Cylindrical Quantum Dot Research Articles

Research on nonlinear optical absorption coefficients of GaAs/Ga1−ηAlηAs quantum dots

The effects of quantum dot (QD) radius, incident light intensity, temperature and other factors on the optical absorption coefficients (OACs) of cylindrical QDs are investigated numerically for typical GaAs/Ga[Formula: see text]Al[Formula: see text]As. We theoretically studied the cylindrical QDs with like-inversely quadratic Hellmann potential and parabolic potential. The expressions for the energy levels and wave functions were obtained by the Nikiforov–Uvarov method, and then the expressions for the OACs were derived by using the density matrix theory and the iterative method. Finally, the results show that the structural and external parameters affect the sensitivity of the optical absorption properties by influencing the matrix elements and energy levels, and the quantum confinement effects affect their applicable frequencies, which makes the QD system easier to tune.

Theoretical Investigation and Improvement of Characteristics of InAs/GaAs Quantum Dot Intermediate Band Solar Cells by Optimizing Quantum Dot Dimensions

Quantum dot (QD)-based solar cells have been the focus of extensive research. One of the critical challenges in this field is optimizing the size and placement of QDs within the cells to enhance light absorption and overall efficiency. This paper theoretically investigates InAs/GaAs QD intermediate band solar cells (QD-IBSC) employing cylindrical QDs. The goal is to explore factors affecting light absorption and efficiency in QD-IBSC, such as the positioning of QDs, their dimensions, and the spacing (pitch) between the centers of adjacent dots. Achieving optimal values to enhance cell efficiency involves modifying and optimizing these QD parameters. This study involves an analysis of more than 500 frequency points to optimize parameters and evaluate efficiency under three distinct conditions: output power optimization, short-circuit current optimization, and generation rate optimization. The results indicate that optimizing the short-circuit current leads to the highest efficiency compared to the other conditions. Under optimized conditions, the efficiency and current density increase to 34.3% and 38.42 mA/cm2, respectively, representing a remarkable improvement of 15% and 22% compared to the reference cell.

Investigation of nonlinear optical rectification within multilayer wurtzite InGaN/GaN cylindrical quantum dots under the impact of temperature and pressure

We have looked numerically at variations of the nonlinear optical rectification (NOR) for an impurity donor localized in a wurtzite Inx0Ga1−x0N/GaN multilayer cylindrical quantum dots (MLCQDs), which is under the effect of hydrostatic pressure and temperature. Using the effective mass approximation, the compact density matrix formalism, we have solved the Schrödinger equation by the finite element method (FEniCS Project) . Firstly, we consider the impact of altering the polarization direction of the incident electromagnetic radiation. Specifically, we explore two distinct scenarios: axial, along the growth axis of the structure, and circular, across the cross-section of the cylinder. From our investigation, we found that the nonlinear optical rectification is determined by two main parameters: the transition energy, which is mainly related to quantum confinement, and the geometrical factor that depends on the distribution of the electron wave functions. The NOR experiences a blueshift with an increase in hydrostatic pressure or indium composition, and a redshift with an increase in temperature or quantum dot radius, where the impurity is located in the center of the upper QD. Furthermore, the results show that the amplitude and resonance peak position of NOR are significantly influenced by varying the impurity’s position and the barrier width.

Hofstadter-like spectrum and magnetization of artificial graphene constructed with cylindrical and elliptical quantum dots

A comparative study of the electronic and magnetic properties of quasi-two-dimensional electrons in an artificial graphene-like superlattice composed of circular and elliptical quantum dots is presented. A complete orthonormal set of basis functions has been implemented for calculation of the energy dispersions, Hofstadter spectra, density of states and orbital magnetization of the considered systems. Our calculations indicate a topological change in the miniband structure due to the ellipticity of the quantum dots, and non-trivial modifications of the electron energy dispersion surfaces with the change of the magnetic flux per unite cell. The ellipticity of the quantum dots leads to an opening of a gap and considerable modifications of the Hofstadter spectrum. The orbital magnetization is shown to reveal significant oscillations with the change of the magnetic flux. The ellipticity of quantum dots has a qualitative impact on the dependencies of the magnetization on both the magnetic flux and the temperature.

