The spectrum of real-space indirect magnetoexcitons

This paper discusses two-dimensional excitons with spatially separated electrons and holes in coupled quantum wells and vertically coupled quantum dots in a transverse magnetic field for a wide range of two characteristic quantities of the problem-the distance between wells or dots d and the magnetic field H. Dispersion relations E _nm(P) are calculated for coupled quantum wells at various values of d and H (where P, the magnetic momentum along the well, is a conserved quantity in the magnetic field). The spectrum is calculated by numerically diagonalizing the Hamiltonian in various bases—either a Coulomb or a Landau basis (the choice of basis is determined by the value of effective magnetic field). Asymptotic energy dependences (with respect to d, H, P) are determined analytically. As the momentum P increases, the exciton spectrum in a weak fixed H crosses over from Coulombic to magnetic, and for large P consists of bands adjacent to the Landau levels. Also discussed is the problem of an electron spatially separated from a charged impurity in coupled quantum wells. Fiz. Tekh. Poluprovodn. 32, 1354–1362 (November 1998)     

Lateral association of vertically coupled quantum dots

The modification produced in the structural and optical properties of vertically coupled In_0.5Ga_0.5As quantum dots in a GaAs matrix by increasing the number of deposited layers of quantum dots has been investigated. It was shown that the deposition of a sequence of In_0.5Ga_0.5As quantum-dot planes separated by narrow (of the order of the height of the quantum dots) GaAs layers gives rise to an interaction between neighboring vertically coupled quantum dots. This interaction shifts the photoluminescence line due to the recombination of nonequilibrium carriers via states of the quantum dots into the region of lower photon energies. Fiz. Tekh. Poluprovodn. 31, 851–854 (July 1997)     

Luminescence properties of InAs/GaAs quantum dots prepared by submonolayer migration-stimulated epitaxy

The results of an investigation of the luminescence properties of an ensemble of InAs quantum dots, obtained by submonolayer migration-stimulated epitaxy on singular and vicinal GaAs(100) surfaces, are reported. The largest width at half-height of the photoluminescence line is observed in samples with a 3° disorientation, indicating that the size-variance of the quantum dots is largest in this case. Quasiequilibrium quantum dots are formed either with a long sample holding time in an arsenic flow or with a larger quantity of deposited indium. Fiz. Tekh. Poluprovodn. 31, 912–915 (August 1997)     

Photoelectric properties of GaAs/InAs heterostructures with quantum dots

The capacitive photovoltage and photoconductivity spectra of GaAs/InAs heterostructures with quantum dots is discussed. For these structures, which were fabricated by metallorganic gas-phase epitaxy, the photosensitivity spectrum has a sawtoothed shape in the wavelength range where absorption by the quantum dots takes place, which is characteristic of a δ-function-like density of states function. The spectra also exhibit photosensitivity bands associated with the formation of single-layer InAs quantum wells in the structure. An expression is obtained for the absorption coefficient of an ensemble of quantum dots with a prespecified size distribution. It is shown that the energy distribution of the joint density of states, the surface density of quantum dots, and the effective cross section for trapping a photon can all be determined by analyzing the photosensitivity spectrum based on this assumption. Fiz. Tekh. Poluprovodn. 31, 1100–1105 (September 1997)     

Influence of composition and anneal conditions on the optical properties of (In, Ga)As quantum dots in an (Al, Ga)As matrix

The optical properties of structures containing InGaAs quantum dots in GaAs and AlGaAs matrices grown by molecular-beam epitaxy are investigated. It is shown that increasing the In content in the quantum dots has the effect of raising the energy of carrier localization and increasing the energy distance between the ground state and the excited states of carriers in the quantum dots. An investigation of the influence of postgrowth annealing on the optical properties of the structures shows that the formation of vertically coupled quantum dots and the use of a wide-gap AlGaAs matrix enhances the thermal stability of the structures. Moreover, high-temperature (830 °C) thermal annealing can improve the quality of the AlGaAs layers in structures with vertically coupled InGaAs quantum dots in an AlGaAs matrix. The results demonstrate the feasibility of using postgrowth annealing to improve the characteristics of quantum dot lasers. Fiz. Tekh. Poluprovodn. 33, 91–96 (January 1999)     

Excitonic molecules trapped by quantum dots and isoelectronic impurities in many-valley semiconductors

Excitonic molecules with two to six excitons trapped by quantum dots or isoelectronic impurity centers are studied theoretically and experimentally for the first time. Such excitonic molecules can exist only in many-valley semiconductors. A model of an excitonic molecule, valid for a large number of valleys in the conduction and valence bands, is examined. The formation kinetics of excitonic molecules is discussed. A material feature of excitonic molecules bound on quantum dots is the existence of tunneling transitions of excitons between quantum dots. Fiz. Tekh. Poluprovodn. 33, 1123–1125 (September 1999)     

