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Journal of Physics Conference Series

Temperature-dependent current-voltage characteristics of niobium SNIS Josephson junctions

V. Lacquaniti, Mikhail A. Belogolovskii, Domenico Andreone, Cristina Cassiago, Natascia De Leo, Matteo Fretto, A. Sosso

Published Jun 1, 2010• 2 citations
Publisher

Abstract

Motivated by a search for a suitable technology to fabricate Josephson junctions with a tunable damping regime, we performed a systematic study of the temperature effect on the critical current in Nb/Al-AlO x -Nb heterostructures with a nanometer-thick Al interlayer. For Al layer thicknesses ranging from 40 to 110 nm, we have observed a transition from hysteretic (below 4.2 K) to non-hysteretic (above 4.2 K) current-voltage curves. Measured supercurrent-vs-temperature characteristics which significantly differ from those of traditional SIS and SNS devices are interpreted in terms of the superconducting proximity effect between Al and Nb films. Thermal stability and good reproducibility of our junctions are demonstrated.

DOI
10.1088/1742-6596/234/4/042018

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Motivated by a search for a suitable technology to fabricate Josephson junctions with a tunable damping regime, we performed a systematic study of the temperature effect on the critical current in Nb/Al-AlO x -Nb heterostructures with a nanometer-thick Al interlayer. For Al layer thicknesses ranging from 40 to 110 nm, we have observed a transition from hysteretic (below 4.2 K) to non-hysteretic (above 4.2 K) current-voltage curves. Measured supercurrent-vs-temperature characteristics which significantly differ from those of traditional SIS and SNS devices are interpreted in terms of the superconducting proximity effect between Al and Nb films. Thermal stability and good reproducibility of our junctions are demonstrated.
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.
Niobium-based junctions are still the best candidates for many small-scale applications of superconductivity and in particular, for the design and the fabrication of programmable voltage standards and D/A converters of fundamental accuracy. Non-hysteretic junctions with a large current density JC, and a reasonable characteristic voltage Vc are required, while, in order to make these circuits available to a wider market, operation at temperatures above 4.2 K would be important. In this work we report our results achieved on Nb/Al-AlOX/Nb junctions whose properties are studied in order to optimize the aforementioned features. Their behavior is analyzed showing how the overdamped characteristic is obtained with a less critical structure compared to other type of junctions. Moreover, using the scattering matrix approach, we have calculated a deviation of superconducting current-phase relation in such a SNIS Josephson junction from sinusoidal one and its effect on the current amplitudes of Shapiro steps as function of temperature. Measurement of the step dependence on temperature are also reported, where we considered the possibility of using bothn= 1 andn= 2 RF step above 4.2 K, for a possible reduction of the number of junctions in array circuits.
We present a systematic study of the temperature-dependent transport properties of Nb- and Al-based superconductor--insulator--thin superconductor--ferromagnet--superconductor (SIsFS) Josephson junctions. By varying the superconducting material and the thickness of the intermediate superconducting layer, we identify three distinct transport regimes: a tunnel-dominated SIS-like regime, a temperature-driven crossover, and a complex weak-link regime. A unified Usadel-based description connects these regimes through the self-consistent pair potential of the intermediate s-layer. While for Al-based devices an SIS-like transport regime is observed, a self-consistent calculation of the s/F/S proximity electrode, combined with perturbative tunnel coupling to the outer electrode, accounts for Nb devices with different thickness of the superconducting interlayer. The calculation places the 10-nm-interlayer sample in a proximity-supported, strongly suppressed s state, close to the SInFS limit in which the interlayer is effectively driven normal (\textit{n}). A reduced nonlinear long-junction description captures the same limiting physics. The evolution of I_cR_N(T) thereby provides a direct experimental signature of the three transport regimes. Beyond the fundamental interest, the results establish practical design guidelines: Nb junctions with sufficiently thick s interlayers are well suited for high-speed cryogenic memory elements and tunable superconducting parametric amplifiers, while Al junctions are natural candidates for coherent superconducting quantum 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.
SNS Nb/Al-AlOx/Nb Josephson junctions have been recently developed with Al layer thicknesses 30- 100 nm , for which non-hysteretic I-V characteristics have been obtained by oxidation ranging from 150 to 200 Pa.s. The Stewart-McCumber parameter of the junctions can be lower or higher than unity, depending on the above-mentioned conditions. Typical Jc values of several tens of kA/cm2 and RnIc values up to 0.5-0.7 mV at 4.2 K have been obtained, suitable for digital applications. Two interesting features of this technology are: i) the absence of external shunt resistance that can allow higher integration and; ii) the reproducibility of technological parameters that is connected to a least dependence of the Josephson critical current on the barrier thickness. In this work, we will present a detailed analysis of speed and jitter that can be expected from basic digital circuits based on such a technology. This analysis is based on the accurate experimental extraction of typical quantities, like the Stewart-McCumber parameter or specific capacitance. In particular, the influence of the temperature of operation on the performance of elementary cells will be assessed. A comparisonwith similar cells based on externally-shunted Josephson junctions will be presented in order to evaluate the ranges of parameters for which this technology exhibits some clear advantages. In particular we will show the technological constraints that need to be met in order to build circuits that can compete with existing technologies
Device-modeling physics of four-layered Nb/Al-ultra-thin AlO x barrier-Nb Josephson structures is discussed, empha sizing practical applications. The temperature effect on the minigap induced in a proximized Al layer and that on a critical voltage across the SNIS junction is numerically simulated. The McCumber-Stewart parameter is found to decreasefromabout2at1.7Ktovalues nearunityat4.2K, becauseofthe temperature suppressedsubgap resistance.Due to the single-valued current-voltage dependence and comparatively high critical voltage values, the overdamped SNIS junctionshave demonstrated well-developed rf-induced featuresinthe current-voltage characteristicsevenfarabove4.3 K, appealing properties for programmable Josephson voltage standard applications.