Abstract:

Surface oxides on elemental superconductors such as niobium play a large role in limiting the performance of superconducting circuits/qubits. Great attention has been focused on cleaning/thinning/removing this oxide with strategies such as chemical passivation or in-situ encapsulation. Thermal dissolution provides an alternative strategy to remove this oxide which, in addition, paves a route to 3D device integration. This surface is a key interface, the structure of which is important to characterize in order to understand device performance limitations. We have investigated the chemical and structural evolution of this surface throughout an ultra high vacuum anneal using depth-sensitive, and X-ray standing wave excited, photoelectron spectroscopy. We find the emergence of an oxygen (interstitial) rich, subsurface layer beneath the always present NbO surface, which may have an undesirable effect on the superconductivity of the niobium.