Investigation of surface induced loss mechanisms in high-quality superconducting Nb resonators

Performance of superconducting qubit devices is limited by two-level system (TLS) defects, predominately found in amorphous interface layers. Reducing microwave loss contributions from these interfaces by proper surface treatments is key to push the device performance forward. We study niobium resonators where the native oxides at the metal-air & substrate-air interface are selectively etched with Hydrofluoric (HF) acid. Although HF treatment right before the low temperature characterization is known to yield an order of magnitude improvement in the resonator’s quality factor, the precise loss contribution from various residual oxides and other defects at the interfaces is still unknown. By combining resonator quality factor measurements with X-ray photoelectron spectroscopy (XPS), electron energy loss spectroscopy (EELS) and other surface characterization techniques, we investigate the reappearance of loss mechanisms introduced by exposure to ambient conditions. This is of particular interest for fabricating air resilient, high-quality superconducting qubit devices as they share the same loss mechanisms with resonators.