Quasi-stationary states of electrons and holes in an open composite cylindrical quantum wire

The energies of the quasi-stationary states of electrons and holes in an open composite cylindrical quantum wire are calculated within the effective-mass approximation by means of the S-matrix theory. Specific calculation is carried out for the HgS/CdS/HgS system. The poles of the S matrix in the complex energy plane are studied. The dependences of the lifetimes of quasiparticles in quasi-stationary resonance states on the longitudinal quasi-momentum and geometric parameters of the nanosystem are obtained. It is shown that the quasiparticle lifetimes in the resonance states exponentially diminish as the longitudinal quasi-momentum increases.     

Prediction of an excitonic ground state in InAs/InSb quantum dots

Using atomistic pseudopotential and configuration-interaction many-body calculations, we predict an excitonic ground state in the InAs/InSb quantum-dot system. For large dots, the conduction band minimum of the InAs dot lies below the valence band maximum of the InSb matrix. Due to quantum confinement, at a critical size calculated here for various shapes, the gap E(g) between InAs conduction states and InSb valence states vanishes. Strong electron-hole correlation effects are induced by the spatial proximity of the electron and hole wave functions, and by the lack of strong (exciton unbinding) screening, afforded by the existence of discrete 0D confined energy levels. These correlation effects overcome E(g), leading to the formation of a biexcitonic ground state (two electrons in InAs and two holes in InSb) being energetically more favorable (by approximately 15 meV) than the dot without excitons.     

Charge and energy transfer in double asymmetric quantum wells with quantum dots

The luminescence and luminescence excitation spectra of CdSe/ZnSe quantum dots are studied in a set of double quantum wells with the ZnSe barrier of width 14 nm, the same amount of a deposited CdSe layer forming a deep well and shallow wells with different depths. It is found that for a certain relation between the depths of shallow and deep wells in this set, conditions are realized under which the exciton channel in the luminescence excitation spectrum of a shallow well dominates in the region of kinetic exciton energies exceeding 10 longitudinal optical phonons above the bottom of the exciton band of the ZnSe barrier. A model is developed for the transfer of electrons, holes, and excitons between the electronic states of shallow and deep quantum wells separated by wide enough barriers. It is shown that the most probable process of electronic energy transfer between the states of shallow and deep quantum wells is indirect tunneling with the simultaneous excitation of a longitudinal optical phonon in the lattice. Because the probability of this process for single charge carriers considerably exceeds the exciton tunneling probability, a system of double quantum wells can be prepared in which, in the case of weak enough excitation, the states of quantum dots in shallow quantum wells will be mainly populated by excitons, which explains experimental results obtained.     

Surface integral determination of built-in electric fields and analysis of exciton binding energies in nitride-based quantum dots

We present a simple analytical approach to calculate the built-in strain-induced and spontaneous piezoelectric fields in nitride-based quantum dots (QDs) and then apply the method to describe the variation of exciton, biexciton and charged exciton energy with dot size in GaN/AlN QDs. We first present the piezoelectric potential in terms of a surface integral over the QD surface, and confirm that, due to the strong built-in electric field, the electrons are localised near the QD top and the holes are localised in the wetting layer just below the dot. The strong localisation and smaller dielectric constant results in much larger Coulomb interactions in GaN/AlN QDs than in typical InAs/GaAs QDs, with the interaction between two electrons, J_ee, or two holes, J_hh, being about a factor of three larger. The electron–hole recombination energy is always blue shifted in the charged excitons, X⁻ and X⁺, and the biexciton, and the blue shift increases with increasing dot height. We conclude that spectroscopic studies of the excitonic complexes should provide a useful probe of the structural and piezoelectric properties of GaN-based QDs.     

