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Applied Physics Letters

Delayed pulses from high-transparency Josephson junctions

Songtao Xu, C.A. Mancini, M. J. Feldman

Published May 14, 2001• 4 citations
Publisher

Abstract

Calculations of the shape and the timing of single-magnetic-flux quanta (SFQs) generated by high-current-density “self-shunted” Josephson junctions are presented. The junction current is assumed to be due in part to multiple Andreev reflections within the high-transparency barrier. The SFQ pulses from these junctions show several differences when compared to those from lower-current-density resistively shunted Josephson junctions, the most important being that the pulses are significantly delayed in the self-shunted junctions, by as much as many times the pulse width.

DOI
10.1063/1.1371969

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Calculations of the shape and the timing of single-magnetic-flux quanta (SFQs) generated by high-current-density “self-shunted” Josephson junctions are presented. The junction current is assumed to be due in part to multiple Andreev reflections within the high-transparency barrier. The SFQ pulses from these junctions show several differences when compared to those from lower-current-density resistively shunted Josephson junctions, the most important being that the pulses are significantly delayed in the self-shunted junctions, by as much as many times the pulse width.
Superconducting digital circuits based on Josephson junctions with amorphous niobium-silicon (a-NbSi) barriers have been designed, fabricated, and tested. Single-flux-quantum (SFQ) shift registers operated with ±30% bias margins, confirming junction reproducibility and uniformity. Static digital dividers operated up to 165 GHz for a single value of bias current, which was only marginally slower than circuits fabricated with externally shunted AlOx-barrier junctions having a comparable critical current density of 4.5 kA/cm2. In comparison, self-shunted a-NbSi junctions enabled a doubling in circuit density. This and their relatively thick 10 nm barriers could increase the yield of complex SFQ circuits.
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.
To increase the integration scale of superconductor electronics, we are developing a new node, titled SFQ7ee, of the fabrication process at MIT Lincoln Laboratory. In comparison to the existing SFQ5ee node, we increased the number of fully planarized superconducting layers to ten and utilized NbN and NbN/Nb kinetic inductors to increase the possible inductor number density above a hundred million inductors per cm2. Increasing the Josephson junction number density to the same level requires implementing self-shunted high-JcJosephson junctions. We investigated the properties of Nb/NbNx/Nb trilayer junctions as a potential replacement of high-JcNb/Al-AlOx/Nb junctions, where NbNxis a disordered, overstoichiometric, nonsuperconducting nitride deposited by reactive sputtering. Dependences of theIcRnproduct and Josephson critical current density, Jc on the NbNx barrier thickness and temperature were studied in the thickness range from 5 nm to 20 nm. The fabricated junctions can be well described by the microscopic theory of SNS junctions, assuming no suppression of the energy gap in Nb electrodes near the NbNxinterfaces and a Cooper pair decay length in the NbNx barrier, ξnTcof about 2.3 nm. Current-voltage characteristics of the junctions are well described by the resistively and capacitively shunted junction (RCSJ) model without excess current. In the studied rangeJc2, the Nb/NbNx/Nb junctions have lower values of the specific resistanceRnA, lower IcRn products, and a stronger dependence of theIcRnon temperature than the self-shunted or critically damped externally shunted Nb/Al-AlOx/Nb junctions with the same critical current density; A is the junction area,Icthe junction critical current, and Rnthe effective shunting resistance.
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.
By combining a long Josephson junction (LJJ) with single-flux-quantum (SFQ) circuits, new digital applications for long junctions can be explored. To fully investigate the dynamics of the hybrid LJJ/SFQ circuits, a numerical model was developed for an unshunted long junction coupled to shunted SFQ Josephson transmission line (JTL) through superconducting strips. The model is based on a one-dimensional array of Josephson junctions that is described by a set of discrete sine-Gordon equations. Simulations support our experimental observation that the first zero-field step can exist when the LJJ is terminated by SFQ elements. The influences on the first zero-field step by the self-inductance, the critical current, and the shunt resistance of JTL cells are studied in detail. Simulation also confirms that each 4/spl pi/ phase leap at the end of the long junction creates two separated SFQ pulses after several overdamped JTL cells.
Self-shunted aluminum Josephson junctions with high-transparency barriers were fabricated using the shadow-evaporation technique and measured at low temperatures, T=25 mK. Due to high junction transparency, the IV-characteristics were found to be of only small hysteresis with retrapping-to-switching current ratio of up to 80%. The observed critical currents were close to the Ambegaokar-Baratoff values (up to 80-100%). Good barrier quality was confirmed by the low subgap leakage currents in the quasiparticle branches, which makes the self-shunted Al junctions promising for application in integrated RSFQ-qubit circuitry.