Showing papers on "Exciton published in 1996"

Band-edge exciton in quantum dots of semiconductors with a degenerate valence band: Dark and bright exciton states

Abstract: We present a theoretical analysis of the band-edge exciton structure in nanometer-size crystallites of direct semiconductors with a cubic lattice structure or a hexagonal lattice structure which can be described within the framework of a quasicubic model. The lowest energy exciton, eightfold degenerate in spherically symmetric dots, is split into five levels by the crystal shape asymmetry, the intrinsic crystal field (in hexagonal lattice structures), and the electron-hole exchange interaction. Transition oscillator strengths and the size dependence of the splittings have been calculated. Two of the five states, including the ground state, are optically passive (dark excitons). The oscillator strengths of the other three levels (bright excitons) depend strongly on crystal size, shape, and energy band parameters. The relative ordering of the energy levels is also heavily influenced by these parameters. The distance between the first optically active state and the optically forbidden ground exciton state increases with decreasing size, leading to an increase of the Stokes shift in the luminescence. Our results are in good agreement with the size dependence of Stokes shifts obtained in fluorescence line narrowing and photoluminescence experiments in CdSe nanocrystals. Mixing of the dark and bright excitons in an external magnetic field allows the direct optical recombination of the dark exciton ground state. The observed shortening of the luminescence decay time in CdSe nanoncrystals in a magnetic field is also in excellent agreement with the theory, giving further support to the validity of our model. \textcopyright{} 1996 The American Physical Society.

Relationship between electroluminescence and current transport in organic heterojunction light‐emitting devices

Abstract: We measure the current–voltage and electroluminescencecharacteristics of single‐heterojunction, vacuum‐deposited organic light‐emitting devices(OLEDs) over a wide range of materials, temperatures, and currents. We find that the current is limited by a large density of traps with an exponential energy distribution below the lowest unoccupied molecular orbital. The characteristic trap depth is 0.15 eV. Furthermore, in metal–quinolate‐based devices,electroluminescence originates from recombination of Frenkel excitons, and its temperature dependence is consistent with the excitons being formed by Coulombic relaxation of the trapped electrons with holes injected from the counter electrode. By semiempirical molecular orbital modeling, we find that the trap distribution obtained from the current–voltage characteristics is consistent with a distribution in the metal–quinolate molecular conformations which result in a continuous, exponential distribution of allowed states below the lowest unoccupied molecular orbital. We discuss the implications of the intrinsic relationship between electroluminescence and current transport in OLEDs for the optimization of efficiency and operating voltage in these devices.

Fine structure splitting in the optical spectra of single GaAs quantum dots.

Abstract: We report a photoluminescence study of excitons localized by interface fluctuations in a narrow GaAs/AlGaAs quantum well. This type of structure provides a valuable system for the optical study of quantum dots. By reducing the area of the sample studied down to the optical near-field regime, only a few dots are probed. With resonant excitation we measure the excited-state spectra of single quantum dots. Many of the spectral lines are linearly polarized with a fine structure splitting of 20--50 \ensuremath{\mu}eV. These optical properties are consistent with the characteristic asymmetry of the interface fluctuations.

Exciton diffusion and dissociation in a poly(p-phenylenevinylene)/c60 heterojunction photovoltaic cell

Abstract: We report measurements of the photovoltaic response of two‐layer photocells formed with layers of the conjugated polymer poly(phenylenevinylene), PPV and fullerene, C60, formed between indium‐tin oxide and aluminum electrodes. Peak quantum efficiencies of up to ∼9% (electrons collected per incident photon) were measured under short‐circuit conditions. We model the photovoltaic response as arising from excitons photogenerated in the PPV layer which are able to diffuse to the interface with the C60 layer where they are ionized. We obtain a value for the exciton diffusion range of 7±1 nm, both from the spectral response and from the absolute efficiency. We demonstrate that the branching ratio for the creation of singlet excitons from absorbed photons is close to unity.

