Paper SS-TuP15
Oxidation of Nb(100) and Kinetics of Surface to Bulk Transport and Extension to Nb₃Sn

Tuesday, October 23, 2018, 6:30 pm, Room Hall B

Niobium (Nb) is commonly used in superconducting radio frequency (SRF) accelerator cavities due to its ultra-low surface resistance (R_s) and high cavity quality factor (Q) at ~ 2 K. Nb cavities are, however, highly susceptible to localized surface heating, resulting in quenching of the superconducting properties. To minimize quenching, much work has been done to understand factors influencing SRF cavity performance for the clean and oxidized Nb surface. In this work, we have investigated the structural evolution of oxidized Nb(100) under ultra-high vacuum (UHV) conditions to elucidate the structural evolution of the (3×1)-O ladder structure following exposure to O₂. Auger electron spectroscopy (AES) was used to determine oxygen coverage and surface structure was determined using scanning tunneling microscopy (STM). The (3×1)-O Nb(100) surface was exposed to O₂ at 300 K and annealed to various substrate temperatures to facilitate oxygen dissolution. Dissolution kinetics elucidated the surface to bulk oxygen transport mechanism. STM showed the decomposition of the ordered (3×1)-O ladder structure during oxygen dissolution, indicating the importance of oxygen concentration on surface structure. As the fundamental limits of Nb SRF cavities are being realized, it is crucial that alternative SRF materials be studied. Nb₃Sn has been identified as a most promising next generation SRF material with higher Q as well as the ability to operate at much higher temperatures, greatly reducing cryogenic infrastructure costs. Ongoing work is focusing on developing preparation methods leading to more homogeneous Nb₃Sn films. In situ Sn doping on (3×1)-O Nb(100) combined with surface sensitive techniques such as AES, XPS, and LT-STM will hopefully allow for the diffusion mechanism for Sn on Nb to elucidated, leading to improved procedures for Sn infusion and materials growth.