The effects of barrier hydrogenation in NbN-based Josephson junctions

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Abstract
NbN/Si/Nb tunnel junctions with hydrogenated amorphous silicon barriers were investigated for several hydrogen concentrations used during barrier deposition. Junctions with critical current densities exceeding 2 kA/cm2 were fabricated using the selective niobium anodization process junction isolation technique. The penetration depth of the NbN-base electrode material was determined to be 200 nm from the critical current magnetic field modulation period. Some junctions had figure of merit, Vm, values exceeding 40 mV which degraded with reduced junction size. Hydrogen doping of the silicon barrier produced little degradation in Vm for up to 50% of hydrogen in the argon-hydrogen sputter gas mixture. The barrier height, as determined by the Simmons model, increased with increasing hydrogen concentrations in the sputter gas, which is attributed to a decrease in the dangling bond density. Barrier heights of 40 to 350 mV were observed.