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Showing papers on "Quantum dot published in 1993"


Journal ArticleDOI
TL;DR: In this paper, the 2D-3D growth mode transition during the initial stages of growth of highly strained InGaAs on GaAs is used to obtain quantum-sized dot structures.
Abstract: The 2D–3D growth mode transition during the initial stages of growth of highly strained InGaAs on GaAs is used to obtain quantum‐sized dot structures. Transmission electron micrographs reveal that when the growth of In0.5Ga0.5As is interrupted exactly at the onset of this 2D–3D transition, dislocation‐free islands (dots) of the InGaAs result. Size distributions indicate that these dots are ∼300 A in diameter and remarkably uniform to within 10% of this average size. The areal dot densities can be varied between 109 and 1011 cm−2. The uniformity of the dot sizes is explained by a mechanism based on reduction in adatom attachment probabilities due to strain. We unambiguously demonstrate photoluminescence at ∼1.2 eV from these islands by comparing samples with and without dots. The luminescent intensities of the dots are greater than or equal to those of the underlying reference quantum wells.

1,482 citations


Journal ArticleDOI
TL;DR: In this paper, a review is concerned with quantum confinement effects in low-dimensional semiconductor systems, focusing on the optical properties, including luminescence, of nanometre-sized microcrystals.
Abstract: This review is concerned with quantum confinement effects in low-dimensional semiconductor systems. The emphasis is on the optical properties, including luminescence, of nanometre-sized microcrysta...

1,030 citations


Journal ArticleDOI
TL;DR: The luminescence in the visible range of porous silicon is analyzed in the hypothesis of quantum confinement and it is concluded that experimental nonradiative processes in porous silicon are more efficient than calculated radiative ones at T=300 K.
Abstract: The luminescence in the visible range of porous silicon is analyzed in the hypothesis of quantum confinement. We calculate the electronic and optical properties of silicon crystallites and wires with sizes between 0 and 4.5 nm. The band-gap energies of such confined systems are in agreement with the photon energies observed in luminescence. We calculate the radiative recombination times of the confined excitons. We conclude that experimental nonradiative processes in porous silicon are more efficient than calculated radiative ones at T=300 K. The high photoluminescence efficiency of porous silicon is due to the small probability of finding a nonradiative recombination center in silicon nanocrystallites. Recently, it has been proposed that the low-temperature dependence of the experimental radiative decay time of the luminescence of porous silicon could be explained by the exchange splitting in the fundamental exciton. We show that the influence of the valley-orbit splitting cannot be excluded. The sharp optical-absorption edge above 3.0 eV is not proof of the molecular origin of the properties of porous silicon because silicon nanostructures present a similar absorption spectrum. We calculate the nonradiative capture of electrons or holes on silicon dangling bonds and show that it is very dependent on the confinement. We find that the presence of one dangling bond at the surface of a crystallite in porous silicon must destroy its luminescent properties above 1.1 eV but can produce a luminescence below 1.1 eV due to a radiative capture on the dangling bond.

860 citations


Journal ArticleDOI
19 Nov 1993-Science
TL;DR: Size-selective precipitation and size-exclusion chromatography cleanly separate the silicon nanocrystals from larger crystallites and aggregates and provide direct evidence for quantum confinement in luminescence.
Abstract: The dynamics and spectroscopy of silicon nanocrystals that emit at visible wavelengths were analyzed. Size-selective precipitation and size-exclusion chromatography cleanly separate the silicon nanocrystals from larger crystallites and aggregates and provide direct evidence for quantum confinement in luminescence. Measured quantum yields are as high as 50 percent at low temperature, principally as a result of efficient oxide passivation. Despite a 0.9—electron-volt shift of the band gap to higher energy, the nanocrystals behave fundamentally as indirect gap materials with low oscillator strength.

