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Journal of Applied Physics

Nonlatching Josephson interferometer made with small-area junctions

Richard E. Howard, Evelyn L. Hu, L. D. Jackel, Linus A. Fetter

Published Nov 1, 1980• 1 citation
Publisher

Abstract

A 1:2:1 Josephson interferometer has been fabricated with small-area, high–current-density tunnel junctions. The short intrinsic RC times of these junctions result in device I-V characteristics with reduced hysteresis suitable for use in nonlatching logic circuits. The control current for this interferometer was fed directly, and asymmetrically, into the base electrode. Critical current versus control current measurements indicate that this device has a current gain greater than unity. The interferometer described represents an exploratory study of circuit design incorporating low-hysteresis junctions.

DOI
10.1063/1.327489

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A 1:2:1 Josephson interferometer has been fabricated with small-area, high–current-density tunnel junctions. The short intrinsic RC times of these junctions result in device I-V characteristics with reduced hysteresis suitable for use in nonlatching logic circuits. The control current for this interferometer was fed directly, and asymmetrically, into the base electrode. Critical current versus control current measurements indicate that this device has a current gain greater than unity. The interferometer described represents an exploratory study of circuit design incorporating low-hysteresis junctions.
This paper describes the design, fabrication and characterization of damped three-junction interferometers suitable for LSI latching logic. The high speed and low power dissipation of Josephson interferometers make them ideal devices for logic applications. However, their inherent high Q gives rise to resonances which appear as current steps in the quasiparticle curve. If not controlled, these current steps can interfere with the proper operation of logic circuits. Studies have shown that these resonances can be effectively damped by shunting the interferometer inductance with a small resistance. In this work the interferometers were made by placing the interferometer loops parallel to the groundplane and clustering the Josephson junctions together. This minimized the loop inductance associated with the counterelectrode which in turn meant that adequate damping could be achieved with metallurgically compatible short resistors connected only to the base electrode. The resultant structure with two overlying control lines each 2.5μm wide had an area of 0.008mm2. The Josephson junctions were 5μm in diameter. The measured damped resonance characteristics and threshold characteristics (Ig(Vg) and Im(Ic) respectively) of these structures agreed very well with theory.
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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.