Chemistry Reference
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evolution of atomic structure of the interface. The simulations are done on canonical
ensemble, i.e., system with constant number of particles at constant temperature.
Full lattice dynamics for all atoms in adsorbed and substrate layers is applied for
complete spatial relaxation of the system. The final equilibrium state is realized
after an average Monte Carlo time of 5
10
5
MC steps per atom. At that state,
for a fixed temperature, the total system energy is minimal and fluctuates around a
constant value. All simulation details are described elsewhere [
10
].
As a model for studying surface diffusion and surface-confined intermixing we
chose Pb/Cu interface. An important reason for this preference is the large number
of experimental data collected for this system, which provides a reliable point of
reference for the computational model [
2
,
16
-
25
]. Extensive studies in the last two
decades demonstrated variety of Pb/Cu mixed equilibrium phases. Although Pb and
Cu are volume-immiscible substances, it was shown that the formation of 2D surface
alloy on the interface is an energetically favorable process [
10
,
18
-
23
]. Hence, in
the framework of present computational and physical models, we will discuss in
detail the surface diffusion on this epitaxial interface.
×
6.4 Surface Diffusion on Smooth Epitaxial Interface: Blocked
Surface Alloying
We have already defined the basic energy regions (
6.1
), (
6.2
), and (
6.3
), where
essential variation of surface diffusion mechanisms is expected. Being directly
related to the system temperature, these regions determine the thermal energy of
adatoms and atomic clusters for migration on epitaxial interface. The atoms in their
random walk have to overcome a variety of energy barriers originating from the spe-
cific surface morphology. At low temperature, the adatom energy does not exceed
the energy barriers for direct embedding inside the substrate, (
6.1
). In that case the
interface structure evolves as a result of atomic migration exclusively on top of the
surface layer. The exchange of atoms with substrate is strongly suppressed and,
therefore, the diffusion generates isolated clusters or large ordered assemblies fol-
lowing the trend to minimize the interface energy. As shown in Fig.
6.2
, attachment
and detachment to the steps and kinks of atomic terraces are also feasible.
Being well worn, the diffusion of single atoms at low temperatures and the result-
ing decoration of steps and kinks are phenomena studied in great detail both theo-
retically and experimentally [
6
-
8
]. That is the reason, in the energy gap defined by
(
6.1
), to focus our attention exclusively on diffusion of 2D atomic assemblies on
smooth epitaxial interface, where fine details of the mechanisms of islands migra-
tion are still an open task. These details relate to the questions (i) How the diffusion
coefficient of an atomic cluster is influenced by cluster size and geometry? (ii) Does
the lattice constant of an atomic cluster on foreign substrate depend on the number
of atoms building the cluster? (iii) How does the variation of cluster/substrate misfit
affect the general cluster diffusion behavior?