718 citations


Journal ArticleDOI
TL;DR: The infinite-U Anderson model is applied to non-equilibrium transport through a quantum dot containing two spin levels weakly coupled to two leads, leading to an observable peak in the differential conductance when the non-Equilibrium bias equals the Zeeman energy.
Abstract: The infinite-U Anderson model is applied to nonequilibrium transport through a quantum dot containing two spin levels weakly coupled to two leads. At low temperatures, the Kondo peak in the equilibrium density of states is split upon the application of a voltage bias. The split peaks, one at the chemical potential of each lead, are suppressed by nonequilibrium dissipation. In a magnetic field, the Kondo peaks shift away from the chemical potentials by the Zeeman energy, leading to an observable peak in the differential conductance when the nonequilibrium bias equals the Zeeman energy.

696 citations


Journal ArticleDOI
TL;DR: In this article, the energy levels for the electron and the hole are calculated with the spherical confinement, the nonparabolicity of the conduction band, and the valence band degeneracy taken into account.
Abstract: CdSe is used as a prototype to show the implications of valence-band degeneracy for the optical properties of strongly quantum-confined nanocrystals. Absorption spectra and photoluminescence spectra obtained under intermediate and strong pulsed excitation show the presence of new structures. The energy levels for the electron and the hole are calculated with the spherical confinement, the nonparabolicity of the conduction band, and the valence band degeneracy taken into account. The oscillator strengths of the dipole-allowed transitions are also calculated. This model is found to be in good agreement with the experimental observations, which originate mainly from the quantization of the energy spectrum of holes with due account given to valence-band degeneracy.

681 citations


Journal ArticleDOI
TL;DR: The behavior of a laterally confined quantum dot in close proximity to a one-dimensional channel in a separate electrical circuit is investigated, finding the activation energy of transport through the dot is much lower than expected.
Abstract: We have investigated the behavior of a laterally confined quantum dot in close proximity to a one-dimensional channel in a separate electrical circuit. When this channel is biased in the tunneling regime the resistance is very sensitive to electric fields, and therefore is sensitive to the potential variations on the dot when it is showing Coulomb blockade oscillations. This effect can be calibrated directly, allowing the Coulomb charging energy to be measured. We also found the activation energy of transport through the dot is much lower than expected.

577 citations


Journal ArticleDOI
Raymond Ashoori1, H. L. Stormer1, J. S. Weiner1, Loren Pfeiffer1, K. W. Baldwin1, Ken W. West1 
TL;DR: Using single-electron capacitance spectroscopy, the magnetic field dependence of the ground state energies of a single quantum dot containing from 0 to 50 electrons is mapped.
Abstract: Using single-electron capacitance spectroscopy, we map the magnetic field dependence of the ground state energies of a single quantum dot containing from 0 to 50 electrons. The experimental spectra reproduce many features of a noninteracting electron model with an added fixed charging energy. However, in detailed observations deviations are apparent: Exchange induces a two-electron singlet-triplet transition, self-consistency of the confinement potential causes the dot to assume a quasi-two-dimensional character, and features develop which are suggestive of the fractional quantum Hall effect.

438 citations


Journal ArticleDOI
TL;DR: The electronic structure of nanocrystalline Si which shows visible photoluminescence is calculated using the density-functional approach for finite structures, and results for clusters suggest that the band gap scales linearly with L, where L is the cluster diameter.
Abstract: The electronic structure of nanocrystalline Si which shows visible photoluminescence is calculated using the density-functional approach for finite structures. Except for geometry this is the same theory as for first-principles band structures of semiconductors and other solids. Our results for clusters ranging up to 706 Si atoms suggest that the band gap scales linearly with ${\mathit{L}}^{\mathrm{\ensuremath{-}}1}$, where L is the cluster diameter. For such clusters it is found that dipole transitions across the gap are symmetry allowed. The finite structures thus show a direct band gap which is considerably larger than the one of bulk silicon. For larger clusters we find a strong decrease of oscillator strength, consistent with the occurrence of the indirect gap in the bulk limit.

