Ellipsoidal Quantum Dot Research Articles

Alloy Fluctuations Act as Quantum Dot-like Emitters in GaAs-AlGaAs Core-Shell Nanowires.

GaAs-AlxGa1-xAs (AlGaAs) core-shell nanowires show great promise for nanoscale electronic and optoelectronic devices, but the application of these nonplanar heterostructures in devices requires improved understanding and control of nanoscale alloy composition and interfaces. Multiple researchers have observed sharp emission lines of unknown origin below the AlGaAs band edge in photoluminescence (PL) spectra of core-shell nanowires; point defects, alloy composition fluctuations, and self-assembled quantum dots have been put forward as candidate structures. Here we employ laser-assisted atom probe tomography to reveal structural and compositional features that give rise to the sharp PL emission spectra. Nanoscale ellipsoidal Ga-enriched clusters resulting from random composition fluctuations are identified in the AlGaAs shell, and their compositions, size distributions, and interface characteristics are analyzed. Simulations of exciton transition energies in ellipsoidal quantum dots are used to relate the Ga nanocluster distribution with the distribution of sharp PL emission lines. We conclude that the Ga rich clusters can act as discrete emitters provided that the major diameter is ≥4 nm. Smaller clusters are under-represented in the PL spectrum, and spectral lines of larger clusters are broadened, due to quantum tunneling between clusters.

Optical nutation in multilayered ellipsoidal quantum dots

The optical nutation signal of two and three ellipsoidal quantum dots (EQD) with finite confinement potential in the presence of polarized laser field is considered. In this regard, the effective-mass approximation within a perturbation scheme is used and the induced effective polarization of atoms in the multilayered ellipsoidal quantum dots is calculated for GaAs/GaAlAs/GaAs structure. The results showed that the optical nutation peak depends not only on the thickness of the intermediate layer but also on the ellipticity constant.

Exchange and Coulomb interactions of two electrons in double ellipsoidal quantum dots

The present study seeks to scrutinize the interactions of two electrons on the electronic properties of double ellipsoidal quantum dots (EQD). In this regard, the effective-mass approximation within a perturbation scheme is used and the Coulomb and exchange energies of the two electrons ellipsoidal quantum dot are calculated for GaAs/GaAlAs/AlAs structure. The results showed that the Coulomb and exchange energies depend not only on the thickness of the intermediate layer but also on the ellipticity constant.

Polarization charges effects on optical properties of double ellipsoidal quantum dot

The present study seeks to scrutinize the optical properties of a double ellipsoidal quantum dot (EQD) containing an electron in the presence of a donor impurity. The effect of surface polarization charges due to impurity on absorption coefficients for different values of dielectric constant is investigated. The variational method is used and for different values of ellipticity constant, the linear, nonlinear and total absorption coefficients are calculated as a function of the incident photon energy. The results show that the absorption coefficients of double EQD increase with increases in the ellipticity constant and decrease as polarization charges are considered.

Electric field and shape effect on the linear and nonlinear optical properties of ellipsoidal finite-potential quantum dots

In this work, the optical properties of ellipsoidal finite-potential quantum dot with an on-center shallow hydrogenic impurity in the presence of an external electric field have been studied. A variational procedure in the framework of perturbation theory was employed within the effective-mass approximation. Numerical results on typical GaAs/AlxGa1−xAs materials show that, besides incident light intensity, the electric field and dot shape have an important influence on the peak positions and magnitudes of linear and third-order nonlinear absorption coefficients and refractive index changes.

Raman study of a hydrogenic impurity in a three-dimensional anisotropic quantum dot

The anisotropy effects on the electron Raman scattering of a hydrogenic impurity have been investigated in a three-dimensional anisotropic quantum dot. The differential cross section is calculated as a function of the diffusion photon energy. The results show that the differential cross section of a hydrogenic impurity in an anisotropic quantum dot is strongly affected by the anisotropy degree and the dot size. We note that the differential cross section of the Raman scattering in an oblate ellipsoidal quantum dot is 1–2 orders of magnitude larger than that in a prolate ellipsoidal quantum dot. As the deviation from the spherical quantum dot increases, there are two different results for a prolate and an oblate ellipsoidal quantum dot.

Observation of Single Electron Transport via Multiple Quantum States of a Silicon Quantum Dot at Room Temperature

Single electron transport through multiple quantum levels is realized in a Si quantum-dot device at room-temperature conditions. The energy spacing of more than triple the omnipresent thermal energy is obtained from an extremely small ellipsoidal Si quantum dot, and high charge stability is attained through a construction of the gate-all-around structure. These properties may move us a step closer to practical applications of quantum devices at elevated temperatures. An in-depth analysis on the transport behavior and quantum structure is presented.

