Extraordinary alignment of Nb films with sapphire and the effects of added hydrogen

We present the results of high-resolution x-ray scattering studies of the structural coherence of niobium [110] films grown by molecular-beam epitaxy on sapphire [112¯0] substrates. In transverse scans of the out-of-plane (110) Bragg peak we find two components, the sharper of which implies mosaicities an order of magnitudes better than bulk single-crystal Nb, and transverse structural coherence lengths exceeding ${10}^4$ Å. In addition, we observe that the planes associated with the sharp component are exactly aligned with the sapphire (112¯0) planes. Upon hydrogen loading of the Nb film, we find evidence for a dramatic increase of the lateral coherence length.