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Physics of the Solid State

Josephson Tunnel Junctions with an Integral Superconductor–Insulator–Normal Metal Shunt

M. S. Shevchenko, Lyudmila V. Filippenko, O. S. Kiselev, V. P. Koshelets

Published May 1, 2022• 2 citations
Publisher

Abstract

Abstract This study is devoted to the investigation of tunneling superconductor–insulator–superconductor (SIS) Josephson junctions with a new type of shunt based on an additional superconductor–insulator–normal metal (SIN) junction placed around the SIS junction. In the course of study, the parameters of such shunted junctions are numerically calculated and their current–voltage characteristics (IVC) are simulated. The designed structures are assembled in real samples and their parameters are studied; the IVC of junctions with different degrees of shunting are measured, and the behavior of these junctions under the influence of high-frequency signals in the subterahertz range is studied.

DOI
10.1134/s1063783422060075

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Abstract This study is devoted to the investigation of tunneling superconductor–insulator–superconductor (SIS) Josephson junctions with a new type of shunt based on an additional superconductor–insulator–normal metal (SIN) junction placed around the SIS junction. In the course of study, the parameters of such shunted junctions are numerically calculated and their current–voltage characteristics (IVC) are simulated. The designed structures are assembled in real samples and their parameters are studied; the IVC of junctions with different degrees of shunting are measured, and the behavior of these junctions under the influence of high-frequency signals in the subterahertz range is studied.
This work is devoted to the study of tunnel Josephson superconductor-insulator-superconductor (SIS) junctions with a new type of shunting based on the usage of an additional superconductor-insulator-normal metal (NIS) junction located around the SIS junction. Numerical calculations of the parameters of such shunted junctions were carried out and modeling of their IVC (current-voltage characteristics) was performed. The designed samples were manufactured, their parameters were studied. To investigate the behavior of junctions under the influence of high-frequency signals in the sub-THz range, their IVCs were measured. Keywords: Superconducting devices, superconductor-insulator-superconductor tunnel junction, Josephson effect, shunting of Josephson junctions.
Josephson junctions (JJs) with non-hysteretic I-V characteristic and high critical parameters are required for many applications; in particular, they are needed to create tunable THz range oscillators. We fabricated and studied shunted superconductor-insulator-superconductor (SIS) Nb-AlOx-Nb tunnel junctions with an area of about 1μm2, estimated their parameters, and compared them with theoretical models. To assess the performance of shunted junctions in the terahertz range, their characteristics were studied under the influence of a high-frequency signal; the estimated value of the characteristic voltage for the shunted junction at a high frequency is about 1 mV. To improve the circuit parameters, a new type of the shunting based on application of NIS junctions was proposed, which reduces the influence of parasitic inductance, expands the frequency range of the generator, and decreases the size of the structure. The JJs with a new type of shunting have been fabricated and studied. The niobium integrated circuits intended for matching the superconducting oscillator and an SIS junction as a microwave detector were designed, fabricated, and tested. Such structures are required for studying the spectral characteristics and analyzing the operation of the oscillator. The developed circuits have demonstrated efficient operation at frequencies from 300 to 700 GHz; the experimental results are in good agreement with the simulated characteristics. The results obtained will be implemented for further development of the THz range oscillators.
Within the framework of the microscopic model of tunneling, we modeled the behavior of the Josephson junction shunted by the superconductor--insulator--normal-metal (SIN) tunnel junction. The electromagnetic impedance of the SIN junction includes both the frequency-dependent damping and reactance, which affect the circuit dynamics. We calculated the dc $I\text{\ensuremath{-}}V$ curves and transient characteristics and show their differences to the characteristics obtained within the simpler models. The correct operation of the basic single-flux-quantum circuits, i.e., the Josephson transmission line and the toggle flip-flop, based on such SIN-shunted Josephson junctions with small damping at low frequencies, has also been modeled.
A sidewall shunting method is demonstrated to realize overdamped (i.e., nonhysteretic) current-voltage (IV) characteristics in superconductor-insulator-superconductor (SIS) junctions. Unlike the conventional layout of shunted SIS junctions, patterning a normal metal shunt on one of the sides of the junction considerably reduces the parasitic inductive path and leaves more area on a chip for active components. Using a conventional optical lithography route we have fabricated sidewall shunted SIS junctions of various sizes from NbN–MgO–NbN trilayers on MgO and oxidized Si substrates. In our experiment the effective shunting resistance is realized by a tunnel contact between the shunt metal and the base electrode via a thin layer of insulating MgO. The results of our electrical measurements verify the overdamped nature of the junctions shunted in this manner.
Abstract At present, microwave transmission lines are characterized by a strong frequency dependence of loss in the subterahertz range. This work is aimed at development, research, and optimization of superconducting integrated circuits designed for matching the impedances of a long Josephson junction oscillator (a so-called “flux-flow oscillator”) and a superconductor–insulator–superconductor (SIS) detector in the subterahertz frequency range. The goal of this study is to improve and approve the numerical calculation methods, which make it possible to describe correctly experimental superconducting structures in a wide frequency range. Numerical calculations of integrated circuits have been performed in order to optimize the topology and parameters of transmission lines. The main parameters of the transmission lines and their influence on the signal propagation are determined. The results of optimization of integrated matching circuits in the range of 450–700 GHz have been experimentally confirmed. Optimization and improvement of transmission lines allow one to design new-generation integrated superconducting detectors and investigate tunnel SIS junctions more thoroughly (including shunted ones) and the properties of heterodyne oscillators based on long Josephson junctions.
Abstract Superconducting circuits based on Josephson junctions have the potential to achieve high speed and ultra-low power consumption, but their integration is limited by the low controllability of Nb-based tunnel junction and the existence of shunt resistors. In this work, we report the fabrication of superconductor/normal metal/superconductor (SNS) Josephson junctions with high reproducibility on oxidized Si substrates. The junctions based on NbN/TaN/NbN trilayers measured at 4.2 K show excellent Josephson properties with a wide range of critical current ( I c ) from 227 to 2000 μ A. The variations of critical current density ( J c ) and characteristic voltage ( V c ) are respectively less than 7.1% and 7.5% in the 2 inch region. The standard deviation of I c is calculated to be less than 1.7% for Josephson arrays with the number of junctions up to 10 000 occupying an area of 0.34 mm 2 . The results provide a guarantee for the applications of NbN-SNS Josephson junctions in superconducting large-scale integrated circuits.