Rapid single flux quantum logic in high temperature superconductor technology

A Josephson junction is the basic element of rapid single flux quantum logic (RSFQ) circuits. A high operating speed and low power consumption are the main advantages of RSFQ logic over semiconductor electronic circuits. To realize complex RSFQ circuits in HTS technology one needs a reproducible fabrication of Josephson junctions with low parameter spread. High quality HTS junctions require a fully epitaxial multilayer structure with clean interfaces and a smooth surface morphology. Neodymium barium copper oxide (NBCO) thin films were used as superconducting electrodes due to their high transition temperature and low surface roughness. NBCO forms a solid solution in which Nd3+ ions can substitute for Ba2+ ions with the formula Nd1+xBa2-xCu3Oy (x=0.12 in our study). Cation disorder and oxygen deficiency affect the structural and electronic properties of grown NBCO films. Target composition, substrate temperature, oxygen partial-pressure and annealing steps are properly chosen to obtain smooth films with a high transition temperature. NBCO is deposited on an STO buffered MgO substrate at a pressure of 0.35 mbar in an Argon and Oxygen (20:1) gas mixture. The substrate temperature was fixed at 8200 C. The deposited film is smooth with a surface roughness of 1-3 nm for a 150 nm thick layer and a transition temperature around 90K.