421 citations


Book
01 Oct 1993
TL;DR: In this paper, the authors present an overview of the background and recent developments in the rapidly growing field of ultrasmall semiconductor microscrystallites, in which the carrier confinement is sufficiently strong to allow only quantized states of the electrons and holes.
Abstract: "Semiconductor Quantum Dots" presents an overview of the background and recent developments in the rapidly growing field of ultrasmall semiconductor microscrystallites, in which the carrier confinement is sufficiently strong to allow only quantized states of the electrons and holes. The main emphasis of this book is the theoretical analysis of the confinement induced modifications of the optical and electronic properties of quantum dots in comparison to extended materials. The book develops the theoretical background material for the analysis of carrier quantum-confinement effects, it introduces different confinement regimes for absolute or center-of-mass motion quantization of the electron-hole-pairs, and it gives an overview of the best approximation schemes for each regime. A detailed discussion of the carrier states in quantum dots is presented, including variational calculations, a configuration interaction approach, and quantum Monte Carlo techniques. Surface polarization instabilities are analyzed which lead to the self-trapping of carriers near the surface of the dots and the influence of spin-orbit coupling on the quantum-confined carrier states is discussed. The linear and nonlinear optical properties of small and large quantum dots are analyzed in detail, including transient optical nonlinearities (photon echo) and two-photon transitions. The influence of the quantum-dot size distribution in many realistic samples is outlined, including the analysis of quantum dot growth laws and universal size distributions. Phonons in quantum dots, as well as the influence of external electric or magnetic fields are discussed. The recent developments dealing with regular systems of quantum dots are reviewed, including a lattice model of quantum dots and quantum dot superlattices.

408 citations


Journal ArticleDOI
TL;DR: In this paper, a controlled synthesis of quantized colloidal CdTe nanocrystals (in aqueous solutions) with narrow size distributions and stabilized against rapid oxidation was achieved by capping the quantum dot particles with 3-mercapto-1,2-propanediol.
Abstract: The controlled synthesis of quantized colloidal CdTe nanocrystals (in aqueous solutions) with narrow size distributions and stabilized against rapid oxidation was achieved by capping the quantum dot particles with 3-mercapto-1,2-propanediol. Nanocrystals (i.e., quantum dots) with mean diameters of 20, 25, 35, and 40 A were produced. Optical absorption spectra showed strong excitonic peaks at the smallest size; the absorption coefficient was shown to follow an inverse cube dependence on particle diameter, while the extinction coefficient per particle remained constant. The quantum yield for photoluminescence increased with decreasing particle size and reached 20% at 20 A. The valence band edges of the CdTe quantum dots were determined by pulse radiolysis experiments (hole injection from oxidizing radicals); the bandgaps were estimated from pulse radiolysis data (redox potentials of hole and electron injecting radicals) and from the optical spectra. The dependence of the CdTe bandgap on quantum dot size was found to be much weaker than predicted by the effective mass approximation; this result is consistent with recently published theoretical calculations by several groups. 36 refs., 5 figs., 1 tab.

Journal ArticleDOI
TL;DR: In this article, a simple model quantum dot cell containing two electrons is analyzed as a candidate for quantum cellular automata implementations and the cell has eigenstates whose charge density is strongly aligned along one of two directions.
Abstract: A simple model quantum dot cell containing two electrons is analyzed as a candidate for quantum cellular automata implementations. The cell has eigenstates whose charge density is strongly aligned along one of two directions. In the presence of the electrostatic perturbation due to a neighboring cell, the ground state is nearly completely aligned (polarized) in one direction only. The polarization is a highly nonlinear function of the perturbing electrostatic fields and shows the strong bistable saturation important for cellular automation function.

Journal ArticleDOI
TL;DR: Modulation spectroscopy is a powerful method for the study and characterization of a large number of semiconductor configurations, including bulk/thin film, microstructures (heterojunctions, quantum wells, superlattices, quantum dots), surfaces/interfaces and actual device structures in addition to semiconductor growth/processing as mentioned in this paper.
Abstract: Modulation spectroscopy is a powerful method for the study and characterization of a large number of semiconductor configurations, including bulk/thin film, microstructures (heterojunctions, quantum wells, superlattices, quantum dots), surfaces/interfaces and actual device structures in addition to semiconductor growth/processing. Furthermore, the influence of external perturbations such as temperature, electric fields, hydrostatic pressure, uniaxial stress, etc. can be investigated. This optical technique utilizes a very general principle of experimental physics, in which a periodically applied perturbation (either to the sample or probe) leads to sharp, derivative-like spectral features in the optical response of the system. Because of the richness of the derivative-like spectra, the information in the lineshape fits, room temperature performance and relative simplicity of operation this method is becoming increasingly more important as a tool to study these materials and structures. This article will review developments in the field during the last decade.

