Search

This study investigates the effect of quantum size and an external magnetic field on the optoelectronic properties of a cylindrical Al x Ga 1 − x As/GaAs-based core/shell nanowire. We used the one-band effective mass model to describe the Hamiltonian of an interacting electron-donor impurity system and employed two numerical methods to calculate the ground state energies: the variational and finite element methods. With the finite confinement barrier at the interface between the core and the shell, the cylindrical symmetry of the system revealed proper transcendental equations, leading to the concept of the threshold core radius. Our results show that the optoelectronic properties of the structure strongly depend on core/shell sizes and the strength of the external magnetic field. We found that the maximum probability of finding the electron occurs in either the core or the shell region, depending on the value of the threshold core radius. This threshold radius separates two regions where physical behaviors undergo changes and the applied magnetic field acts as an additional confinement. [ABSTRACT FROM AUTHOR]

This article presents a theoretical study of the electronic, impurity-related, and nonlinear optical properties of CdS/ZnS quantum dots subjected to electric and magnetic fields. The magnetic field is applied along the z-axis, with the donor impurity always located in the center of the quantum dot. In the case of the electric field, two situations have been considered: applied along the z-axis and applied in the radial direction (central electric field). In both cases, the azimuthal symmetry (around the z-axis) is preserved. In the absence of a magnetic field and considering a central electric field, the system preserves its spherical symmetry both in the presence and in the absence of the donor impurity. The study is carried out in the effective mass approximation and it uses the finite element method to find the eigenfunctions and their corresponding energies, both in the presence and in the absence of the impurity. This work investigates the optical absorption coefficient and the relative change of the refractive index, considering only intraband transitions between l = 0 states (states with azimuthal symmetry concerning the z-axis). Calculations are for z-polarized incident radiation. The study shows that the combined effects of a central electric field and a z-directed magnetic field can give rise to a typical core/shell-like quantum confinement with oscillations of the electron ground state. Additionally, it is shown that the presence of the donor impurity suppresses such oscillations and it is responsible for blue shifts in the optical properties and magnifications of the corresponding resonances. [ABSTRACT FROM AUTHOR]

The optical absorption of the donor impurity in the two-dimensional GaAs quantum ring was studied with the effective mass approximation using the finite element method. The influence of the magnetic field, light intensity, and impurity position on the optical absorption coefficient is analyzed. The results reveal that the application of a magnetic field can increase the peak value of the absorption coefficient and cause a blue shift phenomenon. With the increase of light intensity, the peak splitting in total optical absorption appears. With the increase of the distance between the impurity and the center of the quantum ring, the spacing of the energy level decreases, resulting in a decrease in the peak value of the total absorption coefficients, and a red shift occurs. On the other hand, when the impurity is located between the inner and outer radius, the opposite effects appear as the impurity is far from the center of the quantum ring. [ABSTRACT FROM AUTHOR]

This study investigates the effect of quantum size and an external magnetic field on the optoelectronic properties of a cylindrical AlxGa1−xAs/GaAs-based core/shell nanowire. We used the one-band effective mass model to describe the Hamiltonian of an interacting electron-donor impurity system and employed two numerical methods to calculate the ground state energies: the variational and finite element methods. With the finite confinement barrier at the interface between the core and the shell, the cylindrical symmetry of the system revealed proper transcendental equations, leading to the concept of the threshold core radius. Our results show that the optoelectronic properties of the structure strongly depend on core/shell sizes and the strength of the external magnetic field. We found that the maximum probability of finding the electron occurs in either the core or the shell region, depending on the value of the threshold core radius. This threshold radius separates two regions where physical behaviors undergo changes and the applied magnetic field acts as an additional confinement.

The effect of the magnetic field on the electron states in the Al 0.3 Ga 0.7 As/GaAs/Al 0.3 Ga 0.7 As multilayered quantum dot with and without shallow-donor impurity is under study. The magnetic field dependence of the impurity-related transition energies and the corresponding photoionisation cross-section is calculated, taking into account the excited states. The problem is solved using the effective mass approximation and rectangular potential profiles for wells and barriers using diagonalisation and finite element methods. The results obtained by both methods coincide with high accuracy. It is shown that the transition energies and photoionisation cross-section strongly depend on the impurity position and the magnetic field induction. As the magnetic field induction increases, all the peaks of photoionisation cross-section shift to the region of higher energies in the case of off-centre impurity and shift to lower energies in the case of on-centre impurity. The effect of merging two photoionisation cross-section peaks with increasing magnetic field induction is revealed, leading to a significant increase in the value of impurity photoionisation cross-section. [ABSTRACT FROM AUTHOR]