Optical Properties of Nitride Cylindrical Quantum Dots as Biosensor Transducer

Optical Properties of Nitride Cylindrical Quantum Dots as Biosensor Transducer

Hydrogenic donor-related binding energy and diamagnetic susceptibility in multilayer cylindrical quantum dots under hydrostatic pressure

Hydrogenic donor-related binding energy and diamagnetic susceptibility in multilayer cylindrical quantum dots under hydrostatic pressure

Talbot effect in InAs/GaAs coupled cylindrical quantum dots ensemble

Talbot effect in InAs/GaAs coupled cylindrical quantum dots ensemble

Negative donor impurity polarizability and stability in quantum dot under a lateral electric field

The binding energies of the ground and excited states of negative (D−) and neutral (D0) donor impurities in a parabolic cylindrical quantum dot (CQD) are calculated within single-band effective mass approximations. The quantum variational method is used to solve the time-independent Schrödinger equation for centered and off-centered impurities under the effect of a lateral electric field. Based on the calculated negative or positive binding energies, the stability of D− and D0 is discussed. The results show that at a specific lateral electric field strength, each D− is switched from a bound (stable) to an unbound (unstable) state in the CQD and differs for centered and off-centered D−. Using the calculated energies and wave functions, the response of the CQD static dipole polarizability (SDP) to the lateral electric field strength is investigated. The results show that CQD size, lateral electric field strength, and lateral electric field direction have a noticeable influence on polarizability. The D− polarizability is found to decrease with increasing lateral electric field strength but shows different behavior with increasing CQD radius.

Electronic States in a Cylindrical Quantum Dot under the Influence of Gaussian and Bessel Laser Beams

Electronic States in a Cylindrical Quantum Dot under the Influence of Gaussian and Bessel Laser Beams

Tuning of nonlinear optical characteristics of a cylindrical quantum dot by external fields and structure parameters

ABSTRACT In this present study, we have theoretically investigated the effect of applied external electric field and non-resonant intense laser field, as well as the adjustable physical parameters (A, M, and η) of the system, on the linear and nonlinear of GaAs/GaAlAs cylindrical quantum dot. The confinement potential of the quantum dot is composed of the axial potential with a Razavy-type quantum well in the z-direction, and the cylindrical-type potential in the radial direction. To achieve this goal, the wave functions, and the corresponding eigenvalues of the electron are investigated by resolving the time-independent Schrödinger equation using the diagonalisation technique in terms of the effective mass approximation. The linear and nonlinear optical properties’ expressions have been calculated with the help of the compact density matrix method. Our numerical results show that by changing the A, M, and η parameters, we can see a blueshift or redshift in the total optical absorption coefficients (TOACs) and relative refractive index changes (RRICs). Additionally, they are redshifted, and their extrema increase as the radius increases. The increase in electric field or laser field intensities generates a strong displacement of the resonant peak position towards the higher energies, diminishes the TOACs magnitude, and shrinks RRICs extrema. The model potential used in the computation is important, and the study of it will be practical in the development and research of nanostructures systems.

Modeling of Quantum Dots with the Finite Element Method

Considering the increasing number of experimental results in the manufacturing process of quantum dots (QDs) with different geometries, and the fact that most numerical methods that can be used to investigate quantum dots with nontrivial geometries require large computational capacities, the finite element method (FEM) becomes an incredibly attractive tool for modeling semiconductor QDs. In the current article, we used FEM to obtain the first twenty-six probability densities and energy values for the following GaAs structures: rectangular, spherical, cylindrical, ellipsoidal, spheroidal, and conical QDs, as well as quantum rings, nanotadpoles, and nanostars. The results of the numerical calculations were compared with the exact analytical solutions and a good deviation was obtained. The ground-state energy dependence on the element size was obtained to find the optimal parameter for the investigated structures. The abovementioned calculation results were used to obtain valuable insight into the effects of the size quantization’s dependence on the shape of the QDs. Additionally, the wavefunctions and energies of spherical CdSe/CdS quantum dots were obtained while taking into account the diffusion effects on the potential depth with the use of a piecewise Woods–Saxon potential. The diffusion of the effective mass and the dielectric permittivity was obtained with the use of a normal Woods–Saxon potential. A structure with a quasi-type-II band alignment was obtained at the core size of ≈2.2 nm This result is consistent with the experimental data.