Effects of electric field and shape on the ground state energy of an exciton bound to an ionized donor impurity in ellipsoidal quantum dots

The effects of electric field and shape on the ground state energy of an exciton bound to an ionized donor are studied in a weakly prolate (oblate) GaAs ellipsoidal quantum dot within the effective-mass approximation using a variational method in the framework of perturbation theory. Both the heavy-hole and light-hole excitons are considered. The results show that the energy shift not only depends on the electric field but also on the dot shape and size. The influence of the dot shape on the light hole case is more pronounced than that of the heavy hole case, but the influence of the electric field on the heavy hole case is more pronounced.

Effect of dielectric mismatch on impurity binding energy in double ellipsoidal quantum dots

The present study seeks to scrutinize the effect of polarization charges on the electronic properties of double ellipsoidal quantum dots. In this regard, the effective-mass approximation within a variational scheme is used and the binding energy of hydrogenic impurity located at the center of ellipsoidal quantum dot (EQD) is calculated for GaAs/GaAlAs/AlAs structure. The effect of surface polarization charges due to impurity and self-polarization charges on the binding energy is considered. The results showed that the binding energy depends not only on the thickness of the intermediate layer but also on the ellipticity constant.

Three-dimensional imaging for precise structural control of Si quantum dot networks for all-Si solar cells

All-Si tandem solar cells based on Si quantum dots (QDs) are a promising approach to future high-performance, thin film solar cells using abundant, stable and non-toxic materials. An important prerequisite to achieve a high conversion efficiency in such cells is the ability to control the geometry of the Si QD network. This includes the ability to control both, the size and arrangement of Si QDs embedded in a higher bandgap matrix. Using plasmon tomography we show the size, shape and density of Si QDs, that form in Si rich oxide (SRO)/SiO2 multilayers upon annealing, can be controlled by varying the SRO stoichiometry. Smaller, more spherical QDs of higher densities are obtained at lower Si concentrations. In richer SRO layers ellipsoidal QDs tend to form. Using electronic structure calculations within the effective mass approximation we show that ellipsoidal QDs give rise to reduced inter-QD coupling in the layer. Efficient carrier transport via mini-bands is in this case more likely across the multilayers provided the SiO2 spacer layer is thin enough to allow coupling in the vertical direction.

The Ostwald ripening at nanoengineering of InAsSbP spherical and ellipsoidal quantum dots on InAs (100) surface

We present the results of growth of quasi-ternary InAsSbP spherical and ellipsoidal quantum dots (QDs) on InAs (100) surface by the method of liquid-phase epitaxy. Coarsening of QDs due to coalescence and Ostwald ripening was investigated by atomic-force and scanning electron microscopy. Ellipsoidal QDs prolated in [010] and oblated in [001] directions have been grown. Elongation ratios for the ellipsoidal QDs were measured in all three directions. It is shown that elongation of spherical QDs to ellipsoidal is started at QDs diameter of ∼50 nm. Shape transformation of the QDs’ size distribution function from the Gram-Charlier-like to the Gaussian and then to the Lifshits-Slezov-like distribution was revealed at increasing the nucleation time.

On the possibility of implementation of Kohn’s theorem in the case of ellipsoidal quantum dots

An electron gas in a strongly oblated ellipsoidal quantum dot with impenetrable walls is considered. Influence of the walls of the quantum dot is assumed to be so strong in the direction of the minor axis (the OZ axis) that the Coulomb interaction between electrons in this direction can be neglected and considered as two-dimensional, coupled. On the basis of geometric adiabaticity we show that in the case of a few-particle gas a powerful repulsive potential of the quantum dot walls has a parabolic form and localizes the dot in the geometric center of the structure. Due to this fact, conditions occur to implement the generalized Kohn theorem for this system.

Effect of incident light polarization on optical properties of an ellipsoidal quantum dot

The optical properties of an ellipsoidal quantum dot (EQD) with parabolic confinement potential in the presence of polarized laser field are considered. The linear, nonlinear, total absorption coefficient and refractive index as functions of the incident photon energy, photon intensity and ellipsoidal parameter for the different values of the confinement strength are calculated. The results show that the total absorption coefficient of an EQD increases with increases in the ellipticity parameter and oscillator strength. The relative change in refractive index increases (decreases) with ellipticity parameter (oscillator strength). The results are in agreement with other investigations.

