Quantum Dot Geometry Research Articles

Interplay of morphology, composition, and optical properties of InP-based quantum dots emitting at the 1.55μm telecom wavelength

Results for the development and detailed analysis of self-organized InAs/InAlGaAs/InP quantum dots suitable for single-photon emission at the $1.55\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{m}$ telecom wavelength are reported. The structural and compositional properties of the system are obtained from high-resolution scanning transmission electron microscopy of individual quantum dots. The system is composed of almost pure InAs quantum dots embedded in quaternary InAlGaAs barrier material, which is lattice matched to the InP substrate. When using the measured results for a representative quantum-dot geometry as well as experimentally reconstructed alloy concentrations, a combination of strain-field and electronic-state calculations is able to reproduce the quantum-dot emission wavelength in agreement with the experimentally determined photoluminescence spectrum. The inhomogeneous broadening of the latter can be related to calculated variations of the emission wavelength for the experimentally deduced In-concentration fluctuations and size variations.

Phonon impact on optical control schemes of quantum dots: Role of quantum dot geometry and symmetry

Phonons strongly influence the optical control of semiconductor quantum dots. When modeling the electron-phonon interaction in several theoretical approaches the quantum dot geometry is approximated by a spherical structure, though typical self-assembled quantum dots are strongly lens-shaped. By explicitly comparing simulations of a spherical and a lens-shaped dot using a well-established correlation expansion approach we show that indeed lens-shaped dots can be exactly mapped to a spherical geometry when studying the phonon influence on the electronic system. We also give a recipe to reproduce spectral densities from more involved dots by rather simple spherical models. On the other hand, breaking the spherical symmetry has a pronounced impact on the spatio-temporal properties of the phonon dynamics. As an example we show that for a lens-shaped quantum dot the phonon emission is strongly concentrated along the direction of the smallest axis of the dot which is important for the use of phonons for the communication between different dots.

Symmetric analysis, categorization, and optical spectrum of ideal pyramid quantum dots

Self-assembled quantum dots possess an intrinsic geometric symmetry. Applying group representation theory, we systematically analyze the symmetric properties of the bound states for ideal pyramid quantum dots, which neglect band mixing and strain effects. We label each bound state by its symmetry group’s corresponding irreducible representation and define a concept called the quantum dots’ symmetry category. A class of quantum dots with the same irreducible representation sequence of bound states are characterized as belonging to a specific symmetry category. This category concept generally describes the symmetric order of Hilbert space or wavefunction space. We clearly identify the connection between the symmetry category and the geometry of quantum dots by the symmetry category graph or map. The symmetry category change or transition corresponds to an accidental degeneracy of the bound states. The symmetry category and category transition are observable from the photocurrent spectroscopy or optical spectrum. For simplicity’s sake, in this paper, we only focus on inter-subband transition spectra, but the methodology can be extended to the inter-band transition cases. We predict that from the spectral measurements, the quantum dots’ geometric information may be inversely extracted.

Interdependence of ICD rates in paired quantum dots on geometry.

Using state-of-the-art antisymmetrized multiconfiguration time-dependent Hartree (MCTDH) electron dynamics calculations we study the interdependence of the intermolecular Coulombic decay (ICD) process on the geometric parameters of a doubly-charged paired quantum dot (PQD) model system in the framework of the effective mass approximation (EMA). We find that ICD displays a maximum rate for a certain geometry of the electron-emitting quantum dot, which is simultaneously dependent on both the distance between the quantum dots as well as the photon-absorbing quantum dot's geometry. The rate maximum is shown to be caused by the competing effects of polarization of electron density and Coulomb repulsion. The ICD rate-maximized PQD geometry in GaAs QDs yields a decay time of 102.39 ps. It is given by two vertically-aligned cylindrical QDs with radii of 14.42 nm separated by 86.62 nm. The photon absorbing QD then has a height of 46.59 nm and the electron emitting QD a height of 16.33 nm. © 2017 Wiley Periodicals, Inc.