The exciton spectrum of the cylindrical quantum dot - quantum ring semiconductor nanostructure in an electric field

In the model of effective masses and rectangular potentials for an electron and a hole, the influence of a uniform electric field on the energy spectrum and wave functions of the exciton and the oscillator strengths of interband quantum transitions in the semiconductor (GaAs/AlxGa1-xAs) quantum dot-quantum ring nanostructure is theoretically investigated. The stationary Schrödinger equations for noninteracting quasiparticles in the presence of an electric field cannot be solved analytically. For their approximate solution, the unknown wave functions are sought in the form of an expansion over the complete set of cylindrically symmetric wave functions, and the electron energy is found by solving the corresponding secular equation. The exciton binding energy is found using perturbation theory.
 The dependences of the energy spectra, the wave functions of an electron, hole, and exciton, and the intensity of interband optical quantum transitions on the magnitude of the electric field strength are analyzed.

Theoretical study of the non-parabolicity and size effects on the diamagnetic susceptibility of donor impurity in Si, HgS and GaAs cylindrical quantum dot and quantum disk: applied magnetic field influence is considered

ABSTRACT Through this work, we have investigated the influence of the conduction band non-parabolicity (NP) and magnetic field on the diamagnetic susceptibility for a confined shallow donor impurity in a homogeneous cylindrical quantum dot (CQD) and quantum disk (QDisk); the size effect has been considered. Our calculations have been performed using the variational method which consists of the choice of a trial wave function. The Schrodinger equation has been solved within the effective-mass approximation considering the infinite height potential barrier. The numerical calculations are performed taking into account different semiconductor materials (i.e. Si, HgS and GaAs). Our results reveal that both the size and conduction band NP have a significant impact on the behaviour of the diamagnetic susceptibility for both studied systems (i.e. CQD and QDisk). It has been also noticed that, for both CQD and QDisk, the NP increases the diamagnetic susceptibility (). Furthermore, this effect is remarkable for low band gap semiconductors, especially in large QD (weak radial confinement). However, it is much more important in HgS due to its narrow band gap compared to Si and GaAs materials.

Effect of Gaussian and Bessel laser beams on linear and nonlinear optical properties of vertically coupled cylindrical quantum dots

Vertically coupled quantum dots are one of the most interesting and promising structures, which find application in a wide range of fields. Particularly, they are noteworthy candidates as a source of single photon and entangled photon pair sources, as qubits and quantum gates in quantum computation, etc. Various external perturbations can act as an instrument for the manipulation of the properties of these structures and an intense laser field is a great example of this. In this work, vertically coupled cylindrical quantum dots made of InAs in a GaAs matrix with a modified Pöschl–Teller potential have been considered under intense laser fields with two laser beam profiles: Gaussian and Bessel. The energy levels and wave functions have been obtained for this structure. Linear and third-order nonlinear absorption coefficients, refractive index changes and second and third harmonic generation susceptibilities have been investigated at different temperatures and for different values of the laser parameters.

The electron gas in the core/shell cylindrical quantum dot: Thermodynamic and diamagnetic properties

Thermodynamic and magnetic properties have been investigated for the model of weak-interacting electron gas localized in the core/shell cylindrical quantum dot (QD) in the presence of an axial magnetic field. The radial analog of the two-dimensional potential of Winternitz-Smorodinsky has been considered as the radial confinement potential. It has been shown that gas has diamagnetic properties, wherein the magnetization practically has a linear dependence on the magnetic field. The system entropy, heat capacity and mean energy dependencies on magnetic field have been studied. In particular, it has been shown that with the increase of magnetic field, system entropy increases.

Electron–hole interaction in cylindrical quantum dots

The Coulomb interaction of electron and hole in a cylindrical quantum dot in the frame of the first perturbation theory and the effective mass approximation is studied. The analogue of Brus’s equation for Coulomb interaction of electron–hole pair in a spherical quantum dot is obtained for a cylindrical nanocrystal and an expression for the energy band gap shift is derived, which can be used for explaining of some experimental results obtained for MoS2 and 3C-SiC nanocrystals.