圆柱状量子点量子导线复合系统的激子能量和电子概率分布

建立了圆柱状量子点量子导线复合系统中激子满足的方程，用微扰论求出激子能量.以CdS/HgS/CdS/HgS/CdS圆柱状量子点量子导线复合系统为例，研究了系统中电子的概率分布和系统线度对激子能量的影响.结果表明：系统中电子、空穴以及激子的能量均随量子点高度h₀的增大而减小，电子-空穴相互作用对基态激子能量的影响要大于激发态；电子沿径向方向的概率分布呈起伏状，在轴线和表面附近的概率趋于零，而在R/2附近概率最大；在量子点附近电子沿轴向方向的概率分布呈振荡特征 关键词： 量子点 量子导线 激子 能量     

Excitons in CdS and CdSe semiconducting quantum wires with dielectric barriers

Features of the photoluminescence spectra observed for various polarizations and intensities of the pumping radiation and the kinetics of photoluminescence of the CdS and CdSe nanocrystals grown in hollow nanochannels of an Al₂O₃ matrix are explained in terms of exciton transitions in semiconducting quantum wires with dielectric barriers. The observed exciton transition energies coincide with the values calculated with an allowance for the effects of quantum confinement and the “dielectric enhancement” of excitons. The latter effect is manifested by a significant increase in the Coulomb attraction between electrons and holes (the exciton binding energy exceeds 100 meV) due to a difference between the permittivities of semiconductor and insulator. It is shown that the exciton transition energy remains constant when the quantum wire diameter varies within broad limits. This is related to the fact that a growth in the one-dimensional bandgap width of the quantum wire caused by a decrease in the diameter is compensated by an increase in the exciton binding energy.     

Stark shifts of charged particles in semiconductor quantum wells

 We calculate the effect of a homogeneous electric field on electrons, holes and excitons confined in a quantum well structure consisting of alternate thin layers of well and barrier material. The electric field which acts perpendicular to the quantum well is taken as a perturbation on the quantum well structure confining the charges. The electron and hole energies in the conduction and valence subbands are calculated by solving a one-dimensional Schr?dinger equation. The exciton binding energy is calculated using an improved excitonic model. Results obtained indicate the importance of higher-order excitons in optical transitions at high electric fields. Received: 29 February 1996/Accepted: 19 August 1996     

Effect of a Coulomb well in (In,Ga)As/GaAs quantum wells

Magnetooptical investigation of exciton transitions in high-quality quantum wells of an (In, Ga)As/GaAs heterosystem has been carried out. Investigation of transmission of free-hanging samples detached from the substrate in the magnetic fields of up to 12 T revealed a rich fine structure associated with various heavy-hole and light-hole exciton transitions. In particular, transitions from the excited states of light holes localized in a Coulomb potential produced by an electron along the heterojunction axis (a Coulomb well) have been detected. Taking into account consistently stresses, formation of Landau levels, the binding energies of excitons (diamagnetic excitons), and the effect of a Coulomb well, we have succeeded to describe the experimental results with the use of a self-consistent variational procedure. As a result, new features in the structure of optical transitions have been explained and the effective masses of electrons and holes of excitons formed by both heavy and light holes have been determined with a high accuracy.     

耦合量子阱中激子凝聚的研究进展

近年来，半导体量子阱中激子的玻色一爱因斯坦凝聚研究取得了很大进展．实验上利用耦合量子阱间接激子中电子和空穴在空间上的分离，显著提高了激子的冷却速度和寿命，成功地把激子冷却到1K以下，观察到了激子的准凝聚状态，并且在强激光照射下，发现了随光照强度增强而增大的激子发光环和环上形成的有规则斑点图案，引起了广泛的兴趣和重视．理论研究表明，发光环的出现是电子和空穴在量子阱中的反常输运行为造成的，但环上形成规则斑点的物理机理目前尚不清楚．文章介绍了这方面的实验背景和形成激子环的物理图像，指出了理论研究中存在的问题，并对解决问题的方案进行了讨论．     

The donor bound exciton states in wurtzite GaN quantum dot

Based on the effective mass approximation, the donor bound exciton states in a wurtzite (WZ) GaN/AlGaN quantum dot (QD) are investigated by means of a variational method, including the strong built-in electric field effect due to the spontaneous and piezoelectric polarizations. Numerical results show that the donor bound exciton binding energy is highly dependent on the impurity position and QD size. In particular, we find that the donor bound exciton binding energy is insensitive to dot height when the impurity is located at the right boundary of the WZ GaN/AlGaN QD with large dot height.     