Nonequilibrium condensates and lasers without inversion: Exciton-polariton lasers

Abstract: We analyze elementary properties of exciton and polariton lasers --- devices that generate coherent optical and matter waves using final-state stimulation of exciton-phonon scattering. First we discuss the relation between the conditions for the onset of equilibrium and nonequilibrium excitonic condensates. Provided that the thermal de Broglie wavelength ${\ensuremath{\lambda}}_{\mathit{T}}$ exceeds the exciton Bohr radius ${\mathit{a}}_{\mathit{B}}$, an exciton laser operates without electronic population inversion. In contrast to previous proposals, this is a different type of laser without inversion which utilizes many-body coherences. When the excitonic character of the polariton branch vanishes, a polariton laser becomes indistinguishable from a photon laser. \textcopyright{} 1996 The American Physical Society.

Size dependence of exciton fine structure in CdSe quantum dots

Abstract: We use photoluminescence excitation and fluorescence line narrowing spectroscopies to examine structure observed in the band-edge absorption feature of CdSe quantum dots. We study eight samples ranging from \ensuremath{\sim}15 to \ensuremath{\sim}50 \AA{} in radius to probe the size dependence of this structure. We compare our results with recent theories, which predict band-edge exciton splittings in CdSe dots due to their internal crystal structure, nonspherical shape, and the exchange interaction between the electron and hole. We find reasonable agreement between our data and theory, supporting the observation of exciton fine structure. \textcopyright{} 1996 The American Physical Society.

Homogeneous Linewidths in the Optical Spectrum of a Single Gallium Arsenide Quantum Dot

Abstract: The homogeneous linewidths in the photoluminescence excitation spectrum of a single, naturally formed gallium arsenide (GaAs) quantum dot have been measured with high spatial and spectral resolution. The energies and linewidths of the homogeneous spectrum provide a new perspective on the dephasing dynamics of the exciton in a quantum-confined, solid-state system. The origins of the linewidths are discussed in terms of the dynamics of the exciton in zero dimensions, in particular, in terms of lifetime broadening through the emission or absorption of phonons and photons.

Pseudopotential calculations of nanoscale CdSe quantum dots.

Abstract: A plane-wave semiempirical pseudopotential method with nonlocal potentials and spin-orbit coupling is used to calculate the electronic structure of surface-passivated wurtzite CdSe quantum dots with up to 1000 atoms. The calculated optical absorption spectrum reproduces the features of the experimental results and the exciton energies agree to within \ensuremath{\sim}0.1 eV over a range of dot sizes. The correct form of Coulomb interaction energy with size-dependent dielectric constant is found to be essential for such good agreement. \textcopyright{} 1996 The American Physical Society.

Direct Measurement of Conjugated Polymer Electronic Excitation Energies Using Metal/Polymer/Metal Structures

Abstract: We report electroabsorption measurements of built-in electric fields and internal photoemission measurements of Schottky barriers to determine the charge transfer and single-particle energy gaps of the conjugated polymer poly[2-methoxy, 5-(2\ensuremath{'}-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV). For MEH-PPV, with an exciton absorption peak of 2.25 eV, or results yield a single-particle energy gap of 2.45 eV and a charge transfer energy gap of at least 2.35 eV. Therefore the exciton binding energy is 0.2 eV and the bipolaron binding energy is less than 0.1 eV.

Spectroscopic Determination of Electron and Hole Effective Masses in a Nanocrystalline Semiconductor Film

Abstract: Spectroelectrochemical techniques may be used to determine the absolute energy of the valence and conduction band edges of a transparent nanocrystalline semiconductor electrode. Such determinations have been made for ZnO (wurtzite) and TiO2 (anatase) electrodes constituted from nanocrystallites possessing average radii close to, and substantially larger than, the radius of a bound exciton in the corresponding bulk semiconductor. Electrodes constituted from crystallites whose radii are close to that of a bound exciton exhibit an onset for band gap absorption that is significantly blue-shifted. Those constituted from crystallites whose radii are substantially larger than that of a bound exciton exhibit an absorption onset characteristic of the bulk material. Knowing the absolute energies of band edges, the observed increase in band gap energy for electrodes constituted from confined nanocrystallites may be partitioned between the conduction and valence bands. A subsequent analysis permits determination of t...

Formation of self‐assembling CdSe quantum dots on ZnSe by molecular beam epitaxy

Abstract: We report the formation of self‐assembling CdSe quantum dots during molecular beam epitaxial growth on ZnSe and ZnMnSe. Atomic force microscopy measurements on specimens with uncapped dots show relatively narrow dot size distributions, with typical dot diameters of 40±5 nm, and with a diameter‐to‐height ratio consistently very close to 4:1. Uncapped CdSe dots are unstable with time: their density was observed to drop by an order of magnitude in 10 days, with clear evidence of ripening observed for some dots. Photoluminescence from capped dots indicates exciton localization much stronger than in ZnCdSe/ZnSe quantum wells, due to the additional lateral confinement.

