Low barrier ZrO$_x$-based Josephson junctions

TL;DR

This paper introduces ZrO$_x$ as a new, scalable tunnel barrier material for Nb-based Josephson junctions, demonstrating its structural properties and promising electrical characteristics for superconducting electronics.

Contribution

It proposes and experimentally verifies ZrO$_x$ as a low barrier, scalable tunnel barrier for Nb Josephson junctions, with detailed structural and electrical analysis.

Findings

ZrO$_x$ can form fully crystalline, chemically abrupt barriers with Nb.

ZrO$_x$ exhibits a low tunnel barrier height suitable for superconducting applications.

Fabrication process enables large-scale production of Nb/ZrO$_x$/Nb junctions.

Abstract

The Josephson junction is a crucial element in superconducting devices, and niobium is a promising candidate for the superconducting material due to its large energy gap relative to aluminum. AlO$_x$ has long been regarded as the highest quality oxide tunnel barrier and is often used in niobium-based junctions. Here we propose ZrO$_x$ as an alternative tunnel barrier material for Nb electrodes. We theoretically estimate that zirconium oxide has excellent oxygen retention properties and experimentally verify that there is no significant oxygen diffusion leading to NbO$_x$ formation in the adjacent Nb electrode. We develop a top-down, subtractive fabrication process for Nb/Zr-ZrO$_x$/Nb Josephson junctions, which enables scalability and large-scale production of superconducting electronics. Using cross sectional scanning transmission electron microscopy, we experimentally find that depending on the Zr thickness, ZrO$_x$ tunnel barriers can be fully crystalline with chemically abrupt interfaces with niobium. Further analysis using electron energy loss spectroscopy reveals that ZrO$_x$ corresponds to tetragonal ZrO$_2$. Room temperature characterization of fabricated junctions using Simmons' model shows that ZrO$_2$ exhibits a low tunnel barrier height, which is promising in merged-element transmon applications. Low temperature transport measurements reveal sub-gap structure, while the low-voltage sub-gap resistance remains in the megaohm range.