| | XPS studies have consistently shown that Nb surfaces
for SRF chiefly comprise of a few nm of Nb2O5 on top of
Nb metal, with minor amounts of Nb sub-oxides. Nb
samples after BCP/EP treatment with post-baking at the
various conditions have been examined by using
synchrotron based XPS. Despite the confounding
influence of surface roughness, certain outcomes are
clear. Lower-valence Nb species are always and only
associated with the metal/oxide interface, but evidence for
an explicit layer structure or discrete phases is lacking.
Post-baking without air exposure shows decreased oxide
layer thickness and increased contribution from lower
valence species, but spectra obtained after subsequent air
exposure cannot be distinguished from those obtained prior to baking, though the SRF performance improvement remains. | |