Exciton Delocalization Length in the B850 Antenna of Rhodobacter sphaeroides

Abstract: The properties of elementary excited states in the B850 band of the peripheral light-harvesting antenna (LH2) of the photosynthetic purple bacterium Rhodobacter sphaeroides has been studied at room temperature by means of femtosecond transient absorption experiments combined with computer simulations. Polarized pump−probe kinetics have a fast component of 100 and 65 fs for the anisotropic and isotropic decays, respectively. Direct numerical simulations show that for incoherent hopping-like excitation transfer in the B850 ring of 18 Bchl a molecules at room temperature the fast component of the anisotropy decay is 3 times longer than the corresponding component of the isotropic decay, strongly suggesting that delocalized exciton states are involved in the observed dynamics. To estimate the coherence length of the exciton we have measured absorption difference spectra of LH2 from 810 to 880 nm 2 ps after the excitation into the B800 band with 75 fs laser pulses. Exciton calculations where also monomeric dou...

Exciton localization and temperature stability in self‐organized InAs quantum dots

Abstract: We investigated the temperature effect on exciton localization in self‐organized InAs quantum dots Quenching energy for excitons in reference quantum well and quantum dots was found to be 2 and 7 meV, respectively Thermoactivation energy of electron‐hole emission through a GaAs barrier in the quantum dots was measured as 46 meV We observed an unusual decrease of photoluminescence peak full width at half maximum with temperature, suggesting suppression of nonpredominant size quantum dot emissions due to carrier tunneling between nearby dots

Optical transitions and carrier relaxation in self assembled InAs/GaAs quantum dots

Abstract: We present experimental results concerning optical transitions and carrier dynamics (capture and relaxation) in self assembled InAs/GaAs quantum dot structures grown by metalorganic vapor phase epitaxy.Photoluminescence(PL) measurements at high excitation level reveal optical transitions above the ground state emission. These transitions are found to originate from occupied hole states by solving the quantum doteigenvalue problem. Time‐resolved studies after non‐resonant pulse excitation exhibit a relaxation ladder of the excited carriers from the GaAs barrier down to the ground state of the quantum dots. From both the continuous‐wave measurements and the PL‐decay curves we conclude that the carrier relaxation at non‐resonant excitation is mediated by Coulomb interaction (Auger effect). PL‐decay curves after resonant pulse excitation reveal a longer rise time compared to non‐resonant excitation which is a clear indication of a relaxation bottleneck inside the quantum dots. We interpret the rise time (≊ 400 ps) in this case to originate from relaxation via scattering by acoustic phonons. The PL‐decay time of the ground state emission ≊700 ps is interpreted as the excitonic lifetime of the quantum dot.

Ordered arrays of quantum dots: Formation, electronic spectra, relaxation phenomena, lasing

Abstract: Elastic relaxation on facet edges, renormalization of the surface energy of the facets, and interaction between i&no3 via the strained substrate are the driving forces for self-organization of ordered arrays of uniform coherent three-dimensional is/a& on crystal surfaces. For a (100) surface of a cubic crystal, two-dimensional square lattice of pyramid-like islands (quantum dots) with the periodicity along the directions of the lowest stiffness (OlO) and (OOI) has the minimum energy among different one-dimen- sional and two-dimensional arrays. For the InAs/GaAs(lOO) system, an equilibrium array of dots of the lateral size _ 120-140 A exists in a fixed range of growth parameters. T'he main luminescence peak at 1.1 eV, as well as peaks of excited states coincide in energy with the peaks revealed in the calorimetric absorption spectra regardless of the amount of InAs deposited (2-5 ML). Raman spectra indicate significant strain in InAs dots. The "phonon bottleneck" effect is bypassed via multi-phonon exciton and carrier relaxation. Ultranarrow lines (< 0.15 meV) are observed in cathodoluminescence spectra up to high temperatures. Low threshold current density operation via zero-dimensional states and ultrahigh temperature stability of the threshold current (T, = 450 K) are realized for a quantum dot injection laser. Increase in the gain and significant reduction in the radiative lifetime are possible via the self-organization of vertically-coupled quantum dots (VECODs) arranged in a well ordered artificial three-dimensional tefragonal lattice.