Journal ArticleDOI
TL;DR: In this paper, a general scheme was established within the effective mass approximation to calculate systematically the excitonic energy spectra in a semiconductor quantum dot including the dielectric confinement effect.
Abstract: A general scheme is established within the effective-mass approximation to calculate systematically the excitonic energy spectra in a semiconductor quantum dot including the dielectric confinement effect. This effect is found to appear most pronounced in the quantum-dot structure in comparison with the quantum-well and quantum-wire structures. A formula of the lowest exciton energy in the strong confinement regime is derived and the significance of the dielectric confinement effect is clarified. We investigate the dependence of the binding energy and the oscillator strength of the lowest-energy excitonic state on the quantum-dot radius, the electron-to-hole mass ratio, and the dielectric-constant ratio between the quantum dot and the surrounding medium. The subband mixing effect due to the electron-hole Coulomb interaction gives a finite oscillator strength to excitonic transitions which are forbidden in the absence of the Coulomb interaction. This effect is shown unambiguously in the calculated excitonic energy spectra. Furthermore, the electron-hole exchange interaction in a quantum dot is discussed. The short-range part of the exchange energy is shown to increase in proportion to the inverse of the volume of the quantum dot as the quantum-dot size is reduced. On the other hand, the long-range part of the exchange energy is found to be sensitively dependent on the shape of the quantum dot. In particular, it vanishes for the optically allowed excitonic states in a spherical quantum dot.

Journal ArticleDOI
TL;DR: The behavior of linear arrays of cells composed of quantum dots is examined in this paper, where the electrons in the cell tend to align along one of two axes resulting in a cell 'polarization' which can be used to encode binary information.
Abstract: The behavior of linear arrays of cells composed of quantum dots is examined. Each cell holds two electrons and interacts Coulombically with neighboring cells. The electrons in the cell tend to align along one of two axes resulting in a cell ‘‘polarization’’ which can be used to encode binary information. The ground‐state polarization of a cell is a highly nonlinear function of the polarization of its neighbors. The resulting bistable saturation can be used to transmit binary information along the line of cells, thus forming a binary wire.

Proceedings Article
02 May 1993
TL;DR: Microscopic photoluminescence (PL) reveals a splitting and a blueshift, which depend systematically on dot size, which are attributed to recombination between zero-dimensional electron and hole states.
Abstract: Optical spectroscopy in one-dimensional (ID) and zero-dimensional (OD) semiconductor structures has been applied in recent years to investigate the peaked density of states.1-4 However, inhomogeneous broadening of the photoluminescence (PL) line caused by fluctuations in composition and structure size within arrays of ID and 0D systems made a clear identification of lateral quantization difficult. This inhomogeneous broadening is eliminated in spectroscopy of a single OD system.

Journal ArticleDOI
TL;DR: Nanometer-size semiconductor particles coated with another semiconductor can exhibit unusual and interesting phenomena associated with the redistribution of the electron and hole wave functions.
Abstract: Nanometer-size semiconductor particles coated with another semiconductor can exhibit unusual and interesting phenomena associated with the redistribution of the electron and hole wave functions. Using the band offsets and effective masses, the overlap of the electron and hole wave functions can be altered by changing the core radius of the particles. The theory can incorporate multiple shells, band bending, and charge effects. An efficient method for solving the equations is given.

Journal ArticleDOI
TL;DR: In this article, a wet chemical synthetic route is presented for the preparation of a spherical quantum well, which consists of a core of size-quantized CdS, a monolayer of HgS and approximately four monolayers of CcS as the outermost shell.

Patent
19 Feb 1993
TL;DR: In this paper, the active channel region of a heterojunction field effect device uses alternating layers of pure silicon and germanium, which form a short period superlattice with the thickness of each layer in the super lattice being no greater than the critical thickness for maintaining a strained heterjunction.
Abstract: Carrier mobility in a heterojunction field effect device is increased by reducing or eliminating alloy scattering. The active channel region of the field effect device uses alternating layers of pure silicon and germanium which form a short period superlattice with the thickness of each layer in the superlattice being no greater than the critical thickness for maintaining a strained heterojunction. The gate contact of the field effect device can comprise quantum Si/Ge wires which provide quantum confinement in the growth plane, thereby allowing the field effect device to further improve the mobility by restricting phonon scattering. The structure can be used to improve device speed performance.

