Oxygen Dissolution and Surface Oxide Reconstructions on Nb(100)


The ever-present native oxide layer on niobium plays a fundamental role in the performance of Nb superconducting radio frequency (SRF) cavities for particle accelerators and light sources. Using Nb(111) and Nb(100) as model systems, oxygen dissolution and surface structural evolution as a function of thermal treatments in ultrahigh vacuum (UHV) were studied using combined Auger electron spectroscopy (AES) and scanning tunneling microscopy (STM), providing novel, real space information regarding the complex evolution of the Nb surface oxide. The surface crystallographic orientation was shown to influence oxide surface structure; Nb(111) displayed disordered surface oxide domains while Nb(100) was comprised of well-ordered, (n × 1)-O domains. The temperature-dependent dissolution of the native oxide layer on Nb(100) was characterized to ascertain the energetics for oxygen dissolution, and evolution of the surface oxide superlattice structure following thermal annealing was observed with STM. By understanding the kinetic behavior of oxygen dissolution during thermal annealing and subsequent structural evolution of the Nb(n × 1)-O superlattice on Nb(100), a more thorough understanding of the complex interactions driving chemical composition and cavity surface structure under Nb SRF cavity processing and operating conditions may be understood.

Surface Science