Anisotropic emission of interface fluctuation quantum dots

We calculate energy levels, dipole moments and radiative broadening of interface fluctuation quantum dots. For optically allowed states, the dipole moment grows proportionally to the lateral quantum dot radius while the radiative broadening saturates towards the quantum well radiative broadening for large lateral quantum dot radii. This is accompanied by a change in the angular emission pattern, concentrating emission in forward and backward direction. Optically forbidden states do not couple to light propagating in the growth direction yet they may have a considerable radiative broadening due to spontaneous emission in other directions. Received 20 March 2002 Published online 25 June 2002     

Charged-Exciton Complexes in Quantum Dots

It is known experimentally that stable charged-exciton complexes can exist in low-dimensional semiconductor nanostructures. Much less is known about the properties of such charged-exciton complexes since three-body problems are very difficult to be solved, even numerically. Here we introduce the correlated hyperspherical harmonics as basis functions to solve the hyperangular equation for negatively and positively charged excitons (trions) in a harmonic quantum dot. By using this method, we have calculated the energy spectra of the low-lying states of a charged exciton as a function of the radius of quantum dot. Based on symmetry analysis, the level crossover as the dot radius increases can be fully explained as the results of symmetry constraint.``     

Charged-Exciton Complexes in Quantum Dots

It is known experimentally that stable charged-exciton complexes can exist in low-dimensional semiconductor nanostructures. Much less is known about the properties of such charged-exciton complexes since three-body problems are very difficult to be solved, even numerically. Here we introduce the correlated hyperspherical harmonics as basis functions to solve the hyperangular equation for negatively and positively charged excitons (trions) in a harmonic quantum dot. By using this method, we have calculated the energy spectra of the low-lying states of a charged exciton as a function of the radius of quantum dot. Based on symmetry analysis, the level crossover as the dot radius increases can be fully explained as the results of symmetry constraint.     

Prompt light particle emission in the 36Ar + 58Ni reaction at 95 MeV/nucleon

The prompt component at intermediate velocity of light charged particles is investigated. An improved coalescence model coupled to the intra-nuclear cascade code ISABEL is used to obtain light complex particle energy spectra and multiplicities as a function of impact parameter. The results are compared with experimental data from the ³⁶Ar + ⁵⁸Ni experiment at 95 MeV/nucleon, performed with the INDRA 4π detection system. The calculated prompt component is found to rather well reproduce proton spectra. For complex light charged particles the calculated components well populate the high energy part of spectra. Prompt emission can therefore explain the large transverse energies experimentally observed at mid-rapidity. Received: 27 July 2000 / Accepted: 20 November 2000     

Electron energy state spin-splitting in 3D cylindrical semiconductor quantum dots

In this article we study the impact of the spin-orbit interaction on the electron quantum confinement for narrow gap semiconductor quantum dots. The model formulation includes: (1) the effective one-band Hamiltonian approximation; (2) the position- and energy-dependent quasi-particle effective mass approximation; (3) the finite hard wall confinement potential; and (4) the spin-dependent Ben Daniel-Duke boundary conditions. The Hartree-Fock approximation is also utilized for evaluating the characteristics of a two-electron quantum dot system. In our calculation, we describe the spin-orbit interaction which comes from both the spin-dependent boundary conditions and the Rashba term (for two-electron quantum dot system). It can significantly modify the electron energy spectrum for InAs semiconductor quantum dots built in the GaAs matrix. The energy state spin-splitting is strongly dependent on the dot size and reaches an experimentally measurable magnitude for relatively small dots. In addition, we have found the Coulomb interaction and the spin-splitting are suppressed in quantum dots with small height. Received 15 May 2001 / Received in final form 14 May 2002 Published online 13 August 2002     

The Electron－Hole Pair in a Single Quantum Dot and That in a Vertically Coupled Quantum Dot

The energy spectra of low-lying states of an exciton in a single and a vertically coupled quantum dots are studied under the influence of a perpendicularly applied magnetic field. Calculations are made by using the method of numerical diagonalization of the Hamiltonian within the effective-mass approximation. We also calculated the binding energy of the ground and the excited states of an exciton in a single quantum dot and that in a vertically coupled quantum dot as a function of the dot radius for different vaJues of the distance and the magnetic field strength.     

