Showing papers on "Quantum dot published in 1991"
TL;DR: In this article, it was shown that a two-dimensional quantum confinement (quantum wire) in the very narrow walls between the pores not only explains the change in band gap energy but also may also explain the dissolution mechanism that leads to porous silicon formation.
Abstract: Porous silicon layers grown on nondegenerated p‐type silicon electrodes in hydrofluoric acid electrolytes are translucent for visible light, which is equivalent to an increased band gap compared to bulk silicon. It will be shown that a two‐dimensional quantum confinement (quantum wire) in the very narrow walls between the pores not only explains the change in band‐gap energy but may also be the key to better understanding the dissolution mechanism that leads to porous silicon formation.
1,705 citations
TL;DR: Les resultats analytiques explicites sont obtenus pour the periodicite, l'amplitude, the forme de the raie, et l'energie d'activation des oscillations de the conductance.
Abstract: A linear-response theory is developed for resonant tunneling through a quantum dot of small capacitance, in the regime of thermally broadened resonances. The theory extends the classical theory of Coulomb-blockade oscillations by Kulik and Shekhter to the resonant-tunneling regime. Both the cases of negligible and strong inelastic scattering in the quantum dot are considered. Effects from the non-Fermi-Dirac distribution of electrons among the energy levels (occurring when kT is comparable to the level separation) are fully included. Explicit analytic results are obtained for the periodicity, amplitude, line shape, and activation energy of the conductance oscillations.
1,250 citations
AT&T1
TL;DR: In this paper, a review and analysis of the optical properties of quantum crystallites, with principal emphasis on the electro-optic Stark effect and all optical third order nonlinearity is presented.
Abstract: This is a review and analysis of the optical properties of quantum crystallites, with principal emphasis on the electro-optic Stark effect and all optical third order nonlinearity. There are also introductory discussions on physical size regimes, crystallite synthesis, quantum confinement theory, and linear optical properties. The experiments describe CdSe crystallites, exhibiting strong confinement of electrons and holes, and CuCl crystallites, exhibiting weak confinement of the exciton center of mass. In the CdSe system, neither the Stark effect nor the third order nonlinearity is well understood. The Stark shifts appear to be smaller than calculated, and field inducted broadening also occurs. The third order nonlinearity is only modestly stronger than in bulk material, despite theoretical prediction. Unexpectedly large homogeneous widths, due to surface carrier trapping, in the nominally discrete crystallite excited states appear to be involved. The CuCl system shows far narrower spectroscopic homogeneous widths, and corresponds more closely to an ideal quantum dot in the weak confinement limit. CuCl also exhibits exciton superradiance at low temperature. Surface chemistry and crystallite encapsulation are critical in achieving the predicted large Stark and third order optical effects in II-VI and III-V crystallites.
995 citations
TL;DR: It is proposed that electrons captured from the barriers in the upper levels of quantum boxes are retained in their cascade to the fundamental states for more than nanoseconds, and no luminescence can be obtained from these stored electrons with reasonable assumptions for the hole population.
Abstract: The poor radiative efficiency in quantum-box luminescence is tentatively explained as an intrinsic effect rather than the usually invoked effect of etch damages From the recently calculated decreased relaxation rate in zero-dimensional (0D) systems under 100\char21{}200-nm lateral quantization, we propose that electrons captured from the barriers in the upper levels of quantum boxes are retained in their cascade to the fundamental states for more than nanoseconds Due to the mutual orthogonality of quantum states in a box, no luminescence, or much less than in 2D or 3D, can be obtained from these stored electrons with reasonable assumptions for the hole population Magnetic-confinement experiments in quantum-well lasers support our conclusion A realistic model at low temperature describes more quantitatively the observed strong decay of the radiative efficiency in quantum boxes and pseudowires with decreased lateral dimensions
744 citations
TL;DR: The participation of multiple electronic levels in each conductance peak explains the anomalous temperature dependence of peak heights observed in recent narrow-channel experiments.
Abstract: The conductance through a quantum dot is calculated via an Anderson model of a site weakly coupled to ideal leads with an on-site Coulomb interaction. As the chemical potential is varied, peaks occur periodically in the conductance whenever an electron is added to the site. The participation of multiple electronic levels in each conductance peak explains the anomalous temperature dependence of peak heights observed in recent narrow-channel experiments
504 citations
TL;DR: It is demonstrated that an integer number of electrons pass through the quantum dot each rf cycle, at integer multiples of ef, where f is the rf frequency.
