Function Of Dot Radius Research Articles

Optical Properties of Three‐Electron GaAs/AlxGa1−xAs QDs with Finite Confinement Potential

AbstractIn the case of finite confinement potential, the average energies and corresponding wave functions for the 1s2nl configurations, in which nl = 2s, 2p, 3d, and 4f, of three‐electron GaAs/AlxGa1−xAs quantum dot with and without impurity are computed by using a new variational approach which is a combination of Quantum Genetic Algorithm procedure and Hartree–Fock–Roothaan method. Using the calculated average energies and wave functions, a detailed investigation of the linear, third‐order nonlinear and total absorption coefficients (ACs) and the refractive index changes (RICs) for the quantum dot is performed, and the obtained results are presented as a function of dot radius and photon energies. The results show that the dot radius, the impurity charge, and the height of potential barrier have a strong influence on the average energies and absorption spectra of the system. As the potential barrier height increases, the peak positions of the ACs and RICs shift toward higher energy, and there is a significant increase in the amplitudes of the absorption spectra as the potential barrier height increases. In addition, the electron wave functions begin to enter the quantum well at smaller dot radii with increasing barrier height.

Calculation of electric field gradient in spherical quantum dots

ABSTRACTWe have calculated the ground and excited energy states of the quantum dot as a function of the depth of confining potential well and radius R. Based on the calculated energies and wave functions, the electric field gradient induced by the non-spherical charge at impurity is investigated as a function of dot radius and the confining potential. For Deuterium dot with spherical confining potential, the expectation value of the quadrupole moment operator is carried out. The calculations show that the dot radius, the confining potential and the impurity charge have a great influence on the electric field gradient and the quadrupole moment. It is found that as the confining potential increases, the peak position of the electric field gradient shifts toward smaller dot radii and its amplitude increases with the increase of confining potential.

Effects of built-in fields on the optical gain in a CdO/ZnO quantum dot with the Smorodinsky–Winternitz potential

Exciton energy, with and without the inclusion of built-in internal fields, as a function of inner dot radius is investigated in a CdO/ZnO quantum dot with the Smorodinsky–Winternitz potential. The internal fields are owing to spontaneous and piezoelectric polarizations existing between the inner core and outer shell materials in the heterostructure. The dielectric mismatch effect is included. Numerical computations are carried out to obtain the exciton energy employing the variational formulism within the single band effective mass approximation. The energy shift owing to spontaneous and piezoelectric polarizations is found to be 172 meV for 20 Å quantum dot radius. The oscillator strength and the radiative lifetime as a function of dot radius are found. The optical matrix elements and the corresponding gain are studied with the constant carrier density. The optical properties are investigated using density matrix method. It is found that the substantial increase of optical gain is estimated with the inclusion of built-in internal fields.

Magnetic field effects on oscillator strength, dipole polarizability and refractive index changes in spherical quantum dot

We have computed the ground and excited state energies and the wave functions of a spherical quantum dot with finite potential barrier in the presence of magnetic field. The oscillator strength, the static dipole polarizability and the refractive index changes are investigated as a function of dot radius and magnetic field strength. The results show that the energy state, the oscillator strength, the static dipole polarizability and the refractive index changes are strongly affected by the magnetic field strength and dot size. It is found that while the magnetic field strength increases, the static polarizability decreases strongly.

Finite difference calculation of optical properties of hydrogenic impurity in spherical quantum dot with parabolic confinement

The optical properties of an electron with an impurity in a spherical quantum dot under parabolic confinement are studied and energies, wave functions, binding energies, radial matrix elements, polarizability, susceptibility and oscillator strength have been evaluated. The numerical method used is the finite difference method in the framework of the effective mass approximation. The variation of the energy levels and radial matrix elements have been studied as function of the radius of the GaAs sphere and also as function of the frequency of the harmonic oscillator potential or parabolic potential. In addition we have studied how polarizability, susceptibility and the oscillator strength vary as a function of dot radius and at different parabolic potential frequencies.

