X-ray diffraction study of epitaxial Nb-Ta overlayers on sapphire

We report on a four-circle diffractometer study of the epitaxial structure of Nb-Ta superlattices and overlayers on sapphire. In addition to confirming the previously reported orientation of the single-crystal films with respect to the sapphire substrate, we identify a (1 ×{} 4) in-plane supercell mesh which provides the basis for the high degree of epitaxial ordering in this system. In-plane coherence is shown to increase as a function of film thickness.