Hydrogenic impurity states in zinc-blende GaN/AlN coupled quantum dots

Based on the effective-mass approximation, we have calculated the donor binding energy of a hydrogenic impurity in zinc-blende (ZB) GaN/AlN coupled quantum dots (QDs) using a variational method. Numerical results show that the donor binding energy is highly dependent on the impurity position and coupled QDs structural parameters. The donor binding energy is largest when the impurity is located at the center of quantum dot. When the impurity is located at the interdot barrier edge, the donor binding energy has a minimum value with increasing the interdot barrier width.     

Hydrogenic donor states in wurtzite InGaN/GaN coupled quantum dots

The binding energies of the hydrogenic impurity in wurtzite InGaN coupled quantum dots (QDs) are calculated by means of a variational method, considering the strong built-in electric field induced by the spontaneous and piezoelectric polarizations. Numerical results show that the strong built-in electric field induces an asymmetrical distribution of the donor binding energy with respect to the center of the coupled QDs. When the impurity is located in the center of the left dot, the donor binding energy is largest and insensitive to the barrier height of the wurtzite InGaN coupled QDs.     

Hydrogenic impurity states in zinc-blende InGaN quantum dot

Within the framework of effective-mass approximation, the binding energy of a hydrogenic donor impurity in a zinc-blende (ZB) InGaN/GaN cylindrical quantum dot (QD) is investigated using a variational procedure. Numerical results show that the donor binding energy is highly dependent on impurity position and QD size. The donor binding energy E_b is largest when the impurity is located at the center of the QD. The donor binding energy is decreased when the dot height (radius) is increased.     

Hydrostatic pressure effects on impurity states in InAs/GaAs quantum dot

Within the framework of effective-mass approximation, the hydrostatic pressure effects on the donor binding energy of a hydrogenic impurity in InAs/GaAs self-assembled quantum dot(QD) are investigated by means of a variational method. Numerical results show that the donor binding energy increases when the hydrostatic pressure increases for any impurity position and QD size. Moreover, the hydrostatic pressure has a remarkable influence on the donor binding energy for small QD. Realistic cases, including the impurity in the QD and the surrounding barrier, are considered.     

Electronic states of a hydrogenic impurity in a zinc-blende GaN/AlGaN quantum well

Binding energies of ground and a few low lying excited states of a hydrogenic donor confined in a zinc-blende GaN/AlGaN quantum well are investigated. They are computed within the framework of single band effective mass approximation, by means of a variational approach. The donor states are investigated with the various impurity positions as a function of well width. The calculations have been carried out with the inclusion of conduction band non-parabolicity through the energy dependent effective mass. The variational solutions have been improved by using a two-parametric trial wavefunction. The results seem better and good agreement with the other investigators. To support our results, we observe that the values of variational parameters are consistent when two parameter wave function is used. We find that the inclusion of non-parabolic effects leads to more binding for all the values of well width and is significant for narrow wells. The results are compared with the existing available literature.     

Electronic structure and impurity states in GaN quantum dots

We study the electronic structure of spherical GaN quantum dots (QD's) with a substitutional acceptor impurity at the center. The size-dependent energy spectra are calculated within the sp³s* tight-binding model, which yields a good agreement with the confinement-induced blue shifts observed in undoped QD's. The acceptor binding energy is strongly enhanced in a QD and decreases with increasing size following a scaling law that extrapolates to the bulk experimental value. The size-dependent average radius of the hole orbit is also calculated. The results are in agreement with the available experimental data for Mg impurity in bulk GaN.     

The electric field effect on binding energy of hydrogenic impurity in zinc-blende GaN/AlxGa1−xN spherical quantum dot

Within the framework of effective mass approximation, the binding energy of a hydrogenic donor impurity in zinc-blende GaN/Al_xGa_1−xN spherical quantum dot (QD) is investigated using the plane wave basis. The results show that the binding energy is highly dependent on impurity position, QD size, Al content and external field. The binding energy is largest when the donor impurity is located at the centre of the QD and the binding energy of impurity is degenerate for symmetrical positions with respect to the centre of QD without the external electric field. The maximum of the donor binding energy is shifted from the centre of QD and the degenerating energy levels for symmetrical positions with respect to the centre of QD are split in the presence of the external electric field. The binding energy is more sensitive to the external electric field for the larger QD and lower Al content. In addition, the Stark shift of the binding energy is also calculated.     

Excitons in coupled quantum dots

Properties of excitons in vertically coupled GaAs/AlGaAs quantum dots were investigated using the variational method within the envelope function and effective mass approximations. It was found that when the thickness of the spacer layer becomes less than about one exciton Bohr radius, both the exciton binding energy and the fundamental optical transition energy are reduced compared to those in isolated quantum dots. This is a result of increased space extension of exciton due to the penetration of carrier wave functions into the spacer layer and corresponding reduction in confinement energy which dominates over the Coulomb interaction between the electron and the hole.     

Exciton states in zinc-blende InGaN/GaN quantum dot

Within the framework of effective-mass approximation, exciton states confined in zinc-blende(ZB) InGaN/GaN quantum dot(QD) are investigated by means of a variational approach, considering finite band offsets. The ground-state exciton binding energy and the interband emission energy are investigated as functions of QD structural parameters in detail. Numerical results show clearly that both the QD size and In content of InGaN have a significant influence on the exciton states and interband optical transitions in the ZB InGaN/GaN QD.     

