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International Conference on Applied Superconductivity and Electromagnetic Devices

Characterization of Josephson tunneling junction by lifetime of zero-voltage state and fiske self-resonant step

Hao Li, Gang Li, Jianshe Liu, Tie Fu Li, Wei Chen

Published Oct 1, 2013• 0 citations
Publisher

Abstract

We fabricated high quality Josephson tunneling junctions by self-aligned lift-off process. Parameters including its critical current, normal resistance and capacitance which determine the dynamic energy consumption and speed of the superconducting electronics had been measured. The critical current of a 22 × 22 μm2Junction is 1796 ± 52 μA calculated from the dependent curve between the lifetime of zero-voltage and switching current. The junction capacitance is 24.7 ± 3.9 pF obtained by observing Fiske self-resonant step when applied several Gauss magnetic field.

DOI
10.1109/asemd.2013.6780835

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We fabricated high quality Josephson tunneling junctions by self-aligned lift-off process. Parameters including its critical current, normal resistance and capacitance which determine the dynamic energy consumption and speed of the superconducting electronics had been measured. The critical current of a 22 × 22 μm2Junction is 1796 ± 52 μA calculated from the dependent curve between the lifetime of zero-voltage and switching current. The junction capacitance is 24.7 ± 3.9 pF obtained by observing Fiske self-resonant step when applied several Gauss magnetic field.
Parametric Characterization and Macroscopic Quantum Tunneling of Nb/AlOx/Nb\hbox{Nb}/\hbox{AlO}_{\rm x}/\hbox{Nb}Nb/AlOx​/Nb Josephson Junctions
Parameters ofNb/AlOx/Nb\hbox{Nb}/\hbox{AlO}_{\rm x}/\hbox{Nb}Nb/AlOx​/NbJosephson including its critical current and capacitance which determine the dynamic energy consumption and speed of the superconducting electronics have been measured. The critical current of a22×22 μm222 \times 22 \ \mu\hbox{m}^{2}22×22 μm2junction is1801±52 μA1801 \pm 52 \ \mu\hbox{A}1801±52 μAcalculated from the dependent curve between the lifetime of zero-voltage and switching current. The specific capacitance is0.051±0.008 pF/μm20.051 \pm 0.008 \ \hbox{pF}/\mu\hbox{m}^{2}0.051±0.008 pF/μm2obtained by observing Fiske self-resonant step when applied several Gauss magnetic field. Critical current distributions had been measured as a function of temperature from 900 mK to 40 mK. The distribution widths become independent of temperature below 200 mK, which indicates the transition between thermal activation and macroscopic quantum tunneling of junction phase.
The transport properties of a small superconductor--normal-metal--superconducting tunnel junction can be controlled by a gate electrode coupled capacitively to the central island. We evaluate the critical Josephson current Ic{\mathit{I}}_{\mathit{c}}Ic​ through such a system as a function of the gate voltage Vg{\mathit{V}}_{\mathit{g}}Vg​ taking into account parity effects in the superconductors. The dependence Ic{\mathit{I}}_{\mathit{c}}Ic​(Vg{\mathit{V}}_{\mathit{g}}Vg​) shows resonant singularities and has a qualitatively different character for \ensuremath{\Delta}EC{\mathit{E}}_{\mathit{C}}EC​ and \ensuremath{\Delta}>EC{\mathit{E}}_{\mathit{C}}EC​, \ensuremath{\Delta} being the superconducting gap and EC{\mathit{E}}_{\mathit{C}}EC​=e2{\mathit{e}}^{2}e2/2C being the charging energy. Due to the sweeping of the gate voltage, the system can be driven to states which are metastable with respect to a change of the number of electrons on the central island. Since the lifetime of the metastable states can be macroscopically large, one can observe a remarkable bistability of the Josephson current. A weak magnetic field suppresses the interference of the electrons propagating through the central electrode, and, consequently, the magnitude of the critical current.
The microwave response of the dc quasiparticle tunneling current in Josephson tunnel junctions, where the Josephson current is suppressed by an external magnetic field, has been studied quantitatively in order to clarify its characteristics as a probe for the measurement of the junction capacitance. Extensive experiments for both small and long junctions are carried out for distinguishing between microwave behaviors of lumped and distributed constant junctions. It is shown that the observed voltage dependence of the dc quasiparticle tunneling current modified by an applied rf field is in good agreement with a theoretical result which takes into account the influence of the microwave circuit connected to the junction. The comparison between theory and experiment gives the magnitude of the internal rf field in the junction. Together with the applied rf field, this internal rf field leads to the junction rf impedance which is dominated by the junction capacitance in our experimental condition. In the case of lumped junctions, this experimental rf impedance is in reasonable agreement with the theoretical one with the junction capacitance estimated from the Fiske step of the distributed junction fabricated on the same substrate; the obtained ratio of the experimental impedance to the theoretical one is approximately 0.6–1.7. In the case of distributed junctions, however, experimental values of their characteristic impedances are approximately 0.2–0.3 of theoretical values calculated by assuming the one-dimensional junction model and taking account of the standing-wave effect in the junction.
Josephson junction transistors and 50-junction arrays with linear junction dimensions from 200nm down to 70nm were fabricated from standard Nb∕AlOx∕Nb trilayers. The fabrication process includes electron beam lithography, dry etching, anodization, and planarization by chemical-mechanical polishing. The samples were characterized at temperatures down to 25mK. In general, all junctions are of high quality and their I-U characteristics show low leakage currents and high superconducting energy gap values of Δ≈1.35meV. The characteristics of the transistors and arrays exhibit some features in the subgap area, associated with tunneling of Cooper pairs, quasiparticles, and their combinations due to the redistribution of the bias voltage between the junctions. Total island capacitances of the transistor samples ranged from 1.5fF to 4fF, depending on the junction sizes. Devices made of junctions with linear dimensions below 100nm×100nm demonstrate a remarkable single-electron behavior in both superconducting and normal state. We also investigated the area dependence of the junction capacitances for transistor and array samples.
Transitions from the zero-voltage to the voltage state have been directly measured on large (≃104 μm2) Josephson junctions. By adjusting the current load, the switching times (10–90%) were varied from 60 to 400 psec. Good agreement was found between theoretical and experimental results. The times required to transfer current from junctions into superconductive inductances (10−11−10−10 H) were also investigated. The switching times ranged from 300 to 550 psec and agreed basically with our simple theoretical model. To explain the details of the voltage developed during current transfer, self-pumped junction resonances had to be invoked. Although these times are expected to be smaller in miniaturized devices, they represent to our knowledge the shortest times ever reported for circuits with a dissipation of less than 10−5 W in continuous operation.