Quantum confined stark shift and ground state optical transition rate in [100] laterally biased InAs/GaAs quantum dots (original) (raw)
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Numerical simulation of a coupling effect on electronic states in quantum dots
Lasers operating at 1.3 µm have attracted considerable attention owing to their potential to provide efficient light sources for next-generation high-speed communication systems. InAs/GaAs quantum dots (QDs) were pointed out as a reliable low-cost way to attain this goal. However, due to the lattice mismatch, the accumulation of strain by stacking the QDs can cause dislocations that significantly degrade the performance of the lasers. In order to reduce this strain, a promising method is the use of InAs QDs embedded in InGaAs layers. The capping of the QD layer with InGaAs is able to tune the emission toward longer and controllable wave-lengths between 1.1 and 1.5 µm. In this work, using the effective-mass envelope-function theory, we investigated theoretically the optical properties of coupled InAs/GaAs strained QDs based structures emitting around 1.33 µm. The calculation was performed by the resolution of the 3D Schrödinger equation. The energy levels of confined carriers and the optical transition energy have been investigated. The oscillator strengths of this transition have been studied with and without taking into account the strain effect in the calculations. The information derived from the present study shows that the InGaAs capping layer may have profound consequences as regards the performance of an InAs/GaAs QD based laser. Based on the present results, we hope that the present work make a contribution to experimental studies of InAs/GaAs QD based structures, namely the optoelectronic applications concerning infrared and mid-infrared spectral regions as well as the solar cells.
Optical response of (InGa)(AsSb)/GaAs quantum dots embedded in a GaP matrix
Physical Review B
The optical response of (InGa)(AsSb)/GaAs quantum dots (QDs) grown on GaP (001) substrates is studied by means of excitation and temperature-dependent photoluminescence (PL), and it is related to their complex electronic structure. Such QDs exhibit concurrently direct and indirect transitions, which allows the swapping of Γ and L quantum confined states in energy, depending on details of their stoichiometry. Based on realistic data on QD structure and composition, derived from high-resolution transmission electron microscopy (HRTEM) measurements, simulations by means of k • p theory are performed. The theoretical prediction of both momentum direct and indirect type-I optical transitions are confirmed by the experiments presented here. Additional investigations by a combination of Raman and photoreflectance spectroscopy show modifications of the hydrostatic strain in the QD layer, depending on the sequential addition of QDs and capping layer. A variation of the excitation density across four orders of magnitude reveals a 50 meV energy blueshift of the QD emission. Our findings suggest that the assignment of the type of transition, based solely by the observation of a blueshift with increased pumping, is insufficient. We propose therefore a more consistent approach based on the analysis of the character of the blueshift evolution with optical pumping, which employs a numerical model based on a semi-self-consistent configuration interaction method.
…, 2002
Material layers with a thickness of a few nanometers are common-place in today's semiconductor devices. Before long, device fabrication methods will reach a point at which the other two device dimensions are scaled down to few tens of nanometers. The total atom count in such deca-nano devices is reduced to a few million. Only a small finite number of "free" electrons will operate such nano-scale devices due to quantized electron energies and electron charge. This work demonstrates that the simulation of electronic structure and electron transport on these length scales must not only be fundamentally quantum mechanical, but it must also include the atomic granularity of the device. Various elements of the theoretical, numerical, and software foundation of the prototype development of a Nanoelectronic Modeling tool (NEMO 3-D) which enables this class of device simulation on Beowulf cluster computers are presented. The electronic system is represented in a sparse complex Hamiltonian matrix of the order of hundreds of millions. A custom parallel matrix vector multiply algorithm that is coupled to a Lanczos and/or Rayleigh-Ritz eigenvalue solver has been developed. Benchmarks of the parallel electronic structure and the parallel strain calculation performed on various Beowulf cluster computers and a SGI Origin 2000 are presented. The Beowulf cluster benchmarks show that the competition for memory access on dual CPU PC boards renders the utility of one of the CPUs useless, if the memory usage per node is about 1-2 GB. A new strain treatment for the 1 gekco@jpl.nasa.gov tight-binding models is developed and parameterized for bulk material properties of GaAs and InAs. The utility of the new tool is demonstrated by an atomistic analysis of the effects of disorder in alloys. In particular bulk In x Ga 1−x As and In 0.6 Ga 0.4 As quantum dots are examined. The quantum dot simulations show that the random atom configurations in the alloy, without any size or shape variations can lead to optical transition energy variations of several meV. The electron and hole wave functions show significant spatial variations due to spatial disorder indicating variations in electron and hole localization.
