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IEEE International Superconductive Electronics Conference (ISEC)

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

Hiroyuki Akaike, Tatsunori Funai, Shunya Sakamoto, Akira Fujimaki

Published Jul 1, 2013• 1 citation

Abstract

We have evaluated the electrical characteristics of NbN/Al-AlNx/NbN tunnel junctions fabricated on sapphire-C or fused-silica substrates. NbN/Al-AlNx/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, Rsg/Rn, where respective Rsgand Rnare the sub-gap resistance at 2 mV and junction resistance at 5 mV, was above 15. The gap voltage (Vg) was around 3.3 mV, which was reduced in comparison with Vgvalues of ~4.3 mV for junctions on MgO substrates. The reduced Vgwas caused by superconducting properties of lower NbN layers deposited on the sapphire or fused-silica substrates. Uniformity in critical current (Ic) was relatively good, and the maximum-to-minimum spread in Icwas ±1.7% for 200 junctions on a sapphire substrate and ±2.0% for the junctions on a fused-silica substrate.

DOI: 10.1109/isec.2013.6604313

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Superconductor–insulator–superconductor (SIS) tunnel junctions including magnetic barriers, with structures NbN/AlNiN/NbN or NbN/MgO/NiO/MgO/NbN, were fabricated. The junctions with upper NbN layers of good crystallinity showed a gap voltage ( V g ) higher than 5 mV. In an NbN/Al 0.942 Ni 0.058 N/NbN junction, its Josephson current I c was suppressed to approximately a third of a theoretical value under no magnetic field. Therefore, it would be possible to avoid Josephson noise in magnetic barrier junctions with a much smaller magnetic field than junctions with conventional barriers. The Ni ion area density dependence of I c suppression ratio was identical for both types of junction, suggesting that the area density of magnetic ions is the factor that determines the I c suppression ratio. On the other hand, the extents of degradation of V g and increase in leakage current caused by the increase in Ni ion area density were smaller in MgO/NiO/MgO barrier junctions than in AlNiN barrier junctions. This suggests that the proximity effect of magnetic ions was weakened owing to the separation of Ni ions and NbN layers by the inserted thin MgO layers.