  Nb3Sn has the potential to achieve superior performance in terms of quality
factor, accelerating gradient and operating temperature (4.2 K vs 2 K)
resulting in significant reduction in both capital and operating costs compared
to traditional niobium SRF accelerator cavities. Tin vapor diffusion coating of
Nb3Sn on niobium appears to be a simple, yet most efficient technique so far to
fabricate such cavities. Here, cavity interior surface coatings are obtained by
a two step process: nucleation followed by deposition. The first step is
normally accomplished with Sn/SnCl2 at a constant low temperature (500 {\deg}C)
for several hours. To elucidate the role of this step, we systematically
studied the niobium surface nucleated under varying process conditions. The
surfaces obtained in typical tin/tin chloride processes were characterized
using SEM/EDS, AFM, XPS, SAM and TEM. Examination of the surfaces nucleated
under the standard conditions revealed not only tin particles, but also tin
film on the surfaces resembling the surface obtained by Stranski-Krastanov
growth mode. All the nucleation attempted with SnCl2 yielded better uniformity
of Nb3Sn coating compared to coating obtained without nucleation, which often
included random patchy regions with irregular grain structure. Even though the
variation of nucleation parameters was able to produce different surfaces
following nucleation, no evidence was found for any significant impact on the
final coating.