2008 8th IEEE Conference on Nanotechnology, IEEE-NANO, 2008
Strain and electronic structure of InAs/GaAs quantum dot molecules made up of identical and non-identical vertically stacked quantum dots are compared using the sp 3 d 5 s* nearest neighbor empirical tight binding model. Hydrostatic and biaxial strain profiles strongly impact the local band edges and electronic structure for both identical and non-identical dots. Strain in the lower dot is significantly different as compared to the upper dot in the non-identical system in contrast to the identical system where it is almost the same in both dots. Therefore structural detailed differences are of critical importance and cannot be neglected. Qualitatively, the electronic structure is similar in identical and non-identical dot systems for small separations (below 6nm) and it is significantly different for large separations. The molecular orbitals convert to the dot-localized atomic orbitals at large dot separations in the non-identical system. Non-idealities such as strain and size variations induce an energy splitting in the considered dot ground states. Larger dissimilarity of dots increases e1-e2 and decreases the optical gap of system. This favors the possible use of such system in the construction of the long wavelength optical laser.
We present an atomistic investigation of the influence of strain on the electronic properties of quantum dots QD's within the empirical sp 3 s* tight-binding ETB model with interactions up to second nearest neighbors and spin-orbit coupling. Results for the model system of capped pyramid-shaped InAs QD's in GaAs, with supercells containing 10 5 atoms are presented and compared with previous empirical pseudopotential results. The good agreement shows that ETB is a reliable alternative for an atomistic treatment. The strain is incorporated through the atomistic valence-force field model. The ETB treatment allows for the effects of bond length and bond angle deviations from the ideal InAs and GaAs zinc-blende structure to be selectively removed from the electronic-structure calculation, giving quantitative information on the importance of strain effects on the bound-state energies and on the physical origin of the spatial elongation of the wave functions. Effects of dot-dot coupling have also been examined to determine the relative weight of both strain field and wave-function overlap.
Physical Review B, 2003
We present an atomistic investigation of the influence of strain on the electronic properties of quantum dots (QD's) within the empirical sp 3 s * tight-binding (ETB) model with interactions up to 2nd nearest neighbors and spin-orbit coupling. Results for the model system of capped pyramid-shaped InAs QD's in GaAs, with supercells containing ∼ 10 5 atoms are presented and compared with previous empirical pseudopotential results. The good agreement shows that ETB is a reliable alternative for an atomistic treatment. The strain is incorporated through the atomistic valence force field model. The ETB treatment allows for the effects of bond length and bond angle deviations from the ideal InAs and GaAs zincblende structure to be selectively removed from the electronic-structure calculation, giving quantitative information on the importance of strain effects on the bound state energies and on the physical origin of the spatial elongation of the wave functions. Effects of dot-dot coupling have also been examined to determine the relative weight of both strain field and wave function overlap.
Computational Materials Science, 2021
A comparison between k → · p → and tight-binding methods for the analysis of InAs/GaAs quantum dot bandstructures is presented based on a fully coupled computation of electromechanical effects. Electromechanical effects are addressed using a continuum elastic model for the k → · p → method and a pre-conditioned Valence Force Field algorithm for the tight-binding atomistic calculations. The Valence Force Field method allows the direct identification of the impact of internal strain. Results to ensure model parameter consistency between the two methods are also given by comparing bulk and unstrained quantum-well dispersion relations. The quantum dot size dependence of the bandstructure is investigated based on the models including electromechanical fields. Additionally, the effect of the electromechanical fields is studied for a specific dot size by comparing results with and without electromechanical fields. Good agreement is found for the confined energy levels but model differences...
Mathematical and Computer Modelling, 2013
A new capability of our well-known NEMO 3-D simulator (Ref. Klimeck et al., 2007 [10]) is introduced by carefully investigating the utility of III-V semiconductor quantum dots as infrared photodetectors at a wavelength of 1.2-1.5 µm. We not only present a detailed description of the simulation methodology coupled to the atomistic sp 3 d 5 s * tight-binding band model, but also validate the suggested methodology with a focus on a proof of principle on small GaAs quantum dots (QDs). Then, we move the simulation scope to optical properties of realistically sized dome-shaped InAs/GaAs QDs that are grown by selfassembly and typically contain a few million atoms. Performing numerical experiments with a variation in QD size, we not only show that the strength of ground state interband light transitions can be optimized via QD size-engineering, but also find that the hole ground state wavefunction serves as a control factor of transition strengths. Finally, we briefly introduce the web-based cyber infrastructure that is developed as a governmentfunded project to support online education and research via TCAD simulations. This work not only serves as a useful guideline to experimentalists for potential device designs and other modelers for the self-development of optical TCAD, but also provides a good chance to learn about the science gateway project ongoing in the Republic of Korea.