Effect of Annealing on the Superconducting and Surface Properties of Niobium Thin Films

Niobium (Nb) thin films are a key material in superconducting qubit devices. However, native surface oxides on Nb can introduce defects and dissipation mechanisms that degrade qubit performance and coherence times. In this study, we investigate how argon ion sputtering and subsequent annealing affect the superconducting properties of Nb(110) thin films grown by molecular beam epitaxy (MBE), using low-temperature scanning tunneling microscopy and spectroscopy (STM/STS). Even moderate annealing temperatures of a few hundred degrees Celsius drive oxygen diffusion into the bulk, reducing the critical temperature and superconducting gap, while increasing the upper critical field. Annealing at higher temperatures up to 900 °C remains insufficient to remove oxygen from the bulk. However, annealing above 500 °C yields a metallic, defect-free NbO_x surface with no zero-bias conductance peaks or in-gap states, and a spatially uniform density of states. These results highlight the sensitivity of the electronic and surface structure of Nb to processing conditions.