Showing papers on "Quantum dot published in 1994"
TL;DR: Through the reduction of the number of emitting dots in small mesa structures, it is shown narrow lines in the spectra, each associated with a single InAs dot, indicate short capture and relaxation times into the dots.
Abstract: We present photoluminescence data on InAs quantum dots grown by molecular beam epitaxy on GaAs. Through the reduction of the number of emitting dots in small mesa structures, we evidence narrow lines in the spectra, each associated with a single InAs dot. Beyond the statistical analysis allowed by this technique, our results indicate short capture and relaxation times into the dots. This approach opens the route towards the detailed optical study of high quality easily fabricated single semiconductor quantum dots.
948 citations
TL;DR: In this paper, the authors show that the first quantum dots formed are in the quantum size range (height 30 A, half-base 120 A), that the dispersion on their sizes is remarkably low (±10%), and that they are located fairly regularly (interdot distance 600 A).
Abstract: The deposition of InAs on GaAs proceeds first by two‐dimensional (2D) growth and above a 1.75‐monolayer coverage by the formation of single‐crystal dots on a residual 2D wetting layer. By atomic force microscopy measurements, we show that the first dots formed are in the quantum size range (height 30 A, half‐base 120 A), that the dispersion on their sizes is remarkably low (±10%), and that they are located fairly regularly (interdot distance 600 A). Upon further growth, density and shapes do not change but sizes increase up to double values before coalescence occurs. Self‐organized growth in strained structures is then shown to be a simple and efficient way of building regular quantum dots.
766 citations
TL;DR: In this article, a low threshold, large T/sub o/ injection laser emission via zero-dimensional states in (InGa)As quantum dots is demonstrated, which are formed due to a morphological transformation of a pseudomorphic In/sub 0.5/Ga/sub0.5 /As layer.
Abstract: Low threshold, large T/sub o/ injection laser emission via zero-dimensional states in (InGa)As quantum dots is demonstrated. The dots are formed due to a morphological transformation of a pseudomorphic In/sub 0.5/Ga/sub 0.5/As layer. Laser diodes are fabricated with a shallow mesa stripe geometry.<
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734 citations
TL;DR: Self-assembled lens-shaped InGaAs quantum dots in a suitably designed field-effect-type GaAs/AlAs heterostructure allows us to charge the lowest discrete quantum levels in the dots with single electrons, extracting energy spacings from capacitance as well as infrared transmission spectroscopy at low temperatures and under application of high magnetic fields.
Abstract: Imbedding self-assembled lens-shaped InGaAs quantum dots in a suitably designed field-effect-type GaAs/AlAs heterostructure allows us to charge the lowest discrete quantum levels in the dots with single electrons. Because of their small diameters of about 20 nm the Coulomb charging energy is significantly smaller than the quantization energies. We extract energy spacings of about 41 meV between the $s$-like ground state and the first excited $p$-like state from capacitance as well as infrared transmission spectroscopy at low temperatures and under application of high magnetic fields.
614 citations
TL;DR: In this article, the synthetic procedure, the characterization, and some photophysical properties of a quantum dot quantum well (QDQW) system are described in detail, and theoretical calculations based on the effective mass approximation appropriate to describe the 1s-1s electronic transition of the composite particles are presented.
Abstract: The synthetic procedure, the characterization, and some photophysical properties of a quantum dot quantum well (QDQW) system are described in detail. The novel structures prepared via wet chemical methods consist of a core of size-quantized CdS and a well of 1-3 monolayers of HgS capped by 1-5 monolayers of CdS acting as the outermost shell. Additionally, theoretical calculations based on the effective mass approximation appropriate to describe the 1s-1s electronic transition of the composite particles are presented
474 citations
TL;DR: An avoided crossing around the spin orbit energy in the hole spectra for ∼65 A dots is observed, indicating the importance of valence band complexities in the description of the excited states.
Abstract: We combine a new synthesis with transient optical hole burning to observe and assign the size evolution (19 to 115 \AA{} diameter, \ensuremath{\sigma}5%) of a series of excited states in CdSe quantum dots. We observe an avoided crossing around the spin orbit energy in the hole spectra for \ensuremath{\sim}65 \AA{} dots, indicating the importance of valence band complexities in the description of the excited states. Comparsion with dc Stark data shows that bleach spectra are consistent with localized carrier induced electric fields.
