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

本工作的目的是发展约瑟夫森结作为基本元件，在单通量量子超导集成电路工作在10 K的结被要求具有过阻尼特性和高IcRn产品在10 K。这里，Ic和Rn是一个临界电流和结电阻，分别。为了达到这一目的，我们提出了等离子体氮化NbNx势垒的NbN约瑟夫森结，并研究了结的电学特性。我们得到的结果如下：1.我们已经获得了过阻尼NbN结的IcRn产品高达0.6 mV，在10K.2。对于芯片中的一系列64个结，Ic均匀性在2.9%的标准偏差1-sigma内，并且临界电流密度Jc的批次间变化在14%的1-sigma内。这意味着结可以应用于约瑟夫森集成电路。测得的Jc在几十A/cm ^2到几十kA/cm ^2的范围内。研究了结的导电机理。结果发现，等离子体氮化的NbNx势垒具有局域态，并且准粒子通过两个NbN电极之间的局域态隧穿。研究还发现，这种结表现为超导体-正常层-绝缘体-正常层-超导体（SNINS）结.结的特性很大程度上取决于NbN基层的结晶度和NbNx势垒的形成条件. NbNx势垒中局域态密度的控制对于控制结特性是必不可少的。

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.