Hydrogenic impurity states in wurtzite GaN/AlGaN coupled quantum dots

The ground-state binding energy of a hydrogenic donor impurity in wurtzite (WZ) GaN/AlGaN coupled quantum dots (QDs) is calculated by means of a variational method, considering the strong built-in electric fields caused by the piezoelectricity and spontaneous polarizations. The strong built-in electric fields induce an asymmetrical distribution of the ground-state binding energy with respect to the center of the coupled QDs. If the impurity is located at the low dot, the ground-state binding energy is insensitive to the interdot barrier width of WZ GaN/AlGaN coupled QDs.     

Pressure effects on the donor binding energy in zinc-blende InGaN/GaN quantum dot

Based on the effective-mass approximation, the hydrostatic pressure effects on the donor binding energy of the hydrogenic impurity in zinc-blende (ZB) InGaN/GaN quantum dot (QD) are investigated by means of a variational procedure. Numerical results show that the donor binding energy increases when the hydrostatic pressure increases for any impurity position and QD structure parameter. Moreover, it is found that the hydrostatic pressure has a remarkable influence on the donor binding energy of the hydrogenic impurity located at the vicinity of dot center in ZB InGaN/GaN QD.     

Hydrogenic impurity in double quantum dots

The ground state binding energy and the average interparticle distances for a hydrogenic impurity in double quantum dots with Gaussian confinement potential are studied by the variational method. The probability density of the electron is calculated, too. The dependence of the binding energy on the impurity position is investigated for GaAs quantum dots. The result shows that the binding energy has a minimum as a function of the distance between the two quantum dots when the impurity is located at the center of one quantum dot or at the center of the edge of one quantum dot. When the impurity is located at the center of the two dots, the binding energy decreases monotonically.     

Donor impurity states in direct-gap SiGe quantum well: Applied electric field and quantum size effects

Within the framework of the effective-mass approximation and variational procedure, competition effects between applied electric field and quantum size on donor impurity states in the direct-gap Ge/SiGe quantum well (QW) have been investigated theoretically. Numerical results show that the applied electric field (quantum size) dominates electron and impurity states in direct-gap Ge/SiGe QW with large (small) well width. Moreover, the competition effects also induce that the donor binding energies show obviously different behaviors with respect to electric field in the QW with different well widths. In particular, when the impurity is located at left boundary of the QW, the donor binding energy is insensitive to the variation of well width when well width is large for any electric field case.     

Nonlinear optical rectification of hydrogenic impurity in a disk-like parabolic quantum dot: The role of applied magnetic field

We report a detailed theoretical study of the effect of combined electric and magnetic field on the nonlinear optical rectification of a hydrogenic impurity, confined in a two dimensional disk-like quantum dot, with parabolic confinement potential. We use the compact density matrix formalism and iterative method to obtain nonlinear optical rectification and absorption coefficients. To find energy levels and wave functions, we employ exact diagonalization method in the effective mass approximation. As main result, we found that the transition energy from ground to first excited state redshifts with increasing the magnetic field while blueshifts for transition from ground to second excited state, moreover, for former transition, nonlinear optical rectification coefficient decreases with increasing magnetic field in contrast to that occurs for latter one.     

Photoionization cross section of a donor impurity in a two-dimensional anisotropic quantum dot

A system of an electron with a hydrogenic impurity confined in a two-dimensional anisotropic quantum dot has been investigated. We report a calculation for the binding energy of a donor impurity. The important feature of a donor impurity in a two-dimensional anisotropic quantum dot is obtained via an analysis of the binding energy. The photoionization cross section associated with intersubband transitions has been calculated. The results are presented as a function of the incident photon energy. The results show that the photoionization cross section of a donor impurity in a two-dimensional anisotropic quantum dot is strongly affected by the degree of anisotropy and the size of the quantum dot.     

