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

本研究的目的是研究由高t_c超导体YBa_2Cu_3O_7 （YBCO: s层）和透明导电材料In_2O_3: Sn （ITO: n层）组成的超导器件的性能。基于半导体二极管的一般方程I*exp (eV/nkT)，我们首先分析了由这些材料组成的SN双层的电流-电压（I- v）特性。结果表明，n指数几乎等于1，表明了肖特基型行为而不是pn结行为，势垒高度的温度依赖关系与温度成正比，表明在SN边界处存在界面层。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薄膜比聚合物衬底上的ITO薄膜更稳定，因为在前者的情况下，in原子向衬底的扩散受到抑制。接下来，我们研究了以ITO和In_2O_3作为n层的SNS三层的特性。在这两种类型的结中，I-V曲线在290K时是线性的，在77K时是非线性的，表现出与约瑟夫森器件性质相似的行为。用In_2O_3制备的结的质量较好。此外，大多数含有In_2O_3的SN双层在I-V曲线的相反方向上表现出差分负电阻行为。这可能是由于结的非均匀部分的电子雪崩效应造成的，并且该系统可能用作开关器件。综上所述，我们已经证明了利用ITO薄膜可以制作氧化物超导器件。由于YBCO和ITO层之间可能存在肖特基势垒，因此有望利用这一效应开发出一种新型超导器件。

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.