Coulomb binding of electrons to multiply charged GaSb/GaAs self-assembled quantum dots

We explore the Coulomb binding of electrons to holes confined to type-II GaSb self-assembled quantum dots. We demonstrate that at low laser power electrons are more weakly bound to holes trapped by the dots than to holes in the wetting layer. On the other hand, at high laser power the hydrogenic binding energy of dot excitons increases by more than a factor of two, and so exceeds that of wetting layer excitons. We attribute this to the strong binding of ‘core’ electrons to dots that are highly charged with holes by optical pumping.     

Direct and spatially indirect excitons in GaAs/AlGaAs superlattices in strong magnetic fields

Luminescence and luminescence excitation spectra are used to study the energy spectrum and binding energies of direct and spatially indirect excitons in GaAs/AlGaAs superlattices having different electron and hole miniband widths in high magnetic fields perpendicular to the heterolayers. The ground state of the indirect excitons formed by electrons and holes which are spatially distributed among neighboring quantum wells is found to lie between the ground 1s state of the direct excitons and the threshold of the continuum of dissociated exciton states in the minibands. The indirect excitons have a substantial oscillator strength when the binding energy of the exciton exceeds the scale of the width of the resulting miniband. It is shown that a high magnetic field shifts a system of symmetrically bound quantum wells toward weaker bonding. At high exciton concentrations, spatially indirect excitons are converted into direct excitons through exciton-exciton collisions. Fiz. Tverd. Tela (St. Petersburg) 40, 833–836 (May 1998)     

Self-consistent calculations of exciton, biexciton and charged exciton energies in InGaN/GaN quantum dots

We present theoretical calculations of the variation of exciton energies in truncated conical InGaN quantum dots (QDs) in a GaN matrix with dot size and indium composition. We compute the built-in strain-induced and spontaneous piezoelectric fields using a surface integral method that we have recently derived, and confirm that the built-in fields can be of the order of a few MV/cm, resulting in a spatial separation of the electrons and holes. The ground state wavefunctions of the exciton (X⁰), biexciton (2X⁰) and the two charged excitons (X⁻ and X⁺) are then calculated in the Hartree approximation, using a self-consistent finite difference method. We find that the electron–hole recombination energy is always blue-shifted for the charged excitons X⁻ and X⁺, with a further blue-shift for the biexciton, and this blue-shift increases with increasing indium content. We describe the trends in interband transition energy and the scale of the blue-shift with dot size, shape and composition. We conclude that spectroscopic studies of the exciton, charged excitons and biexciton should provide a useful probe of the structural and piezoelectric properties of GaN-based QDs.     

Direct and spatially indirect excitons in GaAs/AlGaAs superlattices in strong magnetic fields

Luminescence and luminescence excitation spectra are used to study the energy spectrum and binding energies of direct and spatially indirect excitons in GaAs/AlaAs superlattices, with different widths of the electron and hole minibands, located in a high magnetic field perpendicular to the heterolayers. It is found that the ground state of the indirect excitons formed by electrons and holes and spatially separated between neighboring quantum wells lies between the ls ground state of the direct excitons and the continuum threshold for dissociated exciton states in the minibands. Indirect excitons in superlattices have a significant oscillator strength when the binding energy of the exciton exceeds the order of the width of the resulting miniband. The behavior of the binding energy of direct and indirect heavy hole excitons during changes in the tunneling coupling between the quantum wells is established. It is shown that a strong magnetic field, which intensifies the Coulomb interaction between the electron and hole in an exciton, weakens the bond in a system of symmetrically bound quantum wells. The spatially indirect excitons studied here are analogous to first order Wannier-Stark localized excitons in superlattices with inclined bands (when an electrical bias is applied), but in the present case the localization is of purely Coulomb origin. Zh. éksp. Teor. Fiz. 112, 1106–1118 (September 1997)     

Temperature and density dependence of exciton lifetimes in GaAs/AlGaAs multiple quantum wells

The photoluminescence linewidths and excition lifetimes of free excitons in GaAs/AlGaAs multiple quantum wells were systematically investigated as a function of temperature, quantum well width, and carrier density. The experimental results showed that the exciton decay processes were strongly related to the linewidth of the exciton and the exciton binding energy.     

