Observation and mechanism of non-uniform distribution of tin nuclei in preparing vapor diffusion coated Nb3Sn thin film for SRF applications

Growth of high-quality Nb3Sn thin films for superconducting radiofrequency (SRF) applications using the vapor diffusion method requires a uniform distribution of tin nuclei on the niobium (Nb) surface. This study examines the mechanism underlying the observed non-uniform distribution of tin nuclei with tin chloride SnCl2. Electron backscatter diffraction (EBSD) analysis was used to examine the correlation between the nucleation behavior and orientation of niobium grains in the substrate. The findings of the density functional theory (DFT) simulation are in good agreement with the experimental results, showing that the non-uniform distribution of tin nuclei is the result of the adsorption energy of SnCl2 molecules by varied niobium grain orientations. Further analysis indicated that the surface roughness and grain size of niobium also played significant roles in the nucleation behavior. This study provides valuable insights into enhancing the surface pretreatment of niobium substrates during the growth of Nb3Sn thin films using the vapor diffusion method.