421 citations
TL;DR: The clearest signatures of the Kondo effect in transport through a quantum dot are the broadening, shift, and enhancement of the linear-response conductance peaks at low temperatures, and a peak in the nonlinear differential conductance around zero bias.
Abstract: The infinite-U Anderson model is applied to transport through a quantum dot. The current and density of states are obtained via the noncrossing approximation for two spin-degenerate levels weakly coupled to two leads. At low temperatures, the Kondo peak in the equilibrium density of states strongly enhances the linear-response conductance. Application of a finite voltage bias reduces the conductance and splits the peak in the density of states. The split peaks, one at each chemical potential, are suppressed in amplitude by a finite dissipative lifetime. We estimate this lifetime perturbatively as the time to transfer an electron from the higher-chemical-potential lead to the lower-chemical-potential one. At zero magnetic field, the clearest signatures of the Kondo effect in transport through a quantum dot are the broadening, shift, and enhancement of the linear-response conductance peaks at low temperatures, and a peak in the nonlinear differential conductance around zero bias.
417 citations
416 citations
TL;DR: In this paper, the authors present a linear-in-size method that enables calculation of the eigensolutions of a Schrodinger equation in a desired energy window. And they illustrate this method by studying the near-gap electronic structure of Si quantum dots with size up to Si1315H460(≊37 A in diameter) using a plane wave pseudopotential representation.
Abstract: We present a simple, linear‐in‐size method that enables calculation of the eigensolutions of a Schrodinger equation in a desired energy window. We illustrate this method by studying the near‐gap electronic structure of Si quantum dots with size up to Si1315H460(≊37 A in diameter) using a plane wave pseudopotential representation.
409 citations
TL;DR: Using a micron-sized photoluminescence (PL) probe enables us to study single islandlike interface defects of a thin GaAs/AlGaAs quantum well to discuss the spectra and their polarization properties on the basis of a discrete level scheme and the Pauli exclusion principle.
Abstract: Using a micron-sized photoluminescence (PL) probe enables us to study single islandlike interface defects of a thin GaAs/AlGaAs quantum well. The bound exciton ground state locally emits a distinct sharp line. With increasing excitation of this quantum dot level additional transition lines emerge at lower energy. They are attributed to localized biexciton states. The biexciton correlation energy is about 4 meV. A distinct two-photon resonant absorption peak of the biexciton ground state is observed in PL excitation spectroscopy. Its linewidth is only about 30 \ensuremath{\mu}eV. The spectra and their polarization properties are discussed on the basis of a discrete level scheme and the Pauli exclusion principle.
402 citations
TL;DR: In this article, a plane wave basis expansion and an empirical pseudopotential Hamiltonian were applied to Si quantum dots using a realistic surface relaxation geometry, and the dependences of energy gaps and radiative recombination rates on the size, shape, and orientation of the Si quantum dot were investigated.
Abstract: The electronic structure of quantum dots containing N [ge] 1000 atoms is difficult to calculate by conventional molecular methods since the effort scales as N[sup 3]. Our newly developed method allows calculation of eigenstates within a desired [open quotes]energy window[close quotes] and thus has a linear-in-N scaling. This method is applied here to Si quantum dots using a plane wave basis expansion and an empirical pseudopotential Hamiltonian. Hydrogen atoms passivate the surface dangling bonds using a realistic surface relaxation geometry. We investigate the dependences of energy gaps and radiative recombination rates on the size, shape, and orientation of the Si quantum dots. We find that (1) a unified curve exists for band gap vs size of quantum spheres, cubes, and rectangular boxes; (2) the band edge states of Si quantum dots are bulklike, not surfacelike; (3) the band gap is insensitive to the surface orientation and to the overall shape of the quantum dot as long as it is not too prolate; (4) the radiative lifetime is sensitive to the shape and orientation; and (5) effective mass and single band truncated crystal models describe inadequately the electronic structure of Si quantum dots in the size range ([approx lt]40 [angstrom]) studiedmore » here. 52 refs., 8 figs., 2 tabs.« less
TL;DR: The measurements demonstrate photon-assisted tunneling in the mesoscopic regime and find features related to the photon energy whose positions in gate voltage are independent of the microwave power buy vary linearly with frequency.
Abstract: We have measured dc transport through a GaAs/AlGaAs quantum dot in the presence of a microwave signal of frequency $f$. We find features related to the photon energy $\mathrm{hf}$ whose positions in gate voltage are independent of the microwave power buy vary linearly with frequency. The measurements demonstrate photon-assisted tunneling in the mesoscopic regime. A comparison is made with a model that extends Coulomb blockade theory to include photon-assisted tunneling.
