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the cluster to the substrate potential. In contrast, the incommensurate clusters exhibit
floating solid behavior and therefore, faster migration on the surface.
6.5 Surface Diffusion on Stepped Epitaxial Interface: Incomplete
Surface-Confined Intermixing
In the energy regions defined by system temperature below
T
L
(blocked alloying,
(
6.1
)) and above
T
H
(complete alloying, (
6.3
)), the simulation results for the diffu-
sion of single adatoms do not reveal any unusual behavior. The system completely
follows experimental findings of entirely phase separated at
T
<
T
L
or entirely
mixed ordered or disordered interface at
T
T
H
.[
18
,
19
]. Opportunely, this is
not the case of atomic diffusion in the temperature gap
T
L
≤
>
T
H
, where the
atomistic model reveals a number of fine effects. These effects come out by the
variety of energy barriers that the adatom with fixed energy meets at specific sites
on the crystal surface. Consequently, the initial atomic-scale surface morphology
moulds the final equilibrium interface structure. In this section we will discuss diffu-
sion scenarios at the most common types of surface morphology: epitaxial interface
with large, atomically smooth domains and epitaxial interface with terraces having
variable width, edged by different steps and kinks.
T
≤
6.5.1 Diffusion in the Vicinity of Steps of Atomic Terraces
In the temperature region 300 K
500 K the energy of migrating atoms is
below the barrier for direct embedding inside the substrate, (
6.2
). The simulations
at 400 K show lack of any atomic exchange between adsorbed and substrate atoms
on large terraces or smooth, step-free surface areas. This behavior is consistent with
recently reported rather high diffusion energy barrier of 1.17 eV for direct embed-
ding of Pb atoms inside the Cu(111) terrace [
17
]. On the other hand, the low acti-
vation energy of 0.03 eV for adatom surface diffusion favors the atomic transport
on top of the substrate. Hence, in specific temperature window
T
L
≤
T
≤
≤
T
≤
T
H
,
the suppressed direct alloying
E
ATOM
<
DIRECT
in the outermost surface layer
assists the atoms to attach the steps of the atomic terraces, Fig.
6.7
.
We have already pointed out that adatoms have different ability for embedding,
depending on the sites they inhabit on the epitaxial interface. As a result of different
elastic strain in complete surface layer and in terrace with finite size, the energy
barriers for diffusion into the smooth substrate by direct atomic exchange,
E
DB
E
DB
DIRECT
,
and for diffusion across the step of terraces,
E
DB
STEP
, are expected to be different.
Indeed, at equilibrium, close to step edges, the simulations show very distinct Pb
adatom diffusion inside the Cu matrix, Fig.
6.8
. Following initial intensive pro-
cess of intermixing in the vicinity of step, the diffusion within the terrace signifi-
cantly decreases and the concentration of foreign Pb atoms dramatically falls down
after the third atomic row of the Cu terrace. Approaching equilibrium, the system
