Ar ion bombardment effects of NbN/Pb Josephson junctions with plasma oxidized barriers

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Abstract
Details of the relation between Ar ion bombardment effects on NbN surface and I‐V characteristics of NbN/Pb junctions with plasma oxidized barriers have been studied. NbN base electrodes, which behave as a strong coupling superconductor (Tc>15 K, 2Δ/kTc=4.2–4.3), were prepared onto substrates at 300 °C by reactive dc magnetron sputtering. In the NbN/Pb junction fabrication, cathode self‐bias voltage VCSB during Ar sputter cleaning strongly influences the subgap leakage current value, which increases with VCSB . The VCSB also influences normalized normal tunnel resistance ARNN. It is found, in reflection electron diffraction and x‐ray photoelectron spectroscopy studies, that the Ar ion bombardment at VCSB> 200 V on a NbN surface produces an amorphous structure in the surface layer. It is also found that the thickness of amorphous layer increases with Ar ion energy. Based on the experimental results, low subgap leakage current junctions, where Vm exceeds 100 mV at 4.2 K, were fabricated under low energy plasma oxidation conditions, where Vm is the product of Josephson critical current and subgap resistance. Sputter cleaning and plasma oxidation conditions for obtaining high quality junctions are also discussed.