In this study, linear, nonlinear and total optical absorption coefficients related a single shallow donor atom confined in semiconductor core/shell/shell quantum dot heterostructure are researched in detail within the compact density matrix formalism approximation. For this purpose, firstly, the energies and the wavefunctions are computed by the diagonalization method in the effective mass approach. Moreover, the effects of size modulation, donor position and magnetic field are analyzed. The numerical results indicate that the linear and nonlinear parts of the absorption coefficients related with intersubband 1 s → 1 p and 1 p → 1 d donor transitions undergo significant changes. [ABSTRACT FROM AUTHOR]

This article presents a theoretical study of the electronic, impurity-related, and nonlinear optical properties of CdS/ZnS quantum dots subjected to electric and magnetic fields. The magnetic field is applied along the z-axis, with the donor impurity always located in the center of the quantum dot. In the case of the electric field, two situations have been considered: applied along the z-axis and applied in the radial direction (central electric field). In both cases, the azimuthal symmetry (around the z-axis) is preserved. In the absence of a magnetic field and considering a central electric field, the system preserves its spherical symmetry both in the presence and in the absence of the donor impurity. The study is carried out in the effective mass approximation and it uses the finite element method to find the eigenfunctions and their corresponding energies, both in the presence and in the absence of the impurity. This work investigates the optical absorption coefficient and the relative change of the refractive index, considering only intraband transitions between l = 0 states (states with azimuthal symmetry concerning the z-axis). Calculations are for z-polarized incident radiation. The study shows that the combined effects of a central electric field and a z-directed magnetic field can give rise to a typical core/shell-like quantum confinement with oscillations of the electron ground state. Additionally, it is shown that the presence of the donor impurity suppresses such oscillations and it is responsible for blue shifts in the optical properties and magnifications of the corresponding resonances.

We investigated comparatively the electronic and optical properties of GaAs/GaAlAs pseudo-elliptic quantum rings with donor impurity under magnetic field in the absence and the presence of electron spin, taken into account through Rashba and Dresselhaus spin–orbit couplings and Zeeman effect. The energy and absorption spectra were computed for four representative positions of the donor impurity (edges and x(y)-middle of the ring). Both approaches demonstrate the important role played by the impurity position, with the strongest effects corresponding to the impurity placed in the middle of the ring. In the spin absence, the impurity position influences both the energies of the absorption peaks and their oscillatory behaviours in energy and amplitude. In the presence of electron spin, the absorption spectra of the pseudo-elliptic quantum ring become more complex at large magnetic fields as the number of transitions is increased by the splitting of the energy levels and present new distinct features at low and high energies that could be observed experimentally. [ABSTRACT FROM AUTHOR]

This study systematically explored the binding energy and the photoionization cross-section (PCS) of a hydrogenic shallow donor impurity within a GaAs double quantum box structure incorporated into a Ga (1 − x) Al x As matrix under the influence of an external intense magnetic field. The computations are performed using the effective-mass approximation (EMA) and the finite element method (FEM), taking into account a finite potential across all surfaces of the nanosystem. The results emphasize the significant role of the magnetic field, primarily in enhancing the donor binding energy within the double quantum box system. We demonstrate that increasing the magnetic field results in an augmentation of binding energy across various aluminum concentrations. The PCS of a shallow donor impurity undergoes a blue shift when shifted from the barrier center to the box center, attributed to the increased binding energy. When the double quantum box is subjected to a magnetic field effect, the amplitude peaks of PCS show an elevation coupled with a blue shift. By setting magnetic field values at 0 T, 35 T, or 70 T and varying aluminum concentrations, we observe a shift in the PCS towards higher energy levels along with an increase in peak amplitude. We also study the PCS of the donor impurity as a function of photon energy, mainly examining the effects of box width and barrier width modifications in the double quantum box. [ABSTRACT FROM AUTHOR]

We present a theoretical study to investigate the effect of donor impurity on the magnetization (M) and the magnetic susceptibility (χ) of single electron quantum dot (QD) with Gaussian confinement in the presence of a magnetic field. We solve the Hamiltonian of this system, including the spin, by using the exact diagonalization method. The ground state binding energy (BE) of an electron has been shown as a function of QD radius and confinement potential depth. The behaviors of the magnetization and the magnetic susceptibility of a QD have been studied as a function of temperature, confinement potential depth, quantum radius and magnetic field. We have shown the effect of donor impurity on the magnetization and magnetic susceptibility curves of Gaussian quantum dot (GQD). [ABSTRACT FROM AUTHOR]