Coherent Versus Incoherent Energy Transfer and Trapping in Photosynthetic Antenna Complexes

Abstract: In this paper we present a study of a model in which there is energy transfer as well as a special site where an irreversible reaction takes place. This model has an arbitrary ratio of homogeneous broadening versus site interaction energy. This allows us to study the crossover from hopping dynamics to exciton dynamics. We show that for the survival time the hopping (Forster) approximation gives a surprisingly accurate final result even when the energy transfer is excitonic. Fluorescence depolarization however is a sensitive probe for the nature of the energy transfer. We study the number of coherent molecules by considering a generalization of the inverse participation ratio. For LH1, assuming that it is a ring of 16 dimers, we estimate that the excitation is, on the average, delocalized over two dimers. The excitation is localized by phonons.

Study of localized and extended excitons in 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) I. Spectroscopic properties of thin films and solutions

Abstract: Excitonic transitions responsible for low energy absorption and fluorescence in dilute solutions and thin films of the organic molecule 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) have been investigated. Combining the results of concentration-dependent solution and thin film absorption and fluorescence with results on thin film exciton diffusion lengths (presented in the following paper, Paper II), we proposed a comprehensive model for the low energy excitonic states in this archetype molecular crystal. We unambiguously identify an extended free charge transfer state at E F CT = 2.23±0.03 eV which has previously been observed to exhibit effects of quantum confinement in ultrathin layers grown under ultrahigh vacuum. This state self-traps at an energy of E ST CT = 2.11±0.04 eV due to the strong exciton-phonon coupling characteristic of this closely packed organic crystal.

Fundamental optical transitions in GaN

Abstract: A coherent picture for the band structure near the Γ point and the associated fundamental optical transitions in wurtzite (WZ) GaN, including the electron and hole effective masses and the binding energies of the free excitons associated with different valence bands, has been derived from time‐resolved photoluminescence measurements and a theoretical calculation based on the local density approximation. We also determine the radiative recombination lifetimes of the free excitons and neutral impurity (donor and acceptor) bound excitons in WZ GaN and compare ratios of the radiative lifetimes with calculated values of the ratios obtained with existing theories of free and bound excitons.

Enhancement of electron-hole exchange interaction in CdSe nanocrystals: A quantum confinement effect

Abstract: Photoluminescence of CdSe nanocrystals in a glass matrix was investigated at low temperature with size-selective excitation. The emission spectrum consists of a line a few meV below the excitation laser energy (denoted the F line) and a two-phonon replica superimposed on a broadband. The energy difference between the excitation energy and the F-line position increases with decreasing nanocrystal size. From the analysis of the time behavior of the luminescence and the degree of linear polarization, we attribute the F line to the recombination of the optically forbidden A exciton. Radiation recombination is made possible through a phonon-assisted virtual transition to the confined B-exciton state. The experimental degree of linear polarization is in good agreement with the theoretical calculations. The value of the electron-hole exchange energy obtained from the energy separation between the excitation energy and the F line is much larger than the bulk value and reaches 24 meV in 30-\AA{}-diam. nanocrystals. The size dependence of the exchange energy is in good agreement with the theoretical prediction in the limit of small nanocrystals. \textcopyright{} 1996 The American Physical Society.

Exciton fine structure in undoped GaN epitaxial films.

Abstract: We report on photoluminescence experiments on hexagonal GaN epitaxial films grown by hydride and organometallic vapor phase epitaxy on sapphire and 6H-SiC. At low temperatures we observe free and bound exciton recombinations, which allow us to establish the free-exciton binding energy and the localization energies of the excitons bound to neutral donors in undoped films. We demonstrate that the energetic positions of the excitonic recombination lines depend on the layer thickness and the substrate materials on which the layer was deposited. The influence of strain on the valence-band splittings can be quantified when observing the free-exciton transitions onto the different valence bands. The experimental results are compared to a theoretical calculation using a first-principle total-energy pseudopotential method within the local-density formalism. We present evidence for the existence of two shallow donors in GaN. One of them most likely stems from an intrinsic defect. \textcopyright{} 1996 The American Physical Society.

