High-resolution X-ray scattering studies of niobium [10] films grown b
y molecular beam epitaxy on sapphire [1120BAR] substrates have reveale
d novel structural features. In transverse scans of the out-of-plane N
b (110) Bragg peak we find two components, the sharper of which implie
s mosaicities an order of magnitude better than bulk single crystal Nb
. The planes associated with the sharp component are aligned with the
sapphire (1120BAR) planes. Upon hydrogen loading of the Nb film, we fi
nd evidence for a dramatic increase of the lateral coherence length. I
t appears that the addition of hydrogen may allow defects to move to t
he film boundary, removing inhomogeneous strain and thus improving the
epitaxial film quality by acting as a very effective ''cold-annealing
'' agent.