Investigation Of Microwave Loss Induced By Oxide Regrowth In High-Q Niobium Resonators

The coherence of state-of-the-art superconducting qubit devices is predominantly limited by two-levelsystem defects, which are found primarily at amorphous interface layers. Reducing microwave loss from these interfaces by proper surface treatments is key to pushing 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 air. We find that microwave loss in resonators are reduced by approximately an order of magnitude: internal Q factors increase from 1 x 106 to 7 x 106 in the single-photon regime when oxides are removed and device exposure to air is no longer than 16 min. Furthermore, we observe that Nb2O5 is the only surface oxide that grows significantly within the first 200 h, following the extended Cabrera-Mott growth model. In this time, microwave losses scale linearly with the Nb2O5 thickness, with an extracted loss tangent, tan delta Nb2O5, of about 1 x 10-2. Our findings are of particular interest for devices spanning from superconducting qubits, quantum-limited amplifiers, and microwave kinetic inductance detectors to single-photon detectors.