Abstract: We have observed a quantized current in a lateral quantum dot, defined by metal gates in the two-dimensional electron gas (2DEG) of a GaAs/AlGaAs heterostructure. By modulating the tunnel barriers in the 2DEG with two phase-shifted rf signals, and employing the Coulomb blockade of electron tunneling, we produced quantized current plateaus in the current-voltage characteristics at integer multiples of ef, where f is the rf frequency. This demonstrates that an integer number of electrons pass through the quantum dot each rf cycle.
460 citations
TL;DR: This analysis shows that dc current-voltage characteristics of a small-area quantum wells should exhibit an interplay between single-electron charging and energy-quantization effects.
Abstract: Single-electron charging effects similar to those in small-area metallic tunnel junctions should take place in semiconductor heterostructures, in particular, small-area quantum wells. Our analysis shows that dc current-voltage characteristics of such a well should exhibit an interplay between single-electron charging and energy-quantization effects. Relative magnitude of the single-electron charging effects is determined by the same parameter which scales multielectron charging in the conventional (large-area) quantum wells.
370 citations
TL;DR: The laterally confining potential of quantum dots on semiconductors is approximated by a two-dimensional harmonic-oscillator well and the discrete level diagram for two interacting electrons in this potential is calculated in the effective-mass approximation.
Abstract: The laterally confining potential of quantum dots on semiconductors is approximated by a two-dimensional harmonic-oscillator well. The discrete level diagram for two interacting electrons in this potential is calculated in the effective-mass approximation as a function of the dot size and the strength of a magnetic field directed perpendicularly to the dot plane.
317 citations
TL;DR: In this paper, the band gaps, band structure, and excited state (exciton) energies of CdS, GaAs, and GaP semiconductor clusters are calculated using pseudopotentials.
Abstract: The band gaps, band structure, and excited‐state (exciton) energies of CdS, GaAs, and GaP semiconductor clusters are calculated using pseudopotentials. In addition, the sensitivity of the exciton energies to the size, shape, crystal structure, and lattice constant of the unit cell are investigated. The calculated exciton energies of CdS clusters are in excellent agreement with experiment over a wide range of cluster sizes. Also, the exciton states of small CdS clusters are sensitive to whether their crystal structure is zinc blende or hexagonal. Such a sensitivity is absent in large CdS clusters. Furthermore, small GaAs clusters are shown to exhibit anomalous redshift of their absorption spectra, in sharp contrast to CdS and large GaAs clusters whose spectra always shift to blue with decreasing cluster size. Finally, the lowest‐energy non‐Franck–Condon transition in GaP clusters always shifts to blue with decreasing cluster size, whereas the higher‐energy Franck–Condon transition in small clusters exhibits the anomalous redshift. These novel findings reveal that (1) the optical spectroscopy of semiconductor clusters is strongly material and crystal structure dependent; (2) the spectroscopy of small clusters is dramatically different from those of large clusters and bulk; and (3) these effects cannot be explained, even qualitatively, using the effective‐mass approximation.
296 citations
TL;DR: In this article, a tetrahedral quantum dot (TQD) was proposed to make a zero-dimensional electron-hole system, where the TQDs are surrounded by crystallographic facets fabricated using selective area metalorganic chemical vapor deposition (MOCVD) on (111)B GaAs substrates.
Abstract: New GaAs quantum dot structures, called tetrahedral quantum dots (TQDs), are proposed to make a zero‐dimensional electron‐hole system. The TQDs are surrounded by crystallographic facets fabricated using selective area metalorganic chemical vapor deposition (MOCVD) on (111)B GaAs substrates. The calculated energy sublevel structures of zero‐dimensional electrons in a GaAs TQD show large quantum size effects, because electrons are confined three dimensionally. GaAs and AlGaAs tetrahedral facet structures on (111)B GaAs substrates partially etched into a triangular shape were grown using MOCVD. Tetrahedral growth with {110} facets occurs in the triangular areas. The cathodoluminescence intensity map for GaAs tetrahedrons buried in AlGaAs shows the tetrahedral dot array.
212 citations
TL;DR: In this article, the charging effects in a lateral split-gate quantum dot defined by metal gates in the two dimensional electron gas (2 DEG) of a GaAs/AlGaAs heterostructure were studied.
