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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2.3 Low-Dimensional Semiconductor (Nanostrtuctures)<br />
the energy levels are discrete. For this reason, QDs are also referred to as superatoms<br />
or articial atoms. A QD <strong>of</strong> typical size (several nanometers to several tens<br />
<strong>of</strong> nanometers) contains several ten thousands to several tens <strong>of</strong> million atoms.<br />
Quantum dots have generated much interest as a new class <strong>of</strong> human-made materials<br />
with tunable (by varying both the composition and size) energies <strong>of</strong> discrete<br />
atomic-like states [22].<br />
2.3.3 The Excitons Connement in Quantum Dots<br />
An exciton is a bound state <strong>of</strong> an electron and an electron hole which are attracted<br />
to each other by the electrostatic Coulomb force. Excitons are the main mechanism<br />
for light emission in <strong>semiconductor</strong>s. An exciton can form when a photon<br />
is absorbed by a <strong>semiconductor</strong>. This excites an electron from the valence band<br />
into the conduction band. The Coulomb force in excitons act attractively between<br />
the two particles, negatively and positively charged, respectively, and a<br />
stable state can be formed between electron-hole pair. However, this attraction<br />
provides a stabilizing energy balance. Consequently, the exciton has slightly less<br />
energy than the unbound electron and hole. The binding energy is much smaller<br />
and the particle's size much larger than a hydrogen atom. This is because <strong>of</strong> both<br />
the screening <strong>of</strong> the Coulomb force by other electrons in the <strong>semiconductor</strong> ( i.e.,<br />
its dielectric constant), and the small eective masses <strong>of</strong> the excited electron and<br />
hole [23, 24].<br />
In general, excitons can be classied in two basic types: Wannier-Mott excitons<br />
also called free excitons, and Frenkel excitons which are tightly bound<br />
excitons. Wannier-Mott excitons are typically found in <strong>semiconductor</strong> crystals<br />
with small energy gaps and high dielectric constants, but have also been identied<br />
in liquids, such as liquid xenon [24]. In <strong>semiconductor</strong>s, the dielectric constant is<br />
generally large. Consequently, electric eld screening tends to reduce the Coulomb<br />
interaction between electrons and holes. The result is a Wannier exciton, which<br />
has a radius larger than the lattice spacing. As a result, the eect <strong>of</strong> the lattice<br />
potential can be incorporated into the eective masses <strong>of</strong> the electron and hole.<br />
Likewise, because <strong>of</strong> the lower masses and the screened Coulomb interaction, the<br />
binding energy is usually much less than that <strong>of</strong> a hydrogen atom, typically on<br />
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