• Spin-orbit interaction effect on a D 0 centre is studied in a quantum dot in a magnetic field. • Binding energy, magnetization and susceptibility are obtained by a variational method. • Magnetization and Susceptibility are found to be diamagnetic in nature. • Rashba interaction is found to reduce the binding energy and enhance the diamagnetic susceptibility. • Dresselhaus interaction enhances the binding energy but decreases the diamagnetic susceptibility. The binding energy of a hydrogenic-like donor complex D 0 placed in a two-dimensional Gaussian quantum dot GaAs semiconductor is determined incorporating the Rashba and Dresselhaus spin-orbit interactions in the presence of an externally applied magnetic field. A unitary transformation is employed to deal with the spin-orbit interactions and their effects are incorporated up to second order in the coupling coefficients. The resulting Hamiltonian is solved variationally using a simple wave function. The results indicate that the Rashba spin-orbit interaction and the confining potential reduce the energy of the donor complex whereas the magnetic field enhances it. Interestingly enough, the binding energy is enhanced by the Dresselhaus spin-orbital interaction in the presence of a magnetic field and while it is reduced in the absence of it. Finally we study the effect of magnetic field and spin-orbit interactions on the magnetic moment and susceptibility. [ABSTRACT FROM AUTHOR]

The influence of a constant magnetic field and the position of a shallow donor impurity on the energy spectrum of an electron and its probability density distribution in a spherical AlxGa1-xAs/GaAs/AlxGa1-xAs nanosystem has been studied. In the approximation of the effective mass and the model of rectangular wells and barriers, the solutions of the Schrödinger equation have been obtained by the expansion of the ground and excited state wave functions of an electron on the basis of the exact wave functions in the absence of a magnetic field and an impurity have been found. It has been shown that with increasing magnetic field induction, the role of the electron ground state in the nanosystem without impurities is consistently performed by the states with the magnetic quantum number m = 0, - 1, - 2, ... (the Aharonov-Bohm effect). It has been established that the period of such oscillations of the electron ground state does not depend on the potential barriers height. However, the presence of an off-central donor impurity located in a potential well of the nanosystem, results in the vanishing of the Aharonov-Bohm effect. The dependences of the electron energy spectrum and wave functions on the position of an impurity shifted from the nanosystem center along and perpendicular to the magnetic field direction have been obtained. It has been shown that in the first case, the quantum states are characterized by a certain value of the magnetic quantum number m, and the expansion contains only wave functions of states with this value of m. In the second case, the problem loses cylindrical symmetry and new quantum states are formed from the states with different values of all three quantum numbers n, l, m. [ABSTRACT FROM AUTHOR]

The effect of homogeneous magnetic field and location of donor impurity on the electron energy spectrum and distribution of its probability density in spherical core-shell quantum dot is investigated. In the framework of the effective mass approximation and rectangular infinitely deep potential well, the solutions of the Schrodinger equation are found using the matrix method. The wave functions are expanded over the complete set of exact functions obtained without the magnetic field and impurity. It is shown that when the induction of magnetic field increases, the ground state of electron in the nanostructure without impurity or on-center impurity is successively formed by the states with m = 0, −1, −2, … (Aharonov-Bohm effect). When donor impurity is located in the shell of the nanostructure the Aharonov-Bohm effect vanishes. The dependences of electron energy spectrum and its wave functions on the location of impurity, placed along the direction of magnetic field or perpendicularly to it, are studied. It is shown, that in the first case, the quantum states are characterized by the certain value of magnetic quantum number m and the expansion contains the wave functions of the states with it only. In the second case, the cylindrical symmetry of the problem is broken and the new quantum states are formed from the states with different values of all three quantum numbers n , l , m and electron energy spectrum weakly depends on the magnetic field induction. [ABSTRACT FROM AUTHOR]

Theoretical investigations are carried out of binding energies of a donor charge assumed to be located exactly at the center of symmetry of two concentric cylindrical quantum wires. The intrinsic confinement potential in the region of the inner cylinder is modeled in any one of the three profiles: simple parabolic, shifted parabolic or the polynomial potential. The potential inside the shell is taken to be a potential step or potential barrier of a finite height. Additional confinement of the charge carriers is due to the vector potential of the axial applied magnetic field. It is found that the binding energies attain maxima in their variations with the radius of the inner cylinder irrespective of the particular intrinsic confinement of the inner cylinder. As the radius of the inner cylinder is increased further, the binding energies corresponding to either the parabolic or the polynomial potentials attain minima at some critical core-radius. Finally, as anticipated, the binding energies increase with the increase of the parallel applied magnetic field. This behaviour of the binding energies is irrespective of the particular electric potential of the nanostructure or its specific dimensions. [ABSTRACT FROM AUTHOR]

