18 research outputs found
Excitonic structure and pumping power dependent emission blue-shift of type-II quantum dots
In this work we study theoretically and experimentally the multi-particle
structure of the so-called type-II quantum dots with spatially separated
electrons and holes. Our calculations based on customarily developed full
configuration interaction approach reveal that exciton complexes containing
holes interacting with two or more electrons exhibit fairly large antibinding
energies. This effect is found to be the hallmark of the type-II confinement.
In addition, an approximate self-consistent solution of the multi-exciton
problem allows us to explain two pronounced phenomena: the blue-shift of the
emission with pumping and the large inhomogenous spectral broadening, both of
those eluding explanation so far. The results are confirmed by detailed
intensity and polarization resolved photoluminescence measurements on a number
of type-II samples.Comment: 11 pages, 5 figure
Excitonic fine structure of epitaxial Cd(Se,Te) on ZnTe type-II quantum dots
The structure of the ground state exciton of Cd(Se,Te) quantum dots embedded
in ZnTe matrix is studied experimentally using photoluminescence spectroscopy
and theoretically using and configuration interaction
methods. The experiments reveal a considerable reduction of fine-structure
splitting energy of the exciton with increase of Se content in the dots. That
effect is interpreted by theoretical calculations to originate due to the
transition from spatially direct (type-I) to indirect (type-II) transition
between electrons and holes in the dot induced by increase of Se. The trends
predicted by the theory match those of the experimental results very well.The
theory identifies that the main mechanism causing elevated fine-structure
energy in particular in type-I dots is due to the multipole expansion of the
exchange interaction. Moreover, the theory reveals that for Se contents in the
dot , there exist also a {\bf peculiar type of confinement showing
signatures of both type~I and type~II} and which exhibits extraordinary
properties, such as almost purely light hole character of exciton and toroidal
shape of hole states
Type-I to type-II band alignment switching for (In,Ga)(As,Sb)/GaAs/GaP quantum dots overgrown by a thin GaSb capping layer
We study the optical and theoretical properties of (In,Ga)(As,Sb)/GaAs
quantum dots (QDs) embedded in a GaP (100) matrix, which are overgrown by a
thin GaSb capping layer with variable thickness. QD samples are studied by
temperature-dependent photoluminescence, and the results analyzed with the help
of theoretical simulations by eight-band~\textbf{kp}, with multiparticle
corrections using the configuration interaction. We reveal a type-I to type-II
band alignment switching when QDs are overgrown by a GaSb layer with a
thickness larger than one monolayer. Moreover, we observe a temperature driven
blueshift of the quantum dot luminescence, which is explained by decomposing
the spectra into sum of Gaussians. Our analysis reveals that the GaSb overlayer
causes switching of the intensity between - and L-transitions, making
the -indirect electron-hole transition in type-II regime to be
optically more radiant than the -direct one. Finally, we provide
theoretical expectations for the storage time for (In,Ga)(As,Sb)/GaAs/GaP QDs
overgrown by the GaSb layer with an AlP barrier underneath, to be embedded in a
nanomemory device. We find that by increasing the thickness of the GaSb layer
from 0 to 1.5~monolayers (MLs) leads to an increase in the storage time of four
orders of magnitude, from 1 hour to up almost a year, rendering such QDs very
promising candidates as storage units for nanomemory devices
Effect of second order piezoelectricity on excitonic structure of stress-tuned InGaAs/GaAs quantum dots
We study the effects of the nonlinear piezoelectricity and the In
distribution on the exciton energy, the electron-hole electric dipole moment,
and the fine-structure splitting in stress-tunable InGaAs/GaAs quantum dots
integrated onto a piezoelectric actuator. In particular, we investigate in
detail the contributions of various elements of the expansion of the electrical
polarization in terms of externally induced elastic strain on the latter two
important quantum dot properties. Based on the comparison of the effects of
first- and second-order piezoelectricity we provide a simple relation to
estimate the influence of applied anisotropic stress on the quantum dot dipole
moment for quantum dots significantly lattice mismatched to the host crystal
Modeling electronic and optical properties of III-V quantum dots – selected recent developments
Electronic properties of selected quantum dot (QD) systems are surveyed based on the multi-band k·p method, which we benchmark by direct comparison to the empirical tight-binding algorithm, and we also discuss the newly developed “linear combination of quantum dot orbitals” method. Furthermore, we focus on two major complexes: First, the role of antimony incorporation in InGaAs/GaAs submonolayer QDs and In1−xGax AsySb1−y/GaP QDs, and second, the theory of QD-based quantum cascade lasers and the related prospect of room temperature lasing.TU Berlin, Open-Access-Mittel - 2022EC/H2020/956548/EU/Quantum Semiconductor Technologies Exploiting Antimony/QUANTIMONYEC/H2020/731473/EU/QuantERA ERA-NET Cofund in Quantum Technologies/QuantER
Inversion of the exciton built-in dipole moment in In(Ga)As quantum dots via nonlinear piezoelectric effect
We show that anisotropic biaxial stress can be used to tune the built-in
dipole moment of excitons confined in In(Ga)As quantum dots up to complete
erasure of its magnitude and inversion of its sign. We demonstrate that this
phenomenon is due to piezoelectricity. We present a model to calculate the
applied stress, taking advantage of the so-called piezotronic effect, which
produces significant changes in the current-voltage characteristics of the
strained diode-membranes containing the quantum dots. Finally, self-consistent
k.p calculations reveal that the experimental findings can be only accounted
for by the nonlinear piezoelectric effect, whose importance in quantum dot
physics has been theoretically recognized although it has proven difficult to
single out experimentally.Comment: 6 pages, 4 figure
Interplay between multipole expansion of exchange interaction and Coulomb correlation of exciton in colloidal II-VI quantum dots
We study the effect of Coulomb correlation on the emission properties of the ground state exciton in zincblende CdSe/ZnS core-shell and in wurtzite ZnO quantum dots (QDs). We validate our theory model by comparing results of computed exciton energies of CdSe/ZnS QDs to photoluminescence and scanning near-field optical microscopy measurements. We use that to estimate the diameter of the QDs using a simple model based on infinitely deep quantum well and compare the results with the statistics of the atomic force microscopy scans of CdSe/ZnS dots, obtaining excellent agreement. Thereafter, we compute the energy fine structure of exciton, finding striking difference between properties of zincblende CdSe/ZnS and wurtzite ZnO dots. While in the former the fine structure is dominated by the dipole terms of the multipole expansion of the exchange interaction, in the latter system that is mostly influenced by Coulomb correlation. Furthermore, the correlation sizeably influences also the exciton binding energy and emission radiative rate in ZnO dots