Linear and nonlinear absorption coefficients of spherical quantum dot inside external magnetic field

We have calculated the wavefunctions and energy eigenvalues of spherical quantum dot with infinite potential barrier inside uniform magnetic field. In addition, we have investigated the magnetic field effect on optical transitions between Zeeman energy states. The results are expressed as a function of dot radius, incident photon energy and magnetic field strength. The results present that, in large dot radii, the external magnetic field affects strongly the optical transitions between Zeeman states. In the strong spatial confinement case, energy level is relatively insensitive to the magnetic field, and electron spatial confinement prevails over magnetic confinement. Also, while m varies from −1 to +1, the peak positions of the optical transitions shift toward higher energy (blueshift).

Magnetic field induced optical gain in a dilute nitride quaternary semiconductor quantum dot

Effects of magnetic field strength on the electronic and optical properties are brought out in a Ga0.661In0.339N0.0554As0.9446/GaAs quantum dot for the applications of desired wavelength in opto-electronic devices. The band alignment is obtained using band anticrossing model and the model solid theory. The magnetic field dependent electron-heavy hole transition energies with the dot radius in a GaInNAs/GaAs quantum dot are investigated. The magnetic field induced oscillator strength as a function of dot radius is studied. The resonant peak values of optical absorption coefficients and the changes of refractive index with the application of magnetic field strength in a GaInNAs/GaAs quantum dot are obtained. The magnetic field induced threshold current density and the maximum optical gain are found in a GaInNAs/GaAs quantum dot. The results show that the optimum wavelength for fibre optical communication networks can be obtained with the variation of applied magnetic field strength and the outcomes may be useful for the design of efficient lasers based on the group III-N-V semiconductors.

Calculation of Zeeman splitting and Zeeman transition energies of spherical quantum dot in uniform magnetic field

In this study we report a detailed theoretical investigation of the effect of an external magnetic field on the 1s-, 2p-, 3d- and 4f-energy states of a spherical quantum dot. We treat the contribution of the diamagnetic term as a perturbation and discuss the effect of the diamagnetic term on the 1s-, 2p-, 3d- and 4f-energy states. We also have calculated the Zeeman transition energies between 2p→1s and 3d→2p states with m=0, ±1 and 0, ±1, ±2 as a function of dot radius and the magnetic field strength. The results show that the magnetic field, impurity charge and dot radius have a strong influence on the energy states and the Zeeman transitions. It is found that the energies of the electronic states with m<0 addition of the diamagnetic term firstly decrease toward a minimum, and then increase with the increasing magnetic field strength. We have seen that as magnetic field intensity is adjusted, frequency of the emitted light can be changed for Zeeman transitions.

Graphene quantum dot with a hydrogenic impurity

We investigate the electronic energy levels of a circular graphene quantum dot with a hydrogenic impurity placed at the center of the dot. We have adopted a variational scheme in such a way that the trial wave functions take into account the carrier confinement due to the dot geometry as well as the effect of the magnetic field. We found that the presence of impurity strongly modifies the electronic spectrum of the graphene quantum dot. We also calculated the critical value of the impurity strength as a function of dot radius.

Graphene quantum dot with a hydrogenic impurity

We investigate the electronic energy levels of a circular graphene quantum dot with a hydrogenic impurity placed at the center of the dot. We have adopted a variational scheme in such a way that the trial wave functions take into account the carrier confinement due to the dot geometry as well as the effect of the magnetic field. We found that the presence of impurity strongly modifies the electronic spectrum of the graphene quantum dot. We also calculated the critical value of the impurity strength as a function of dot radius.

Effect of hydrostatic pressure and polaronic mass of the binding energy in a spherical quantum dot

Simultaneous effect of hydrostatic pressure and polaronic mass on the binding energies of the ground and excited states of an on-center hydrogenic impurity confined in a GaAs/GaAlAs spherical quantum dot are theoretically investigated by the variational method within the effective mass approximation. The binding energy is calculated as a function of dot radius and pressure. Our findings proved that the hydrostatic pressure led to the decrease of confined energy and the increase of donor binding energy. Conduction band non-parabolicity and the polaron masses are effective in the donor binding energy which is significant for narrow dots not in the confined energy. The maximum donor binding energy achieved by the polaronic mass in the ground and excited states are 2%–19% for the narrow dots. The confined and donor binding energies approach zero as the dot size approaches infinity.

