Role of nanoscale surface defects on Sn adsorption and diffusion behavior on oxidized Nb(100)

Abstract

Nanoscale structural defects such as grain boundaries, atomic dislocations, and surface roughness inhibit the stoichiometrically homogeneous growth of Nb3Sn on Nb. This is a critical technological bottleneck for the implementation of next-generation Nb3Sn superconducting radio frequency cavities, as thin film inhomogeneities are known to degrade superconducting properties that are essential for reaching optimal cavity performance. To determine the influence of structural defects on Nb3Sn film growth, low and moderate surface defect densities were intentionally induced onto a (3 × 1)-O Nb(100) substrate, which serves as a model system to study atomic-scale Sn adsorption and diffusion. Scanning tunneling microscopy shows that, while initial Sn adsorption behavior at room temperature differs between the low and moderate defect density Nb(100) surfaces, the overall diffusion pathways at elevated temperatures are guided by the underlying oxide structure with variations resulting from increased nanoscale surface defects. The (3 × 1)-O Nb(100) surface with a moderate defect density also demonstrates enhanced Sn thermal stability, with the Sn desorption threshold occurring between 850 and 900 °C, approximately 50 °C higher than desorption from both the low defect density and pristine thin oxide surfaces. This suggests that structural surface defects may stabilize adsorbed Sn species on oxidized Nb at the elevated temperatures utilized in Nb3Sn alloy growth procedures. Auger electron spectroscopy shows no significant difference in surface composition following Sn deposition at varying coverages on the pristine and defect-induced (3 × 1)-O Nb(100) surfaces. This indicates that the surface and near-surface composition are not influenced by the presence of nanoscale surface defects despite slight attenuations in Sn diffusion pathways on defected substrates. These results provide the first in situ visualization of Sn adsorption and diffusion behavior on oxidized Nb at the nanoscale, revealing the significance of the underlying Nb oxide surface structure and defect density on Nb3Sn film growth and, ultimately, cavity performance.