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Applied Physics Letters

One-dimensional parallel Josephson-junction arrays as a tool for diagnostics

Herre S. J. van der Zant, R. A. M. Receveur, Terry P. Orlando, Alan W. Kleinsasser

Published Oct 17, 1994• 28 citations
Publisher

Abstract

We propose and demonstrate the use of underdamped, one-dimensional parallel arrays of Josephson junctions as a tool for circuit diagnostics. By measuring the Fiske modes and the critical current in a magnetic field, we determined the self- and nearest-neighbor inductances as well as the capacitances of single junctions. We have used this technique to find the capacitance of Nb-Al2Ox-Nb junctions for critical current densities of 0.3–20 kA/cm2. We find that the specific capacitance increases by about a factor of 2 over this range. This increase has important consequences for the design of single-flux-quantum circuits and SQUIDs. Measurement of the junction capacitance for critical current densities of 100 kA/cm2 is possible, but requires submicrometer junctions with dimensions of the order of 0.3 μm.

DOI
10.1063/1.113041

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One-dimensional parallel Josephson-junction arrays as a tool for circuit diagnostics

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We propose and demonstrate the use of underdamped, one-dimensional parallel arrays of Josephson junctions as a tool for circuit diagnostics. By measuring the Fiske modes and the critical current in a magnetic field, we determined the self and nearest neighbor inductances as well as the capacitances of single junctions. We have used this technique to find the capacitance of Nb-Al/sub 2/O/sub x/-Nb junctions for critical current densities of 0.3-20 kA/cm/sup 2/. We find that the specific capacitance increases by about a factor of 2 over this range. This increase has important consequences for the design of single-flux-quantum circuits and SQUIDs. Measurement of the junction capacitance for critical current densities of 100 kA/cm/sup 2/ is possible, but requires submicron junctions with dimensions of the order of 0.3 /spl mu/m.>

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IEICE Transactions on Electronics•Jan 1, 2011•0 citations
We propose and demonstrate the use of underdamped, one-dimensional parallel arrays of Josephson junctions as a tool for circuit diagnostics. By measuring the Fiske modes and the critical current in a magnetic field, we determined the self- and nearest-neighbor inductances as well as the capacitances of single junctions. We have used this technique to find the capacitance of Nb-Al2Ox-Nb junctions for critical current densities of 0.3–20 kA/cm2. We find that the specific capacitance increases by about a factor of 2 over this range. This increase has important consequences for the design of single-flux-quantum circuits and SQUIDs. Measurement of the junction capacitance for critical current densities of 100 kA/cm2 is possible, but requires submicrometer junctions with dimensions of the order of 0.3 μm.
We propose and demonstrate the use of underdamped, one-dimensional parallel arrays of Josephson junctions as a tool for circuit diagnostics. By measuring the Fiske modes and the critical current in a magnetic field, we determined the self and nearest neighbor inductances as well as the capacitances of single junctions. We have used this technique to find the capacitance of Nb-Al/sub 2/O/sub x/-Nb junctions for critical current densities of 0.3-20 kA/cm/sup 2/. We find that the specific capacitance increases by about a factor of 2 over this range. This increase has important consequences for the design of single-flux-quantum circuits and SQUIDs. Measurement of the junction capacitance for critical current densities of 100 kA/cm/sup 2/ is possible, but requires submicron junctions with dimensions of the order of 0.3 /spl mu/m.>
Measurement of Specific Capacitance for Nb/Al–AlO x_{x}x​/Nb Josephson Junctions in Single-Flux Quantum Circuit Applications
In this study, we investigate different technologies to measure the specific capacitances C8of Nb/Al-AlOx/Nb Josephson junctions. They involve measuring Fiske mode resonance in long Josephson junctions, superconducting quantum interference device (SQUID) resonance, and Josephson plasma resonance. We find a logarithmic relationship between the junction C8and its critical current density Jcin the range 0.1-2 kA/cm2using the Fiske mode resonance method. Our measured C8values using SQUID resonance and Josephson plasma resonance are compared with the results obtained by other groups via analyzing the major errors in the C8calculation. We discuss the accuracy in evaluating C8for all three techniques and suggest using Fiske mode resonance as a relatively stable method for C8extraction for the future design of our single-flux quantum circuit.
The specific capacitance Cs of Nb/AlOx/Nb Josephson tunnel junction has been measured by means of the superconducting quantum interference device (SQUID) resonance technique. We have investigated the junctions with critical current densities Jc in the range of 0.1–18 kA/cm2 and found that 1/Cs linearly depends on a logarithm of Jc. This suggests that the barrier thickness is moderately uniform in the junction area and increases continuously during growth. The results also show that increasing Jc reduces time constants of the junction.
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.
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.
We describe the fabrication processes and electrical characteristics of two types of NbN junctions. One is a self-shunted NbN/NbNx/AlN/NbN Josephson junction, which is expected to improve the density of integrated circuits; the other is an underdamped NbN/AlNx/NbN tunnel junction with radical-nitride AlNx barriers, which has highly controllable junction characteristics. In the former, the junction characteristics were changed from underdamped to overdamped by varying the thickness of the NbNx layer. Overdamped junctions with a 6-nm-thick NbNx film exhibited a characteristic voltage of Vc =0.8mV and a critical current density of Jc =22A/cm2 at 4.2K. In the junctions with radical-nitride AlNx barriers, Jc could be controlled in the range 0.01-3kA/cm2 by varying the process conditions, and good uniformity of the junction characteristics was obtained.