Energy spectrums of bilayer triangular phosphorene quantum dots and antidots

We theoretically investigate the confined states of the bilayer triangular phosphorene dots and antidots by means of the tight-binding approach. The dependence of the energy levels on the size, the type of the boundary edges, and the orientation of the dots and antidots, and the influences of the electric and magnetic fields on the energy levels, are all completely analyzed. It is found that the energy level numbers of the bilayer dots and antidots are determined by the energy levels in two layers. The external electric field can effectively tune the energy levels of the edge states in both layers to move in opposite directions. With the increase of the magnetic field, the magnetic energy levels can approach the Landau levels of the phosphorene monolayer, the phosphorene bilayer, or both, depending on the specific geometry of the monolayer-bilayer hybrid phosphorene quantum dots. This research should be helpful for the overall understanding of the electronic properties of the multilayer hybrid phosphorene nanostructures and designing the corresponding phosphorene devices.

Electron Raman Scattering and Raman Gain in Pyramidal Semiconductor Quantum Dots.

The Raman scattering related with conduction band states in semiconductor pyramidal quantum dots is theoretically investigated. The electron Raman differential cross section and Raman gain coefficient are calculated making use of analytically determined quantum states. The energy spectrum is obtained within the effective mass approximation. The features of the Raman differential cross section are discussed in terms of their dependence on the changes of the quantum dot geometry. The variation of the Raman gain coefficient as a function of the quantum dot size and shape is also analyzed.

Impact of Phonons on Dephasing of Individual Excitons in Deterministic Quantum Dot Microlenses.

Optimized light–matter coupling in semiconductor nanostructures is a key to understand their optical properties and can be enabled by advanced fabrication techniques. Using in situ electron beam lithography combined with a low-temperature cathodoluminescence imaging, we deterministically fabricate microlenses above selected InAs quantum dots (QDs), achieving their efficient coupling to the external light field. This enables performing four-wave mixing microspectroscopy of single QD excitons, revealing the exciton population and coherence dynamics. We infer the temperature dependence of the dephasing in order to address the impact of phonons on the decoherence of confined excitons. The loss of the coherence over the first picoseconds is associated with the emission of a phonon wave packet, also governing the phonon background in photoluminescence (PL) spectra. Using theory based on the independent boson model, we consistently explain the initial coherence decay, the zero-phonon line fraction, and the line shape of the phonon-assisted PL using realistic quantum dot geometries.

Coulomb Mediated Hybridization of Excitons in Coupled Quantum Dots.

We report Coulomb mediated hybridization of excitonic states in optically active InGaAs quantum dot molecules. By probing the optical response of an individual quantum dot molecule as a function of the static electric field applied along the molecular axis, we observe unexpected avoided level crossings that do not arise from the dominant single-particle tunnel coupling. We identify a new few-particle coupling mechanism stemming from Coulomb interactions between different neutral exciton states. Such Coulomb resonances hybridize the exciton wave function over four different electron and hole single-particle orbitals. Comparisons of experimental observations with microscopic eight-band k·p calculations taking into account a realistic quantum dot geometry show good agreement and reveal that the Coulomb resonances arise from broken symmetry in the artificial semiconductor molecule.

Elliptical quantum dots as on-demand single photons sources with deterministic polarization states

In quantum information, control of the single photon's polarization is essential. Here, we demonstrate single photon generation in a pre-programmed and deterministic polarization state, on a chip-scale platform, utilizing site-controlled elliptical quantum dots (QDs) synthesized by a top-down approach. The polarization from the QD emission is found to be linear with a high degree of linear polarization and parallel to the long axis of the ellipse. Single photon emission with orthogonal polarizations is achieved, and the dependence of the degree of linear polarization on the QD geometry is analyzed.

Tuning of zero energy states in quantum dots of Silicene and bilayer graphene by electric field

Electronic properties of triangular and hexagonal nano-scale quantum dots (QDs) of Silicene and bilayer graphene are studied. It is shown that the low-energy edge-localized electronic states, existing within the size-quantized gap are easily tunable by electric field. The appearance and field evolution of the electronic gap in these zero energy states (ZES) is shown to be very sensitive to QD geometry that permits to design the field-effect scalable QD devices with electronic properties on-demand.

