NbN multilayer technology on R-plane sapphire

A new NbN multilayer technology has been developed on 3 inch diameter R-plane sapphire substrates, for combining on-chip fast RSFQ circuits with GHz bandwidth optical links, The circuits take advantage of two high quality (110) NbN layers sputtered epitaxially on sapphire at 600°C and selectively patterned: a 400 nm thick layer (λ _L~250 nm at 6 K) acts for the ground-plane and microbridge photodetectors are made of a 3.5-8 nm thick NbN epilayer with T_c above 11 K. Innovative dielectrics formed of 10 nm thick MgO sputtered on top of 200 mm SiO₂ layers are found to improve significantly the superconductivity of NbN junction electrode lines deposited below 300°C. Good quality, hysteretic 2 μm² area, NbN/MgO/NbN junctions with high J_c (up to 50 kA/cm²) are obtained with very large gap voltage (6.20 mV) and low sub-gap leakage current (V_m>15 mV) at 4.2 K. At 11 K such junctions are found self-shunted (J_c~10 k4/cm² ) with R_nI_c above 0.5 mV and with low J_c spread in arrays. J_c can be adjusted (reduced) without any detrimental effect on the junction quality or spread by annealing at 250°C