Abstract: We have studied charging effects in a lateral split-gate quantum dot defined by metal gates in the two dimensional electron gas (2 DEG) of a GaAs/AlGaAs heterostructure. The gate structure allows an independent control of the conductances of the two tunnel barriers separating the quantum dot from the two 2 DEG leads, and enables us to vary the number of electrons that are localized in the dot. We have measured Coulomb oscillations in the conductance and the Coulomb staircase in current-voltage characteristics and studied their dependence on the conductances of the tunnel barriers. We show experimentally that at zero magnetic field charging effects start to affect the transport properties when both barrier conductances are smaller than the first quantized conductance value of a point contact at 2e2/h. The experiments are described by a simple model in terms of electrochemical potentials, which includes both the discreteness of the electron charge and the quantum energy states due to confinement.
TL;DR: The first application of valence-band photoemission to a quantum-dot system and the spectra are qualitatively similar to those obtained for bulk cadmium sulfide, but show a shift in the valences-band maximum with size.
Abstract: We report the first application of valence-band photoemission to a quantum-dot system. Photoemission spectra of cadmium sulfide quantum dots, ranging in size from 12 to 35 A radius, were obtained using photon energies of 20 to 70 eV. The spectra are qualitatively similar to those obtained for bulk cadmium sulfide, but show a shift in the valence-band maximum with size.
TL;DR: The first example of a dilute magnetic semiconductor confined in all three dimensions (DMS quantum dot) was reported in this paper, where Zn0.93Mn 0.07S clusters of not, vert, similar 25 A diameter were successfully synthesized inside a glass matrix and fully characterized by chemical analysis, x-ray diffraction, extended xray absorption fine structure (EXAFS), and photoluminescence spectroscopy.
TL;DR: In this paper, the authors used anodization techniques to process porous surface regions in p-type Czochralski Si and in P-type Si0.85Ge0.15 epitaxial layers grown by molecular beam epitaxy.
Abstract: We have used anodization techniques to process porous surface regions in p-type Czochralski Si and in p-type Si0.85Ge0.15 epitaxial layers grown by molecular beam epitaxy. The SiGe layers were unrelaxed before processing. We have observed strong near-infrared and visible light emission from both systems. Analysis of the radiative and nonradiative recombination processes indicate that the emission is consistent with the decay of excitons localized in structures of one or zero dimensions.
TL;DR: In this paper, strong photoluminescence with sub-band-gap photon energies has been observed in fine Si particles prepared by the gas-evaporation technique, the band tail covering the visible light region.
Abstract: Strong photoluminescence with sub‐band‐gap photon energies has been observed in fine Si particles prepared by the gas‐evaporation technique. After surface oxidation, the Si particles show above‐band‐gap photoluminescence, the band tail covering the visible light region. The amount of the increased apparent band gap (0.3 eV) estimated from this blueshift can be explained by a quantum‐size effect expected to be observed in Si quantum dots with a diameter of 50 A.
TL;DR: In this paper, a simple yet physically reasonable model is presented which describes an arbitrary number of interacting particles in a quantum dot and exact analytic expressions are obtained for the energy spectrum as a function of particle number and magnetic field.
Abstract: A simple, yet physically reasonable, model is presented which describes an arbitrary number of interacting particles in a quantum dot. Exact analytic expressions are obtained for the energy spectrum as a function of particle number and magnetic field
TL;DR: Four well-resolved one-dimensional subbands in the excitation spectra are found, whose constant spacing confirms quantitatively that there is quantum confinement in the parabolic well predicted by continuum elasticity theory.
Abstract: We have created one-dimensional quantum wells (quantum wires) by laterally straining a GaAs quantum well with patterned carbon stressors 180 nm in width. We find four well-resolved one-dimensional subbands in the excitation spectra, whose constant spacing of 2.4 meV confirms quantitatively that there is quantum confinement in the parabolic well predicted by continuum elasticity theory. The lateral width of the electron ground state is 35 nm
TL;DR: The calculated resonance spectrum reflects and explains all features observed in recent high-resolution measurements and demonstrates the evidence of many-particle effects in systems where the generalized Kohn's theorem is no longer applicable.
Abstract: The magneto-optical response to far-infrared radiation of quantum dots containing two electrons is studied theoretically. The symmetry-breaking effects of deviations from a strictly parabolic confinement potential are emphasized. We demonstrate the evidence of many-particle effects in systems where the generalized Kohn's theorem is no longer applicable. The calculated resonance spectrum reflects and explains all features observed in recent high-resolution measurements.