The Electron-Hole Pair in a Single Quantum Dot and That in a Vertically Coupled Quantum Dot

The energy spectra of low-lying states of an exciton in a single and a vertically coupled quantum dots arestudied under the influence of a perpendicularly applied magnetic field. Calculations are made by using the method ofnumerical diagonalization of the Hamiltonian within the effective-mass approximation. We also calculated the bindingenergy of the ground and the excited states of an exciton in a single quantum dot and that in a vertically coupledquantum dot as a function of the dot radius for different values of the distance and the magnetic field strength.     

极性晶体量子点中强耦合激子的内部激发态

在有效质量近似下,采用线性组合算符和幺正变换方法研究了极性晶体量子点中强耦合激子的内部激发态性质,导出了极性晶体量子点中强耦合激子的基态能量、第一内部激发态能量和激发能量随量子点半径的变化关系。对TlCl晶体进行数值计算,结果表明,量子点中强耦合激子的基态能量、第一内部激发态能量和激发能量随量子点半径的减小而增大。     

NEGATIVE DONOR CENTER QUANTUM DOTS IN MAGNETIC FIELDS

The method of few-body physics is applied to treat a D^{-<\sup> center quantum dot system in a magnetic field. The magnetic field is applied in the z direction. Using this method, we investigate the energy spectra of low-lying states of D-<\sup> center quantum dots as a function of magnetic field. The dependence of the binding energies of the ground-state of the D-<\sup> center are calculated as a function of the dot radius with a few values of the magnetic field strength and compared with other results.}     

Spin configuration transformation in laterally coupled few-electron artificial molecules

Artificial molecules, namely laterally coupled quantum dots with a three-dimensional spherical confinement potential well of radius R and depth V ₀, were studied by the unrestricted Hartree-Fock-Roothaan (UHFR) method. By varying the distance d between the centers of the two coupled quantum dots, the transition from the strong coupling situation to the weak one is realized. Hund's rule, suitable for a single quantum dot is destroyed in certain conditions in the artificial molecule. For example, in the few-electron system of the strongly coupled quantum-dot molecule, a transformation of spin configuration has been found. Received 8 March 2002 / Received in final form 29 May 2002 Published online 17 September 2002     

Electronic properties of model quantum-dot structures in zero and finite magnetic fields

We have computed electronic structures and total energies of circularly confined two-dimensional quantum dots and their lateral dimers in zero and finite uniform external magnetic fields using different theoretical schemes: the spin-density-functional theory (SDFT), the current-and-spin-density-functional theory (CSDFT), and the variational quantum Monte Carlo (VMC) method. The SDFT and CSDFT calculations employ a recently-developed, symmetry-unrestricted real-space algorithm allowing solutions which break the spin symmetry. Results obtained for a six-electron dot in the weak confinement limit and in zero magnetic field as well as in a moderate confinement and in finite magnetic fields enable us to draw conclusions about the reliability of the more approximative SDFT and CSDFT schemes in comparison with the VMC method. The same is true for results obtained for the two-electron quantum dot dimer as a function of inter-dot distance. The structure and role of the symmetry-breaking solutions appearing in the SDFT and CSDFT calculations for the above systems are discussed. Received 16 October 2001 and Received in final form 17 January 2002     

Negative Trions Trapped by a Spherical Parabolic Quantum Dot

In this paper, a negatively charged exciton trapped by a spherical parabolic quantum dot has been investigated. The energy spectra of low-lying states are calculated by means of matrix diagonalization. The important feature of the low-lying states of the negatively charged excitons in a spherical quantum dot is obtained via an analysis of the energy spectra.     

Electronic structure of two-electron quantum dot with parabolic potential

In this study, we investigate the parabolic potential effects on the ground and excited energy states of two-electron quantum dot with impurity inside an infinite spherical confining potential well. The wave function and energy eigenvalues were calculated using a modified variational optimization procedure based mainly on quantum genetic algorithm and Hartree–Fock–Roothaan method. The results show that the parabolic potential and impurity charge have a strong effect on the energy states and ionization energies. It is worth pointing out that as impurity charge increases, the ionization energy rises, but the ionization dot radius decreases. On the other hand, as parabolic potential increases, the ionization energy decreases, but the ionization dot radius increases.     

Size effects of an exciton–acceptor complex in quantum dots

In this study, a detailed investigation of the size effects of an exciton–acceptor complex in a disc-like quantum dot has been carried out by using the matrix diagonalization method and the compact density-matrix approach. We calculate the binding energy and the oscillator strength of intersubband quantum transition from the ground state into the first excited state as a function of the dot radius. Based on the computed energies and wave functions, the linear, third-order and total optical absorption coefficients as well as the refractive index have been examined between the ground and the first excited states. We find that the all absorption spectra and refractive index changes are strongly affected by the quantum dot size. However, for two cases of a smaller dot and a larger dot, the results of quantum size effects on the optical absorptions are opposite.     

