I present a systematic study of self-assembled InAs/InP and InAs/GaAs quantum dot single-particle and many-body properties as a function of the quantum dot-surrounding matrix valence band offset. I ...use an atomistic, empirical tight-binding approach and perform numerically demanding calculations for half-million-atom nanosystems. I demonstrate that the overall confinement in quantum dots is a non-trivial interplay of two key factors: strain effects and the valence band offset. I show that strain effects determine both the peculiar structure of confined hole states of lens type InAs/GaAs quantum dots and the characteristic 'shell-like' structure of confined hole states in the commonly considered 'low-strain' lens type InAs/InP quantum dot. I also demonstrate that strain leads to single-band-like behavior of hole states of disk type ('indium flushed') InAs/GaAs and InAs/InP quantum dots. I show how strain and valence band offset affect quantum dot many-body properties: the excitonic fine structure, an important factor for efficient entangled photon pair generation, and the biexciton and charged exciton binding energies.
A method for inclusion of strain into the tight-binding Hamiltonian is presented. This approach bridges from bulk strain to the atomistic language of bond lengths and angles, and features a diagonal ...parameters shift in a form suitable for atomistic calculation of million atom nanosystems with a small number of empirical parameters. I illustrate this method by calculating electronic and optical properties of self-assembled InAs/(InP,GaAs) lens-shaped quantum dots. Avery different structure of confined quantum dots states is shown, depending on the matrix material and inclusion of strain effects. Results are compared with the well-established empirical pseudopotential method, and reasonable agreement is found.
We use an atomistic model to consider the effect of shape symmetry breaking on the optical properties of self-assembled InAs/GaAs quantum dots. In particular, we investigate the energy level ...structure and optical activity of the lowest energy excitons in these nanostructures. We compare between quantum dots with twofold rotational and two reflections (C sub(2)v) symmetry and quantum dots in which this symmetry was reduced to one reflection only (C sub(s)) by introducing a facet between the quantum dots and the host material. We show that the symmetry reduction mostly affects the optical activity of the dark exciton. While in symmetric quantum dots, one of the dark exciton eigenstates has a small dipole moment polarized along the symmetry axis (growth direction) of the quantum dot, in nonsymmetric ones, the two dark excitons' dipole moments are predominantly cross-linearly polarized perpendicular to the growth direction and reveal pronounced polarization anisotropy. Our model calculations agree quantitatively with recently obtained experimental data.
Quantum dots with light-hole ground states could find numerous applications including faster quantum bit operations or coherent conversion of photons into electron spins. Typically, however, holes ...confined in epitaxial quantum dots are of heavy-hole character. I show, by use of atomistic tight-binding theory, that the hole ground state undergoes a transition from heavy holelike to light holelike with increasing height of a nanowire InAs/InP quantum dot. The fine structure of the light-hole exciton consists of a dark ground state and three bright states. Two of the bright states are quasidegenerate and are in-plane polarized, whereas, the third energetically higher bright state is polarized in the perpendicular out-of-plane direction. The light-hole exciton fine structure is robust against alloying.
Main goal of the study was present the results of some respirometric measurements of activated sludge biodegrading the substrate in the wastewater originated in selected sections of the dairy ...processing line. The following dairy production effluents were analyzed in the research: the pumping station wastewater (combined wastewater from all the sections of the dairy factory), the apparatus room wastewater, the butter section wastewater, the milk reception point wastewater, the cheese section wastewater and the cottage cheese section wastewater. Apart from that, sweet and sour whey, which are secondary products of hard cheese and cottage cheese production, respectively, was the subject of the research. The amount of organic matter being oxidized during a 5-day measurement session was calculated on 1
g of the activated sludge biomass. The research was conducted at the temperature of 20
°C and 35
°C at the applied sludge loading rate of
A′
=
0.2
g BOD
g
−1 dry mass
d
−1, which ensured complete biodegradation.
The results indicated a correlation between a technological process of dairy processing, an ultimate outcome of which was the wastewater analyzed, and dairy wastewater biodegradability. The results confirmed that all dairy processing effluents can be treated together, with the exception of whey, whose complex biodegradation demands may cause too much burden to any wastewater treatment technological system and thus should be managed within a separate installation.
The bright exciton splitting in nanosystems and its origins are of primary importance for quantum-dot-based entangled-photon-pair generation. In this paper, I investigate excitonic energies and fine ...structure for million-atom InAs/InP quantum dots using many-body theory in conjunction with the empirical tight-binding method. Whereas the phenomenological theories relate the fine-structure splitting to quantum-dot-shape asymmetry, using an atomistic approach I demonstrate that for certain elongated quantum-dot shapes the bright exciton splitting can be significantly reduced. I demonstrate that strain effects play an essential role as the main contribution to the bright exciton splitting in InAs/InP quantum dots and observe highly reduced fine-structure splitting for high-symmetry quantum dots without wetting layer. I report the "intrinsic" fine-structure splitting, due to the underlying crystal lattice, to be generally significantly larger than the values predicted by the empirical pseudopotential calculations. Finally, I study excitonic properties of alloyed InAsP quantum dots and demonstrate that alloying effects can significantly reduce fine-structure splitting even in significantly elongated quantum dots.
A transition from a cylindrical quantum dot to a highly elongated quantum dash is theoretically studied here with an atomistic approach combining empirical tight binding for single particle states ...and a configuration interaction method for excitonic properties. Large nanostructure shape anisotropy leads to a peculiar trend of bright exciton splitting, which is quenched at a certain point with further shape elongation, contradicting predictions of simplified models. Moreover, strong shape elongation promotes pronounced optical activity of the dark exciton, which can reach a substantial 1% of the bright exciton intensity without application of any external fields. An atomistic calculation is augmented with an elementary phenomenological model expressed in terms of light-hole exciton admixture increasing with shape deformation. Also, excitonic complexes X−, X+, and XX are studied, and the correlations due to the presence of higher excited states are identified as key factors affecting excitonic binding energies and the fine structure.
Excitons in alloyed nanowire quantum dots exhibit unique spectra, as shown here using atomistic calculations. The bright exciton splitting is triggered solely by alloying, despite the cylindrical ...quantum dot shape reaching over 15μeV. These results are contrary to previous theoretical predictions; however, they are in line with experimental data. This splitting can nonetheless be tuned by an electric field to go below the 1μeV threshold. The dark exciton optical activity is also strongly affected by alloying that reaches a notable 1/3500 fraction of the bright exciton and has a large out-of-plane polarized component.
•Radiative and nonradiative processes of recombination in GaNP(As) layers were studied.•Thermally activated processes from photoluminescence thermal quenching were identified.•Increasing nitrogen ...content leads to an unexpected nonmonotonic behavior of the low temperature Stokes shift.
Recombination processes in GaNP(As) alloys were studied by an analysis of photoluminescence spectra in wide temperature and excitation power ranges. Temperature dependent photoluminescence studies shown carrier localization effects characteristic of Highly Mismatched Alloys group of semiconductors. Low temperature GaNP(As) emission was attributed to localized and delocalized exciton recombination by power dependent photoluminescence studies. Also, a non-monotonic dependence of the Stokes shift and emission intensity as a function of nitrogen content were found and attributed to a deactivation of localization centers due to the N content increase induced conduction band edge downward shift. Two activation energies were determined from photoluminescence thermal quenching analysis. In GaNP these energies were found to be approximately 13 meV and 50–35 meV, the second decreased with the increase in N concentration. For constant N content and varied As in the GaNPAs layers the two activation energies decreased between 13–8 meV and 50–30 meV, both with As increase.
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