Electric field effects on optical properties in zinc-blende InGaN/GaN quantum dot

The effect of electric field on exciton states and optical properties in zinc-blende (ZB) InGaN/GaN quantum dot (QD) are investigated theoretically in the framework of effective-mass envelop function theory. Numerical results show that the electric field leads to a remarkable reduction of the ground-state exciton binding energy, interband transition energy, oscillator strength and linear optical susceptibility in InGaN/GaN QD. It is also found that the electric field effects on exciton states and optical properties are much more obvious in QD with large size. Moreover, the ground-state exciton binding energy and oscillator strength are more sensitive to the variation of indium composition in InGaN/GaN QD with small indium composition. Some numerical results are in agreement with the experimental measurements.     

Electric field effect in the spin dynamics of self-assembled InAs/GaAs quantum dots

We identify fundamental mechanisms of electron and hole dynamics in self-organized InAs/GaAs quantum dots (QDs) subject to vertical electric fields by photocurrent investigations. We propose a spin–flip mechanism involving a spin exchange between neighboring QDs. The spin–flip process is revealed in the photocurrent dynamics when the exciton population increases unexpectedly with reverse bias.     

Impurity states of electrons in quantum dots in external magnetic fields

The influence of isolated impurity atoms on the electron energy spectrum in a parabolic quantum dot in quantizing magnetic field is studied. The impurity potential is approximated by a Gaussian separable operator which allows one to obtain the exact solution of the problem. We demonstrate that in the electron energy spectrum there is a set of local levels which are split from the Landau zone boundaries in the upward or downward direction depending on the impurity type. We have calculated the local level positions, the wave functions of electrons in bound states, and the residues of the electron scattering amplitudes by impurity atoms at the poles.     

Four-wave mixing in coupled semiconductor quantum dots

We present a theoretical analysis of four-wave mixing in coupled quantum dots subject to inhomogeneous broadening. For the biexciton transitions, a clear signature of interdot-coupling appears in the spectra. The possibility of experimental observation is discussed.     

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.     

Electric field effects on the states of an exciton in square cross-section quantum well wires

The Letter studies the role of the external electric field on the binding energy of the exciton states in square cross-section quantum well wires. Using the effective-mass approximation within a variational scheme and expanding the wave function into Fourier series, we calculate the binding energies of the ground state as well as that of the excited states as the functions of the geometry and the strength of the applied electric field. In the presence of an electric field, it is found that for the ground state the Stark effect is redshift, and for the first and the second excited state the binding energy are split into two levels which will change in contrary situation along with the increasing of the strength of the applied electric field.     

Stability of neutral and negative donor impurity in a semiconductor cylindrical quantum dot

The stability of neutral (D⁰) and negative charged donor (D⁻) on- and off-center in anisotropic cylindrical quantum dot (CQD) is studied by use of a variational approach. Two-parameter anisotropic trial wave function which includes electron-correlation effects is utilized, to explore strong and weak confinement regions. A comparison between one and two-parameter trial wave functions results is introduced. The finite barrier height and the CQD dimensions, dependence of the “stability and the binding energy” of the D⁰ and the D⁻ is obtained. It has been shown that the donor's stability dependent on CQD dimensions and the confinement potential in strong confinement region but in weak confinement region, the stability of D⁰ and D⁻ is dependent strongly on the quantum dot (QD) radius R. It has been found that the donors D⁰ and D⁻ off-center are less stable than the on-center impurities, and also the off-center donors more stable in small CQDs. It has shown that the stability of D⁻ depends on the energy of the excess electron.     

Exciton in wurtzite GaN/AlxGa1−xN coupled quantum dots

Based on effective-mass approximation, we present a three-dimensional study of the exciton in GaN/Al_xGa_1−xN vertically coupled quantum dots (QDs) by a variational approach. The strong built-in electric field due to the piezoelectricity and spontaneous polarization is considered. The relationship between exciton states and structural parameters of wurtzite GaN/Al_xGa_1−xN coupled QDs is studied in detail. Our numerical results show that the strong built-in electric field in the GaN/Al_xGa_1−xN strained coupled QDs leads to a marked reduction of the effective band gap of GaN QDs. The exciton binding energy, the QD transition energy and the electron-hole recombination rate are reduced if barrier thickness L^AlGaN is increased. The sizes of QDs have a significant influence on the exciton state and interband optical transitions in coupled QDs.     

The multilayered spherical quantum dot under a magnetic field

The binding energy of an impurity located at the center of multilayered spherical quantum dot (MSQD) is reported as a function of the dot and barrier thickness for different alloy compositions under the influence of a magnetic field. Within the effective mass approximation, the binding energy has been calculated using the fourth order Runge-Kutta method without magnetic field. A variational approach has been employed if a magnetic field is present. The binding energy in MSQD with equal dot and barrier thickness is calculated. It is shown that the binding energy in MSQD differs from that of a single quantum dot. Also, the geometry is dominant on the binding energy for thin MSQDs, but the magnetic field becomes more effective for thick MSQDs.