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

Theoretical Background of Semiconductor Nanostructures
On other hand, Fig. 2.1(a) shows a direct bandgap semiconductor materials
such as compound semiconductors of group III and V in the periodic table (e.g.,
GaAs or InAs, etc.). If the electron given some energy in the form of heat, voltage,
or absorption of a photon. When an electron is raised to the conduction band, it
leaves behind in the valence band a particle called a hole, which represents the
absence of an electron and is treated as a positive particle. These two particles
are called exciton (electron-hole pair). Between those two bands is the bandgap,
a region of forbidden energies for the electrons (bandgap). Electrons that have
been excited to the conduction band will eventually return to the valence band,
recombining with a hole. This represents the natural tendency of a system to
return into the lowest energy state possible. When an electron recombines with
a hole, conservation of energy dictates that their dierence in energy (which is
the energy dierence between the bottom of the conduction band and the top of
the valence band, i.e. the bandgap energy) must be conserved. Depending on the
type of material, this energy dierence might result either in light (emission of a
photon) or it is mostly transferred to the lattice vibrations and dissipated as heat
(emission of a phonon). Therefore, an electron raised to the conduction band, will
recombine in a very short time 10 −9 s with a valence band hole to emit a photon
of energy hν equal to the bandgap E g of the material [15]. Thus, the probability
of radiative recombination is very high in direct bandgap semiconductors.
Despite that silicon has very excellent electronic, thermal and mechanical
properties. However due to its indirect bandgap silicon exhibits poor optoelectronic
properties. The light emission in indirect bandgap silicon can be explained
in terms of phonon-assisted exciton recombination across the bandgap [13, 15]. In
the bulk material, this phonon-assisted optical transition is very weak and dominates
many other non-radiative processes resulting in a huge drop in the light
emission eciency. Therefore, bulk silicon is not suitable for the implementation
of optoelectronic devices, which is needed for the new demand of ultra-high
speed chips (super chips). To date, the semiconductor optoelectronics industry
has been dominated by the III-V compound semiconductors because of their high
eciency in optical transitions primarily due to their direct fundamental energy
bandgap. However, the integration of III-V optoelectronic devices with silicon
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