Origin of flat morphology and high crystallinity of ultrathin bismuth films

We have investigated the origin of atomically flat and single-crystalline growth of Bi films on Si(1 1 1)-7 x 7 through comparative experiments using Si(1 1 1)-beta-root 3 x root 3-Bi as a control system. On the Si(1 1 1)-7 x 7 substrate, the majority of initial nuclei stabilize with pseudocubic (PC) paired layers analogous to the black phosphorus (BP) structure, and grow in a strong two-dimensional fashion that results in a textured but atomically flat surface morphology. After the coalescence of the BP-like grains at a nominal thickness of 4 monolayers (ML), a tiny number of minority hexagonal (HEX) bulk crystal nuclei, aligned commensurately with the substrate 7 x 7 lattice. cause the textured BP-like PC film to transform into a single-crystalline bulk-like HEX film. On the Si(1 1 1)-beta-root 3 x root 3-Bi substrate, however, the BP-like structure breaks Lip into a conventional bulk-like PC structure and the HEX nucleation is suppressed up to as thick as similar to 6 ML. Therefore, the morphology and crystallinity of the films are simply rough and polycrystalline, respectively. (c) 2007 Elsevier B.V. All rights reserved.