Electric-dipole-induced spin resonance in a lateral double quantum dot incorporating two single-domain nanomagnets

On-chip magnets can be used to implement relatively large local magnetic field gradients in nanoelectronic circuits. Such field gradients provide possibilities for all-electrical control of electron spin qubits where important coupling constants depend crucially on the detailed field distribution. We present a double quantum dot (QD) hybrid device laterally defined in a GaAs/AlGaAs heterostructure which incorporates two single-domain nanomagnets. They have appreciably different coercive fields which allows us to realize four distinct configurations of the local inhomogeneous field distribution. We perform dc transport spectroscopy in the Pauli-spin blockade regime as well as electric-dipole-induced spin resonance (EDSR) measurements to explore our hybrid nanodevice. Characterizing the two nanomagnets we find excellent agreement with numerical simulations. By comparing the EDSR measurements with a second double QD incorporating just one nanomagnet we reveal an important advantage of having one magnet per QD: It facilitates strong field gradients in each QD and allows us to control the electron spins individually for instance in an EDSR experiment. With just one single-domain nanomagnet and common QD geometries EDSR can likely be performed only in one QD.

Polarization properties of single zinc-blende GaN/AlN quantum dots

We study by microphotoluminescence the polarization properties of single zinc-blende GaN/AlN quantum dots fabricated by two growth processes: the droplet epitaxy technique and the Stranski-Krastanov growth mode. A statistical analysis of their polarization properties both at the mesoscopic and nanoscopic scales indicates that it is possible to modify the phase and amplitude of their valence band mixing by switching on and off the influence of AlN antiphase domains through the growth process. $k\phantom{\rule{0.16em}{0ex}}.p$ calculations show that the strain effects and quantum dot geometries resulting from the growth process play a major role in shaping their polarization properties.

Structural and emission properties of InGaAs/GaAs quantum dots emitting at 1.3 μm

A combined experimental and theoretical study of InGaAs/GaAs quantum dots (QDs) emitting at 1.3 μm under the influence of a strain-reducing InGaAs quantum well is presented. We demonstrate a red shift of 20–40 nm observed in photoluminescence spectra due to the quantum well. The InGaAs/GaAs QDs grown by metal organic vapor phase epitaxy show a bimodal height distribution (1 nm and 5 nm) and indium concentrations up to 90%. The emission properties are explained with combined tight-binding and configuration-interaction calculations of the emission wavelengths in conjunction with high-resolution scanning transmission electron microscopy investigations of QD geometry and indium concentrations in the QDs, which directly enter the calculations. QD geometries and concentration gradients representative for the ensemble are identified.

Effects of strain on the band alignment and the optical gain of a CdTe/ZnTe quantum dot

The band lineups at the strained layer interfaces, in ZnxCd1–xTe/ZnTe quantum dot nanostructure for various Zn alloy content, are computed using model solid theory with an example of CdTe/ZnTe interface. The effects of strain, due to hydrostatic and biaxial strain, and the internal electric fields, due to the spontaneous and piezoelectric polarization, are taken into consideration. The dielectric mismatch, through the effective potential, is introduced between the dielectric constants of the dot and the barrier materials. The interband emission energy as a function of dot radius is computed for various Zn alloy content. The optical gain spectra of heavy hole exciton for various Zn concentration are studied. Calculations are obtained for different confinement potentials with the inclusion strain effect. Our results show that (i) the potential taking into account the effects of PB potential due to the dielectric mismatch enhances the exciton binding energy and (ii) the geometry of quantum dot, the strain effects and the Zinc alloy content have great influences on the electrical and optical properties of the dot. Our results are in good agreement with the previous investigators.

