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Applied Physics Letters

Niobium nitride Josephson tunnel junctions with magnesium oxide barriers

Akira Shoji, Masahiro Aoyagi, S. Kosaka, Fujitoshi Shinoki, Hisao Hayakawa

Published Jun 1, 1985• 137 citations
Publisher

Abstract

Niobium nitride-niobium nitride Josephson tunnel junctions have been fabricated using amorphous magnesium oxide (a-MgO) films as barriers. These junctions have excellent tunneling characteristics. For example, a large gap voltage (Vg=5.1 mV), a large product of the maximum critical current and the normal tunneling resistance (IcRn=3.25 mV), and a small subgap leakage current (Vm=45 mV, measured at 3 mV) have been obtained for a NbN/a-MgO/NbN junction. The critical current of this junction remains finite up to 14.5 K.

DOI
10.1063/1.95774

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Japanese Journal of Applied Physics•Mar 1, 2008•0 citations
Niobium nitride-niobium nitride Josephson tunnel junctions have been fabricated using amorphous magnesium oxide (a-MgO) films as barriers. These junctions have excellent tunneling characteristics. For example, a large gap voltage (Vg=5.1 mV), a large product of the maximum critical current and the normal tunneling resistance (IcRn=3.25 mV), and a small subgap leakage current (Vm=45 mV, measured at 3 mV) have been obtained for a NbN/a-MgO/NbN junction. The critical current of this junction remains finite up to 14.5 K.
We report the electrical properties of all niobium nitride(NbN) Josephson tunnel junctions with magnesium oxide(MgO) films as barriers in the temperature range 4.2-15 K. NbN/MgO/NbN Josephson tunnel junctions have large gap voltages(Vg=4.8-5.4 mV), large products of the maximum critical currents and the normal tunneling resistances(IcRn=3.15-3.35 mV), and small subgap leakage currents(Vm=13-138 mV) at 4.2 K. The junction parameters decreased with increase in temperature, but reasonably large junction parameters(for example, Vg=4.9 mV, IcRn=2.67 mV, and Vm=22 mV) have been obtained even at 9 K. The magnetic penetration depth in NbN electrodes has been measured up to 15 K using two-junction interferometers which consist of NbN/MgO/NbN junctions, NbN ground planes, and NbN control lines.
All-NbN Josephson tunnel junctions with sputter-deposited magnesium oxide barriers have been fabricated using single-crystal NbN films for the base electrodes. Fabricated Josephson junctions have shown good tunneling characteristics with large gap voltages (5.6-5.8 mV), narrow gap widths (0.1-0.2 mV, from 30 to 70%), and small subgap leakage currents (V/sub m/=20-30 mV, measured at 3 mV). The results of a measurement of a subgap structure for a fabricated junction suggest that the excess leakage currents of fabricated junctions are due to multiparticle tunneling through locally thin areas in the MgO barriers.
We report the fabrication of all-refractory superconductor-insulator-superconductor tunnel junctions of the form NbN/MgO/NbN. The MgO insulating barrier was deposited by e-beam evaporation. High quality junctions were fabricated with sum gaps of 5.2 meV, and a small subgap leakage parameter (Vm=30 mV, measured at 3mV). These devices are for eventual use as quasiparticle mixer elements in millimeter/submiltimeter wave heterodyne receivers. Fabrication techniques and current-voltage characteristics are discussed. We also propose a new growth mode for MgO films on NbN.
Niobium/aluminum-oxide/niobium Josephson tunnel junctions have been fabricated using electron beam evaporation to form all-metal films in a high vacuum system. A quality factor, V m (the product of the maximum dc Josephson current and the subgap resistance defined at 2 mV) between 36–37 mV, has been obtained for the best junctions of a 6 nm-thick Al layer by cooling down the surface of the base electrode for 100 minutes under a high background pressure, before the deposition of aluminum, after the formation of the base niobium electrode. An array of 80 series-connected junctions has also been fabricated using the selective niobium anodization process and the planarization process. The current-voltage characteristics of the array showed a standard deviation of the maximum dc Josephson current of 3.3%.
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.