Low barrier Nb/ZrO_x/Nb Josephson junctions

Superconducting qubits are leading candidates for quantum computing, in part because they can be precisely designed and scalably fabricated, with increasingly long coherence lifetimes. Among materials candidates for improving Josephson junctions, niobium has several advantages over aluminum including a higher critical temperature and a wider operational frequency range. Many Nb-based Josephson junctions have adapted AlO_x as the tunnel barrier, which is already established in Al-based junctions. While AlO_x is easily integrated into Al-based junctions by oxidizing the Al electrode during an additive fabrication process, this approach is less advantageous for Nb-based junction fabrication, because it requires additional Al deposition followed by oxidization. On the other hand, NbO_x, the native oxide form of Nb, could be used as a tunnel barrier in Nb-based Josephson junctions, it is known to have high dielectric loss [1, 2] and is not suitable as a natural tunnel barrier. In our study, we explore ZrO_x as a candidate for tunnel barriers in Nb-based Josephson junctions. We find that ZrO_x has an order-of-magnitude lower barrier height compared to AlO_x. This is potentially advantageous for creating pinhole-free junctions that use a thicker oxide, or for making junctions with a very high critical current density. Additionally, we discuss our implementation of airbridge top electrodes with a cylindrical junction area, which may be helpful for characterization of sidewall effects.



[1] Verjauw, J. et al, Phys. Rev. Appl. 16, 014018 (2021)

[2] Altoe, M.V.P. et al, PRX Quantum 3, 020312 (2022)