Investigation of surface oxide thermal evolution and removal on Nb thin films

Surface oxides present in superconducting quantum devices are a source of microwave loss, which can limit qubit coherence through dielectric loss and relaxation through two-level systems hosted in the often-amorphous oxide. Several methods of thinning or removing these oxides have been proposed, such as in-situ prevention with surface capping, in-vacuum thermal or sputter removal, and encapsulation [1], or surface passivation [2]. Here, we investigate the evolution of niobium surface oxides during thermal removal in ultra-high vacuum using several X-ray scattering and spectroscopy techniques. We find the formation of an oxygen-rich (interstitial) subsurface region in single-crystal thin films. We will discuss the effects of thin film grain structure on the resulting oxide evolution and how grain boundaries affect the pathways through which oxygen dissolves into the niobium. Finally, we will touch on the efficacy of this method for oxide removal in superconducting devices.

1. npj Quantum Inf, 10, 43 (2024).

2. ACS Appl.Mater. Interfaces, 15, 2319−2328 (2023)