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

摘要

Recent progress of high-temperature superconduc- tor Josephsonjunction technology is reviewed in the light of the futureapplication to digital circuits. Among various types of Josephsonjunc- tions so far developed, ramp--edge--type junctions with abarrier layer composed of oxide materials in the vicinity ofmetal-insulator transi- tion seem to offer a unique opportunity tofulfill all the requirements for digital circuit applications byvirtue of their small junction dimen- sions, overdamped propertiesand relatively high I_cR_n, product values at the temperature ofaround 30--40 K. Recently developed interface en- gineered junctionscan be classified as junctions of this type. These junctions alsoraise an interesting problem in physics concerning the possibility ofresonant tunneling of Cooper pairs via localized states in thebarrier. From the viewpoint of practical applications, the improve-ment of the spread of the junction parameters is still a seriouschal- lenge to the present fabrication technology. Although interfaceengi- neered junctions seem to be most promising in this regard atpresent, lσ spread of around 8/100 in the present fabricationtechnology is far from satisfactory for the fabrication oflarge--scale integrated circuits. The detailed understanding of thebarrier formation mechanism in the interface engineered junction isindispensable not only for advancing this particular fabricationtechnology but also for improving other junc- tion technologyutilizing ramp-edge structures.