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

1.采用原子层金属有机化学气相沉积方法制备了氧化物超导体和绝缘体异质结构的超薄膜。采用基于超声换能器和椭圆偏振光谱的在线监测技术，提高了薄膜制备的重复性。约瑟夫森结和平面型器件的结合增强了超导场效应。在用扫描探针显微镜光刻技术制备的YBCO薄膜器件中，观察到了由本征约瑟夫森结引起的异常电流-电压特性。在5K.4时，获得了2.5 mV的IcRn产物值。研究了实现高IcRn积值的过阻尼约瑟夫森结的两种可能方法。由准粒子电流通过隧道内的定域态形成的具有并联电阻的超导体-绝缘体-超导体结构的使用比使用超导体-正常金属-超导体结构更有前途。充分阐明了PrBa2Cu3O7(PBCO)中局域态的物理性质及其与输运性质的关系。Co掺杂PBCO势垒厚度在6~10 nm范围内的斜边型结具有良好的约瑟夫森特性，在4.2K下实现了最大IcRn乘积值超过2 mV。初步制作了包括RS触发器和分频器在内的单磁通量子逻辑电路并进行了测试。采用斜边型约瑟夫森结和埋地基板的分频电路在12K下的工作频率可达200 GHz。

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.