TL;DR: In this article, the authors review the latest developments on the growth and properties of self-assembling quantum dot structures and present structural results based on atomic force and transmission electron microscopy studies of the quantum dot arrays obtained by MBE and MOCVD growth.
TL;DR: In this paper, the authors report about optical experiments on electric field tunable AlAs/GaAs coupled quantum well structures in the regime of the electric field induced ε-Gamma-X transition.
Abstract: We report about optical experiments on electric field tunable AlAs/GaAs coupled quantum well structures in the regime of the electric field induced \ensuremath{\Gamma}-X transition. Using the energetically tunable X-point state in the AlAs layer as an internal energy spectrometer and charge reservoir we are able to map out the electronic states in the neighboring GaAs quantum well in great detail. In spatially resolved and bias voltage dependent photoluminescence experiments we find sets of extremely narrow emission lines below the fundamental band gap energy of the GaAs quantum well. The new emission lines are shown to originate from natural quantum dots which are formed by well width fluctuations of the GaAs quantum well.
TL;DR: In this paper, the electronic properties of shallow band-edge surface traps in nanometer-size CdSe quantum dots are probed using fluorescence-line-narrowing spectroscopy.
Abstract: The electronic properties of shallow band-edge surface traps in nanometer-size CdSe quantum dots are probed using fluorescence-line-narrowing spectroscopy. We find large changes in electron-hole-pair radiative lifetimes and couplings to LO phonons as the temperature is changed from 1.75 to 10 K. We attribute these changes to the localization of the photogenerated hole at the surface of the dots, accompanied by thermally activated motion between these surface localized states. A simple model based on the observed exciton--LO-phonon couplings is constructed to estimate the extent of hole localization in the luminescing state. A size-dependent study (20--80 \AA{} diameter) indicates that surface effects diminish rapidly with increasing size.
TL;DR: It is found that, for strong confining potentials, the edge of a $
u=1$ liquid is described by the $Z_F=1 $ Fermi Liquid theory, even in the presence of interactions, a consequence of the chiral nature of the system.
Abstract: We study the transition between sharp and smooth density distributions at the edges of quantum Hall liquids in the presence of interactions. We find that, for strong confining potentials, the edge of a \ensuremath{
u}=1 liquid is described by the ${\mathit{Z}}_{\mathit{F}}$=1 Fermi-liquid theory, even in the presence of interactions, a consequence of the chiral nature of the system. When the edge confining potential is decreased beyond a point, the edge undergoes a reconstruction and electrons start to deposit a distance \ensuremath{\sim}2 magnetic lengths away from the initial quantum Hall liquid. Within the Hartree-Fock approximation, a new pair of branches of gapless edge excitations is generated after the transition. We show that the transition is controlled by the balance between a long-ranged repulsive Hartree term and a short-ranged attractive exchange term. Such a transition also occurs for quantum dots in the quantum Hall regime and should be observable in resonant tunneling experiments. We find that the edge theory for sharp edges also applies to smooth edges (i.e., reconstructed edges) once the additional pairs of edge branches are included. Electron tunneling into the reconstructed edge is also discussed.
TL;DR: In this article, the Stranski-Krastanow growth mode was used for the direct deposition of strained InGaAs-dot structures with a diameter of about 15 nm on GaAs surfaces by metalorganic chemical vapor deposition.
Abstract: We report the direct deposition of strained InGaAs‐dot structures with a diameter of about 15 nm on GaAs surfaces by metalorganic chemical vapor deposition growth. High resolution scanning electron micrographs show highly uniform quantum‐sized dots formed by the Stranski–Krastanow growth mode. The sharp photoluminescence emission band of buried dot structures indicates efficient carrier capture and a homogeneous heterointerface. The average dot size and area dot density can be controlled accurately by growth temperature, and InGaAs deposition thickness, respectively.
TL;DR: In this article, the optical properties of porous Si films produced by electrochemical dissolution of Si are reviewed and an inverse relationship between the optical gap energy and the average nanoparticle size has been obtained demonstrating the quantum confinement of electron-hole pairs in Si nanocrystallites.
TL;DR: In this article, the optical characterization of the strained InGaAs/GaAs quantum dots (QDs) was reported, showing that the onset energy of thermal quenching in ∼20nm−diam QDs is enhanced by a factor of ∼2 as compared to a quantum well (QW), due to the additional confinement.