The second-order nonlinear optical susceptibility in a cylindrical quantum dot (CQD) of G a A s / G a 0.6 A l 0.4 A s with a donor impurity inside under the influence of a uniform magnetic field applied in the axial direction of the cylinder is studied theoretically. Using the variational method, the energies and the wave functions of the 1 S − l i k e , 2 P − − l i k e , and 2 P + − l i k e impurity states were found when it moves inside of the CQD. It was found that there is a degeneration of the energy of the impurity for the 2 P − − l i k e and 2 P + − l i k e states in the absence of magnetic field, producing two resonant peaks for the second-harmonic generation (SHG) for any position of the impurity. This degeneration is split only when a magnetic field is applied, indicating a Zeeman effect in the system. Thus the variation of the intensity of the magnetic field allows tuning the condition of resonance of two photons. It was also found that the increase in the radial position of the impurity produces an increase in the intensity of the peaks of the optical rectification (OR) and SHG coefficients, therefore a variation of the position of the impurity in a CQD allows modulating the intensity of the second-order nonlinear optical response. [ABSTRACT FROM AUTHOR]

An investigation of the nonlinear optical rectification of a GaAs two-dimensional disc-shaped quantum ring with an off-center donor impurity under magnetic field has been performed by using a variational method in the effective mass approximation. The two-dimensional quantum ring was described by a pseudo-harmonic potential. The results are presented as functions of the incident photon energy for the different values of the impurity position and the magnetic field. It is found that the nonlinear optical rectification spectra are strongly affected by the position of the off-center impurity and the magnetic field. [ABSTRACT FROM AUTHOR]

The effects of an off-centre donor impurity on the non-linear optical absorption, second and third harmonic generation in a GaAs two-dimensional disc-shaped quantum ring under magnetic field are investigated within the compact density-matrix formalism and the effective mass approximation. The results reveal that: (i) the absorption spectra extend on larger energy intervals at the increment of the magnetic field; (ii) the possibility of generating second harmonic response from the system is demonstrated for an impurity placed on the repulsive part of the confining potential; (iii) both second harmonic and third harmonic coefficients are one order of magnitude larger for an impurity placed on the repulsive part of the potential and are blue-shifted by the increment of the magnetic field regardless the impurity position. [ABSTRACT FROM AUTHOR]

Using the finite element method, the effect of magnetic fields on the donor states and transition energies in a InAs/GaAs quantum dot coupled to its wetting layers is investigated. Results are obtained for different impurity locations. We found that the diamagnetic shift of the ground state energy increases monotonously with the applied field and can be described by a simple function which interpolates between the low and high magnetic-field behavior. Frequencies associated to the transitions between the S -like ground state and P − ( P + ) excited states range in terahertz region and show a magnetic field-induced red (blue) shift, irrespectively of the impurity position. [ABSTRACT FROM AUTHOR]

Using the fractal dimension method, we calculate the ground state energy for a D0 donor impurity in a GaAs/GaAlAs cylindrical nanotube under the presence of a uniform magnetic field applied along the symmetry axis. We analyze the effect of two quantum well on the energy. We express the wave function as a product of combination of the 1s and 2px,y wave functions with an unknown envelope function that depends only on the electron-ion separation. Curves of the D0 ground state The results for the binding energies, as a function of the impurity position, were obtained for different potential shapes and heights.Due to the presence of the additional barrier, these results show an additional peak. Also, we found that the magnetic field increases the binding energies of the donors located close to the axis, whereas for the donors located far from the axis the effect is the opposite. [ABSTRACT FROM AUTHOR]

Abstract: The binding energy and diamagnetic susceptibility χ dia are investigated for a shallow donor confined to move in a spherical Inhomogeneous Quantum Dots “IQD” in the presence of a magnetic field. The calculation was performed with the use of a variational method in the effective mass approximation. We describe the effect of the quantum confinement by an infinite deep potential. The results for a spherical Inhomogeneous Quantum Dots made out of [Ga1− x Al x As (Core)/GaAs (Well)/Ga1− x Al x As (Shell)] show that the diamagnetic susceptibility and the binding energy increase with the magnetic field. There are more pronounced for large spherical layer. The binding energy and the diamagnetic susceptibility depend strongly on the donor position. We remark that the diamagnetic susceptibility presents a minimum corresponding to a critical value of the ratio of the inner radius to the outer radius , this critical value is important for nanofabrication techniques. [Copyright &y& Elsevier]

