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Applied Physics Letters

Overdamped Josephson junctions with Nb/AlOx/Al/AlOx/Nb structure for integrated circuit application

Masaaki Maezawa, Akira Shoji

Published Jun 30, 1997• 68 citations
Publisher

Abstract

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.

DOI
10.1063/1.119246

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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.
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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.
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
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