TL;DR: In this paper, the Schrodinger and Poisson equations are self-consistently solved in a finite quantum box which includes the whole metal-insulator-semiconductor structure.
Abstract: A general method for the study of quantum effects in accumulation layers is presented. The Schrodinger and Poisson equations are self‐consistently solved in a finite quantum box which includes the whole metal‐insulator‐semiconductor structure. An appropriate choice of the boundary conditions allows the achievement of box‐independent results. For the first time, the electrostatical potential and quantum energy levels of an accumulated n‐type semiconductor are fully self‐consistently calculated without considering the electric‐quantum limit approximation. Hence, being able to treat the problem even at room temperature, we report results in the whole range from liquid‐helium temperature to room temperature and beyond. This has been possible because our method allows the calculation of both bound and mobile electron states and their introduction into the Poisson equation on equal footing. The effect of the penetration of the wave functions into the oxide has been determined, and it has been demonstrated that the consideration of an infinite semiconductor‐insulator interface barrier leads to more serious errors than previously estimated by other authors. Having included the oxide‐metal interface into the quantum box, we also propose a simple method to calculate the tunnel current which flows through the insulator. Although the contribution of many subbands has to be added up to obtain the total current, oscillations in the Fowler–Nordheim current‐voltage characteristic, which are due to reflection resonances at the insulator‐anode interface, are clearly observed. Initially conceived for the accumulation layer problem, the presented method is obviously valid for treating inversion layers as well.
TL;DR: InAs microclusters embedded in a single-crystal GaAs matrix by molecular-beam epitaxy are synthesized and it is shown that the optical response of excitons attached to the InAs dots is determined by the zero-dimensional symmetry of the system.
Abstract: We directly synthesize InAs microclusters embedded in a single-crystal GaAs matrix by molecular-beam epitaxy. Fractional monolayers of InAs are deposited on terraced (001) GaAs surfaces and subsequently overgrown with GaAs. Growth conditions are adjusted in situ by reflection high-energy electron diffraction to those favoring step-flow nucleation of both Ga and In adatoms. The resulting microscopic structural configuration is studied by double-crystal x-ray diffractometry and high-resolution electron microscopy. These experiments reveal that InAs growth takes place in fact by nucleation of In adatoms on step edges. An array of isolated InAs clusters of subnanometer size (quantum dots) is thereby formed within the GaAs matrix. The interface of the InAs clusters is in registry with the surrounding GaAs matrix and is thus defect-free. Several spectroscopic techniques, such as transmission, cw photoluminescence, photoluminescence excitation, and picosecond photoluminescence, are applied to get insight into the optical properties of this system. We show that the optical response of excitons attached to the InAs dots is determined by the zero-dimensional symmetry of the system. This effect is most evident when comparing the spontaneous emission of InAs dots and InAs planes, which in either case results from the relaxation of excitons to the emitting state followed by their radiative recombination. The reduced translational symmetry causes a progressive release of wave-vector conservation, thus modifying the selection rules that uniquely determine the interaction of excitons with phonons (relaxation) and photons (recombination).
TL;DR: In this article, the Friedel sum rule was used to predict the transmission probability of an electron through a quantum dot if the temperature is lower than the Kondo temperature, i.e., the characteristic energy of the spin fluctuations divided by Boltzmann constant.
Abstract: Electron transport through a quantum dot is discussed from the point of view of the Friedel sum rule. When an odd number of electrons are in the dot, the localized moment may be formed because of the Coulomb repulsion between the electrons. In this situation, it is predicted that the transmission probability of an electron through the dot is almost unity if the temperature is lower than the Kondo temperature, i.e., the characteristic energy of the spin fluctuations divided by Boltzmann constant.
TL;DR: On generalise le theoreme de Kohn aux structures confinees harmoniquement aux structures par des potentiels paraboliques externes.
Abstract: We consider theoretically the long-wavelength magneto-optical response of an electron system, harmonically confined in two (quantum wire) or three (quantum dot or disk) spatial dimensions by external parabolic potentials. In particular, we prove explicitly that the resonance frequencies of such systems are independent of electron number and exactly the same as the corresponding bare resonance frequencies. This is the generalization of Kohn's theorem to harmonically confined structures. We discuss a number of recent experimental results in light of this exact result.
TL;DR: In this paper, CdS quantum dots were prepared in glass films obtained from tetramethoxysilane (TMOS) or from TEOS of typical thickness of 0.1-0.2 μm.