Gain in injection lasers based on self-organized quantum dots

The analytical form of the dependence of the gain on pump current density for lasers with an active region based on self-organized quantum dots is derived in a simple theoretical model. The proposed model is shown to faithfully describe experimental data obtained for laser diodes based on InGaAs quantum dots in an AlGaAs/GaAs matrix, as well as InAs quantum dots in an InGaAs/InP matrix. The previously observed gain saturation and switching of the lasing from the ground state to an excited state of the quantum dots are studied. The influence of the density of quantum-dot arrays on the threshold characteristics of lasers based on them is examined on the basis of this model. Fiz. Tekh. Poluprovodn. 33, 215–223 (February 1999)     

Gain characteristics of quantum-dot injection lasers

The current dependence of the optical gain in lasers based on self-organized InGaAs quantum dots in a AlGaAs/GaAs matrix is investigated experimentally. A transition from lasing via the ground state of quantum dots to lasing via an excited state is observed. The saturated gain in the latter case is approximately four times greater than for the ground state. This result is attributable to the fourfold degeneracy of the excited level of quantum dots. The effect of the density of the quantum-dot array on the threshold characteristics is investigated. A lower-density array of dots is characterized by a lower threshold current density in the low-loss regime, because the transmission current is lower, while dense quantum-dot arrays characterized by a high saturated gain are preferable at high threshold gains. Fiz. Tekh. Poluprovodn. 33, 1111–1114 (September 1999)     

Spin injection in GaAs/GaSb quantum-well heterostructures

GaAs/GaSb type-II quantum-dot heterostructures were grown by molecular-beam epitaxy. The circularly polarized photoluminescence of these samples in a magnetic field up to 4.7 T in the Faraday configuration was investigated. It was found that the emission from quantum dots in a magnetic field is σ⁻-polarized, which corresponds to the electron-spin component along the magnetic-field vector of +1/2. The degree of polarization increases with increasing excitation intensity. The observed effect is explained in terms of spin injection from the GaSb matrix, where spin orientation appears owing to the Zeeman splitting of the conduction band. An increase in the degree of polarization occurs due to a reduction in the charge-carrier radiative lifetime in type-II quantum dots with increasing excitation level.     

Photoluminescence of InSb quantum dots in GaAs and GaSb matrices

The photoluminescent properties of quantum dots formed in the deposition of an InSb thin film (1–3 monolayers) on GaAs(100) and GaSb(100) surface are investigated. The results indicate the importance of As-Sb substitution reactions in the formation of quantum dots on a GaAs surface. Fiz. Tekh. Poluprovodn. 31, 68–71 (January 1997)     

Characterization of GaAs/ InxGa1−x As quantum-dot heterostructures by electrical and optical methods

Results of electrical and optical studies of GaAs/In_xGa_1−x As heterostructures are reported. The aim of these studies was to identify the quantum dots and develop a technology of their growth by spontaneous transformation of an In_xGa_1−x As layer. The surface charge at the depth of the quantum dots and their surface density as a function of the deposition time of this narrow-band material are estimated by C-V profiling. A photoluminescence study of the quantum dots revealed peculiarities of the filling of their electron states at various excitation levels. The influence of Coulomb interactions on the optical properties of the quantum dots is discussed. Fiz. Tekh. Poluprovodn. 32, 111–116 (January 1998)     

Continuous stimulated emission at T=293 K from separate-confinement heterostructure diode lasers with one layer of InAs quantum dots grown on vicinal GaAs(bd001) surfaces misoriented in the [010] direction in the active region

The electroluminescence and stimulated emission of lasers with one layer of InAs quantum dots (QD’s) grown in a single molecular-beam epitaxial process on vicinal GaAs(001) surfaces misoriented in the direction [010] by 2, 4 and 6° are investigated. It is discovered that an increase in the misorientation angle leads to a blue shift and a decrease in the full width at half maximum (FWHM) of the electroluminescence spectrum. This effect is attributed to a decrease in the size of the quantum dots and improvement in their size uniformity. A strong dependence of the threshold current density on the width of the spontaneous luminescence spectrum is discovered. The room-temperature threshold current density of the lasers with one layer of quantum dots and the spontaneous luminescence spectrum having the smallest FWHM (54 meV) equals 210 A/cm². Fiz. Tekh. Poluprovodn. 32, 1482–1486 (December 1998)     

X-Ray diffraction analysis of multilayer InAs-GaAs heterostructures with InAs quantum dots

