Effects of charged defects on the electronic and optical properties of self-assembled quantum dots (original) (raw)
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Physical Review B, 2011
We study the effect of elastic anisotropic biaxial strain, induced by a piezoelectric actuator, on the light emitted by neutral excitons confined in different kinds of epitaxial quantum dots. We find that the light polarization rotates by up to ∼80 • and the fine structure splitting (FSS) varies nonmonotonically by several tens of μeV as the strain is varied. These findings provide the experimental proof of a recently predicted strain-induced anticrossing of the bright states of neutral excitons in quantum dots. Calculations on model dots qualitatively reproduce the observations and suggest that the minimum reachable FSS critically depends on the orientation of the strain axis relative to the dot elongation.
Semiconductor Science and Technology, 2014
The effect of the lattice-mismatch strain and of the charge carrier confinement profile, on the optical properties of thermally annealed self-assembled In x Ga 1−x As/GaAs quantum dots (QDs), is theoretically analyzed by using a recently developed 40-band k.p model. First, to evaluate the composition and size of QDs as a function of thermal annealing conditions, we model the In/Ga interdiffusion by a Fickian diffusion. Second, we investigate the decrease of the strain effects on the carrier confinement potentials with annealing by solving the Schrödinger equation separately for electrons and holes. It is clearly found that the strain strongly modifies the QD potential profile, leading to a different electron and hole energy distribution. Finally, we carry on a comparison between theoretical calculations and photoluminescence (PL) experimental results performed in thermal annealed samples. A good agreement is obtained for the energy blueshift and the linewidth narrowing of the PL spectra measured on annealed QD ensemble. These results prove the relevance of the present approach to describe the optoelectronic properties of the nanostructures through the post-growth thermal annealing treatment.
GaAs quantum dots under quasi-uniaxial stress: experiment and theory
arXiv (Cornell University), 2022
The optical properties of excitons confined in initially-unstrained GaAs/AlGaAs quantum dots are studied as a function of a variable quasi-uniaxial stress. To allow the validation of state-of-theart computational tools for describing the optical properties of nanostructures, we determine the quantum dot morphology and the in-plane components of externally induced strain tensor at the quantum dot positions. Based on these experimental parameters, we calculate the strain-dependent excitonic emission energy, degree of linear polarization, and fine-structure splitting using a combination of eight-band k • p formalism with multiparticle corrections using the configuration interaction method. The experimental observations are quantitatively well reproduced by our calculations and deviations are discussed.
Two-step strain analysis of self-assembled InAs/GaAs quantum dots
Semiconductor Science and Technology, 2006
Strain effects on optical properties of self-assembled InAs/GaAs quantum dots grown by epitaxy are investigated. Since a capping layer is added after the self-assembly process of the quantum dots, it might be reasonable to assume that the capping layer neither experiences nor affects the induced deformation of quantum dots during the self-assembly process. A new two-step model is proposed to analyse the three-dimensional induced strain fields of quantum dots. The model is based on the theory of linear elasticity and takes into account the sequence of the fabrication process of quantum dots. In the first step, the heterostructure system of quantum dots without the capping layer is considered. The mismatch of lattice constants between the wetting layer and the substrate is the driving source for the induced elastic strain. The strain field obtained in the first step is then treated as an initial strain for the whole heterostructure system, with the capping layer, in the second step. The strain from the two-step analysis is then incorporated into a steady-state effective-mass Schrödinger equation. The energy levels as well as the wavefunctions of both the electron and the hole are calculated. The numerical results show that the strain field from this new two-step model is significantly different from models where the sequence of the fabrication process is completely omitted. The calculated optical wavelength from this new model agrees well with previous experimental photoluminescence data from other studies. It seems reasonable to conclude that the proposed two-step strain analysis is crucial for future optical analysis and applications.
Electron-hole symmetry breakings in optical fine structures of single self-assembled quantum dots
Physica E: Low- …, 2009
The effects of electron–hole (e-h) symmetry breaking on the optical fine structure splitting (FSS) of single excitons in individual InGaAs/GaAs self-assembled quantum dots are experimentally and theoretically studied. The measured FSSs of small InGaAs/GaAs self-assembled quantum dots show a monotonic decrease with increasing emission energy and eventually almost vanish (t10meV) in the high energy regime. A theory based on 3D asymmetric parabolic model for e–h exchange interaction in combination with 3D finite difference simulations for Ga-diffused InGaAs/GaAs QDs is developed to explore the underlying physics. The reduced FSSs in the high emission energy regime are shown closely related to the e-h wave function symmetry breaking which is especially significant in highly Ga- diffused quantum dots. The Ga-diffusion induced e–h asymmetry reduces the e–h wave function overlap and results in the feature of reduced fine energy splitting.
Physical Review B, 2003
Atomistic pseudopotential many-body calculations of excitonic ͑X͒ recombination in charged, selfassembled In x Ga 1Ϫx As/GaAs dots predict and explain remarkable trends. ͑i͒ The redshift of the exciton energy upon negative charging is rapidly reduced with increasing the In content and increasing the dot height. The opposite behavior is observed upon positive charging. ͑ii͒ The recombination peak energies of different charge states show intriguing symmetries and alignments, e.g., X Ϫ aligns with X 2Ϫ and X 3Ϫ aligns with X 4Ϫ. ͑iii͒ The X 3Ϫ spectrum shows that a triplet initial state is lower in energy for flat dots ͑yielding two spectral lines͒, whereas the singlet state is lower in energy for taller dots ͑yielding a single line͒. These trends are explained theoretically in terms of a crossover occurring at a critical In concentration and dot height at which the electron wave functions becomes more localized than the hole wave functions.
Insight into optical properties of strain-free quantum dot pairs
Journal of Nanoparticle Research, 2011
Self-assembled GaAs/AlGaAs quantum dot pairs (QDPs) are grown by molecular beam epitaxy using high temperature droplet epitaxy technique. A typical QDP consists of dual-size quantum dots as observed based on atomic force microscopy image. The average height of quantum dot is 5.7 nm for the large quantum dots and 4.6 nm for the small ones. The average peak-to-peak distance of the two dots is about 75 nm. The optical properties of GaAs QDPs are studied by measuring excitation powerdependent and temperature-dependent photoluminescence. Unique photoluminescence properties have been observed from both excitation power-dependent and temperature-dependent measurements. Excitation power-dependent as well as temperature-dependent PL measurements have suggested lateral exciton transfer in the QDPs.
Fabrication and photoluminescence of quantum dots induced by strain of self-organized stressors
Solid-State Electronics, 1996
Quantum dots have been fabricated from InGaAs/GaAs quantum wells by growing self-organized InP islands on the surface of the structure. The strain from the island-stressors modifies locally the band edges of the quantum well, forming quantum dots. The deposition conditions of InP are optimized to ensure a narrow size distribution of InP islands with a density of about IO9 cm-* measured by atomic force microscopy. The lateral strain-induced confinement of carriers produces distinct energy levels as seen in photoluminescence measurements. The ground state transition is redshifted by 64-105 meV from the quantum well peak, depending on the top barrier thickness. Five narrow excited state transitions separated by up to 19 meV are observed between the ground state peak and the quantum well peak at increasing excitation intensity. The narrow linewidth and high photoluminescence efficiency of the dots indicates the very high optical quality of the structure.