Visualization of Sn Adsorption and Diffusion Behavior on (3×1)-O/Nb(100) with Low and Moderate Defect Densities

Nb₃Sn is a promising next-generation material for superconducting radio frequency (SRF) cavities, but a mechanistic understanding of optimal Nb₃Sn film growth has not been realized; this is needed to achieve optimized alloy growth for high-performance Nb₃Sn SRF cavities. Using in situ ultra-high vacuum surface preparation and characterization techniques, Sn adsorption and diffusion behavior was visualized at the nanoscale on a low (ρ_L) and moderate (ρ_M) defect density (3×1)-O/Nb(100) surface. Scanning tunneling microscopy data revealed distinct Sn adsorption behavior at room temperature on pristine, ρ_L, and ρ_M surfaces. Elevated surface temperatures induced Sn diffusion pathways primarily guided by the underlying oxide structure. The ρ_M surface displayed enhanced Sn thermal stability, highlighting the role of structural defects in stabilizing adsorbed Sn species at elevated temperatures. Auger electron spectroscopy showed consistent surface composition of the ρ_L and ρ_M substrates, suggesting that surface and near-surface composition is not influenced by nanoscale defects. This work provides the first in situ nanoscale visualization of Sn adsorption and diffusion on oxidized Nb and illustrates the significance of the underlying Nb oxide morphology on Sn adsorption and diffusion.