Optimizing Nb₃Sn Growth for SRF Applications: Nanoscale Morphological and Electronic Characterization of Intermetallic Adlayers on a Highly Ordered Nb Oxide

To achieve improved accelerating gradients operating above the temperature of liquid helium, efforts are underway to coat Nb superconducting radiofrequency cavities with high-quality Nb₃Sn films. The enhanced performance of Nb₃Sn coated cavities is contingent upon the growth of smooth, homogeneous A15 Nb₃Sn grains. Nb-Sn growth studies probe the interplay between the underlying Nb oxide morphology, Sn coverage, and substrate heating conditions on Sn wettability, intermediate surface phases, and Nb₃Sn grain growth dynamics. Using a well-characterized (3×1)-O/Nb(100) single crystal substrate, Sn was deposited with sub-monolayer precision and the Sn/Nb interface was analyzed and heated in situ. Scanning tunneling microscopy and spectroscopy data detail thermally induced Sn diffusion behavior leading to the formation of electronically and structurally distinct Sn adlayers. Photoelectron spectroscopy data further elucidate the impact of growth conditions on the near-surface intermetallic binding motifs with oxygen. This experimental work, supported by concomitant adsorption energy calculations of Sn adatoms sites on the (3×1)-O, detail morphological and electronic consequences of the surface-mediated intermetallic dynamics driving optimal Nb₃Sn formation.