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Physica C Superconductivity

Fabrication and characterization of high-quality all-NbN Josephson tunnel junctions for superconductive quantum interference devices

Quansheng Liu, Huiwu Wang, Qiyu Zhang, Wei Peng, Zhen Wang

Published May 5, 2017• 12 citations
Publisher
DOI
10.1016/j.physc.2017.05.002

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Scientific Reports•Aug 13, 2020•31 citations
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.
A description of the fabrication technology of high-quality tunnel SIS junctions is presented, with following characteristics: energy gap in superconductors Vg=3.2-3.4 mV, tunnel current density up to J 35 kA/cm2, quality factor Rj/Rn (ratio of subgap resistance to normal-state resistance) up to 30, junction area up to 1 μm2. The SIS junctions are integrated into the NbTiN|SiO2|Al microstrip line. Keywords: superconducting devices, superconductor--insulator--superconductor tunnel junction, plasma etching.
In the field of superconducting electronics, a π phase shifter based on a ferromagnetic Josephson junction is expected to provide various advantages to classical and quantum superconducting devices. Here we report niobium nitride (NbN)-based ferromagnetic π junctions on a silicon (Si) substrate with a titanium nitride (TiN) buffer layer, which have applications to flux-bias-free flux quantum bits (qubits) and classical digital logic elements. We fabricated and characterized NbN/aluminum nitride (AlN)/NbN Josephson junctions, NbN/copper nickel (CuNi)/NbN ferromagnetic Josephson junctions, and superconducting quantum interference devices (SQUIDs) consisting of these junctions on the Si substrate. The fabricated NbN/AlN/NbN junctions showed a high junction quality suitable for qubit applications. Furthermore, the magnetic field dependence of the SQUID's critical current indicated that the NbN/CuNi/NbN junction worked as a π phase shifter on the Si substrate.