Magneto-trions in a GaMnAs/Ga0.6Al0.4As Quantum Dot

Magneto bound exciton and the charged exciton in a GaMn 0.02 As / Ga 0.6 Al 0.4 As quantum dot are reported with the spatial confinement effect. The numerical calculations are carried out with the inclusion of exchange interaction between the carrier and the magnetic impurities. The binding energies of exciton and the trions and the optical transition energy are obtained as a function of dot radius. Numerical computations are followed using exact diagonalization method. The spin polaronic energy of the exciton and the charged excitons are obtained using a mean field theory in the presence of magnetic field strength. The magnetization of Mn ion impurities as a function of dot radius is investigated. The effective g-factor of conduction (valence) band electron (hole) is obtained in the GaMnAs quantum dot. The magnetic field induced size dependence of effective Landé g-factor is computed. The result shows that (i) the geometrical dependence on sp-d exchange interaction in the GaMn 0.02 As / Ga 0.6 Al 0.4 As quantum dot has great influence with the geometrical confinement, (ii) the monotonic behavior of effective g-factor with the reduction of dot radius is observed, (iii) the Landé factor is more sensitive if the geometrical confinement effect is included and (iv) the value of effective g-factor increases when the spatial confinement is enhanced for all the dot radii. Our results show that the effective Landé g-factor can be manipulated negative to positive values in the GaMn 0.02 As / Ga 0.4 Al 0.6 As quantum dot.

Electron capture processes in quantum dots due to one-and two-phonon assisted transitions: The role of optical phonon confinement

Electron capture induced by carrier heating in spherical quantum dot-quantum well (QD-QW) structure is studied theoretically. The capture rate (CR) in one- and two-polar- optical-phonon-mediated capture processes has been studied by taking into account the phonon confinement. We have derived the analytic expressions for carrier CRs which can be conveniently applied to practical calculations for the spherical quantum dot systems. The numerical results of the CR as function of dot radius, lattice temperature and electron density in GaAs/AlAs/vacuum and CdSe/ZnS/H2O QD systems are obtained and discussed.

Dependence of heavy hole exciton binding energy and the strain distribution in GaAs1−xBix/GaAs finite spherical quantum dots on Bi content in the material

The ground state binding energy of heavy hole excitons confined in GaAs1−xBix/GaAs spherical quantum dots is calculated as a function of dot radius and the Bi content using a Variational method based on 1-s hydrogenic wave functions with effective mass approximation. The parameter shows strong dependence on the Bi mole fraction x, particularly at smaller values of the dot radii. The strain associated with the quantum dot is found to decrease exponentially with increase in dot radius and shows a linear increase with Bi composition.

Calculation of hyperfine interaction in spherical quantum dot

In this study, we have calculated the unperturbed wavefunctions and energy eigenvalues of the ground and excited states of a spherical quantum dot, GaAs/AlxGa1−xAs, by using quantum genetic algorithm and Hartree–Fock Roothaan method. Hyperfine coupling constant and hyperfine energy of 1s, 2p, 3d and 4f levels are carried out as a function of dot radius. The results show that the hyperfine constant and hyperfine energy varies rapidly in the strong and medium confining regions as dot radius decreases. It is worth pointing out that dot radius, impurity charge and angular momentum have a strong influence on the hyperfine energy. It is also found that hyperfine energy and hyperfine splitting vary with aluminium concentration ratio x.

Effect of Phonon Confinement on Optical Phonon-Mediated Carrier Capture into CdSe/ZnS Quantum Dots

Electron capture induced by carrier heating in the CdSe/ZnS spherical quantum dot–quantum well structure is studied theoretically. The capture rate has been calculated by taking into account the phonon confinement effect. Numerical results for the capture rate as a function of dot radius, lattice temperature, and electron density in the CdSe/ZnS/H2O quantum dot (QD) system are obtained and discussed. It has been shown that the capture rate of an electron from the barrier region to the quantum dot ground state due to the emission of confined or interface optical phonons exhibits strong resonances versus dot radius. Our results reveal that the capture time is larger than 1 ns across broad ranges of quantum dot radius. We have found the increase of the capture rate due to emission of LO1 as well as IO/SO phonons with increasing temperature. However, for structures with lower electron densities (n ≤ 1016 cm−3), the monotonic behavior of the capture rate in case of IO/SO phonon interactions is broken and a local maximum in the temperature dependence appears. In contrast to the GaAs/AlAs spherical quantum dot–quantum well structure, the capture rate in CdSe/ZnS/H2O QDs is very strongly dependent on the electron density.

