Influence of an Al 2 O 3 capping layer on the thermal reduction of the native niobium oxide: An in situ x-ray reflectivity study

Superconducting radio-frequency cavities, critical components of modern particle accelerators and quantum computing hardware, fundamentally rely on the surface properties of niobium. However, native oxide formation and impurity uptake in the near-surface region can degrade superconducting performance and increase radio-frequency losses during operation. In this study, we present a systematic x-ray reflectivity investigation of (110) niobium single-crystal surfaces with and without an atomic layer deposited {Al}₂{O}₃ capping layer under ultrahigh-vacuum conditions up to 650{ }^∘C . Our results reveal a temperature-dependent reduction of the native niobium oxide layers in both capped and uncapped samples, with similarities in the overall behavior but clear differences in the reduction pathways. The {Al}₂{O}₃ capping layer modifies the reduction process, prevents oxide regrowth upon air exposure, and protects the niobium surface against impurity uptake during thermal treatment. These findings demonstrate that {Al}₂{O}₃ capping is an effective strategy to suppress native oxide formation during thermal cycling, offering clear benefits for the performance and operational lifetime of superconducting radio-frequency cavities and related quantum computing technologies.