Microscopic Modeling of Novel Semiconductor Heterostructure Properties
Nowadays, semiconductor-based technology is part of everyday lives of many people around the world. This is most visible in the frequent use of computers and smartphones. By using clouds, messenger services and social networks among other things, enormous amounts of data are transmitted globally...
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|Summary:||Nowadays, semiconductor-based technology is part of everyday lives of many
people around the world. This is most visible in the frequent use of computers
and smartphones. By using clouds, messenger services and social
networks among other things, enormous amounts of data are transmitted
globally. For this purpose, laser signals that propagate through fiber-optic
cables are being used. At this, the wavelengths that can be used for transmission,
are determined by the absorption and dispersion properties of the
propagation medium. Wavelengths in the near-infrared range of the
electromagnetic spectrum are suited for this purpose.
Conventional light-emitting heterostructures that consist of nanometer-thick
semiconductor layers and rely on spatially direct recombination of charge
carriers in the same layer, are not ideally suited for emission in the near-infrared.
This stems from Auger-losses, which increase with increasing wavelength
and are significant for bandgap energies corresponding to wavelengths
in the near-infrared. Hence, alternatives are needed.
Promising alternatives are provided by heterostructures that rely on spatially
indirect recombination of charge carriers. In such heterostructures,
electrons and holes are confined in layers of different semiconductor
materials. This allows to use semiconductor materials with comparatively
large bandgaps and to still generate light with a wavelength in the near-infrared
of the electromagnetic spectrum. Moreover, using two different
materials for charge carrier confinement increases the number of possible
designs for such structures and thus offers more flexibility.
Generally, the confinement of electrons and holes in different semiconductor
layers is accompanied by lowered electron-hole wavefunction overlap in comparison
to structures that rely on spatially direct charge carrier recombinations.
This leads to lowered optical transition rates and can be compensated
to a certain extent by careful optimization of the optical properties
of these heterostructures.
This thesis presents research results that contribute to the optimization
of heterostructures that rely on spatially indirect recombination of electrons
and holes. For this purpose, it was focused on heterostructures where
(InGa)As was used to achieve electron confinement and Ga(AsSb) was used
to achieve hole confinement. At this, both materials were grown on GaAs
as a substrate.
The results presented in this thesis are either based on calculations using the
reliable many-body theory from the semiconductor Bloch and luminescence
equations in combination with the k.p-theory or on density functional theory
calculations. In many respects, the results gained from the calculations
replace the investigative, experimental growth and subsequent experimental
characterization of properties of such heterostructures. In the investigated
heterostructures, charge transfer and recombination processes take place
through internal interfaces. Properties of the internal interfaces can be studied
using interface specific excitations. One of those is the charge-transfer
exciton. This thesis presents certain results from a detailed experiment-theory
investigation of the formation and decay of charge transfer excitons.
The presented results are based on bandstructure calculations with the k.p-theory
and the semiconductor Bloch approach.
The density functional theory calculations carried out in the framework of
this thesis were used to calculate the valence band offsets between GaAs and
Ga(AsSb) in strained heterostructures. This allows for drawing conclusions
on the band alignment in the corresponding heterostructure.
During the density functional calculations the problem appeared that
the Ga(AsSb) bandgaps vanish at certain Sb concentrations in the ternary
semiconductor compound. Related to this, for Sb concentrations exceeding
a critical value the calculated valence band offsets diverged. These problems
could be resolved by introducing the method of half-occupations to
the calculations of the valence band offsets. The presented approach for
the calculation of valence band offsets has the potential to be applicable for
other semiconductor materials as well.|
|Physical Description:||108 Pages|