Stable and low loss oxide layer on {\alpha}-Ta (110) film for superconducting qubits

TL;DR

This paper demonstrates a stable, low-loss amorphous tantalum oxide layer on { extalpha}-Ta (110) films that enhances superconducting qubit coherence times and stability, offering a promising passivation approach for quantum computing devices.

Contribution

It introduces a self-limiting formation process of stable tantalum oxide on { extalpha}-Ta (110) films and evaluates its low dielectric loss for superconducting qubits.

Findings

Oxide layer forms in a self-limiting process with stable thickness.

Resonators with this oxide show high quality factors (>2x10^6).

Oxide stability persists over months in air.

Abstract

The presence of amorphous oxide layers can significantly affect the coherent time of superconducting qubits due to their high dielectric loss. Typically, the surface oxides of superconductor films exhibit lossy and unstable behavior when exposed to air. To increase the coherence time, it is essential for qubits to have stable and low dielectric loss oxides, either as barrier or passivation layers. In this study, we highlight the robust and stable nature of an amorphous tantalum oxide layer formed on {\alpha}-Ta (110) film by employing chemical and structural analyses. Such kind of oxide layer forms in a self-limiting process on the surface of {\alpha}-Ta (110) film in piranha solution, yielding stable thickness and steady chemical composition. Quarter wavelength coplanar waveguide resonators are made to study the loss of this oxide. One resonator has a Qi of 3.0x10^6 in the single photon region. The Qi of most devices are higher than 2.0x10^6. Moreover, most of them are still over 1x10^6 even after exposed to air for months. Based on these findings, we propose an all-tantalum superconducting qubit utilizing such oxide as passivation layers, which possess low dielectric loss and improved stability.