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Applied Physics Letters

Niobium nitride Josephson tunnel junctions with oxidized amorphous silicon barriers

Fujitoshi Shinoki, Alexandre Shoji, S. Kosaka, S. Takada, Hisao Hayakawa

Published Feb 15, 1981• 44 citations
Publisher

Abstract

High-quality NbN/NbN Josephson tunnel junctions have been fabricated by rf sputtering. Tunneling barriers are formed by oxidizing amorphous silicon films produced with rf glow discharge deposition. For patterning the base and counter electrode, sputtered ZnO films are employed as a resist mask. The juncitons are found to have large gaps (2ΔNbN) =4.4 mV), to have low leakage current, and to be quite stable for storage and thermal cyclings.

DOI
10.1063/1.92308

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Applied Physics Letters•Dec 1, 1982•70 citations
High-quality NbN/NbN Josephson tunnel junctions have been fabricated by rf sputtering. Tunneling barriers are formed by oxidizing amorphous silicon films produced with rf glow discharge deposition. For patterning the base and counter electrode, sputtered ZnO films are employed as a resist mask. The juncitons are found to have large gaps (2ΔNbN) =4.4 mV), to have low leakage current, and to be quite stable for storage and thermal cyclings.
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 have developed a new integration method for Josephson tunnel junctions based on high Tc superconducting compound materials such as NbN. In this method, a sputtered ZnO film has been employed as a resist mask which can be used in high substrate temperatures during the film deposition. It has been found that fine patterns of the refractory superconducting film can be obtained by this method and the patterning processes have no interference with other integration processes. We have applied this method to patterning base electrode of NbN and formed high quality NbN-oxide-Pb Josephson tunnel junctions by using an rf plasma oxidation. The fabricated junctions have been found to have a relatively large gap ( ∼ 4.0 mV ) and low current leak in the subgap region, and to be quite stable for thermal cyclings and storage. We have also fabricated quantum interference memory devices by integrating these junctions and the operation has been successfully demonstrated.
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.
All hard Josephson tunnel junctions, whose base and counter electrodes are composed of double-layered niobium nitride (NbN) and niobium (Nb) films, have been successfully fabricated by isolating a junction sandwich formed on a whole silicon wafer with a reactive ion etching technique. The reactive ion etching technique has been used for patterning both base and counterelectrodes, and self-aligning definition of junction areas has been performed. The fabricated junctions show good quality single-particle tunneling characteristics and excellent uniformity in critical currents.