Abstract: We report on the optical characterization of the strained InGaAs/GaAs quantum dots (QDs). The temperature dependence of the photoluminescence (PL) indicates that the onset energy of the thermal quenching in ∼20‐nm‐diam QDs is enhanced by a factor of ∼2 as compared to a quantum well (QW), due to the additional confinement. At low temperature, an increased carrier lifetime is observed for the QDs as compared to a reference QW (880 vs 330 ps). The carrier lifetime in the QDs was found to be independent of the temperature for T<30 K. In addition to this different dynamics of the localized excitons, we find that in the steady state PL and PL excitation, there is virtually no overlap between the emission and the absorption energies.
TL;DR: In this paper, three different size distributions of Ge quantum dots (>200, 110, and 60 A) have been synthesized via the ultrasonic mediated reduction of mixtures of chlorogermanes and organochlorogenes by a colloidal sodium/potassium alloy in heptane.
Abstract: Three different size distributions of Ge quantum dots (>~200, 110, and 60 A) have been synthesized via the ultrasonic mediated reduction of mixtures of chlorogermanes and organochlorogermanes (or organochlorosilanes) by a colloidal sodium/potassium alloy in heptane, followed by annealing in a sealed pressure vessel at 270 °C. The quantum dots are characterized by transmission electron microscopy, x-ray powder diffraction, x-ray photoemission, infrared spectroscopy, and Raman spectroscopy. Colloidal suspensions of these quantum dots were prepared and their extinction spectra are measured with ultraviolet/visible (UV/Vis) and near infrared (IR) spectroscopy, in the regime from 0.6 to 5 eV. The optical spectra are correlated with a Mie theory extinction calculation utilizing bulk optical constants. This leads to an assignment of three optical features to the E(1), E(0'), and E(2) direct band gap transitions. The E(0') transitions exhibit a strong size dependence. The near IR spectra of the largest dots is dominated by E(0) direct gap absorptions. For the smallest dots the near IR spectrum is dominated by the Gamma25-->L indirect transitions.
TL;DR: In this article, a few mololayers of InAs are heteroepitaxially grown on GaAs substrate by molecular beam epitaxy and structural and optical properties are investigated.
Abstract: A few mololayers of InAs is heteroepitaxially grown on GaAs substrate by molecular‐beam epitaxy. Structure and optical properties are investigated. Reflection high‐energy electron‐diffraction observation reveals that an InAs layer forms a three‐dimensional structure with specific facets after two‐dimensional growth. The transmission electron microscope observation shows that these structures have structural anisotropy in the growth plane. Photoluminescense spectroscopy shows that the luminescence from the InAs structures exhibits the polarization property caused by the quantum dot effect of the structural anisotropy.
TL;DR: In this paper, a microcrystalline PbS phase has been thermally produced from an oxide glass host, and X-ray linewidth measurements have shown that the average particle size can be controlled in the range 8-30 nm, depending on the thermal development schedule.
Abstract: A microcrystalline PbS phase has been thermally produced from an oxide glass host. X-ray linewidth measurements have shown that the average particle size can be controlled in the range 8–30 nm, depending on the thermal development schedule. Quantum confinement behavior has been observed based on the measurement of sub-band structure of the absorption edge. Spectral shifts of the sub-bands were found to be consistent with the inverse square of the particle size, consistent with a quantum confinement origin. A lower-concentration PbSe phase has also been obtained from the same base-glass composition showing similar optical effects.
TL;DR: In this paper, a macroscopic continuum model coupling the mechanical vibrational amplitude and electrostatic potential is applied to obtain the optical vibrational modes in quantum-dot structures, and a unified method of solution (valid for any type of nanostructure) that reduces the four coupled second-order differential equations to Helmholtz's and Laplace's equations is described.
Abstract: A macroscopic continuum model coupling the mechanical vibrational amplitude and electrostatic potential is applied to obtain the optical vibrational modes in quantum-dot structures. A unified method of solution (valid for any type of nanostructure) that reduces the four coupled second-order differential equations to Helmholtz's and Laplace's equations is described. Analytical solutions for the vibrational amplitude and the Fr\"ohlich-type electron-phonon interaction are given for quantum dots with spherical geometry. The existence of surface modes and their relation to the matching boundary conditions are studied. A qualitative discussion of the ir and Raman activity of the calculated modes is given together with a comparison with the few existing experimental data.
