Improving the efficiency, performance, and accessibility of particle accelerator infrastructure is contingent upon implementing next-generation materials for superconducting radiofrequency (SRF) cavities. The superconducting properties of niobium-3-tin (Nb₃Sn) justify this alloy as the ideal candidate for enhancing the accelerating performance of existing Nb SRF cavities. Nb₃Sn films grown via vapor diffusion on the interior of Nb surfaces extensive material defects that quench superconductivity at high accelerating gradients, limiting the potential of Nb₃Sn-coated SRF technology.

This thesis explores how the Nb surface morphology guides Sn diffusion and Nb₃Sn film growth. Experimental studies are conducted in an ultrahigh vacuum (UHV) chamber equipped with in situ surface sensitive microscopy and spectroscopy characterization techniques to visualize the substrate-mediated behavior during the alloy growth process. UHV preparation of Nb surfaces prevented the rapid formation of the native Nb pentoxide and enabled an investigation of the reactive monoxide Nb surfaces that dominate in the elevated temperature and low-pressure furnace environment during film growth. A variety of Nb substrates with varying roughness and crystallinity were utilized to examine Sn adsorption, diffusion, and incorporation behavior.

An electron beam evaporator in the UHV system introduces Sn vapor on Nb substrates ranging from sub-monolayer (ML) to micron thick coverages. Atomistic studies of sub-ML Sn on an oxidized Nb monoxide (NbO) surface identified NbO nanoscale defect sites that guide Sn lateral diffusion and stabilize Sn adlayers at the substrate temperatures relevant to Nb₃Sn formation. By increasing the density of NbO surface defect sites, we observed a suppression of Sn desorption that was necessary for Nb₃Sn nucleation. Sn deposition studies were scaled up to demonstrate how the intricate balance of the Nb binding sites, Nb temperature, and Sn flux drive alloy nucleation, particularly during the initial and final steps of the film growth procedure. Outcomes of this work have elucidated the complex substrate-Sn interplay during Nb₃Sn growth and detailed the relationship between experimental Nb₃Sn growth conditions, stoichiometric defect morphology, and the superconducting quenching behavior during SRF operation.