135 research outputs found
Re-entrant Layer-by-Layer Etching of GaAs(001)
We report the first observation of re-entrant layer-by-layer etching based on
{\it in situ\/} reflection high-energy electron-diffraction measurements. With
AsBr used to etch GaAs(001), sustained specular-beam intensity oscillations
are seen at high substrate temperatures, a decaying intensity with no
oscillations at intermediate temperatures, but oscillations reappearing at
still lower temperatures. Simulations of an atomistic model for the etching
kinetics reproduce the temperature ranges of these three regimes and support an
interpretation of the origin of this phenomenon as the site-selectivity of the
etching process combined with activation barriers to interlayer adatom
migration.Comment: 11 pages, REVTeX 3.0. Physical Review Letters, in press
Ab initio study of step formation and self-diffusion on Ag(100)
Using the plane wave pseudopotential method we performed density functional
theory calculations on the stability of steps and self-diffusion processes on
Ag(100). Our calculated step formation energies show that the {111}-faceted
step is more stable than the {110}-faceted step. In accordance with
experimental observations we find that the equilibrium island shape should be
octagonal very close to a square with predominately {111}-faceted steps. For
the (100) surface of fcc metals atomic migration proceeds by a hopping or an
exchange process. For Ag(100) we find that adatoms diffuse across flat surfaces
preferentially by hopping. Adatoms approaching the close-packed {111}-faceted
step edges descend from the upper terrace to the lower level by an atomic
exchange with an energy barrier almost identical to the diffusion barrier on
flat surface regions. Thus, within our numerical accuracy (approx +- 0.05 eV)
there is no additional step-edge barrier to descent. This provides a natural
explanation for the experimental observations of the smooth two-dimensional
growth in homoepitaxy of Ag(100). Inspection of experimental results of other
fcc crystal surfaces indicates that our result holds quite generally.Comment: 10 pages, 9 figures. Submitted to Phys. Rev B (October 31, 1996
Atomic Scale Modelling of Two-Dimensional Molecular Self-Assembly on a Passivated Si Surface
International audienceThe self-assembly of two-dimensional (2D) molecular structures on a solid surface relies on the subtle balance between non covalent intermolecular and molecule-surface forces. The energetics of 2D molecular lattices forming different patterns on a passivated semiconductor surface are here investigated by a combination of atomistic simulation methods. Density-functional theory provides structure and charges of the molecules, while metadynamics with empirical forces provides a best guess for the lowest-energy adsorption sites of single molecules and dimers. Subsequently, molecular dynamics simulations of extended molecular assemblies with empirical forces yield the most favorable lattice structures at finite temperature and pressure.The theoretical results are in good agreement with scanning tunneling microscopy observations of self-assembled molecular monolayers on a B-doped Si(111) surface, thus allowing to rationalize the competition of long-range dispersion forces between the molecules and the surface. Such a result demonstrates the interest of this predictive approach for further progress in supramolecular chemistry on semiconductor surface
High-resolution LEED profile analysis and diffusion barrier estimation for submonolayer homoepitaxy of Ag/Ag(100)
High Resolution Sharp Computational Methods for Elliptic and Parabolic Problems in Complex Geometries
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