Epitaxially grown NbN/AlN/NbN Josephson junctions with sidewall spacer structure for superconducting qubit

Aluminum (Al)-based Josephson junctions have been widely used in superconducting qubits in the past decade. However, the microscopic two-level systems (TLSs) in the amorphous aluminum oxide (AlO_x) tunnel barrier could limit qubit lifetimes. Epitaxially grown niobium nitride (NbN)-based junctions are a promising alternative to solve this problem because the epitaxial AlN tunnel barrier in which the lower TLS density is expected can be used [1]. On the other hand, their complex fabrication process, which includes the planarization of the interlayer dielectric SiO₂, may reduce their intrinsic coherence properties. In this work, we focused on the sidewall spacer structure used in niobium (Nb)-based qubits [2]. In this process, the lossy SiO₂ is present only near the junction sidewalls, and thus the volume of SiO₂ is much less than that in the conventional structures. Furthermore, the sidewall spacer structure makes it easier to remove SiO₂ using buffered hydrogen fluoride in the final process, leading to the reduction of the lossy etching residues. We successfully demonstrated sidewall spacer passivated NbN/AlN/NbN junctions with a small subgap leakage and a wide range of current density. By optimizing the thickness of the AlN barrier and etching conditions, we obtained electrical properties of junctions suitable for qubit applications. We will also discuss superconducting qubit by this process.

[1] Z. Wang et al., Appl. Phys. Lett. 102, 142604 (2013).

[2] A. Anferov et al., Phys. Rev. Appl. 21, 024047 (2024).