Growth of rare earth single crystals by MBE: The epitaxial relationship between hcp rare earth and bcc niobium.

With the newly developed metal MBE technique, high quality rare-earth (RE) single-crystal films of yttrium (Y) and gadolinium (Gd) were successfully grown on a bcc Nb single crystal film which serves as a buffer layer to the sapphire substrates. With in-situ RHEED characterizations, the hcp RE (0001) close- packed plane was found to grow epitaxial not only on the Nb (111) surface of similar symmetry but also on the (110) close- packed plane of Nb in the so called Nishiyama-Wasserman orientation. However, the re-growth of a Nb crystal on the RE (0001) surface only yielded the (110) orientation at comparable deposition temperatures. These epitaxial relationships determined at the interfaces further suggest the possibility of fabricating an ultrathin, coherent crystalline superlattice in the Nb(110)/RE (0001) system.