The growth and structure of epitaxial niobium on sapphire

A review of the experimental and theoretical work on the growth and structure of epitaxial niobium on sapphire is presented. The review focuses on growth methods by molecular beam epitaxy (MBE) and sputtering techniques. The experimental results come from a broad spectrum of techniques, including electron microscopy, electron and X-ray diffraction, and surface microscopy. Theoretical studies provide models for the structure of the Nb-Al2O3 interface and the nature and distribution of misfit dislocations. The results from growth of niobium on a number of sapphire surface planes are critically discussed and analysed, from the first stages of growth to the ensemble structure and as functions of the substrate. The resulting film characteristics are discussed as a function of substrate quality and temperature and of film thickness. The discussion attempts to define some outstanding questions that must be answered to fully understand this system. (C) 2001 Elsevier Science B.V. All rights reserved.