  We present an analysis of the Nb3Sn surface layers grown on a bulk niobium
(Nb) coupon prepared at the same time and by the same vapor diffusion process
used to make Nb3Sn coatings on 1.3 GHz cavities. Tunneling spectroscopy reveals
a well-developed, homogeneous superconducting density of states at the surface
with a gap value distribution centered around 2.7 meV and superconducting
critical temperature (Tc) up to 16.3 K. Scanning Electron microscopy (STEM)
performed on cross section of the sample's surface region shows a 2 microns
thick Nb3Sn surface layer. The elemental composition map exhibits a Nb over Sn
ratio of 3 and reveals the presence of buried sub-stoichiometric regions that
have a ratio f 5. Synchrotron x-ray diffraction experiments indicate a
polycrystalline Nb3Sn film and confirm the presence of Nb rich regions that
occupy about a third of the coating volume. These low Tc regions could play an
important role in the dissipation mechanism occurring during RF tests of
Nb3Sn-coated cavities and open the way for further improving a very promising
alternative to pure Nb cavities for particle accelerators.
