The binding energy of a hydrogenic impurity in self-assembled double quantum dots

The binding energy of a hydrogenic impurity in self-assembled double quantum dots is calculated via the finitedifference method. The variation in binding energy with donor position, structure parameters and external magnetic field is studied in detail. The results found are： （i） the binding energy has a complex behaviour due to coupling between the two dots; （ii） the binding energy is much larger when the donor is placed in the centre of one dot than in other positions; and （iii） the external magnetic field has different effects on the binding energy for different quantum-dot sizes or lateral confinements.

Simultaneous Effects of Hydrostatic Pressure and Conduction Band Non-parabolicity on Binding Energies and Diamagnetic Susceptibility of a Hydrogenic Impurity in Spherical Quantum Dots

Simultaneous effects of conduction band non-parabolicity and hydrostatic pressure on the binding energies of 1S, 2S, and 2P states along with diamagnetic susceptibility of an on-center hydrogenic impurity confined in typical GaAs/Alx- Ga1-x As spherical quantum dots are theoretically investigated using the matrix diagonalization method. In this regard, the effect of band non-parabolieity has been performed using the Luttinger-Kohn effective mass equation. The binding energies and the diamagnetic susceptibility of the hydrogenic impurity are computed as a function of the dot radius and different values of the pressure in the presence of conduction band non-parabolicity effect. The results we arrived at are as follows： the incorporation of the band edge non-parabolicity increases the binding energies and decreases the absolute value of the diamagnetic susceptibility for a given pressure and radius; the binding energies increase and the magnitude of the diamagnetic susceptibility reduces with increasing pressure.

Spectrum and Binding Energy of an Off-Center Hydrogenic Donor in a Spherical Quantum Dot

Off-center impurity effects in a spherical quantum dot are theoretically studied by degenerate perturbationmethod in strong confinement.The energy levels and binding energies are computed for the typical GaAs material asfunction of the donor position.The numerical results show the quantum size effect.We note that the energy levels andbinding energies are not only related to the position of donor and the strength of confinement,but also related to thefold of degenerate states.We can see obviously that gaps will appear among the degenerate states and the splitting ofenergy levels and binding energies will appear as the position of the impurity is shifted away off the center.

Size dependence of biexciton binding energy in single InAs/GaAs quantum dots

This paper studies the size dependence of biexciton binding energy in single quantum dots （QDs） by using atomic force microscopy and micro-photoluminescence measurements. It finds that the biexciton binding energies in the QDs show ＂binding＂ and ＂antibinding＂ properties which correspond to the large and small sizes of QDs, respectively. The experimental results can be well interpreted by the biexciton potential curve, calculated from the exciton molecular model and the Heitler London method.

Effects of hydrostatic pressure and external electric field on the impurity binding energy in strained GaN/Al_xGa_(1-x)N spherical quantum dots

The binding energy and Stark effect energy shifts of a shallow donor impurity state in a strained GaN/AlxGa1-xN spherical finite-potential quantum dot (QD) are calculated using a variational method based on the effective mass approximation. The binding energy is computed as a function of dot size and hydrostatic pressure. The numerical results show that the binding energy of the impurity state increases, attains a maximum value, and then decreases as the QD radius increases for any electric field. Moreover, the binding energy increases with the pressure for any size of dot. The Stark shift of the impurity energy for large dot size is much larger than that for the small dot size, and it is enhanced by the increase of electric field. We compare the binding energy of impurity state with and without strain effects, and the results show that the strain effects enhance the impurity binding energy considerably, especially for the small QD size. We also take the dielectric mismatch into account in our work.

Impurity-related electronic properties in quantum dots under electric and magnetic fields

This paper presents a systematic study of the ground-state binding energies of a hydrogenic impurity in quantura dots subjected to external electric and magnetic fields. The quantum dot is modeled by superposing a lateral parabolic potential, a Gaussian potential and the energies are calculated via the finite-difference method within the effectivemass approximation. The variation of the binding energy with the lateral confinement, external field, position of the impurity, and quantum-size is studied in detail. All these factors lead to complicated binding energies of the donor, and the following results are found： （1） the binding energies of the donor increase with the increasing magnetic strength and lateral confinement, and reduce with the increasing electric strength and the dot size; （2） there is a maximum value of the binding energies as the impurity placed in different positions along the z direction; （3） the electric field destroys the symmetric behaviour of the donor binding energies as the position of the impurity.

Shallow Donor Impurity Ground State in a GaAs/AlAs Spherical Quantum Dot within an Electric Field

Using the configuration-integration methods （CI） [Phys. Rev. B 45 （1992） 19], we report the results of the Hydrogenie-impurity ground state in a GaAs/AIAs spherical quantum dot under an electric field. We discuss the variations of the binding energies of the Hydrogenic-impurity ground state as a function of the position of impurity D, the radius R of the quantum dot, and also as a function of electric field F. We find that the ground energy and binding energy of impurity placed anywhere depend strongly on the position of impurity. Also, electric field can largely change the Hydrogenic-impurity ground state only limiting to the big radius of quantum dot. And the differences in energy level and binding energy are observed from the center donor and off-center donor.

