Chemistry Reference
In-Depth Information
the results in a physical model that implies cluster diffusion with size-dependent
cluster/substrate misfit. The second problem is devoted to diffusion phenomena in
the vicinity of atomic terraces on stepped or vicinal surfaces. Here, we develop a
computational model that refines important details of diffusion behavior of adatoms
accounting for the energy barriers at specific atomic sites (smooth domains, terraces,
and steps) located on the crystal surface. The dynamic competition between energy
gained by mixing and substrate strain energy results in diffusion scenario where
adatoms form alloyed islands and alloyed stripes in the vicinity of terrace edges.
Being in agreement with recent experimental findings, the observed effect of stripe
and island alloy formation opens up a way regular surface patterns to be configured
at different atomic levels on the crystal surface. The complete surface alloying of the
entire interface layer is also briefly discussed with critical analysis and classification
of experimental findings and simulation data.
6.1 Introduction
The problem of surface diffusion and surface-confined intermixing is of funda-
mental importance for contemporary material science [
1
-
5
]. Besides the purely
academic challenge, the attention to this phenomenon is inspired by the strong
dependence of basic physical properties of electronic devices on the sharpness at
atomic level of the interface between materials. That is why detailed knowledge
of the fine atomic structure and thermodynamic properties of the epitaxial layer
is crucial for design and fabrication of interfaces with preferred exotic features.
Since surface diffusion is basic phenomenon controlling nanostructure formation
on surfaces, this study deals with a variety of diffusion scenarios that cause 2D
pattern formation on epitaxial interface. Accounting for energy barriers at specific
sites on the crystal surface, the computational model presented opens up a way the
entire interface to be manipulated at atomic level in great detail [
4
]. In this case the
fine-tuning of the adatom energy through temperature variation can direct the basic
mass-transport mechanisms in a way that promotes or restricts the evolution of spe-
cific nanoscale surface patterns. In the next sections we will review and analyze the
diffusion energy barriers and the resulting diffusion processes on atomically smooth
domains and stepped (vicinal) interfaces.
6.2 Surface Imperfections and Diffusion Energy Barriers
on Epitaxial Interface
Detailed analysis of the general picture of surface morphology on atomic scale
reveals different types of surface diffusion barriers
E
DB
for adsorbed particles
[
4
,
6
-
8
]. Being caused by surface imperfections as steps, kinks, domain bound-
aries, defects, or vacancies, these barriers involve a variety of activation energies
which control diffusion on flat, atomically smooth surface domains; diffusion in
the vicinity of steps of atomic terraces; and diffusion inside the outermost surface
