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arXiv:1710.02707 · Low Temperature Physics

Intrinsically shunted Josephson junctions for electronics applications

M. Belogolovskii, E. Zhitlukhina, V. Lacquaniti, N. De Leo, M. Fretto, A. Sosso

Published Jul 1, 2017• 27 citations
arXivHTMLPDF

Abstract

Conventional Josephson metal-insulator-metal devices are inherently underdamped and exhibit hysteretic current-voltage response due to a very high subgap resistance compared to that in the normal state. At the same time, overdamped junctions with single-valued characteristics are needed for most superconducting digital applications. The usual way to overcome the hysteretic behavior is to place an external low-resistance normal-metal shunt in parallel with each junction. Unfortunately, such solution results in a considerable complication of the circuitry design and introduces parasitic inductance through the junction. This paper provides a concise overview of some generic approaches that have been proposed in order to realize internal shunting in Josephson heterostructures with a barrier that itself contains the desired resistive component. The main attention is paid to self-shunted devices with local weak-link transmission probabilities that are so strongly disordered in the interface plane that transmission probabilities are tiny for the main part of the transition region between two super-conducting electrodes, while a small part of the interface is well transparent. We discuss the possibility of realizing a universal bimodal distribution function and emphasize advantages of such junctions that can be considered as a new class of self-shunted Josephson devices promising for practical applications in superconducting electronics operating at 4.2 K.