Optical anisotropy of a donor in ellipsoidal quantum dots

A system of an electron with a hydrogenic impurity confined in anisotropic quantum dots with ellipsoidal shape has been investigated. The linear and nonlinear optical absorptions as well as refractive index changes associated with intersubband transitions has been calculated. The results are presented as a function of the incident photon energy. The results show that the optical properties of a donor in ellipsoidal quantum dots are strongly affected by the anisotropy degree and the dot size. The dot anisotropy is shown to play a fundamental role in determining the dot properties.

Nucleation features and energy levels of type-II InAsSbP quantum dots grown on InAs(100) substrate

The nucleation features and measurements of holes energy levels of the InAsSbP type-II quantum dots (QDs) with respect to the InAs valence band edge by employing magnetospectroscopy and a photo- and electroluminescence measurements are reported. Three samples are prepared for investigations. The first and second samples consist of unencapsulated spherical and ellipsoidal QDs, respectively. Third sample is a n-InAs/p-InAsSbP heterostructure with QDs embedded into the p-n junction interface. The measured value for the holes first state is ∼33 meV above the InAs valence band. The hysteresis of 0.483 pF and contra-directional oscillations on the first sample’s capacitance-voltage characteristic is detected.

Direct interband absorption of light in a strongly oblate truncated ellipsoidal quantum dot's ensemble

Within the framework of adiabatic approximation the energy levels and direct interband absorption of light in a strongly oblate truncated ellipsoidal quantum dot's ensemble are studied. Analytical expressions for the particle energy spectrum and absorption threshold frequencies in the regime of strong size quantization are obtained. Selection rules for quantum transitions are revealed. To facilitate the comparison of obtained results with the probable experimental data, size dispersion distribution of growing quantum dots by the minor semiaxe by two experimentally realizing distribution functions have been taken into account. Distribution functions of Lifshits-Slezov and Gaussian have been considered.

Electronic structure of asymmetric vertically coupled InAs/GaAs quantum dots

In this paper, the electronic structure of an asymmetric self-assembled vertically coupled quantum dots heterostructure has been investigated. The structure consists of two ellipsoidal quantum dot (QDs) caps made with InAs embedded in a wetting layer InAs and surrounded by GaAs. Using the strain dependent k·p theory, the energy of the two lowest states of a single electron/hole which is confined within the coupled QD structure has been calculated. As a result, it can be estimated the energy gap for different geometry parameters and for tuning the external magnetic field. The numerical results show that the energy gap is very sensitive to the size asymmetry of the structure and to the small separation distance of the dots but less sensitive to the existence of an external magnetic field and large interdot distance.

MAGNETO-ABSORPTION IN ELLIPSOIDAL QUANTUM DOT

The absorption of light in ellipsoidal quantum dot in the presence of a magnetic field is discussed using perturbation theory. Also the same problem is discussed using the normal modes. Quantum dot absorption coefficient is calculated – as well as threshold frequency of absorption – as a function of applied magnetic field. Theoretical results are compared with experimental data obtained by magneto-luminescence method in In0.53Ga0.47As quantum dot (M. Bayer et al, PRB vol.53, pp. 15810–15814, 1996).

The nonlinear optical rectification of an ellipsoidal quantum dot with impurity in the presence of an electric field

We have performed theoretical calculation of second-order nonlinear optical rectification (OR) coefficient in a typical GaAs/AlGaAs QD with ellipsoidal confinement potential in the presence of an impurity and an applied electric field. Using an appropriate coordinate transformation and the perturbation theory, we have investigated the OR coefficient as a function of incident photon energy. Calculation results show that the values of OR coefficient increase with an increase of applied electric field. However, the values decrease with increases in confinement strength and ellipticity constant. Additionally, the presence of a donor impurity shifts the OR coefficient peak positions to higher energies (blueshift), contrary to that of an acceptor impurity.

Exciton bound to an ionized donor impurity in GaAs/Ga 1− xAl xAs ellipsoidal finite-potential quantum dots under hydrostatic pressure

In the framework of perturbation theory, a variational method is used to study the ground state of a donor bound exciton in a weakly prolate GaAs/Ga 1− x Al x As ellipsoidal finite-potential quantum dot under hydrostatic pressure. The analytic expressions for the Hamiltonian of the system have been obtained and the binding energy of the bound exciton is calculated. The results show that the binding energy decreases as the symmetry of the dot shape reduces. The pressure and Al concentration have a considerable influence on the bound exciton. The binding energy increases monotonically as the pressure or Al concentration increases, and the influence of pressure or Al concentration is more pronounced for small quantum dot size.