  Point contact tunneling (PCT) spectroscopy measurements are reported over
wide areas of cm-sized cut outs from niobium superconducting RF cavities. A
comparison is made between a high-quality, conventionally processed (CP) cavity
with a high field Q drop for acceleration field E $>$ 20 MV/m and a nitrogen
doped (N-doped) cavity that exhibits an increasing Q up to fields approaching
15 MV/m. The CP cavity displays hot spot regions at high RF fields where Q-drop
occurs as well as unaffected regions (cold spots). PCT data on cold spots
reveals a near ideal BCS density of states (DOS) with gap parameters, $\Delta$
as high as 1.62 meV, that are among the highest values ever reported for Nb.
Hot spot regions exhibit a wide distribution of gap values down to $\Delta
\sim$ 1.0 meV and DOS broadening characterized by a relatively large value of
pair-breaking rate, $\Gamma$, indicating surface regions of significantly
reduced superconductivity. In addition, hot spots commonly exhibit Kondo
tunneling peaks indicative of surface magnetic moments attributed to a
defective oxide. N-doped cavities reveal a more homoegeneous gap distribution
centered at $\Delta \sim$ 1.5 meV and relatively small values of
$\Gamma/\Delta$. The absence of regions of significantly reduced
superconductivity indicates that the N interstitials are playing an important
role in preventing the formation of hydride phases and other macroscopic
defects which might otherwise severely affect the local, surface
superconductivity that lead to hot spot formation. The N-doped cavities also
display a significantly improved surface oxide, i.e., increased thickness and
tunnel barrier height, compared to CP cavities. These results help explain the
improved performance of N-doped cavities and give insights into the origin of
the initial increasing Q with RF amplitude.