DOI
10.1063/1.4995622

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Conventional Josephson metal-insulator-metal devices are inherently underdamped and exhibit hysteretic current-voltage response due to a very high subgap resistance compared to that in the normal state. At the same time, overdamped junctions with single-valued characteristics are needed for most superconducting digital applications. The usual way to overcome the hysteretic behavior is to place an external low-resistance normal-metal shunt in parallel with each junction. Unfortunately, such solution results in a considerable complication of the circuitry design and introduces parasitic inductance through the junction. This paper provides a concise overview of some generic approaches that have been proposed in order to realize internal shunting in Josephson heterostructures with a barrier that itself contains the desired resistive component. The main attention is paid to self-shunted devices with local weak-link transmission probabilities that are so strongly disordered in the interface plane that transmission probabilities are tiny for the main part of the transition region between two super-conducting electrodes, while a small part of the interface is well transparent. We discuss the possibility of realizing a universal bimodal distribution function and emphasize advantages of such junctions that can be considered as a new class of self-shunted Josephson devices promising for practical applications in superconducting electronics operating at 4.2 K.
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.
An interesting feature of Superconductor Normal metal Superconductor - Josephson junctions for digital applications is due to their non-hysteretic current-voltage characteristics in a broad temperature range below Tc. This allows to design Single-Flux-Quantum (SFQ) cells without the need of external shunts. Two advantages can be drawn from this property: first the SFQ cells can be more compact which leads to a more integrated solution towards nano-devices and more complex circuits; second the absence of electrical parasitic elements associated with the wiring of resistors external to the Josephson junctions increases the performance of SFQ circuits, in particular regarding the ultimate speed of operation. For this purpose Superconductor Normal metal Insulator-Superconductor Nb/Al-AlOx/Nb Josephson junctions have been recently developed at INRiM with aluminum layer thicknesses between 30 and 100 nm. They exhibit non-hysteretic current-voltage characteristics with RnIc values higher than 0.5 mV in a broad temperature range and optimal Stewart McCumber parameters at 4.2 K for RSFQ applications. The main features of obtained SNIS junctions regarding digital applications are presented.
Improved Characteristics of Intrinsically Shunted Nb/Al−AlOx−Nb{\rm Nb/Al}-{\rm AlO}_{x}-{\rm Nb}Nb/Al−AlOx​−Nb Josephson Junctions
Josephson junctions with non-hysteretic current-voltage characteristics form the basis of most superconducting electronic circuits including RSFQ logic and programmable Josephson voltage standards. In contrast to conventional SIS devices, Nb/Al-AlOx-Nb (SNIS) junctions with an additional comparatively thick Al interlayer and a comparatively thin barrier AlOxare intrinsically shunted at T ≥ 4.2 K . In this contribution, we provide experimental and theoretical arguments proving that this finding is mainly explained by a broad distribution of highly-conductive barrier transparencies with a significant effect from nearly ballistic modes. An additional advantage of the proposed SNIS junctions is possibility to tune the critical voltage value by modifying Nb and/or Al film thicknesses. With observations of wide Shapiro steps up to 1.25 V at 6.3 K we show that this type of Josephson junctions can be successfully used at temperatures above 4.2 K. The presence of well-developed quantized voltage features even at 7.2 K means that Nb/Al-AlOx-Nb devices can successfully operate far above the liquid helium temperature and, in principle, are compatible with two-stage cryocoolers.
Abstract We report on the electrical properties of NbN/TaN/NbN Josephson junctions grown on thermally oxidized silicon substrates, along with the design and fabrication of superconducting single-flux-quantum (SFQ) circuits based on these NbN superconductor/normal metal/superconductor (SNS) junctions. The critical current density ( J c ) of the junctions was found to be relatively sensitive to the barrier thickness, decreasing from 108.0 ± 8.1 kA cm −2 for a 15 nm barrier to 12.8 ± 1.9 kA cm −2 for a 30 nm barrier. For a J c of approximately 24.5 ± 2.1 kA cm −2 and a barrier thickness of 25 nm, the NbN SNS junctions are self-shunted and exhibit nonhysteretic current–voltage ( I – V ) characteristics. Especially for junctions with diameter ( φ ) ranging from 0.8 to 1.6 μ m, their critical current ( I c ) falls within the range of 110–450 μ A, making them suitable for SFQ circuits. By considering the impact of excess current and incorporating it as an additional term in the conventional resistively and capacitively shunted junction model, the I – V curves of NbN SNS junctions can be precisely described, successfully minimizing the deviation between simulations and test results. The DC-SFQ and SFQ-DC interface circuits can both operate normally, and the bias margins of cell circuits such as Josephson transmission line, confluence buffer, D flip-flop, and splitter are greater than 40%. Compared to Nb superconductor/insulator/superconductor junctions, their self-shunting characteristics and relatively thick 25 nm barriers can also enhance the integration of circuits and increase the yield to complex circuits.
Optimization of the performance of Nb-based overdamped Josephson junctions for applications to superconducting electronics is a major issue today. The use of cryocoolers, thus avoiding liquid helium refrigeration, with junctions working at temperatures higher than 4.2 K, could greatly increase the use of these devices in applications such as fundamental metrology and measurement sensors. The question we address here is how to maximize the characteristic voltage and minimize the temperature influence on the current-voltage curves of overdamped junctions enabling their use at T > 4.2K. To optimize the properties of a single Josephson heterostructure, we propose a type of overdamped junction that consists of a relatively thin insulating layer and a thick (tens of nm) normal-metal film between two superconducting electrodes. Measurements of the dependence of the critical current and characteristic voltage as function of temperature for dierent electrodes configurations show how it is possible to improve the design of these junctions.
Double-barrier Josephson junctions, with outer electrodes of superconducting Nb and a central thin layer of non-superconducting Al, are being developed as self-shunted junctions for high-frequency superconducting integrated circuits. This paper presents a simplified phenomenological model of the behaviour of such junctions (often called SINIS, for superconductor-insulator-normal-insulator-superconductor) based on the resistively shunted junction model, using Josephson coupling between the layers and the time-dependent Ginzburg-Landau equation to describe the dynamics of the weak induced energy gap in the Al layer. This analysis shows how non-hysteretic I - V characteristics can be obtained but suggests some limitations in their high-frequency performance. Predictions for critical current generally agree with prior experiments. The technological implications of this model are discussed.