Abstract: We consider the effects of electric and magnetic fields as well as of hydrostatic pressure on the donor binding energy in InAs Pöschl–Teller quantum rings. The ground state energy and the electron wave function are calculated within the effective mass and parabolic band approximations, using the variational method. The binding energy dependencies on the electric field strength and the hydrostatic pressure are reported for different values of quantum ring size and shape, the parameters of the Pöschl–Teller confining potential, and the magnetic field induction. The results show that the binding energy is an increasing or decreasing function of the electric field, depending on the chosen parameters of the confining potential. Also, we have observed that the binding energy is an increasing/decreasing function of hydrostatic pressure/magnetic field induction. Likewise, the impurity binding energy behaves as an increasing/decreasing function of the inner/outer radii of the quantum ring nanostructure. [Copyright &y& Elsevier]

Abstract: Within the effective-mass approximation we calculate the energies of a donor impurity in an elliptical quantum ring subjected to a magnetic field. The energies are found to exhibit Aharonov–Bohm oscillations depending on the magnetic field and the eccentricity. As the eccentricity increases, the energies decrease and the period increases. [Copyright &y& Elsevier]

Abstract: We have used a variational procedure within the envelope-function and parabolic-band approximations to investigate the effects of hydrostatic pressure and crossed electric and magnetic fields on shallow-donor states in quantum wells. The donor variational envelope wave function is obtained through a hydrogenic 1s-like wave function and an expansion in a complete set of trigonometrical functions, and a detailed study is performed of the dependence of the donor binding energies on the applied hydrostatic pressure and applied in-growth direction electric and in-plane magnetic fields. [Copyright &y& Elsevier]

The effect of a uniform longitudinal magnetic field on the binding energy and photoionization cross-section of a hydrogen-like donor impurity is studied for a semiconductor quantum well-wire approximated by a cylindrical well of finite depth. The selection rules and analytical expressions for the photoionization cross-section are obtained depending on the magnetic field induction, impurity position, and light wave polarization. [ABSTRACT FROM AUTHOR]

In this study, the binding energies of an on-center and off-center hydrogenic donor impurity in a spherical quantum dot with finite confinement potential under an applied external magnetic field are investigated in detail by using the matrix diagonalization method in the effective mass approximation. Depending on the calculated energies and wave functions, the linear, third-order nonlinear and total optical absorption coefficients between the ground and several excited states are examined by the iterative method in the compact density matrix approximation for the two-level system. The combined effect of donor position, magnetic field and dot size on binding energies and optical absorption coefficients is observed. The results show that these effects cause significant changes on donor binding energy and optical absorption coefficients. • Spherical quantum dot with finite confinement potential under magnetic field. • 1s, 2s, 1p, 2p, 1d and 2d donor impurity states. • Matrix diagonalization method in the effective mass approximation. • The binding energies of an on-center and off-center hydrogenic donor impurity. • The linear, third-order nonlinear and total optical absorption coefficients. • The combined effect of donor position, magnetic field and dot size. [ABSTRACT FROM AUTHOR]

The current paper provides a theoretical study of the binding energy of an off-center donor impurity in wurtzite I n G a N / G a N multilayer cylindrical quantum dots (MCQDs). The Schrödinger equation related to the shallow donor impurity has been solved by the finite element method (FEM) using a parabolic confinement potential in the radial direction and a square one in the z direction within the effective-mass and a parabolic conduction band approximation. Furthermore, we have studied the external perturbations on the donor binding energy such as: hydrostatic pressure, magnetic and electric fields as well as the effect of strain induced by the lattice mismatch. Moreover, the binding energy has been calculated as a function of the geometric parameters of the structure, indium concentration x in the I n x G a 1 − x N alloy and impurity position z 0. Our main findings show that the binding energy is more influenced by the electric field for large sizes, by the hydrostatic pressure and the magnetic field for all sizes, especially for wide radius R , barrier L b and thin L d. Additionally, the results show that the lattice mismatch has an impact on the binding energy, mostly at low pressure. [ABSTRACT FROM AUTHOR]