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