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

这项工作的目的是开发约瑟夫森结作为工作在10K下的单磁通量子超导集成电路的基本元件，要求结具有过抑制特性和10K下的高IcRn积。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约瑟夫森结，并研究了这种结的电学特性。我们所取得的成果如下：1.在10K时，我们获得了高达0.6 mV的过抑制NbN结。对于芯片中的一系列结，IC的均匀性在1-Sigma的标准偏差2.9%以内，临界电流密度Jc的运行到Run的变化在1-Sigma的14%以内。这意味着这些结可以应用于约瑟夫森集成电路。JC在几十安/厘米^2到几十卡/厘米^2.4的范围内获得。对结的导电机理进行了研究。结果表明，离子氮化NbNx势垒中存在局域态，准粒子在两个NbN电极之间通过局域态隧穿。研究还发现，这些结表现为超导体-正常层-绝缘层-正常层-超导体(SNINS)结。结的特性强烈依赖于NbN基层的结晶度和NbNx势垒的形成条件。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.