A Study on Josephson Junctions with Controlled Barrier Layrs Operating at 10 K for Integrated Circuit Application

这项工作的目的是发展约瑟夫森结作为工作在10k下的单通量量子超导集成电路的基本元件。结被要求具有过阻尼特性和在10 K时的高IcRn产物。这里，Ic和Rn分别是临界电流和结电阻。为了达到这一目的，我们提出了具有等离子体氮化NbNx势垒的NbN Josephson结，并研究了该结的电学特性。我们得到的结果如下。我们已经获得了过阻尼的NbN结，其IcRn产物在10K.2时高达0.6 mV。芯片中64个结的Ic均匀性在2.9%的标准差（1 σ）内，临界电流密度（Jc）的运行变化在14%的标准差（1 σ）内。这意味着这些连接可以应用于Josephson ics。得到的Jc值在几十a /cm^2到几十kA/cm^2.4之间。研究了这些结的传导机理。发现等离子体氮化NbNx势垒中存在局域态，准粒子通过局域态在两个NbN电极之间隧穿。还发现该结表现为超导体-正常层-绝缘体-正常层-超导体（SNINS）结。结特性在很大程度上取决于NbN基层的结晶度和NbNx势垒形成条件。控制NbNx势垒中的局域态密度是控制结特性的关键。

The purpose of this work is to develop Josephson junctions as basic elements in single flux quantum superconducting integrated circuits operating at 10 K.The junctions are required to have overdamped characteristics and the high IcRn products at 10 K.Here, the Ic and Rn are a critical current and a junction resistance, respectively. In order to attain the purpose, we have proposed NbN Josephson junctions with plasma-nitrided NbNx barriers and investigated the electrical characteristics of the junctions. The results we have obtained are listed as follows.1. We have obtained overdamped NbN junctions with the IcRn product as large as 0.6 mV at 10K.2. The Ic uniformity was within the standard deviation, 1-sigma, of 2.9% for a series of 64 junctions in a chip and the run-to-run variation in the critical current density, Jc, was within 1-sigma of 14%. This means that the junctions can be applied to Josephson ICs.3. The Jc was obtained in the range of a few tens A/cm^2 to a few ten kA/cm^2.4. The conduction mechanism of the junctions was investigated. It was found that the plasma-nitrided NbNx barrier has localized states in it and that quasi-particles tunnel via localized states between both of the NbN electrodes. It was also found that the junctions behave as a Superconductor-Normal layer-Insulator-Normal layer-Superconductor (SNINS) junction.5. The junction characteristics depend strongly on the crystallinity of the base NbN layers and the NbNx barrier formation conditions.6. The control of the localized state density in the NbNx barriers is essential for controlling the junction characteristics.