Superconducting radio frequency (SRF) technology is the key enabler for current and
future high-energy and high-beam-power accelerators. Performance of SRF cavities for
accelerators is characterized by their quality factor, Q0, a measure of their efficiency of
operation, and the maximum accelerating field, Eacc, that they can sustain before their
quality factor degrades. Chemical and structural defects at the surface and in the nearsurface
region of niobium SRF cavities can negatively or positively affect cavity
performance. In this talk I discuss our investigations of the surface structure, chemistry,
and oxidative states of single-crystal niobium samples as well as technical grade
polycrystalline SRF cavity samples using scanning tunneling microscopy (STM), atomic
force microscopy (AFM), Auger electron spectroscopy (AES) and x-ray photoelectron
spectroscopy (XPS). These tools allow us to obtain atomically resolved top-down images
of the niobium surface, to take surface sensitive chemical composition and oxidative state
measurements, and to profile the concentrations of chemical contaminants and dopants as
a function of depth into the niobium surface. Inspired by recent discoveries at the Fermi
National Accelerator Laboratory that involve the effects of nitrogen-doping that enhance
the operating characteristics of accelerator cavities, I discuss our efforts to explore the
possible relationship between nitrogen-doping and nanoscale niobium hydride surface
precipitation. Ordered hydride phases are known to form on cavity surfaces during
cooldown to operational temperatures. They are non-superconducting and correlate
strongly with diminished cavity performance. It is possible that nitrogen-doping serves
as a hydrogen “trap” preventing hydrides from forming at the niobium surface during
cooldown. I also present initial results on depth-profiling experiments performed on
nitrogen-doped cavity samples provided by Fermi National Accelerator Laboratory.