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arXiv:cond-mat/0411607 · Applied Physics Letters

Josephson junctions with nonlinear damping for rapid single-flux-quantum - qubit circuits

Alexander B Zorin, M. Khabipov, D Balashov, R. Dolata, F.-I. Buchholz, J. Niemeyer

Published Jan 7, 2005• 15 citations
arXivHTMLPDF

Abstract

We demonstrate that shunting of superconductor-insulator-superconductor (S-I-S) Josephson junctions by superconductor-insulator-normal metal (S-I-N) structures having pronounced nonlinear I–V characteristics can remarkably modify the Josephson dynamics. In the regime of Josephson generation the phase behaves as an overdamped coordinate, while in the superconducting state the damping and current noise are strikingly small, that is vitally important for application of such junctions for readout and control of Josephson qubits. Superconducting Nb/AlOx/Nb junction shunted by Nb/AlOx/AuPd junction of S-I-N type was fabricated and, in agreement with our model, exhibited nonhysteretic I–V characteristics at temperatures down to at least 1.4 K.

DOI
10.1063/1.1852076

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Applied Physics Letters•Aug 28, 2000•5 citations
We demonstrate that shunting of superconductor-insulator-superconductor (S-I-S) Josephson junctions by superconductor-insulator-normal metal (S-I-N) structures having pronounced nonlinear I–V characteristics can remarkably modify the Josephson dynamics. In the regime of Josephson generation the phase behaves as an overdamped coordinate, while in the superconducting state the damping and current noise are strikingly small, that is vitally important for application of such junctions for readout and control of Josephson qubits. Superconducting Nb/AlOx/Nb junction shunted by Nb/AlOx/AuPd junction of S-I-N type was fabricated and, in agreement with our model, exhibited nonhysteretic I–V characteristics at temperatures down to at least 1.4 K.
An interesting feature of Superconductor Normal metal Superconductor - Josephson junctions for digital applications is due to their non-hysteretic current-voltage characteristics in a broad temperature range below Tc. This allows to design Single-Flux-Quantum (SFQ) cells without the need of external shunts. Two advantages can be drawn from this property: first the SFQ cells can be more compact which leads to a more integrated solution towards nano-devices and more complex circuits; second the absence of electrical parasitic elements associated with the wiring of resistors external to the Josephson junctions increases the performance of SFQ circuits, in particular regarding the ultimate speed of operation. For this purpose Superconductor Normal metal Insulator-Superconductor Nb/Al-AlOx/Nb Josephson junctions have been recently developed at INRiM with aluminum layer thicknesses between 30 and 100 nm. They exhibit non-hysteretic current-voltage characteristics with RnIc values higher than 0.5 mV in a broad temperature range and optimal Stewart McCumber parameters at 4.2 K for RSFQ applications. The main features of obtained SNIS junctions regarding digital applications are presented.
Within the framework of the microscopic model of tunneling, we modeled the behavior of the Josephson junction shunted by the superconductor--insulator--normal-metal (SIN) tunnel junction. The electromagnetic impedance of the SIN junction includes both the frequency-dependent damping and reactance, which affect the circuit dynamics. We calculated the dc $I\text{\ensuremath{-}}V$ curves and transient characteristics and show their differences to the characteristics obtained within the simpler models. The correct operation of the basic single-flux-quantum circuits, i.e., the Josephson transmission line and the toggle flip-flop, based on such SIN-shunted Josephson junctions with small damping at low frequencies, has also been modeled.
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.
A sidewall shunting method is demonstrated to realize overdamped (i.e., nonhysteretic) current-voltage (IV) characteristics in superconductor-insulator-superconductor (SIS) junctions. Unlike the conventional layout of shunted SIS junctions, patterning a normal metal shunt on one of the sides of the junction considerably reduces the parasitic inductive path and leaves more area on a chip for active components. Using a conventional optical lithography route we have fabricated sidewall shunted SIS junctions of various sizes from NbN–MgO–NbN trilayers on MgO and oxidized Si substrates. In our experiment the effective shunting resistance is realized by a tunnel contact between the shunt metal and the base electrode via a thin layer of insulating MgO. The results of our electrical measurements verify the overdamped nature of the junctions shunted in this manner.
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.
For a study of the supercurrent–phase relationship, we have fabricated Nb/Al/AlOx/Al/AlOx/Al/Nb-based Josephson junctions of low critical current density. At 4.2 K, junctions of a nominal area of 10×10 μm2 showed a nonhysteretic dc current–voltage characteristic with a critical current of about 10 μA. Similar junctions were part of washer-type rf superconductor quantum interference devices. These samples were inductively coupled to high-quality tank circuits for measurements at 4.2 K or at higher temperatures up to values close to the critical temperature of niobium. In this range, the current–phase relation, as deduced from impedance measurements of the phase-biased junction, did not deviate significantly from the sinusoidal shape.