Investigation of microwave loss induced by oxide regrowth in high-Q Nb resonators

TL;DR

This study demonstrates that removing native oxides from niobium resonators significantly reduces microwave loss, with losses increasing linearly as Nb2O5 oxide regrows over time, impacting superconducting quantum device performance.

Contribution

It provides detailed insights into oxide regrowth dynamics and quantifies the loss tangent associated with Nb2O5 in high-Q Nb resonators.

Findings

Losses reduced by an order of magnitude after oxide removal.

Nb2O5 growth follows Cabrera-Mott model within 200 hours.

Microwave losses scale linearly with Nb2O5 thickness.

Abstract

The coherence of state-of-the-art superconducting qubit devices is predominantly limited by two-level-system defects, found primarily at amorphous interface layers. Reducing microwave loss from these interfaces by proper surface treatments is key to push the device performance forward. Here, we study niobium resonators after removing the native oxides with a hydrofluoric acid etch. We investigate the reappearance of microwave losses introduced by surface oxides that grow after exposure to the ambient environment. We find that losses in quantum devices are reduced by an order of magnitude, with internal Q-factors reaching up to 7 $\cdot$ 10$^6$ in the single photon regime, when devices are exposed to ambient conditions for 16 min. Furthermore, we observe that Nb2O5 is the only surface oxide that grows significantly within the first 200 hours, following the extended Cabrera-Mott growth model. In this time, microwave losses scale linearly with the Nb$_2$O$_5$ thickness, with an extracted loss tangent tan$\delta$ = 9.9 $\cdot$ 10$^{-3}$. Our findings are of particular interest for devices spanning from superconducting qubits, quantum-limited amplifiers, microwave kinetic inductance detectors to single photon detectors.