TL;DR: In this article, the two-dimensional layer-by-layer growth mode is shown to be limited to the growth of 1−ML•thick and, in part, 2−ML−thick quantum wells.
Abstract: Ga0.5In0.5P/InP quantum‐sized structures, grown by metalorganic vapor phase epitaxy, have been optically characterized by photoluminescence, cathodoluminescence, and photoluminescence excitation spectroscopy. Additional structural information has been obtained by atomic force microscopy. We find that the two‐dimensional layer‐by‐layer growth mode is limited to the growth of 1‐ML‐thick and, in part, 2‐ML‐thick quantum wells. The transition towards three‐dimensional Stranski–Krastanow island growth occurs before the second monolayer of InP is completed. To further study the dynamics of the island formation, growth interruptions were introduced between the InP deposition and the subsequent growth of the upper GaInP barrier. The two types of coherent islands show a quantum confinement in vertical direction, corresponding to about 2‐ and 3‐ML‐thick and about 9‐ and 10‐ML‐thick InP strained quantum wells.
TL;DR: It is shown that if a double-barrier mesoscopic p-i-n heterojunction is driven by an alternating voltage source, then Coulomb blockade and quantum confinement effects together can suppress the quantum fluctuations usually associated with electron and hole injection processes in semiconductors.
Abstract: We show that if a double-barrier mesoscopic p-i-n heterojunction is driven by an alternating voltage source, then Coulomb blockade and quantum confinement effects together can suppress the quantum fluctuations usually associated with electron and hole injection processes in semiconductors. It is therefore possible to generate heralded single-photon states without the need for a high-impedance current source. Since the frequency of the alternating voltage source determines the repetition rate of the single-photon states and the magnitude of the junction current, the present scheme promises high precision photon-flux and current standards.
TL;DR: In this paper, a detailed effective mass calculation of the energy levels in InAs quantum dots embedded in GaAs is presented, and the results of a separable approximate treatment with a more complete numerical approach are compared.
TL;DR: In this paper, a 1.3-μm emitting self-formed In 0.5 Ga 0.9 As quantum dots on GaAs substrates by supplying InAs and GaAs monolayers alternately during atomic layer epitaxy.
Abstract: We grew 1.3-μm emitting self-formed In 0.5 Ga 0.5 As quantum dots on GaAs substrates by supplying InAs and GaAs monolayers alternately during atomic layer epitaxy. The dots were 20 nm in diameter and 10 nm in height, and were surrounded by In 0.5 Ga 0.9 As in the lateral direction and by GaAs perpendicular to the dots. Diamagnetic energy shifts of excitons in the dots clearly demonstrated three-dimensional quantum confinement
TL;DR: The dynamics of carrier-induced absorption changes in CdSe quantum dots are investigated with femtosecond spectroscopy and a redshift of the lowest optical transition is observed in the initial phase of carrier relaxation.
Abstract: The dynamics of carrier-induced absorption changes in CdSe quantum dots are investigated with femtosecond spectroscopy. After excitation with 4-eV photons a redshift of the lowest optical transition is observed in the initial phase of carrier relaxation. This shift is attributed to a biexciton effect where two electron-hole pairs interact via the Coulomb potential.
TL;DR: In this paper, a threshold for continuum absorption of CdSe nanocrystals at 77 K was established and all samples have a discrete manifold of quantum confined electronic excitations, which results in substantially reduced luminescence efficiency.
Abstract: Luminescence excitation spectra are employed to study the electronic states of CdSe nanocrystals ranging in size from 9 to 26 A radius at 77 K. These studies show that all samples have, in addition to the discrete manifold of quantum confined electronic excitations, a threshold for continuum absorption. Absorption into this continuum results in substantially reduced luminescence efficiency.
TL;DR: The electron addition spectrum in a class of Hubbard-like models which describe arrays of coupled quantum dots is investigated, finding that interdot tunneling leads to a sequence of two phase transitions separating a region of collective Coulomb blockade from a region where the Coulombs blockade of individual dots is maintained.
Abstract: We investigate the electron addition spectrum in a class of Hubbard-like models which describe arrays of coupled quantum dots. Interdot tunneling leads to a sequence of two phase transitions separating a region of collective Coulomb blockade from a region where the Coulomb blockade of individual dots is maintained and a region where the Coulomb blockade is destroyed altogether. Observable experimental consequences of our theory are discussed.