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

通过工程界面势垒的微观控制改进三明治型高温超导约瑟夫森结的性能

基本信息

批准号：
16560278
负责人：
TERADA Norio
金额：
$ 2.37万
依托单位：
Kagoshima University
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (C)
财政年份：
2004
资助国家：
日本
起止时间：
2004 至 2005
项目状态：
已结题

项目摘要

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

为了进一步提高夹心型高温超导约瑟夫森结的性能和重复性，我们尝试在YBa2Cu3Oy(YBCO)高温超导基电极表面制备超光滑(粗糙度约2 nm)、热稳定的阻挡层。本项目采用90度离轴溅射法制备YBCO电极，这是因为这种方法适合大面积均匀沉积。作为第一步，为了获得c轴取向的YBCO薄膜的超平坦表面，优化了生长温度、气压和气体流量等生长条件。一系列的实验表明，在生长的表面上，充分的活性氧供应和表面扩散的加强相结合，应该是促进二维生长和表面光滑的有效方法。提高生长温度…可以促进扩散。衬底表面缺陷密度更真实、更降低。生长温度越高，YBCO的表面越光滑。在770℃的温度下，在商用的SrTiO3和LaSrAlTaO6衬底上生长的c-YBCO薄膜的峰谷粗糙度约为5 nm，接近c-YBC溅射薄膜的表面平整度。由于生长温度受到某些组分的再蒸发温度的限制，因此很难在机械和/或化学降解的衬底表面上实现足够的扩散，例如商业衬底。在再蒸发温度以下，采用由原子平坦的阶梯和单位晶胞高度的阶梯组成的阶梯-阶梯衬底，可以有效地促进YBCO非c轴组分的扩散和抑制其生长。在750℃的阶梯形衬底上生长的C-YBCO薄膜具有高于82K的高超导临界温度和良好的表面平整度，峰谷粗糙度小于2.5 nm，观察到一个或两个单位晶胞高度的台阶。这是c-YBCO溅射薄膜中最光滑的表面之一，满足了三明治结基电极表面的几何要求。为了得到平坦的c-YBCO薄膜，采用了Ar离子束辐照的表面工程方法。对工程表面的结晶、电子结构进行了一系列表征，并对其在随后的高温过程中的再结晶机理进行了研究。原位实验表明，随着入射离子动能的增加，非晶态结构改性区的初始厚度增加。向YBCO的再结晶开始于非晶区和底层YBCO区之间的界面，并向表面扩散。用低能~500 eV离子设计的表面在630℃下原位热处理1小时，很容易转变为YBCO结构。另一方面，对于1keV离子工程表面，即使在730℃下热处理3小时后，绝缘特性仍保持不变。这些结果表明，工程化高能辐照和缩短上电极沉积过程的持续时间相结合是制备三明治结的有效方法。对于c-YBCO/界面工程势垒/c-YBCO夹心型结，采用1keV离子表面工程的c-YBCO电极获得了RSJ型的电流-电压特性，而低能量辐照工程导致了超导短路，建立了YBCO基极表面超光滑，c-YBCO/界面工程势垒/c-YBCO三明治结热稳定的均匀工程势垒的制备工艺。首次阐明了工程区再结晶为超导相的微观机制。最后，通过实现结的优良特性，证明了这些对器件过程的直接和微观控制的有效性。较少

项目成果

期刊论文数量（68）

专著数量（0）

科研奖励数量（0）

会议论文数量（0）

专利数量（0）

Fabrication of (Cu, C)Ba_2CuO_y superconducting thin films by RF magnetron sputtering

射频磁控溅射法制备(Cu,C)Ba_2CuO_y超导薄膜

DOI：
发表时间：
2005
期刊：
Institute of Physics (in press)
影响因子：
0
作者：
仁木栄;寺田教男;K.Ohki;K.Kikunaga;H.Wakamatsu
通讯作者：
H.Wakamatsu

In-situ characterization of Thermal Stability and Depth Profile of Electronic Structure of Engineered Surfaces of c-axis YBCO Films

c 轴 YBCO 薄膜工程表面的热稳定性和电子结构深度剖面的原位表征

DOI：
发表时间：
2004
期刊：
Institute of Physics 181
影响因子：
0
作者：
T.Okuda;N.Terada et al.;N.Terada;仁木栄;S.H.Kong;T.Okuda;S.Niki;K.Ohki;S.Miyanomae;K.Ohki
通讯作者：
K.Ohki

Fabrication of (Cu, C)Ba2CuOy superconducting thin films by RF magnetron sputtering

射频磁控溅射法制备(Cu,C)Ba2CuOy超导薄膜