Journal ArticleDOI
TL;DR: In this paper, the minority electron concentration as a function of temperature in the range 100-300 K has been studied and confirmed the long-standing theoretical prediction that quantum confinement should convert Bi from a semimetal to a semiconductor at a critical thickness on the order of 300 \AA{}.
Abstract: Field- and temperature-dependent magnetotransport measurements on Bi layers grown by molecular-beam epitaxy have been analyzed by mixed-conduction techniques. In the thin-film limit, the net hole density scales inversely with layer thickness while the mobility scales linearly. By studying the minority electron concentration as a function of temperature in the range 100--300 K, we have unambiguously confirmed the long-standing theoretical prediction that quantum confinement should convert Bi from a semimetal to a semiconductor at a critical thickness on the order of 300 \AA{}.

Journal ArticleDOI
01 Apr 1993-EPL
TL;DR: In this article, the thermopower of a quantum dot, defined in the two-dimensional electron gas in a GaAs-AlxGa1-xAs heterostructure, was investigated using a current heating technique.
Abstract: The thermopower of a quantum dot, defined in the two-dimensional electron gas in a GaAs-AlxGa1-xAs heterostructure, is investigated using a current heating technique. At lattice temperatures kBT much smaller than the charging energy e2/C, and at small heating currents, sawtoohlike thermopower oscillations are observed as a function of gate voltage, in agreement with a recent theory. In addition, a remarkable sign reversal of the amplitude of the thermopower oscillations is found in the non-linear regime at large heating currents.

Journal ArticleDOI
TL;DR: In this article, a model quantum dot cells are investigated as potential building blocks for quantum cellular automata architectures and the effects of nonzero temperature on the response of a model cell are investigated.
Abstract: Model quantum dot cells are investigated as potential building blocks for quantum cellular automata architectures. Each cell holds a few electrons and interacts Coulombically with nearby cells. In acceptable cell designs, the charge density tends to align along one of two cell axes. Thus, a cell ‘‘polarization,’’ which can be used to encode binary information, is defined. The polarization of a cell is affected in a very nonlinear manner by the polarization of its neighbors. This interaction is quantified by calculating a cell–cell response function. Effects of nonzero temperature on the response of a model cell are investigated. The effects of multiple neighbors on a cell are examined and programmable logic gate structures based on these ideas are discussed.

Journal ArticleDOI
TL;DR: It is shown that transport occurring via transitions between ground states with different numbers of electrons can be suppressed by the occupation of excited states.
Abstract: Coulomb blockade effects are investigated in lateral transport through a quantum dot defined in a two-dimensional electron gas. Tunneling through excited states of the quantum dot is observed for various tunneling barriers. It is shown that transport occurring via transitions between ground states with different numbers of electrons can be suppressed by the occupation of excited states. Measurements in a magnetic field parallel to the current give evidence for tunneling processes involving states with different spin.

Journal ArticleDOI
TL;DR: In this article, a method for calculating the electronic states and optical properties of multidimensional semiconductor quantum structures is described, which is applicable to heterostructures with confinement in any number of dimensions: e.g. bulk, quantum wells, quantum wires and quantum dots.
Abstract: A method for calculating the electronic states and optical properties of multidimensional semiconductor quantum structures is described. The method is applicable to heterostructures with confinement in any number of dimensions: e.g. bulk, quantum wells, quantum wires and quantum dots. It is applied here to model bulk and multiquantum well (MQW) InGaAsP active layer quaternary lasers. The band parameters of the quaternary system required for the modeling are interpolated from the available literature. We compare bulk versus MQW performance, the effects of compressive and tensile strain, room temperature versus high temperature operation and 1.3 versus 1.55 pm wavelength operation. Our model shows that: compressive strain improves MQW laser performance. MQW lasers have higher amplification per carrier and higher differential gain than bulk lasers, however, MQW performance is far from ideal because of occupation of non-lasing minibands. This results in higher carrier densities at threshold than in bulk lasers, and may nullify the advantage of MQW lasers over bulk devices for high temperature operation. >