Linear and Nonlinear Optical Properties of Spherical Quantum Dots:Effects of Hydrogenic Impurity and Conduction Band Non-Parabolicity

Simultaneous effects of an on-center hydrogenic impurity and band edge non-parabolicity on intersubband optical absorption coefficients and refractive index changes of a typical GaAs/AlxGa1-xAs spherical quantum dot are theoretically investigated, using the Luttinger-Kohn effective mass equation. So, electronic structure and optical properties of the system are studied by means of the matrix diagonalization technique and compact density matrix approach, respectively. Finally, effects of an impurity, band edge non-parabolicity, incident light intensity and the dot size on the linear, the third-order nonlinear and the total optical absorption coemcients and refractive index changes are investigated. Our results indicate that, the magnitudes of these optical quantities increase and their peaks shift to higher energies as the influences of the impurity and the band edge non-parabolicity are considered. Moreover, incident light intensity and the dot size have considerable effects on the optical absorption coefficients and refractive index changes.

Influence of strain on hydrogenic impurity states in a GaN/ Al_xGa_(1-x)N quantum dot

Within the effective-mass approximation, we calculated the influence of strain on the binding energy of a hydrogenic donor impurity by a variational approach in a cylindrical wurtzite GaN/AlxGa1-xN strained quantum dot, including the strong built- in electric field effect due to the spontaneous and piezoelectric polarization. The results show that the binding energy of impurity decreases when the strain is considered. Then the built-in electric field becomes bigger with the Al content increasing and the bin...

Optical properties of core/shell spherical quantum dots

In this study,the effects of quantum dot size on the binding energy,radiative lifetime,and optical absorption coefficient of exciton state in both GaN/AlxGa1-xN core/shell and AlxGa1-xN/GaN inverted core/shell quantum dot structures are studied.For the GaN/AlxGa1-xN core/shell structure,the variation trend of binding energy is the same as that of radiation lifetime,both of which increase first and then decrease with the increase of core size.For AlxGa1-xN/GaN inverted core/shell structure,the binding energy decreases first and then increases with core size increasing,and the trends of radiation lifetime varying with core size under different shell sizes are different.For both structures,when the photon energy is approximately equal to the binding energy,the peak value of the absorption coefficient appears,and there will be different peak shifts under different conditions.

Properties of Unsymmetrical Parabolic Confinement Potential Quantum Dot Qubit

On the condition of electric-LO phonon strong coupling in unsymmetrical parabolic confinement potential quantum dot （QD）, we obtain the eigenenergies of the ground state and the first-excited state, the eigenfunctions of the ground state, and the first-excited state by using variational method of Pekar type. This system in QD may be employed as a two-level quantum system-qubit. When the electron is in the superposition state of the ground state and the first-excited state, we obtain the time evolution of the electron density. The relations both the probability density of electron and the period of oscillation with the electron-LO-phonon coupling strength, the confinement strengths in the xy-plane and the z-direction are discussed.

Ionized Acceptor Bound Exciton States in Wurtzite GaN/Al_xGa(1-x)N Cylindrical Quantum Dot

Based on the framework of the effective-mass approximation, the ionized acceptor bound exciton （A- X） binding energy and the emission wavelength are investigated for a cylindrical wurtzite （WZ） GaN/A1x Ga1-xN quantum dot （QD） with finite potential barriers by means of a variational method. Numerical results show that the binding energy and the emission wavelength highly depend on the QD size, the position of the ionized acceptor and the Al composition x of the barrier material AIxGal-xN. The binding energy and the emission wavelength are larger when the acceptor is located in the vicinity of the left interface of the QD. In particular, the binding energy of （ A-, X） complex is insensitive to the dot height when the acceptor is located at the left boundary of the QD. The ionized acceptor bound exciton binding energy and the emission wavelength are both increased if Al composition x is increased.

Effects of Electric Field on Electronic States in a GaAs/GaAlAs Quantum Dot with Different Confinements

Binding energies of shallow hydrogenic impurity in a GaAs/GaAlAs quantum dot with spherical confinement, parabolic confinement and rectangular confinement are calculated as a function of dot radius in the influence of electric field. The binding energy is calculated following a variational procedure within the effective mass approximation along with the spatial depended dielectric function. A finite confining potential well with depth is determined by the discontinuity of the band gap in the quantum dot and the cladding. It is found that the contribution of spatially dependent screening effects are small for a donor impurity and it is concluded that the rectangulax confinement is better than the parabolic and spherical confinements. These results are compared with the existing literature.

Binding Energy and Photoionization of Hydrogenic Impurities in GaAs/Ga_(1-x)Al_xAs Quantum Well Wires

The binding energy and the photon energy dependence of the photoionization cross-section are calculated for a hydrogenic impurity in GaAs/Ga 1-xAl xAs quantum well wires.The correlation between confined and non-confined direction of the wire in the variational wave function is taken into account.The results show that the photoionization cross-sections are affected by the width of the wire and that their magnitudes are larger than those in infinite potential quantum well wires.In comparison with previous's results,the variational wave function improves the binding energy and decreases the value of photoionization cross-sections of the hydrogenic impurities,which makes the results more reasonable.

