All Niobium Nitride Josephson Junction with Hydrogenated Amorphous Silicon Barrier and its Application to the Logic Circuit

Josephson tunnel junctions with all niobium nitride (NbN) electrodes were fabricated with sputtered amorphous silicon (a-Si:H) films as tunnel barriers whose surfaces are oxidized by an rf-plasma oxidation. The specific capacitance of the fabricated junction strongly decreased with increase in the thickness of the a-Si:H film. The specific capacitance of less than 4 µF/cm 2 was obtained for the a-Si:H thickness of about 2 nm. A logic circuit composed of four-junction logic (4JL) gates was fabricated using these junctions and was operated with the minimum logic delay of 9 ps/gate.