Nonpolar GaN quantum dots: Impact of dot size and shape on the electron and hole wave function overlap

AbstractIn this work we analyze the spatial overlap of electron and hole ground state wave functions in a‐plane GaN/AlN quantum dots. Special attention is paid to the impact of the quantum dot geometry, when taking Coulomb effects into account. Our analysis shows, comparing systems with almost identical volumes, that the shape of the quantum dot base is more important than the dimension of the dot along the c‐axis.This initially surprising result is related to the change of the potential profile inside the nanostructure. Furthermore, our analysis confirms that Coulomb effects play an important role when describing the optical properties of nonpolar nitride‐based QD structures. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Excitonic fine-structure splitting in telecom-wavelength InAs/GaAs quantum dots: Statistical distribution and height-dependence

The variation of the excitonic fine-structure splitting is studied for semiconductor quantum dots under the influence of a strain-reducing layer, utilized to shift the emission wavelength of the excitonic transition into the telecom-wavelength regime of 1.3–1.5 μm. By means of a sp3s*-tight-binding model and configuration interaction, we calculate wavelength shifts and fine-structure splittings for various quantum dot geometries. We find the splittings remaining small and even decreasing with strain-reducing layer composition for quantum dots with large height. Combined with an observed increased emission efficiency, the applicability for generation of entanglement photons is persistent.

Strain induced optical properties of exciton in a CdTe/ZnTe quantum dot

The influence of strain on the binding energies of heavy and light hole is obtained in a ZnxCd1−xTe/ZnTe quantum dot taking into account the phonon confinement effect. The band offsets are calculated using model-solid approach. The dielectric mismatch effect is introduced between the dielectric constants of dot and the barrier. The strain induced energies of excitonic transitions in a CdZnTe quantum dot with ZnTe barrier are brought out considering the internal electric field induced by the spontaneous and piezoelectric polarizations. Calculations have been obtained using Bessel function as an orthonormal basis for different confinement potentials of barrier height, strain induced linear and third-order nonlinear optical absorption coefficients and the changes of refractive index with the incident photon energy are observed. Our results show that the exciton binding energy is enhanced with the inclusion of potential taking into account the effects of dielectric mismatch and the geometry of quantum dot with various Zinc alloy content has a great influence on the optical properties of the dot.

Spin–orbit-induced hole spin relaxation in InAs and GaAs quantum dots

We study the effect of valence band spin–orbit interactions (SOI) on the acoustic phonon-assisted spin relaxation of holes confined in quantum dots (QDs). Heavy hole–light hole (hh–lh) mixing and all the spin–orbit terms arising from zinc-blende bulk inversion asymmetry (BIA) are considered on equal footing in a fully three-dimensional Hamiltonian. We show that hh–lh mixing and BIA have comparable contributions to the hole spin relaxation in self-assembled QDs, but BIA becomes dominant in gated QDs. Simultaneously accounting for both mechanisms is necessary for quantitatively correct results in quasi-two-dimensional QDs. The dependence of the hole spin relaxation on the QD geometry and spin splitting energy is drastically different from that of electrons, with a non-monotonic behavior which results from the interplay between SOI terms. Our results reconcile contradictory predictions of previous theoretical works and are consistent with experiments.

Charge trapping in the system of interacting quantum dots

We analyzed the localized charge dynamics in the system of N interacting single-level quantum dots (QDs) coupled to the continuous spectrum states in the presence of Coulomb interaction between electrons within the dots. Different dots geometry and initial charge configurations were considered. The analysis was performed by means of Heisenberg equations for localized electron pair correlators.We revealed that charge trapping takes place for a wide range of system parameters and we suggested the QDs geometry for experimental observations of this phenomenon. We demonstrated significant suppression of Coulomb correlations with the increasing QDs number. We found the appearance of several time scales with the strongly different relaxation rates for a wide range of the Coulomb interaction values.

The conduction band absorption spectrum of interdiffused InGaAs/GaAs quantum dot infrared photodetectors

We report on a theoretical study of the relationship between interdiffusion and the conduction band optical absorption of In(Ga)As/GaAs quantum dots. Quantum dot geometries are progressively interdiffused based on Fick’s model and the quantum dot strain, band structure and optical absorption cross-section are calculated numerically. Quantifying the effects of interdiffusion on quantum dot optical absorption is important for applications that utilize post-growth techniques such as selective area intermixing.