Fabnication of Superconductor/Insulator/Superconductor Trilayer Tunnel Junction Devices by Atomic Layer MOCVD

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1. Ultrathin films of oxide superconductors and insulator heterostructures were prepared by the atomic layer metalorganic chemical vapor deposition method. Reproducibility of thinfilms fabrication was enhanced by using in situ monitoring based on ultrasonic transducers and spectroscopic ellipsometry.2. Superconducting field effect was enhanced by the combination of Josephson junctions and planar type devices.3. Anomolous current voltage characteristics due to intrinsic Josephson junctions were observed in YBCO thinfilm devices fabricated by scanning probe microscopy lithography. IcRn product value of 2.5mV were obtained at 5K.4. Two possible approaches for realizing overdamped Josephson junctions with a high IcRn product value were investigated. The use of an superconductor-insulator-superconductor structure with a shunt resistor that was formed by quasiparticle current paths via localized states in the tunnel bamer was concluded to be more promising than the use of an superconductor-normal metal-superconductor structure. The physical nature of localized states in PrBa2Cu3O7 (PBCO) and its relevance to transport properties were fully clarified. Ramp-edge-type junctions with a Co-doped PBCO barrier ranging from 6 nm to 10 nm in thickness exhibited excellent Josephson characteristics, and the maximum IcRn product value exceeding 2 mV at 4.2 K has been realized. Preliminary single-flux-quantum logic circuits including an RS- flip flop and a frequency divider have been fabricated and tested. The frequency divider circuit utilizing ramp-edge-type Josephson junctions with a buried groundplane was confirmed to operate correctly up to 200 GHz in frequency at 12 K.