251 research outputs found
Scaling of Heteroepitaxial Island Sizes
Monte Carlo simulations of an atomistic solid-on-solid model are used to
study the effect of lattice misfit on the distribution of two-dimensional
islands sizes as a function of coverage in the submonolayer
aggregation regime of epitaxial growth. Misfit promotes the detachment of atoms
from the perimeter of large pseudomorphic islands and thus favors their
dissolution into smaller islands that relieve strain more efficiently. The
number density of islands composed of atoms exhibits scaling in the form
\mbox{)} where is the average island size. Unlike the
case of homoepitaxy, a rate equation theory based on this observation leads to
qualitatively different behavior than observed in the simulations.Comment: 10 pages, LaTeX 2.09, IC-DDV-94-00
Density Functional Theory of Epitaxial Growth of Metals
This chapter starts with a summary of the atomistic processes that occur
during epitaxy. We then introduce density functional theory (DFT) and describe
its implementation into state-of-the-art computations of complex processes in
condensed matter physics and materials science. In particular we discuss how
DFT can be used to calculate parameters of microscopic processes such as
adsorption and surface diffusion, and how they can be used to study the
macroscopic time and length scales of realistic growth conditions. This meso-
and macroscopic regime is described by the ab initio kinetic Monte Carlo
approach. We discuss several specific theoretical studies that highlight the
importance of the different diffusion mechanisms at step edges, the role of
surfactants, and the influence of surface stress. The presented results are for
specific materials (namely silver and aluminum), but they are explained in
simple physical pictures suggesting that they also hold for other systems.Comment: 55 pages, 20 figures, to be published "Growth of Ultrathin Epitaxial
Layers", The Chemical Physics of Soild Surfaces, Vol. 8, Eds D. A. King and
D. P. Woodruff (Elsevier Science, Amsterdam, 1997
Study of Strain and Temperature Dependence of Metal Epitaxy
Metallic films are important in catalysis, magneto-optic storage media, and
interconnects in microelectronics, and it is crucial to predict and control
their morphologies. The evolution of a growing crystal is determined by the
behavior of each individual atom, but technologically relevant structures have
to be described on a time scale of the order of (at least) tenths of a second
and on a length scale of nanometers. An adequate theory of growth should
describe the atomistic level on very short time scales (femtoseconds), the
formation of small islands (microseconds), as well as the evolution of
mesoscopic and macroscopic structures (tenths of seconds).
The development of efficient algorithms combined with the availability of
cheaper and faster computers has turned density functional theory (DFT) into a
reliable and feasible tool to study the microscopic aspects of growth phenomena
(and many other complex processes in materials science, condensed matter
physics, and chemistry). In this paper some DFT results for diffusion
properties on metallic surfaces are presented. Particularly, we will discuss
the current understanding of the influences of strain on the diffusion (energy
barrier and prefactor) of a single adatom on a substrate.
A DFT total energy calculation by its nature is primarily a static
calculation. An accurate way to describe the spatial and temporal development
of a growing crystal is given by kinetic Monte Carlo (KMC). We will describe
the method and its combination with microscopic parameters obtained from ab
initio calculations. It is shown that realistic ab initio kinetic Monte Carlo
simulations are able to predict an evolving mesoscopic structure on the basis
of microscopic details.Comment: 25 pages, 6 figures, In: ``Morphological Organisation during
Epitaxial Growth and Removal'', Eds. Z. Zhang, M. Lagally. World Scientific,
Singapore 1998. other related publications can be found at
http://www.rz-berlin.mpg.de/th/paper.htm
Level Set Approach to Reversible Epitaxial Growth
We generalize the level set approach to model epitaxial growth to include
thermal detachment of atoms from island edges. This means that islands do not
always grow and island dissociation can occur. We make no assumptions about a
critical nucleus. Excellent quantitative agreement is obtained with kinetic
Monte Carlo simulations for island densities and island size distributions in
the submonolayer regime.Comment: 7 pages, 9 figure
Submonolayer Epitaxy Without A Critical Nucleus
The nucleation and growth of two--dimensional islands is studied with Monte
Carlo simulations of a pair--bond solid--on--solid model of epitaxial growth.
The conventional description of this problem in terms of a well--defined
critical island size fails because no islands are absolutely stable against
single atom detachment by thermal bond breaking. When two--bond scission is
negligible, we find that the ratio of the dimer dissociation rate to the rate
of adatom capture by dimers uniquely indexes both the island size distribution
scaling function and the dependence of the island density on the flux and the
substrate temperature. Effective pair-bond model parameters are found that
yield excellent quantitative agreement with scaling functions measured for
Fe/Fe(001).Comment: 8 pages, Postscript files (the paper and Figs. 1-3), uuencoded,
compressed and tarred. Surface Science Letters, in press
Epitaxial Growth Kinetics with Interacting Coherent Islands
The Stranski-Krastanov growth kinetics of undislocated (coherent)
3-dimensional islands is studied with a self-consistent mean field rate theory
that takes account of elastic interactions between the islands. The latter are
presumed to facilitate the detachment of atoms from the islands with a
consequent decrease in their average size. Semi-quantitative agreement with
experiment is found for the time evolution of the total island density and the
mean island size. When combined with scaling ideas, these results provide a
natural way to understand the often-observed initial increase and subsequent
decrease in the width of the coherent island size distribution.Comment: 4 pages, 4 figure
Influence of adatom interactions on second layer nucleation
We develop a theory for the inclusion of adatom interactions in second layer
nucleation occurring in epitaxial growth. The interactions considered are due
to ring barriers between pairs of adatoms and binding energies of unstable
clusters. The theory is based on a master equation, which describes the time
development of microscopic states that are specified by cluster configurations
on top of an island. The transition rates are derived by scaling arguments and
tested against kinetic Monte-Carlo simulations. As an application we reanalyze
experiments to determine the step edge barrier for Ag/Pt(111).Comment: 4 pages, 4 figure
Implication of the overlap representation for modelling generalized parton distributions
Based on a field theoretically inspired model of light-cone wave functions,
we derive valence-like generalized parton distributions and their double
distributions from the wave function overlap in the parton number conserved
s-channel. The parton number changing contributions in the t-channel are
restored from duality. In our construction constraints of positivity and
polynomiality are simultaneously satisfied and it also implies a model
dependent relation between generalized parton distributions and transverse
momentum dependent parton distribution functions. The model predicts that the
t-behavior of resulting hadronic amplitudes depends on the Bjorken variable
x_Bj. We also propose an improved ansatz for double distributions that embeds
this property.Comment: 15 pages, 8 eps figure
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