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

3.2 The Concept of Epitaxy
In many case S c is less than unity and it may be a small fraction in cases when
the adsorption energy of atoms on the substrate is low, or the substrate temperature
is high. Assuming α is unity, all the arriving atoms are accommodated on
the substrate surface and achieve thermodynamics equilibrium. This does not
mean, that they will stay there permanently. They still have a nite probability
related to the substrate temperature of acquiring sucient energy to overcome
the attractive forces and leave the substrate [33]. If aggregation of adatoms, does
not occur, all the adatoms will eventually be re-evaporated. Thus, the S c can be
almost zero even when α is unity.
There are two main types of adsorption that can occur during MBE. The rst
is physical adsorption, which refers to the case where there is no electron transfer
between adsorbate and adsorbent by forming a van der Waal's bond with a surface
atom "physisorption". The second is is resulted by forming a covalent or ionic
bond with a surface atom, which refers to the case when electron transfer, i.e.,
chemical reaction, take places between adsorbate and adsorbent "chemisorption"
[33]. The molecules stick to the surface via weak physisorption, which enables
the migration. As the physisorbed state is only a precursor state, there is no way
back from the chemisorbed state to the much weaker physisorbed state or even
re-evaporation into the reactor vacuum. The rate at which adatoms adsorbed to
the surface can be described by an exponential law (Eq. 3.3):
R ads ∝ ν a
exp −E ads
K B T
(3.3)
Where ν a is the adsorption frequency, E ads describes the necessary energy to
overcome the electrostatics potential, K B is Boltzmann constant and T is the
substrate temperature [33]. However, most substrates have complicated reconstructions
and the bonding is highly directional. Therefore, the probability of
adsorption to some sites is higher than other. Assuming defect-free surfaces, a
number of theoretical and experimental works has been carried out to nd the
most stable adsorption sites. Adsorbed atoms may diuse from one site to another
via thermally activated hopping , which can be expressed by Eq. 3.4:
D ∝ a 2 K s ∝ a 2 exp −E d
K B T
(3.4)
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