Journal ArticleDOI
TL;DR: In this article, the interface between the semiconductor and the oxide layer serves as a potential well less than 10 nm wide, and while electrons remain free to wander in the plane of the interface, their motion in the perpendicular direction is quantized by this very narrow well.
Abstract: For three decades individual transistors in integrated semiconductor circuits have been getting smaller and smaller. Soon they will be approaching the 100‐nanometer regime, where the classical description of diffusive electron motion breaks down and quantum concepts become important, bringing about fundamental changes in electronic and optical properties. Already in the widely used silicon MOSFET transistors, the interface between the semiconductor and the oxide layer serves as a potential well less than 10 nm wide. While electrons remain free to wander in the plane of the interface, their motion in the perpendicular direction is quantized by this very narrow well. Such two‐dimensional electron systems, best realized in high‐mobility modulation‐doped semiconductor heterostructures, have been found over the years to exhibit new and quite unexpected quantum phenomena, like the integral and fractional quantum Hall effects.

Journal ArticleDOI
TL;DR: The evidence for magnetic field induced spin and angular momentum transitions in the strongly interacting artificial atom is presented.
Abstract: The evolution of charging energies of few-electron artifical atoms (quantum dots) with magnetic field is calculated and compared with results of the single-electron capacitance spectroscopy of Ashoori et al. The evidence for magnetic field induced spin and angular momentum transitions in the strongly interacting artifical atom is presented.

Patent
25 Mar 1993
TL;DR: In this article, a semiconductor light emitting/detecting device has a first doped silicon layer, an intrinsic silicon epitaxial layer formed on the first DBS, at least one quantum dot embedded within the intrinsic silicon element, and a second DBS layer created on the second intrinsic element.
Abstract: A semiconductor light emitting/detecting device has a first doped silicon layer, an intrinsic silicon epitaxial layer formed on the first doped silicon layer, at least one quantum dot embedded within the intrinsic silicon epitaxial layer, and a second doped silicon layer formed on the second intrinsic silicon epitaxial layer.

Journal ArticleDOI
TL;DR: In this paper, a detailed spectroscopic study of highly porous silicon was carried out, and the observation of momentum-conserving phonon satellites in resonantly excited photoluminescence (PL) spectra enabled the luminescent material in porous silicon to be unambiguously identified as crystalline silicon.

Journal ArticleDOI
TL;DR: In this paper, the ground state of a parabolically confined quantum dot in the limit of very strong magnetic fields where the electron system is completely spin-polarised and all electrons are in the lowest Landau level is discussed.
Abstract: In this article we discuss the ground state of a parabolically confined quantum dot in the limit of very strong magnetic fields where the electron system is completely spin-polarised and all electrons are in the lowest Landau level. Without electron-electron interactions the ground state is a single Slater determinant corresponding to a droplet centred on the minimum of the confinement potential and occupying the minimum area allowed by the Pauli exclusion principle. Electron-electron interactions favour droplets of larger area. We derive exact criteria for the stability of the maximum density droplet against edge excitations and against the introduction of holes in the interior of the droplet. The possibility of obtaining exact results in the strong magnetic field case is related to important simplifications associated with broken time-reversal symmetry in a strong magnetic field.

Journal ArticleDOI
TL;DR: Approximate expressions based on the notion that the size of a dot is determined by a competition between confinement and interaction energies are shown to be consistent with exact diagonalization studies for small quantum dots.
Abstract: We consider the magnetic field dependence of the chemical potential for parabolically confined quantum dots in a strong magnetic field. Approximate expressions based on the notion that the size of a dot is determined by a competition between confinement and interaction energies are shown to be consistent with exact diagonalization studies for small quantum dots. Fine structure is present in the magnetic field dependence which cannot be explained without a full many-body description and is associated with ground-state level crossings as a function of confinement strength or Zeeman interaction strength. Some of this fine structure is associated with precursors of the bulk incompressible states responsible for the fractional quantum Hall effect