Molecular beam epitaxial growth of III-V semiconductor ... - KOBRA

3.2 The Concept of Epitaxy
the probability of desorption. In addition, metallic droplets of group III species
can liquefy on the substrate surface. The growth rates are determined by the
group III uxes and the desorption. The V/III ratio must therefore be determined
by the ux of the group V elements.
The occurrence of the epitaxial growth modes depends on various parameters
of which the most important are the thermodynamic driving force and the mist
between substrate and layer. In MBE growth the substrate surfaces are held in
UHV chambers while being exposed to a vapor of molecules or atoms of the growing
material. The thermodynamics and kinetic factors determined the growth.
The classical thermodynamic approach to epitaxial thin lm growth which led
to the denition of the so-called growth modes. This thermodynamics approach
is used to determine growth modes of thin lms close to equilibrium [35]. The
growth mode characterizes the nucleation and growth process. There is a direct
correspondence between the growth mode and the lm morphology, which gives
the structural properties such as perfection, atness and interface abruptness of
the layers. The kinetic description of growth in which the lm morphology is
the result of the microscopic path taken by the system during growth. This path
is determined by the of rates of the single atom, cluster, or molecule displacements
as compared to the deposition, desorption, and dissociation rates. It is
determined by the kinetics of the transport and diusion processes on the surface
[32].
The competition between the lm and substrate surface energies resulted from
the growth dynamics and growth conditions, will determine the growth mode of
the epitaxial growth process close to equilibrium. However, the MBE growth
process is a kinetically dominated process and thermal equilibrium conditions are
only partially fullled. The growing lms using MBE technique usually not in
thermodynamics equilibrium and kinetics eects have to be considered. Because
of the limited surface diusion, the deposited material cannot rearrange itself
to minimize the surface energy. The high supersaturation of the deposit leads
to a large nucleation rate, and kinetics will lead to the occurrence of dierent
growth modes [35]. Therefore, the behavior of deposited species is determined by
a number of kinetic parameters. Among them are the surface diusion coecient
(D s ) of the adatoms, the sticking probability of an adatom arriving the edge to
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