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Binary-divided arrays with intrisically overdamped Nb/Al-AlOx/Nb (SNIS) junctions driven at 70 GHz

V. Lacquaniti, Natascia De Leo, Matteo Fretto, A. Sosso, Franz Mueller, J. Kohlmann

Published Jun 1, 2010• 0 citations
Publisher

Abstract

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.

DOI
10.1109/cpem.2010.5543468

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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.
Binary-divided 1 V arrays were fabricated for programmable Josephson voltage standards in close cooperation between INRIM and PTB. The arrays, consisting of 8192 overdamped Nb/Al-AlO(x)Nb SNIS Josephson junctions, were successfully operated at microwave frequencies around 70 GHz. While the characteristic voltage of the junctions ranged from 0.25 to 0.6 mV, the current margins were larger than 1 mA for the Shapiro step n = 1. In addition, flat Shapiro steps of higher order (n = 3 and 4) were generated at increased microwave power levels due to the large characteristic voltage, demonstrating the small parameter spread of the SNIS junctions. Operation at temperatures up to 6-7 K has also been verified, measuring quantized voltages up to 1.25 V on the n = 1 step.
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.
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.
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.
We have investigated two-dimensional arrays of overdamped Josephson junctions that have been fabricated in Nb/Al/sub 2/O/sub 3//Nb-technology, together with microwave coupling structures. Different arrays have shown to emit coherent microwave radiation by using microwave-coupled on-chip detectors. Low-temperature scanning electron microscopy is used to study the coupling of the microwave radiation to an on-chip load. From the appearance of Shapiro steps in the current vs. voltage curves of detector junctions for different degrees of disorder in the array-junctions, margins of the spread in the critical currents of the Josephson junctions for coherent oscillation can be drawn.