A study of fabrication for oxide-superconductor devices by using transparent-conductive oxides

本文研究了由高温超导体YBa_2Cu_3O_7（YBCO：S-layr）和超导材料In_2O_3：Sn（ITO：N-layr）组成的超导器件的性能。本文从半导体二极管的一般方程I*exp（eV/nkT）出发，首次分析了由这些材料组成的SN双层膜的电流-电压（I-V）特性。结果表明，n的指数近似等于1，表明其为肖特基型行为而不是pn结行为;势垒高度的温度依赖性与温度成正比，表明在SN界面处存在界面层。我们还对ITO/衬底界面进行了化学分析，发现氧化物衬底上的ITO膜比聚合物衬底上的ITO膜更稳定，因为在氧化物衬底上的ITO膜抑制了In原子向衬底中的扩散。在这两种类型的结中，I-V曲线在290 K下是线性的，在77 K下是非线性的，表现出定性地类似于约瑟夫森器件的行为。在用In_2O_3的情况下，接头的质量似乎是上级的。此外，大部分含In_2O_3的SN双层膜在I-V曲线的反方向上表现出微分负阻特性。这可能是由于在结的不均匀部分的电子雪崩效应，该系统可能被用作开关器件。总之，我们已经表明，氧化物超导器件可以用ITO膜。由于YBCO与ITO层间可能存在肖特基势垒，利用这一效应有望开发出新型的超导器件。

The purpose of this study is to investigate properties of superconducting devices composed of a high-T_C superconductor YBa_2Cu_3O_7 (YBCO : S-layr) and a transparent-conductive material In_2O_3 : Sn (ITO : N-layr) . Based on the general equation I*exp (eV/nkT) of semiconducting diodes, we have first analyzed current-voltage (I-V) characteristics of SN bi-layrs consisted of these materials. It has been shown that the exponent n is nearly equal to unity, indicating the Schottky type behavior rather than the pn junction behavior, and that temperature dependence of the barrier height is proportional to temperature, implying the existance of the interfacial layr at the SN boundary. We have also made chemical analyzes for the ITO/substrate interface, and have found that ITO films on oxide substrates are more stable than those on polymer substrates in the sense that diffusion of In atoms into substrates is suppressed in the former case.Next we have investigated characteristics of SNS tri-layrs in which ITO and In_2O_3 are used as N-layrs. In both types of junctions, I-V curves are linear at 290K and nonlinear at 77K showing behavior qualitatively similar to Josephson devices. Quality of the junctions seems to be superior in the case of junctions with In_2O_3. In addition, most of SN bi-layrs with In_2O_3 show differential negative resistance behavior in the reverse direction of I-V curves. This may be attributed from electron avalanche effects in the nonuniform parts of the junctions, and the system is possibly used as a switching device.In conclusion, we have shown that oxide superconducting devices can be made by using ITO films. Since there may exist the Schottky barrier between YBCO and ITO layrs, a new type of superconducting devices is expected to be developed with the aid of this effect.