Multilayer InAs-GaAs structures with an array of vertically aligned InAs quantum dots in a GaAs matrix, grown by molecular-beam epitaxy, were investigated by crystal truncation rods and high-resolution x-ray diffractometry methods. It was shown that the formation of scattering objects such as vertically aligned quantum dots in the structures strongly influences the mechanism of diffraction scattering of x-rays and changes the spatial distribution of the diffracted radiation. This is explained by the appearance of additional long-range order in the lateral arrangement of the scattering objects in the periodic structures, by the curving of the crystallographic planes in the periodic part of the structure, and by the quasiperiodicity of the deformation profile due to the vertically coupled quantum dots. The observed spatial distribution of the diffracted intensity can be explained qualitatively on the basis of a new model where the scattering layers with quantum dots consist of defect-free, coherently coupled, InAs and GaAs clusters. Fiz. Tekh. Poluprovodn. 33, 1359–1368 (November 1999)     

The effect of a longitudinal magnetic field on electronic intersubband transitions in asymmetric heterostructures

Infrared absorption spectra are calculated for electronic intersubband transitions in asymmetric heterostructures subjected to a longitudinal magnetic field. The systems discussed include a single quantum well in a transverse electric field, a “stepped” quantum well, and a double well with tunnel-coupled excited levels. The longitudinal magnetic field causes collisionless broadening of the absorption peaks (because the effect of the magnetic field on the dispersion law of an electronic state varies from state to state, which modifies the transitions between them) and absorption of normally incident light (due to changes in the selection rules). Fiz. Tekh. Poluprovodn. 31, 1121–1125 (September 1997)     

Mn2+‐Doped CdSe Quantum Dots: New Inorganic Materials for Spin‐Electronics and Spin‐Photonics

Recent advances in the chemistry of colloidal semiconductor nanocrystal doping have led to new materials showing fascinating physical properties of potential technological importance. This article provides an overview of efforts to dope one of the most widely studied colloidal semiconductor nanocrystal systems, CdSe quantum dots, with one of the most widely studied transition‐metal dopant ions, Mn²⁺, and describes the major new physical properties that have emerged following successful synthesis of this material. These properties include spin‐polarizable excitonic photoluminescence, magnetic circular dichroism, exciton storage, and excitonic magnetic polaron formation. A brief survey of parallel advances in the characterization of analogous self‐assembled Mn²⁺‐doped quantum dots grown by molecular beam epitaxy is also presented, and the physical properties of the colloidal quantum dots are shown to compare favorably with those of the self‐assembled quantum dots. The rich variety of physical properties displayed by colloidal Mn²⁺‐doped CdSe quantum dots highlights the attractiveness of this material for future fundamental and applied research.     

Lasing at a wavelength close to 1.3 µm in InAs quantum-dot structures

The feasibility of lasing at a wavelength close to 1.3 μm is demonstrated in InAs quantum-dot structures placed in an external InGaAs/GaAs quantum well. It is shown that the required wavelength can be attained with the proper choice of thickness of the InAs layer deposited to form an array of three-dimensional islands and with a proper choice of mole fraction of InAs in the InGaAs quantum well. Since the gain attained in the ground state is insufficient, lasing is implemented through excited states in the temperature interval from 85 K to 300 K in a structure based on a single layer of quantum dots. The maximum attainable gain in the laser structure can be raised by using three rows of quantum dots, and this configuration, in turn, leads to low-threshold (70 A/cm²) lasing through the ground state at a wavelength of 1.26 μm at room temperature. Fiz. Tekh. Poluprovodn. 33, 1020–1023 (August 1999)     

Quantum-dot lasers: Principal components of the threshold current density

Injection heterolasers based on quantum dots grown by molecular-beam epitaxy have been investigated. It is shown that the room-temperature threshold current density can be lowered to 15 A/cm² by decreasing the nonradiative recombination and increasing the degree of carrier localization. The density of states in structures with vertically coupled quantum dots was investigated by the electroabsorption method. Fiz. Tekh. Poluprovodn. 31, 1106–1108 (September 1997)     

Long-time photoluminescence kinetics of InAs/AlAs quantum dots in a magnetic field

The influence of a magnetic field on the low-temperature photoluminescence (PL) kinetics of InAs/AlAs quantum dots is studied. It is found that the PL decay becomes faster upon application of the magnetic field. The results obtained are explained in the context of a model that considers the fine structure of exciton levels and their Zeeman splitting in the magnetic field.     

Injection heterolaser based on an array of vertically aligned InGaAs quantum dots in a AlGaAs matrix

Arrays of vertically aligned InGaAs quantum dots in a AlGaAs matrix have been investigated. It is shown that increasing the band gap of the matrix material makes it possible to increase the localization energy of quantum dots relative to the edge of the matrix band, as well as the states of the wetting layer. The use of an injection laser as the active region makes it possible to decrease the thermal filling of higher-lying states, and thereby decrease the threshold current density to 63 A/cm² at room temperature. A model explaining the negative characteristic temperature section observed at low temperatures is proposed. The model is based on the assumption that a transition occurs from nonequilibrium to equilibrium filling of the states of the quantum dots. Fiz. Tekh. Poluprovodn. 31, 483–487 (April 1997)