Charged magneto-exciton states in semiconductor quantum dots

By embedding a layer of self-assembled quantum dots into a field-effect structure, we are able to control the exciton charge in a single dot. We present the results of photoluminescence experiments as a function of both charge and magnetic field. The results demonstrate a hierarchy of energy scales determined by quantization, the direct Coulomb interaction, the electron–electron exchange interaction, and the electron–hole exchange interaction. For excitons up to the triply charged exciton, the behavior can be understood from a model assuming discrete levels within the quantum dot. For the triply charged exciton, this is no longer the case. In a magnetic field, we discover a coherent interaction with the continuum states, the Landau levels associated with the wetting layer.     

Quasistationary states of an electron in a spherical β-HgS/β-CdS/β-HgS nanoheterosystem

The electron scattering matrix for spherically symmetric states in a spherical β-HgS/β-CdS/β-HgS nanoheterosystem is calculated. The positions of the energy levels and the lifetimes of an electron in the corresponding states are found as functions of the geometric parameters of the system and analyzed. Fiz. Tverd. Tela (St. Petersburg) 41, 2081–2083 (November 1999)     

Control of structural and excitonic properties of self-organized InAs/GaAs quantum dots

A systematic dependence of excitonic properties on the size of self-organized InAs/GaAs quantum dots is presented. The bright exciton fine-structure splitting changes from negative values to more than 0.5 meV, and the biexciton binding energy varies from antibinding to binding, as the height of truncated pyramidal dots increases from 2 to above 9 InAs monolayers. A novel mode of metalorganic vapor phase epitaxy was developed for growing such quantum dots with precise shape control. The dots consist of pure InAs and feature heights varying in steps of complete InAs monolayers. Such dot ensembles evolve from a strained, rough two-dimensional layer with a thickness close to the critical value for the onset of the 2D–3D transition. Dots with a common height represent subensembles with small inhomogeneous broadening. Tuning of subensemble emission energy is achieved by varying the mean lateral extension of the respective QDs. Detailed knowledge of the structural properties of individual dots enable realistic k·p calculations to analyze the origin of the observed excitonic properties. The binding energies of charged and neutral excitons increase due to correlation by the gradually increasing number of bound states for increasing dot size. The monotonously increasing magnitude of the fine-structure splitting with dot size is shown to be caused by piezoelectricity. The identification of key parameters allows to tailor exciton properties, providing a major step towards the development of novel applications.     

Coherent spin precession of electrons and excitons in charge tunable InP quantum dots

Coherent spin precession of electrons and excitons is observed in charge tunable InP quantum dots under the transverse magnetic field by means of time-resolved Kerr rotation. In a quantum dot doped by one electron, spin precession of the doped electron in the quantum dot starts out of phase with spin precession of the doped electrons in a GaAs substrate just after a trion is formed and persists for more than 2 ns even after the trion recombines. Simultaneously spin precession of a trion (hole) starts. Observation of spin precession of both a doped electron and a trion (hole) confirms creating coherent superposition of an electron and a trion as the initialization process of spin of doped electrons in quantum dots. In a neutral quantum dot, the exciton spin precession starts out of phase with spin precession of the doped electrons in a GaAs substrate and the precession frequency does not converge to 0 at the zero field limit. It contains the electron–hole exchange interaction and corresponds to the splitting between bright and dark excitons under the transverse magnetic field.     

Magnetic-field effects on exciton–polaron in a CdSe quantum disk

We study theoretically the interaction between excitons and longitudinal optical (LO) phonons in a cylindrical disk-like semiconductor quantum dot under an applied magnetic field. Due to the intensity of the interaction in the strong coupling regime, a composite quasi-particle called exciton–polaron is formed. We focus on the effect of the disk size and an external magnetic field on the exciton–phonon interaction energy and the exciton–polaron modes. The numerical computation for a CdSe quantum disk have shown that the exciton–phonon interaction energy is very significant and is even dominant when the disk height is small, which leads to a large Rabi splitting between the exciton–polaron modes. We investigate also the effect of the temperature on the integrated photoluminescence (PL) intensity, and show that at relatively high temperature the LO phonons have a noticeable effect on it. This physical parameter also shows a great dependence on quantum disk size and on magnetic field.