Microwave characterization of tantalum superconducting resonators on silicon substrate with niobium buffer layer

TL;DR

This study demonstrates that adding a niobium buffer layer improves the quality and superconducting properties of tantalum thin films on silicon, leading to high-Q microwave resonators suitable for quantum electronics.

Contribution

It introduces a method to grow high-quality tantalum films with enhanced microwave resonator performance using a niobium buffer layer on silicon substrates.

Findings

Internal quality factor approaches 2×10^7 at high photon numbers

TLS loss primarily caused by amorphous silicon at the interface

Quality factor increases below 200 mK, indicating TLS interactions

Abstract

Tantalum thin films sputtered on unheated silicon substrates are characterized with microwaves at around 10 GHz in a 10 mK environment. We show that the phase of tantalum with a body-centered cubic lattice ($\alpha$-Ta) can be grown selectively by depositing a niobium buffer layer prior to a tantalum film. The physical properties of the films, such as superconducting transition temperature and crystallinity, change markedly with the addition of the buffer layer. Coplanar waveguide resonators based on the composite film exhibit significantly enhanced internal quality factors compared with a film without the buffer layer. The internal quality factor approaches $2\times 10^7$ at a large-photon-number limit. While the quality factor decreases at the single-photon level owing to two-level system (TLS) loss, we have identified the primary cause of TLS loss to be the amorphous silicon layer at the film-substrate interface, which originates from the substrate cleaning before the film deposition rather than the film itself. The temperature dependence of the internal quality factors shows a marked rise below 200 mK, suggesting the presence of TLS-TLS interactions. The present low-loss tantalum films can be deposited without substrate heating and thus have various potential applications in superconducting quantum electronics.