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IEEE Transactions on Magnetics

Current-voltage characteristics of nanoampere Josephson junctions

Ronald H. Ono, Michael W. Cromar, Richard L. Kautz, Robert J. Soulen, Jack H. Colwell, William E. Fogle

Published Mar 1, 1987• 32 citations
Publisher

Abstract

We have studied the current-voltage characteristics of small area tunnel junctions at temperatures below 1 K. The junctions were made in an edge geometry with a Nb base electrode and had areas less than .05 μm2and critical currents in the nA range. Although the measured I-V characteristics resemble those of ordinary hysteretic junctions, the supposed zero-voltage portion of the curve proved to have a finite slope and to deviate from zero voltage. For these junctions it is apparently possible for occasional 2π phase slips to occur without switching to the usual voltage state. This behavior can be explained either by macroscopic quantum tunneling or by a model in which the effective shunt conductance of the junction is frequency dependent.

DOI
10.1109/tmag.1987.1064932

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We have studied the current-voltage characteristics of small area tunnel junctions at temperatures below 1 K. The junctions were made in an edge geometry with a Nb base electrode and had areas less than .05 μm2and critical currents in the nA range. Although the measured I-V characteristics resemble those of ordinary hysteretic junctions, the supposed zero-voltage portion of the curve proved to have a finite slope and to deviate from zero voltage. For these junctions it is apparently possible for occasional 2π phase slips to occur without switching to the usual voltage state. This behavior can be explained either by macroscopic quantum tunneling or by a model in which the effective shunt conductance of the junction is frequency dependent.
Low βc tunnel junctions have current-voltage characteristics which deviate from the intrinsic quasiparticle characteristic because of the ac Josephson effect. This effect has previously been analyzed for the special case of a pointlike junction in zero applied field. We demonstrate that deviations still occur when the critical current is suppressed by an applied field to a small fraction of its zero field value. The effects are significant for junctions whose length l is comparable to the Josephson penetration length λJ. These are often the junctions of practical interest. We present a simple model to explain the physical origin of these deviations, and also present detailed computer simulations using up to nine segments to model the distributed junction. For l = λJ, and an applied magnetic field which corresponds to the first minimum of the junction threshold characteristic, we find time-averaged voltages which deviate from the quasiparticle characteristic by up to 5%. Their magnitude is proportional to (l/λJ)2 and inversely proportional to the square of the applied field. This dependence can be used experimentally to confirm the observation of the effect.
Motivated by a search for a suitable technology to fabricate Josephson junctions with a tunable damping regime, we performed a systematic study of the temperature effect on the critical current in Nb/Al-AlO x -Nb heterostructures with a nanometer-thick Al interlayer. For Al layer thicknesses ranging from 40 to 110 nm, we have observed a transition from hysteretic (below 4.2 K) to non-hysteretic (above 4.2 K) current-voltage curves. Measured supercurrent-vs-temperature characteristics which significantly differ from those of traditional SIS and SNS devices are interpreted in terms of the superconducting proximity effect between Al and Nb films. Thermal stability and good reproducibility of our junctions are demonstrated.
We have measured the distribution in current for the onset of voltage in small-area Josephson junctions for temperatures down to \ensuremath{\sim}1.6 K. The lifetime of the 〈V〉=0〈V〉=0〈V〉=0 state for our highest current density junctions becomes temperature independent for ${k}_{\mathrm{B}}T\ensuremath{\lesssim}\frac{\ensuremath{\hbar}{\ensuremath{\omega}}_{0}}{20}$, a TTT much less than the WKB prediction (${\ensuremath{\omega}}_{0}$ is the junction resonant frequency). This is consistent with the Caldeira-Leggett theory, which includes damping effects on the quantum tunneling rate.
Dynamical simulations and scaling arguments are used to study the current-voltage $(I\ensuremath{-}V)$ characteristics of a two-dimensional model of resistively shunted Josephson-junction arrays in presence of percolative disorder, at zero external field. Two different limits of the Josephson-coupling concentration ppp are considered, where pc{p}_{c}pc​ is the percolation threshold. For p>pcp>{p}_{c}p>pc​ and zero temperature, the $I\ensuremath{-}V$ curves show power-law behavior above a disorder-dependent critical current. The power-law behavior and critical exponents are consistent with a simple scaling analysis. At pc{p}_{c}pc​ and finite temperature T,T,T, the results show the scaling behavior of a T=0T=0T=0 superconducting transition. The resistance is linear but vanishes for decreasing TTT with an apparent exponential behavior. Crossover to nonlinearity appears at currents proportional to ${T}^{1+{\ensuremath{\nu}}_{T}},$ with a thermal-correlation length exponent ${\ensuremath{\nu}}_{T}$ consistent with the corresponding value for the diluted XY\mathrm{XY}XY model at pc.{p}_{c}.pc​.
We discuss the fabrication and dc electrical characteristics of small-area (1–6 μm2) superconducting tunnel junctions with Ta or Nb base electrodes and Pb or Pb0.9Bi0.1 counterelectrodes. These junctions have very small subgap leakage currents, a ‘‘sharp’’ current rise at the sum-gap voltage, and show strong quantum effects when used as microwave mixers. The use of a low-energy (∼150 eV) ion cleaning process and a novel step-defined fabrication process that eliminates photoresist processing after base electrode deposition are discussed. Tunnel barriers formed by dc glow discharge oxidation were the most successful. Tunnel barrier formation by thermal oxidation and ion-beam oxidation is also discussed. An oxidized Ta overlayer (∼7 nm thick) was found to improve the characteristics of Nb-based junctions. The electrical characteristics of junctions with different electrode and barrier materials are presented and discussed in terms of the physical mechanisms that lead to excess subgap current and to a width of the current rise at the sum-gap voltage.