Showing papers by "David Vanderbilt published in 1989"

Surface doping and stabilization of Si(111) with boron.

Abstract: We have investigated the incorporation of boron into the Si(111) (\ensuremath{\surd}3\ifmmode\times\else\texttimes\fi{}\ensuremath{\surd}3)R30\ifmmode^\circ\else\textdegree\fi{} surface from low boron concentration up to (1/3 monolayer, using tunneling microscopy and spectroscopy and first-principles total-energy calculations. Surprisingly, we find that a (\ensuremath{\surd}3\ifmmode\times\else\texttimes\fi{}\ensuremath{\surd}3)R30\ifmmode^\circ\else\textdegree\fi{} structure composed almost entirely of Si adatoms on a Si double layer can be stabilized by boron doping in near surface layers. Moreover, when boron atoms are in the surface layers, the adatom site is unfavorable compared with the site underneath the adatom, unlike other group-III elements adsorbed on the Si(111) surface.

Adatoms on Si(111) and Ge(111) surfaces

Abstract: First-principles calculations of energy and stress are performed on adatom-covered Si(111) and Ge(111) surfaces. The presence of adatoms is found to lower the surface energy and cause a large change in surface stress. While the 1\ifmmode\times\else\texttimes\fi{}1 surfaces are under a weak compressive stress, the \ensuremath{\surd}3\ifmmode\times\else\texttimes\fi{}\ensuremath{\surd}3 and 2\ifmmode\times\else\texttimes\fi{}2 adatom-covered surfaces are under a strong tensile stress. Calculations at high plane-wave cutoff unambiguously identify the 2\ifmmode\times\else\texttimes\fi{}2 top-site geometry as the energetically preferred adatom configuration. Relaxed geometries are presented and compared with x-ray structural measurements of adatoms in the Si(111)-7\ifmmode\times\else\texttimes\fi{}7 structure. Vibrational mode frequencies and eigenvectors of the adatom unit are determined from a comprehensive set of frozen-phonon calculations for the Si 2\ifmmode\times\else\texttimes\fi{}2 surface; we find two symmetric modes that are strongly localized at the surface, in agreement with electron-energy-loss--spectroscopy measurements. It is found that the 2\ifmmode\times\else\texttimes\fi{}2 adatom-covered surfaces have three surface bands, and the dispersion relations are calculated along symmetry directions in the surface Brillouin zone. The surface band structures are in good agreement with angle-resolved photoelectron-spectroscopy data for the 7\ifmmode\times\else\texttimes\fi{}7-Si(111) surface.

Origins of stress on elemental and chemisorbed semiconductor surfaces

Abstract: First-principles calculations of stress and energy have been performed on 1\ifmmode\times\else\texttimes\fi{}1 substitutional and \ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 adatom-covered Si(111) and Ge(111) surfaces, using a variety of column-III, -IV, and -V adsorbates. Trends in surface stresses are understood in terms of three contributing factors: the relative atomic size of the adsorbate and substrate atoms, the chemical nature of the adsorbate species, and the bonding topology of the surface reconstruction.

Anharmonic elastic and phonon properties of Si

Abstract: A unified framework is suggested for the discussion of anharmonic phonon coupling constants and anharmonic elastic constants in diamond-structure materials. A summary is given, within this framework, of those anharmonic constants which have previously been determined experimentally or theoretically for silicon. New local-density total-energy calculations for X-point phonons in Si are used to add to this database of known anharmonic constants. It is proposed that empirical models for interatomic potentials should be constrained to fit this database. A generalized Keating model which has been fitted in this way, with two- and three-body couplings of third and fourth order, is presented. It can be used to calculate arbitrary anharmonic phonon couplings through fourth order.

Atomic scale calculations in materials science

Abstract: A wide variety of properties of materials are determined by processes which take place on an atomic scale. In recent years, theoretical developments and advances in computer power have made it possible to examine these properties and processes directly, using a number of different calculational and simulation techniques. The papers here include representatives of major theoretical approaches, from first-principles studies of electronic and structural properties to simulations of extended crystal defects using empirical potentials. Applications are made to metals, semiconductors and insulators, in the form of crystals, defects, glasses, and liquids. This book presents an introduction to this research area as well as a record of the present status.

Elastic stress domains on the Si(100) surface

Abstract: It is proposed that surfaces of crystals that reconstruct with broken orientational symmetry and exhibit an anisotropic stress tensor are unstable to the formation of elastic stress domains. The ground state of such a surface is not uniform, but corresponds to a mosaic pattern of the different domains. This result is applied together with microscopic calculations of surface stress to study the Si(100) surface. A structural transition from single‐layer to double‐layer steps for vicinal Si(100) surfaces is also studied.