Photoluminescence of charged magneto-excitons in InAs single quantum dots

We calculated the photoluminescence spectra of charged magneto-excitons in single two-dimensional parabolic quantum dots, using an unrestricted Hartree–Fock method. The calculated luminescence spectra explain well the observed red shifts of transition energies of InAs/GaAs single quantum dot by additional electron capture in a dot. The magnetic-field-induced transition of the ground state configuration of trapped electrons causes drastic change in the photoluminescence spectra. The dependence of photoluminescence intensities of charged excitons on the excess energies of photogenerated carriers above the bulk GaAs energy gap is studied phenomenologically, by calculating the steady state electron population probability in a dot.     

A bound polaron in a spherical quantum dot

The binding energy of a bound polaron in a spherical quantum dot has been investigated by using the variational method. The influence of LO and SO phonons have taken into consideration. Result shows that the phonon contribution to the binding energy is dependent on the size of the quantum dot as well as the position of the impurity in the quantum dot. Numerical calculation on the ZnSe quantum dot shows that such contribution is about 5% to 20% of the total binding energy. Received: 13 October 1997 / Revised: 4 March 1998 / Accepted: 26 May 1998     

Radiative life time of an exciton confined in a strained GaN/Ga1-xAlxN cylindrical dot: built-in electric field effects

The binding energy of an exciton in a wurtzite GaN/GaAlN strained cylindrical quantum dot is investigated theoretically.The strong built-in electric field due to the spontaneous and piezoelectric polarizations of a GaN/GaAlN quantum dot is included.Numerical calculations are performed using a variational procedure within the single band effective mass approximation.Valence-band anisotropy is included in our theoretical model by using different hole masses in different spatial directions.The exciton oscillator strength and the exciton lifetime for radiative recombination each as a function of dot radius have been computed.The result elucidates that the strong built-in electric field influences the oscillator strength and the recombination life time of the exciton.It is observed that the ground state exciton binding energy and the interband emission energy increase when the cylindrical quantum dot height or radius is decreased,and that the exciton binding energy,the oscillator strength and the radiative lifetime each as a function of structural parameters (height and radius) sensitively depend on the strong built-in electric field.The obtained results are useful for the design of some opto-photoelectronic devices.     

The nonlinear optical properties of a magneto-exciton in a strained Ga_(0.2)In_(0.8)As/GaAs quantum dot

The magnetic field-dependent heavy hole excitonic states in a strained Ga0.2In0.8As/GaAs quantum dot are investigated by taking into account the anisotropy,non-parabolicity of the conduction band,and the geometrical confinement.The strained quantum dot is considered as a parabolic dot of InAs embedded in a GaAs barrier material.The dependence of the effective excitonic g-factor as a function of dot radius and the magnetic field strength is numerically measured.The interband optical transition energy as a function of geometrical confinement is computed in the presence of a magnetic field.The magnetic field-dependent oscillator strength of interband transition under the geometrical confinement is studied.The exchange enhancements as a function of dot radius are observed for various magnetic field strengths in a strained Ga0.2In0.8As/GaAs quantum dot.Heavy hole excitonic absorption spectra,the changes in refractive index,and the third-order susceptibility of third-order harmonic generation are investigated in the Ga0.2In0.8As/GaAs quantum dot.The result shows that the effect of magnetic field strength is more strongly dependent on the nonlinear optical property in a low-dimensional semiconductor system.     

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.     

Electron states,effective masses and transverse effective charge of InAs quantum dots

The electron energy levels, direct energy band gaps, electron and hole effective masses as well as the transverse effective charge of InAs spherically shaped quantum dots have been studied as a function of the quantum dot radius considered as varying from 1 to 10 nm. The direct energy band-gap as well as the electron and heavy hole effective masses decrease non-linearly with increasing the quantum dot radius. Nevertheless, the transverse effective charge is found to increase with increasing the quantum dot radius. It is concluded that the quantum confinement has a strong influence on all the studied physical quantities for quantum dot radius below 6 nm. The results of the present contribution show that more opportunities can be offered to tailor desired optoelectronic properties surpassing those presented by bulk InAs materials.