Electronic States of a Shallow Hydrogenic Donor Impurity in Different Shaped Semiconductor Quantum Wells

The shallow hydrogenic donor impurity states in square, V-shaped, and parabolic quantum wells are studied in the framework of effective-mass envelope-function theory using the plane wave basis. The first four impurity energy levels and binding energy of the ground state are more easily calculated than with the variation method. The calculation results indicate that impurity energy levels decrease with the increase of the well width and decrease quickly when the well width is small. The binding energy of the ground state increases until it reaches a maximum value, and then decreases as the well width increases. The results are meaningful and can be widely applied in the design of various optoelectronie devices.

Magnetic field and temperature dependence of the properties of the ground state of the strong-coupling bound magnetopolaron in quantum rods with hydrogenic impurity

Magnetic field and temperature dependence of the properties of the ground state of the strong-coupling bound magnetopolaron in quantum rods （QRs） with hydrogenic impurity is studied by means of the Huybrechts- Lee-Low-Pines transformation method and the quantum statistical theory. The expressions for the ground-state energy and the mean number ofphonons of the magnetopolaron are derived. Results of the numerical calculations show that the bound state of the magnetopolaron cannot be formed when the value of the aspect ratio of the QR, the dielectric constant ratio, the electron-phonon coupling strength or the temperature parameter is small. The larger the deviation of the value of aspect ratio e′ from 1 is, the more it is unfavorable to the stability of the ground state of the magnetopolaron. When the magnetopolaron is in the bound state, the absolute value of its ground-state energy and its mean number ofphonons increase with an increase of the dielectric constant ratio and confinement strength of QRs, but decrease with an increase in the cyclotron frequency of the external magnetic field and the temperature. The absolute value of the ground-state energy and the mean number of phonons of the magnetopolaron decrease with decreasing e′ when e′ 〈 1, but decrease with increasing e′ when e′ 〉 1. They get the maximum value at e′=1.

Optical properties of ionized donor-bound excitons confined in strained wurtzite ZnO/Mg_xZn_(1-x)O quantum dots

Within the framework of the effective-mass approximation and the dipole approximation, considering the three-dimensional confinement of the electron and hole and the strong bulk-in electric field （BEF） in strained wurtzite ZnO/Mgo.25Zno.750 quantum dots （QDs）, the optical properties of ionized donor-bound excitons （D^＋, X） are investigated theoretically using a variational method. The computations are performed in the case of finite band offset. Numerical results indicate that the optical properties of （D^＋, X） complexes sensitively depend on the donor position, the QD size and the BEF. The binding energy of （D^＋, X） complexes is larger when the donor is located in the vicinity of the left interface of the QDs, and it decreases with increasing QD size. The oscillator strength reduces with an increase in the dot height and increases with an increase in the dot radius. Furthermore, when the QD size decreases, the absorption peak intensity shows a marked increment, and the absorption coefficient peak has a blueshift. The strong BEF causes a redshift of the absorption coefficient peak and causes the absorption peak intensity to decrease remarkably. The physical reasons for these relationships have been analyzed in depth.

Stark Effect Dependence on Hydrogenic Impurities in GaAs Parabolic Quantum-Well Wires

The ground-state and lowest excited-state binding energies of a hydrogenic impurity in GaAs parabolic quantum-well wires （Q WWs） subjected to external electric and magnetic fields are investigated using the finite-difference method within the quasi-one-dimensional effective potential model. We define an effective radius Pen of a cylindrical QWW, which can describe the strength of the lateral confinement. For the ground state, the position of the largest probability density of electron in x-y plane is located at a point, while for the lowest excited state, is located on a circularity whose radius is Pen. The point and circularity are pushed along the left haft of the center axis of the quantum-well wire by the electric field dire ted along the right half. When an impurity is located at the point or within the circularity, the ground-state or lowest excited-state binding energies are the largest; when the impurity is apart from the point or circularity, the ground-state or lowest excited-state binding energies start to decrease.

Magnetic-Field Effects on Donor Impurity States in a Quantum Well

Green’s function technique is used to obtain the solution of Shredinger equation for impurity states in a quantum well (QW) under the magnetic field. Binding energy of impurity states is defined as poles of the wave function. We studied effects of the magnetic field magnitude and impurity position on the binding energy. The calculations were performed for both ground and excited states. The dependences of binding energies versus impurity position and magnetic field are presented for GaAs/Al0.3Ga0.7As QW.

Bipolaron in different configuration of quantum confinement

The authors used Landau-Pekar variational method to investigate a strong-coupling singlet optical bipolaron in different configuration of quantum confinement. Numerical and analytical results showed that when configuration changes from quantum dot and wire to well, confinement shows different effect on the formation of a bipolaron. In contrast to a bipolaron in a quantum dot or wire, the binding energy of a bipolaron in a quantum well increases with increasing con-finement, indicating that confinement favors bipolaron formation in a quantum well.