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

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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