  Nitrogen doping study on niobium (Nb) samples used for the fabrication of
superconducting radio frequency (SRF) cavities was carried out. The samples'
surface treatment was attempted to replicate that of the Nb SRF cavities, which
includes heavy electropolishing (EP), nitrogen doping and the subsequent EP
with different amounts of material removal. The surface chemical composition of
Nb samples with different post treatments has been studied by XPS. The chemical
composition of Nb, O, C and N was presented before and after Gas Cluster Ion
Beam (GCIB) etching. No signals of poorly superconducting nitrides NbNx was
found on the surface of any doped Nb sample with the 2/6 recipe before GCIB
etching. However, in the depth range greater than 30nm, the content of N
element is below the XPS detection precision scope even for the Nb sample
directly after nitrogen doping treatment with the 2/6 recipe.
