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 degreesC and selectively patterned: a 400 nm thick layer lambda (L)similar to 250 nm at 6K) 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 nm SiO2 layers are found to improve significantly the superconductivity of NbN junction electrode lines deposited below 300 degreesC. Good quality, hysteretic 2 mum(2) area, NbN/MgO/NbN junctions with high J(c) (up to 50 kA/cm(2)) 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)similar to 10 kA/cm(2)) with RnIc 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 degreesC.