Hydrogenic impurity bound polaron in a parabolic quantum dot with arbitrary electron-phonon coupling strength

A variational approach is employed to obtain the ground and the first excited state binding energies of an electron bound to a hydrogenic impurity in a polar semiconductor quantum dot (QD) with symmetric parabolic confinement in both two and three-dimensions. We perform calculations for the entire range of the electron-phonon coupling constant and the Coulomb binding parameter and for arbitrary confinement length. It is found that the binding energy of ground and first excited state is larger in a two-dimension (2D) dot than in a three-dimension (3D) dot and this trend is more pronounced with the increase of the electron-phonon coupling constant for the same value of the Coulomb binding parameter and confinement length. Furthermore, the ground and the first excited state binding energy increases with increasing the Coulomb binding parameter in both 2D and 3D QDs for the same electron-phonon coupling constant.     

Hydrostatic pressure effect on the donor impurity states in asymmetric multiple quantum wells

Based on the effective-mass approximation, hydrostatic pressure effect on the donor binding energy in zinc blende (ZB) InGaN/GaN asymmetric multiple quantum wells (AMQWs) is investigated variationally. Numerical results show that the hydrostatic pressure increases the donor binding energy for any impurity position. Moreover, the hydrostatic pressure effect is more noticeable if the impurity is localized inside the wide well of the AMQWs. For any hydrostatic pressure, the donor binding energy is distributed asymmetrically with respect to the center of the AMQWs. In particular, the donor binding energy of impurity located at the center of the wide well of the AMQWs is insensitive to the increment of the inter-well barrier width if the inter-well barrier width is large.     

Hydrogenic impurity states in zinc-blende InxGa1−xN/GaN in cylindrical quantum well wires

Within the framework of effective-mass approximation, the binding energy of a hydrogenic donor impurity in zinc-blende (ZB) In_xGa_1−x  N/GaN cylindrical quantum well wires (CQWWs) is investigated using variational procedures. Numerical results show that the ground-state donor binding energy _Eb$E_b$ is highly dependent on the impurity position and the CQWWs structure parameters. The donor binding energy for a shallow donor impurity located at the center of the CQWWs is the largest. As the impurity position changes from the center of the wire to its edge, the donor binding energy gets smaller. Also, we have found that In concentration is a very important value to tailor the system, since the binding energies close to binding energy maxima are strongly dependent on In content.     

Binding energy and diamagnetic susceptibility of an on-center hydrogenic donor impurity in a spherical quantum dot placed at the center of a cylindrical nano-wire

The binding energy and diamagnetic susceptibility of an on-center hydrogenic donor impurity in an InAs spherical quantum dot placed at the center of a GaAs cylindrical nano-wire have been investigated using finite element method in the framework of the effective mass approximation. The binding energy and diamagnetic susceptibility are calculated as a function of the dot radius, nano-wire radius and nano-wire height. The results show that as the dot radius increases (I) for a dot radius smaller than some critical value, the effect of the spherical confinement on the energy levels becomes negligible and the energies remain constant, for a dot radius larger than some specific value, the energy levels decrease (II) the ground and the first excited state binding energies increase, reach a maximum and then decrease (III) the ground state diamagnetic susceptibility increases, reach a maximum and then decreases (IV) the first excited state diamagnetic susceptibility increases, indicating two maxima and then decreases. The effects of the nano-wire dimensions on the binding energy and diamagnetic susceptibility have also been studied. We found that the binding energy and diamagnetic susceptibility decrease reach a minimum value and then increase as the nano-wire radius increases. Finally we found that as the height of the nano-wire increases the ground state binding energy decreases, reaches a minimum value and then increases but the first excited state binding energy decreases and reaches a constant value.     

Thermoelectric transport through a quantum dot with a magnetic impurity

We study the thermoelectric effect in a small quantum dot with a magnetic impurity in the Coulomb blockade regime.The electrical conductance,thermal conductance,thermopower,and the thermoelectrical figure of merit(FOM)are calculated by using Green’s function method.It is found that the peaks in the electrical conductance are split by the exchange coupling between the electron entering into the dot and the magnetic impurity inside the dot,accompanied by the decrease in the height of peaks.As a result,the resonances in the thermoelectric quantities,such as the thermal conductance,thermopower,and the FOM,are all split,opening some effective new working regions.Despite of the significant reduction in the height of the electrical conductance peaks induced by the exchange coupling,the values of the FOM and the thermopower can be as large as those in the case of zero exchange coupling.We also find that the thermoelectric efficiency,characterized by the magnitude of the FOM,can be enhanced by adjusting the left–right asymmetry of the electrode–dot coupling or by optimizing the system’s temperature.     