Near-field optical spectroscopy of localized excitons in strained CdSe quantum dots

Abstract: Control of the growth dynamics during the epitaxy of coherently strained ZnSe/CdSe quantum structures results in a varied interfacial texture that broadly defines two qualitatively different regimes for exciton localization. An island growth mode produces quantum dot regions in which the lateral confinement of excitons is directly revealed through the observation of resolution-limited (full width at half maximum of 0.8 meV) emission peaks in near-field photoluminescence spectra. By contrast, layer-by-layer growth produces potential fluctuations at length scales small compared to the exciton diameter, so that the localization of excitons is driven by a random interfacial potential with a smooth density of states.

Excitonic emissions from hexagonal GaN epitaxial layers

Abstract: Excitonic photoluminescence (PL) peaks from hexagonal GaN epilayers were investigated making a connection with the analysis of the photoreflectance spectra. Free exciton emissions associated with transitions from the conduction (Γ7c) band to the A (Γ9v) and B (Γ7uv) valence bands are dominant above 100 K. Values of the full widths at half maximum of them were smaller than the thermal energy kBT up to room temperature, which suggests the dominance of excitons in the PL spectra.

Influence of dispersive exciton motion on the recombination dynamics in porous silicon

Abstract: An extended photoluminescence (PL) study of porous silicon is presented. Different PL techniques have been used: continuous wave excited (cw) PL, selectively excited PL, excitation spectroscopy of the PL, time decay of the PL, and time resolved PL. These measurements have been performed on a set of samples of various porosities and at various temperatures. Strong experimental evidence is found for the influence of disorder and of dispersive motion of excitons on the recombination dynamics. The data are interpreted in the framework of the trap‐controlled hopping mechanism for the dispersive motion of excitons in a disordered array of Si nanocrystals.

Coherent dynamics of radiatively coupled quantum-well excitons.

Abstract: The optical response of radiatively coupled semiconductor multiple-quantum-well structures is investigated theoretically. It is shown that the transverse optical field leads to a coupling of exciton states within each well which causes a radiative decay and a mixing of excitonic resonances. Simultaneously, the field-induced long-ranged coupling between different wells leads to collective effects that are very sensitive to the detailed geometry of the structure. For a quantum-well spacing equal to an integer multiple of half the optical wavelength inside the medium, it is predicted that the collective effects cause a stimulated decay of electronic excitations that should be observable in either transmission or reflection geometry. On the other hand, in a quarter wavelength structure, the light-induced coupling causes an interwell energy transport and a splitting of the excitonic resonances, that should be observable as quantum beats in the time-resolved transmitted or reflected signal.

Effective-mass theory for InAs/GaAs strained coupled quantum dots

Abstract: In the framework of effective-mass envelope-function theory, the optical transitions of InAs/GaAs strained coupled quantum dots grown on GaAs (100) oriented substrates are studied. At the Gamma point, the electron and hole energy levels, the distribution of electron and hole wave functions along the growth and parallel directions, the optical transition-matrix elements, the exciton states, and absorption spectra are calculated. In calculations, the effects due to the different effective masses of electrons and holes in different materials are included. Our theoretical results are in good agreement with the available experimental data.

Exciton region reflectance of homoepitaxial GaN layers

Abstract: Reflection measurements in the exciton region of GaN homoepitaxial layers grown by metalorganic chemical vapor deposition (MOCVD) on GaN single crystals are reported. At low temperature (4.2 K) three free exciton lines have been found with energies; EA=3.4776 eV, EB=3.4827 eV, and EC=3.502 eV. The spin‐orbit parameter Δso=19.7±1.5 meV and the crystal field parameter Δcr=9.3±0.3 meV have been obtained. From temperature dependence of exciton spectra the energy gap dependence has been found: E(T)=E(0)−λ/[exp(β/T)−1] (λ=0.121 eV, β=316 K).

Binding of quasi two-dimensional biexcitons

Abstract: Summary form only given. In this presentation we report on a determination of the biexciton binding energies in GaAs-AlGaAs quantum wells of different widths and the results of a theoretical calculation of the ratio of the biexciton binding energy to that of the exciton. We determine the binding energies using spectrally resolved transient four-wave mixing (TFWM) and photoluminescence (PL).