Optical properties of cylindrical quantum dots with diluted magnetic semiconductors structure

The optical properties of diluted magnetic semiconductor cylindrical quantum dot caused by interband transitions are investigated. The behavior of a quantum dot as a function of the energy of an incident photon is studied for various values of temperature, magnetic field, and structural parameters. It is shown that a change in the distance between the energies of electrons and holes in identical quantum states affects the maximum of the absorption coefficient. According to the results obtained, an increase in temperature increases the absorption maximum and shifts it toward lower energy. As the magnetic field increases, the absorption maximum decreases and shifts toward higher energy. In addition, it was found that the absorption threshold frequency varies linearly at high temperatures and nonlinearly at low temperatures depending on the magnetic field.

Photoionization cross section of donor single dopant in multilayer quantum dots under pressure and temperature effects

The shallow donor single dopant confined in the multilayer cylindrical quantum dots is investigated numerically within the framework of effective-mass approximation. The Schrödinger equation that governs the wave function of a complicated system is solved numerically by the finite element method using the Python programming language. The results indicate that the photoionization cross section (PCS) is similar to the Gauss function curve, the height of the curve's peak is proportional to oscillator strength, and the center peak position is related to the impurity binding energy. These two last parameters are dependent on the distribution of probability density across each part of the system. The probability density is tuned by the electron confinement and optical transition. The augmentation of dot radius or temperature decreases the electron confinement, which causes a redshift of PCS, and the presence of hydrostatic pressure blue shifts the PCS. On the other side, As the dot radius and hydrostatic pressure are increased, the oscillator's strength increases, raising values near the critical energy, and vice versa for temperature.

Optical Properties of Cylindrical Quantum Dots with Hyperbolic-Type Axial Potential under Applied Electric Field.

In this paper, we have researched the electronic and optical properties of cylindrical quantum dot structures by selecting four different hyperbolic-type potentials in the axial direction under an axially-applied electric field. We have considered a position-dependent effective mass model in which both the smooth variation of the effective mass in the axial direction adjusted to the way the confining potentials change and its abrupt change in the radial direction have been considered in solving the eigenvalue differential equation. The calculations of the eigenvalue equation have been implemented considering both the Dirichlet conditions (zero flux) and the open boundary conditions (non-zero flux) in the planes perpendicular to the direction of the applied electric field, which guarantees the validity of the results presented in this study for quasi-steady states with extremely high lifetimes. We have used the diagonalization method combined with the finite element method to find the eigenvalues and eigenfunction of the confined electron in the cylindrical quantum dots. The numerical strategies that have been used for the solution of the differential equations allowed us to overcome the multiple problems that the boundary conditions present in the region of intersection of the flat and cylindrical faces that form the boundary of the heterostructure. To calculate the linear and third-order nonlinear optical absorption coefficients and relative changes in the refractive index, a two-level approach in the density matrix expansion is used. Our results show that the electronic and, therefore, optical properties of the structures focused on can be adjusted to obtain a suitable response for specific studies or goals by changing structural parameters such as the widths and depths of the potentials in the axial direction, as well as the electric field intensity.

The intensity and direction of the electric field effects on off-center shallow-donor impurity binding energy in wedge-shaped cylindrical quantum dots

• In the effective mass approximation, the finite difference method is adopted. • Electric field effect on donor binding energy in a wedge quantum dot is obtained. • The geometrical parameters effects (radius, height and opening) have been affected. • The six confined electronic states dependent on dot shape have been examined. Taking into account an infinite confinement potential, including the influence of external electric field applied in different directions, we have systematically examined the different factors that lead to changes in the ground state binding energy between the hydrogen impurity and the electron, which are confined in a wedge-shaped cylindrical quantum dot (WSCQD) of radius R , height H and azimuthal angle θ m . The resolution of the Schrödinger equation in three dimensions is achieved by the numerical finite difference method. The binding energy is calculated for several quantum dot geometry, various electric field direction, and different position of the impurity located on the first rectangular surface or inside the WSCQD. We have obtained that the direction of the electric field has an important influence on the binding energy of the off-center impurity. We have discussed in detail the weak effect of the radial electric field in the critical value of the WSCQD azimuthal angle θ m = 76.20 ∘ for R = 30 nm and H = 50 nm. The competition between the different effects, such as the electric field direction, the impurity position, and the no-symmetry of the WSCQD, on the electron-impurity distance, is physically interpreted in this paper. To make a more self-contained work, the finite confinement potential effects have also been discussed. Calculations have also been validated by using the finite element method.