficient diffusions were hardly achievable over the mechanically and/or chemically, degraded surfaces of substrates, such as commercial ones. Usage of so-called step-and-terrace substrates which consist of atomically flat terrace and unit-cell height steps was effective to promote the diffusions and suppression of outgrowth of non-c-axis component of YBCO at a temperature below the re-evaporation. c-YBCO films grown on the step-and-terrace substrates at 750 degree Celsius exhibits both high superconducting critical temperature above 82 K and an excellent surface smoothness with a peak-to-valley roughness less than 2.5 nm, where one or two unit cell height steps were observed. This is one of the smoothest surfaces of c-YBCO sputtered films, and satisfies geometric requirement for surfaces of base electrode of sandwich type junction.For the flat c-YBCO obtained, surface engineering by means of Ar ion beam irradiation was adopted. Series of characterizations about crystalline, electronic structure of the engineered surfaces and a mechanism of re-crystallization of it in the subsequent high temperature process have been carried out. The in-situ experiments revealed initial thickness of the modified region with amorphous structure became thicker with an increase of kinetic energy of the impinging ions. The re-crystallization to the YBCO started at the interface between the amorphous and underlying YBCO regions and propagated to the surface. The surfaces engineered by ions with a low energy 〜 500 eV were easily converted to the YBCO structure by in-situ annealing at 630 degree Celsius for 1 hour. On the other hand, for the surface engineered with 1 keV ions, insulating feature remained even after an annealing at 730 degree Celsius for 3 hours. These results mean that combination of the engineering high energy irradiation and shortening of duration of deposition process of the upper electrode should be useful to fabricate the sandwich junctions. For the c-YBCO/interface-engineered barrier/c-YBCO sandwich type junctions, a RSJ type current-voltage characteristics has been successfully achieved by adopting the surface engineering of c-YBCO electrode with 1 keV ions, whereas the engineering with a low energy irradiation resulted in superconducting short.As mentioned, the fabrication processes of ultra-smooth surface of YBCO base electrode, and uniform and thermally stable engineered barriers for c-YBCO/interface-engineered barrier/c-YBCO sandwich type junctions has been established. Microscopic mechanism of the re-crystallization of the engineered region to superconducting phase has been clarified for the first time. Finally, Usefulness of these direct and microscopic controls of device-process has been demonstrated by realizing an excellent properties of the junctions. Less