TL;DR: Etude des oscillations collectives longitudinales d'un gaz d'electrons bidimensionnel (2DEG) confine sur un disque (point quantique) dans un champ magnetique perpendiculaire constant changent lorsque l'on fait varier le potentiel de confinement and le nombre d-electrons.
Abstract: We investigate the longitudinal collective oscillations of a two-dimensional interacting electron gas (2DEG) confined to a disk (quantum dot) in a perpendicular constant magnetic field. We show how the exact results known for a parabolically confined 2DEG change when the confining potential and the electron number are varied. Considering reasonable corrections to a parabolic confinement potential, we are able to explain all features of the far-infrared spectrum of quantum dots observed in recent experiments.
TL;DR: Calcul des fonctions d'onde et des courants dans un point quantique circulaire en presence d'un champ magnetique perpendiculaire entre les courants de the mecanique quantique and les trajectoires de the particule classique.
Abstract: The wave functions and currents in a circular quantum dot in a perpendicular magnetic field are calculated. The current in condensed (high-field) eigenstates is composed of concentric rings of current flowing in opposite directions. The current flow near the dot center flows in the direction opposite that expected from the Lorentz force. It is this inner circulation that is responsible for the ``reverse'' current flow associated with edge states. The correspondence between the quantum-mechanical currents and classical-particle trajectories is examined.
TL;DR: In this article, the authors summarized recent studies on absorption, luminescence, nonlinear optical effects and electroabsorption on wide-gap II-VI nano-crystals embedded in a glass matrix.
Abstract: The paper summarizes recent studies on absorption, luminescence, nonlinear optical effects and electro-absorption on wide-gap II-VI nano-crystals embedded in a glass matrix.
TL;DR: Using metalorganic vapor phase epitaxy to grow GaInAs/InP layers on masked InP substrates patterned with submicron square holes, the authors fabricated in situ buried quantum dot arrays for the first time.
Abstract: Using metalorganic vapor phase epitaxy to grow GaInAs/InP layers on masked InP substrates patterned with submicron square holes, we have fabricated in situ buried quantum dot arrays for the first time. Starting with mask openings ≥150 nm × 150 nm and utilizing the natural crystal habits to form low‐index plane facetted pyramids inside the holes, highly regular GaInAs quantum dots embedded in InP are obtained in a single growth step. As verified by cathodoluminescence, the dots exhibit very high luminescence efficiencies, even at room temperature, owing to the absence of air‐exposed or etch‐damaged heterointerfaces.
TL;DR: In this article, a lateral quantum dot defined in the two-dimensional electron gas (2DEG) of a GaAs/AlGaAs heterostructure was used to realize a quantum dot turnstile device.
Abstract: We have performed RF experiments on a lateral quantum dot defined in the two dimensional electron gas (2DEG) of a GaAs/AlGaAs heterostructure. The small capacitance of the quantum dot gives rise to single-electron charging effects, which we employed to realize a quantum dot turnstile device. By modulating the tunnel barriers between the quantum dot and the 2DEG leads with two phase-shifted RF signals, we pass an integer number of electrons through the quantum dot per RF cycle. This is demonstrated by the observation of quantized current plateaus at multiples ofef in current-voltage characteristics, wheref is the frequency of the RF signals. When an asymmetry is induced by applying unequal RF voltages, our quantum dot turnstile operates as a single-electron pump producing a quantized current at zero bias voltage.
TL;DR: In this article, the authors investigate the possibility of generating spontaneous anti-ferroelectric polarization in an array of elongated quantum dots, which they call "quantum dashes" and suggest ways in which this phenomenon might be observed and point out possible technological applications.
Abstract: We investigate the possibility of generating a spontaneous anti‐ferroelectric polarization in an array of elongated quantum dots, which we call ‘‘quantum dashes.’’ We suggest ways in which this phenomenon might be observed and point out possible technological applications resulting from this phenomenon.
TL;DR: Calcul des proprietes a l'equilibre and de the reponse dynamique des structures de points quantiques avec ou sans champ magnetique, en partant d'un potentiel de confinement de forme parabolique.
Abstract: The equilibrium properties and dynamic response of quantum-dot structures with and without magnetic fields are calculated, starting from a confining potential of parabolic shape. Within our analytical theory we can calculate all eigenmodes of the systems. Our calculation can explain the experimentally observed mode spectrum of quantum-dot structures containing about 200 electrons in a disk of radius R\ensuremath{\approxeq}150 nm.