Interplay between size and impurity position of doped quantum dot

We explore the effect of on and off-center repulsive impurities on regular parabolic dots and their impact on the level structures coupled with a variation in the dot size. Information entropy and probability density are invoked to monitor the pattern of evolution of the electronic states. We have considered Gaussian impurity centers of repulsive type. We have found that the dot size and impurity position interplay in a subtle way to modulate the spatial arrangement of impurity doped dot energy levels. The information entropy profile evinces emergence of resonance-like situations and the probability contour delineates the alteration in the spatial extension of the system for different dot sizes and impurity locations.     

Electron Raman scattering of a hydrogenic impurity in a disc-shaped quantum dot

We have carried out the theoretical calculation of the differential cross section for the electron Raman scattering process associated with a hydrogenic impurity in a disc-shaped quantum dot (QD). We consider the impurity states confined in a disc-shaped QD with parabolic potential in the presence of an external electric field. Effects of the electric field and the confinement strength on the differential cross section are investigated. We make a comparison about the Raman intensity between with and without the Coulomb interaction. We found that the differential cross section of the hydrogenic impurity in a disc-shaped QD dependent strongly on the confinement strength, the external electric field intensity and the Coulomb interaction.     

Nonlinear optical rectification of a hydrogenic impurity in a disc-like quantum dot

An investigation of the nonlinear optical rectification of a hydrogenic impurity, which is in a two-dimensional disc-like quantum dot (QD) with parabolic confinement potential, has been performed by using the perturbation method in the effective mass approximation. Both the electric field and the confinement effects on the energy are investigated in detail. The results are presented as a function of the incident photon energy for the different values of the confinement strength and the electric field. It is found that the nonlinear optical properties of hydrogenic impurity states in a disc-like QD are strongly affected by the confinement strength and the electric field.     

Optical properties of an off-center hydrogenic impurity in a spherical quantum dot with Gaussian potential

Optical absorption coefficients and refractive index changes associated with intersubband transition of an off-center hydrogenic impurity in a spherical quantum dot (QD) with Gaussian confinement potential are theoretically investigated. Our results show that the optical absorption coefficients in a spherical QD are 2–3 orders of magnitude higher than those in quantum wells and are 2–3 orders smaller than those in a disk-like QD. It is found that the optical absorptions and the optical refractive index are strongly affected not only by the confinement barrier height, dot radius but also by the position of the impurity.     

Electric field effect on the Raman scattering of a hydrogenic impurity in spherical quantum dot

Using the perturbation method and the effective mass approximation, we studied the combined effects of impurity and external electric field on Raman scattering in a spherical quantum dot with a parabolic potential. Based on the computed energies and wave functions, the differential cross-section involved in this process is investigated, and the selection rules are also calculated. Our results suggest that the scattering intensity is strongly affected by the impurity and external electric field considered in this work.     

Effect of conduction band parabolicity on the spectrum and binding energy of a hydrogenic impurity in GaAs spherical quantum dots

The energy levels and binding energies of a hydrogenic impurity in GaAs spherical quantum dots with radius R are calculated by the finite difference method. The system is assumed to have an infinite confining potential well with radius R, which can be viewed as a hard wall boundary condition. The parabolicity of the conduction band profile for GaAs material can be viewed as a parabolic potential well. The energy levels and binding energies are depended dramatically on the radius of the quantum dot and the parabolic potential well. The results show that parabolic potential can remarkably alter the energy level ordering and binding energy level ordering of hydrogenic impurity states for the quantum dot with a smaller radius R.     

Two-electron impurity in the parabolic quantum dot: Uncertainty relation and perturbation approach

Two-electron impurity states in parabolic confinement have been investigated. We have estimated the ground-state energy value, using the Heisenberg uncertainty relation. Using variational methods the ground state energy and wave function of the single-electron impurity problem have been achieved. The dependence of ground-state energy and Coulomb electron–electron interaction energy correction on the QD size is studied. The dependence of the state exchange time on the QD radius has been calculated. It has been shown that the presence of the impurity leads to the appearance of negative values of the energy of the system on the one hand, and to the saturating character of the state exchange time.