Interband optical properties in wide band gap group-III nitride quantum dots

Size dependent emission properties and the interband optical transition energies in group-III nitride based quantum dots are investigated taking into account the geometrical confinement. Exciton binding energy and the optical transition energy in <TEX>$Ga_{0.9}In_{0.1}N$</TEX>/GaN and <TEX>$Al_{0.395}In_{0.605}N$</TEX>/AlN quantum dots are studied. The largest intersubband transition energies of electron and heavy hole with the consideration of geometrical confinement are brought out. The interband optical transition energies in the quantum dots are studied. The exciton oscillator strength as a function of dot radius in the quantum dots is computed. The interband optical absorption coefficients in GaInN/GaN and AlInN/AlN quantum dots, for the constant radius, are investigated. The result shows that the largest intersubband energy of 41% (10%) enhancement has been observed when the size of the dot radius is reduced from <TEX>$50{\AA}$</TEX> to <TEX>$25{\AA}$</TEX> of <TEX>$Ga_{0.9}In_{0.1}N$</TEX>/GaN (<TEX>$Al_{0.395}In_{0.605}N$</TEX>/AlN) quantum dot.

Magneto-polaron induced intersubband optical transition in a wide band gap II–VI semiconductor quantum dot

Effects of LO-phonon contribution on the electronic and the optical properties are investigated in a Cd0.8Zn0.2Se/ZnSe quantum dot in the presence of magnetic field strength. The magneto-polaron induced hydrogenic binding energy as a function of dot radius in the wide band gap quantum dot is calculated. The oscillator strength and the spontaneous lifetime are studied taking into account the spatial confinement, magnetic field strength and the phonon contribution. Numerical calculations are carried out using variational formulism within the single band effective mass approximation. The optical properties are computed with the compact density matrix method. The magneto-polaron induced optical gain as a function of photon energy is observed. The results show that the optical telecommunication wavelength in the fiber optic communications can be achieved using CdSe/ZnSe semiconductors and it can be tuned with the proper applications of external perturbations.

Linear and nonlinear absorption coefficients of spherical two-electron quantum dot

In this study, optical properties of two-electron quantum dot confined by an infinite spherical potential surface have been investigated. Linear, nonlinear and total absorption coefficients of S→P, P→D and D→F dipole-allowed transitions between singlet–singlet and triplet–triplet states have been calculated as a function of dot radius and photon energy. The results show that the change of dot radius and incident optical intensity effects the peak positions and amplitudes of linear and nonlinear absorption coefficients. Besides, it has been found that the absorption coefficients of transitions between triplet states are stronger than those of the singlet states, and also triplet absorption transitions occur at higher energies.

Magneto-biexciton in a group III–V diluted magnetic quantum dot

Binding energies of biexciton are computed in a GaMnAs / GaAlAs quantum dot with the effects of geometrical confinement, exchange interaction between the charge carrier and the magnetic impurities and the magnetic field. The size dependence of the binding energy of the biexciton in the presence of magnetic field is brought out. The optical transition energy, in the presence of magnetic field strength, is discussed for various magnetic impurities in a GaMn x As quantum dot. Numerical calculations are performed using variational technique. The spin polaronic energy of the biexciton with the effect of spatial confinement is carried out taking into account the mean field approximation in the presence of magnetic field strength. The magnetization of magnetic ion impurities as a function of dot radius is computed in a GaMn 0.02 As quantum dot. The effective g-factor as a function of spatial confinement is found in the GaMnAs quantum dot. The results show that the shift in spin polaron has more influence for the larger dot radius and the nonlinearity to linear behavior of g-factor for a particular dot size is achieved due to the sign reversal of Zeeman splitting.