Recent Progress of High-Temperature Superconductor Josephson Junction Technology for Digital Circuit Applications


IEICE TRANSACTIONS on Electronics   Vol.E83-C    No.1    pp.49-59
Publication Date: 2000/01/25
Online ISSN: 
Print ISSN: 0916-8516
Type of Manuscript: Special Section INVITED PAPER (Special Issue on Superconductive Devices and Systems)
Category: Digital Applications
high-temperature superconductors,  Josephson junction,  SFQ circuit,  ramp-edge-type structure,  resonant tunneling,  

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Recent progress of high-temperature superconductor Josephson junction technology is reviewed in the light of the future application to digital circuits. Among various types of Josephson junctions so far developed, ramp-edge-type junctions with a barrier layer composed of oxide materials in the vicinity of metal-insulator transition seem to offer a unique opportunity to fulfill all the requirements for digital circuit applications by virtue of their small junction dimensions, overdamped properties and relatively high IcRn product values at the temperature of around 30-40 K. Recently developed interface engineered junctions can be classified as junctions of this type. These junctions also raise an interesting problem in physics concerning the possibility of resonant tunneling of Cooper pairs via localized states in the barrier. From the viewpoint of practical applications, the improvement of the spread of the junction parameters is still a serious challenge to the present fabrication technology. Although interface engineered junctions seem to be most promising in this regard at present, 1σ spread of around 8% in the present fabrication technology is far from satisfactory for the fabrication of large-scale integrated circuits. The detailed understanding of the barrier formation mechanism in the interface engineered junction is indispensable not only for advancing this particular fabrication technology but also for improving other junction technology utilizing ramp-edge structures.