Fabrication of NbN/Al-AlNx/NbN tunnel junctions on several kinds of substrates

We have evaluated the electrical characteristics of NbN/Al-AlN_x/NbN tunnel junctions fabricated on sapphire-C or fused-silica substrates. NbN/Al-AlN_x/NbN multilayers were prepared by reactive dc-magnetron sputtering deposition of lower and upper NbN layers, and rf-magnetron sputtering deposition of the Al layer, followed by radical-nitridation. The junctions showed low sub-gap leakage currents in the current-voltage characteristics at 4.2 K. The quality parameter, R_sg/R_n, where respective R_sg and R_n are the sub-gap resistance at 2 mV and junction resistance at 5 mV, was above 15. The gap voltage (V_g) was around 3.3 mV, which was reduced in comparison with V_g values of ~4.3 mV for junctions on MgO substrates. The reduced V_g was caused by superconducting properties of lower NbN layers deposited on the sapphire or fused-silica substrates. Uniformity in critical current (I_c) was relatively good, and the maximum-to-minimum spread in I_c was ±1.7% for 200 junctions on a sapphire substrate and ±2.0% for the junctions on a fused-silica substrate.