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IEEE Transactions on Applied Superconductivity

Engineering Overdamped Niobium-Based Josephson Junctions for Operation Above 4.2 K

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

Published Jun 1, 2009• 27 citations
Publisher

Abstract

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.

DOI
10.1109/tasc.2009.2019072

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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.
Overdamped Nb/Al-AlOx/Nb Josephson junctions are an intermediate state between the SIS and SNS Josephson junctions. With a proper choice of fabrication parameters, stable and reproducible nonhysteretic curent-voltage characteristics have been obtained Featuring critical current densities up to 70 kA/cm2and characteristic voltages up to 450 μV at 4.2 K, overdamped junctions are suitable for many aplications ranging from RSFQ electronic circuits to programmable and ac voltage standards. Tests on single junctions arrays of series-connected junctions have shown a suitable behaviour when driven by a microwave with frequencies around 75 GHz. The quantized steps have proven to be stable and flat at nV level. These junctions also shown an improved temperature dependence, which can be interesting in view of the development instrumentation with an integrated quantum voltage reference operated in a closed cycle refrigerator.
We present characteristics of overdamped Josephson junctions consisting of Nb/AlOx/Al/ AlOx/Nb structures. The junctions were fabricated using a well-developed Nb/AlOx/Nb-junction technology and showed well-defined Josephson characteristics at 4.2 K. The characteristic voltage Vc [the product of the critical current Ic and the effective normal resistance Rn(eff)] of junctions, which determines high-frequency performance of the junction, was in the range of 0.1–0.5 mV, and the critical current density Jc in the range of 102–104 A/cm2. Maximum-to-minimum variations in Ic over a wafer were ±4% for junctions with Vc=0.15 mV and ±13% for junctions with Vc=0.5 mV.
Nb/Al-AlOx/Nb Josephson junctions of few micrometers size, derived by the basic hysteretic SIS process, have shown stable overdamped current voltage, I-V, characteristic with Ic of 1–10 mA and Vc of 100–600 μV at 4.2 K. The moderate dependence on temperature of these parameters has allowed to use them at temperatures above 4.2 K, near the Nb transition temperature, measuring quantized voltage steps several hundreds μA wide on single and 10 junctions arrays at T ≫ 8 K. This paves the way to their use in cryocooler setup of reduced cost and power consumption for a future generation of programmable and AC voltage standard.
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.
Binary-divided IV arrays with 8192 overdamped Nb/Al-AlOx/Nb Josephson junctions, fabricated within a joint cooperation between INRIM and PTB, have been tested at 4.2 K and in a temperature range from 6 to 7 K. We measured the microwave-induced step amplitude and checked the uniformity of junction parameters and microwave response on the whole array and sub-segments. The characteristic voltage Vc of the fabricated junctions varies from 0.25 to 0.6 mV. For the lower limit of Vc we could demonstrate at 4.2 K proper operation of the 1 V circuits with current margins larger than 1 mA for all array segments. In the case of larger values for Vc the arrays can be operated at the second and third Shapiro step, when radiated with microwaves from 69 to 74 GHz. Unsloped 3rd harmonic steps whose width ranged from 0.3 mA up to and above 1 mA have been measured on all subsegments providing voltages up to 3.73 V at 73.7 GHz across the whole array. Devices with high values of Vc (at 4.2 K) have been also successfully operated at 6-7 K, by optimizing the first Shapiro step. Quantized voltages up to 1.24 V with current margins between 0.5 and 0.8 mA have been observed.
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.