Showing papers on "Exciton published in 1999"

Electronic and optical properties of strained quantum dots modeled by 8-band k⋅p theory

Abstract: We present a systematic investigation of the elastic, electronic, and linear optical properties of quantum dot double heterostructures in the frame of eight-band $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ theory. Numerical results for the model system of capped pyramid shaped InAs quantum dots in GaAs (001) with ${101}$ facets are presented. Electron and hole levels, dipole transition energies, oscillator strengths, and polarizations for both electron-hole and electron-electron transitions, as well as the exciton ground-state binding energy and the electron ground-state Coulomb charging energy are calculated. The dependence of all these properties on the dot size is investigated for base widths between 10 and $20$ nm. Results for two different approaches to model strain, continuum elasticity theory, and the Keatings valence force field model in the linearized version of Kane, are compared to each other.

Improved quantum efficiency for electroluminescence in semiconducting polymers.

Abstract: Some conjugated polymers have luminescence properties that are potentially useful for applications such as light-emitting diodes, whose performance is ultimately limited by the maximum quantum efficiency theoretically attainable for electroluminescence1, 2,. If the lowest-energy excited states are strongly bound excitons (electron–hole pairs in singlet or triplet spin states), this theoretical upper limit is only 25% of the corresponding quantum efficiency for photoluminescence: an electron in the π*-band and a hole (or missing electron) in the π-band can form a triplet with spin multiplicity of three, or a singlet with spin multiplicity of one, but only the singlet will decay radiatively3. But if the electron–hole binding energy is sufficiently weak, the ratio of the maximum quantum efficiencies for electroluminescence and photoluminescence can theoretically approach unity. Here we report a value of ∼50% for the ratio of these efficiencies (electroluminescence:photoluminescence) in polymer light-emitting diodes, attained by blending electron transport materials with the conjugated polymer to improve the injection of electrons. This value significantly exceeds the theoretical limit for strongly bound singlet and triplet excitons, assuming they comprise the lowest-energy excited states. Our results imply that the exciton binding energy is weak, or that singlet bound states are formed with higher probability than triplets.

Temperature quenching of photoluminescence intensities in undoped and doped gan

Abstract: This work discusses the temperature behavior of the various photoluminescence (PL) transitions observed in undoped, n- and p-doped GaN in the 9-300 K range. Samples grown using different techniques have been assessed. When possible, simple rate equations are used to describe the quenching of the transitions observed, in order to get a better insight on the mechanism involved. In undoped GaN, the temperature dependence of band edge excitonic lines is well described by assuming that the A exciton population is the leading term in the 50-300 K range. The activation energy for free exciton luminescence quenching is of the order of the A rydberg, suggesting that free hole release leads to nonradiative recombination. In slightly p-doped samples, the luminescence is dominated by acceptor related transitions, whose intensity is shown to be governed by free hole release. For high Mg doping, the luminescence at room temperature is dominated by blue PL in the 2.8-2.9 eV range, whose quenching activation energy is in the 60-80 meV range. We also discuss the temperature dependence of PL transitions near 3.4 eV, related to extended structural defects. (C) 1999 American Institute of Physics. [S0021-8979(99)05619-4].

Exciton diffusion and dissociation in conjugated polymer/fullerene blends and heterostructures

Abstract: We investigate the exciton dynamics in composite systems of conjugated polymers and fullerene molecules by photoluminescence (PL) and femtosecond transient absorption experiments. In solid mixtures (blends) we find a strong concentration-dependent quenching of the polymer PL. This is attributed to an efficient electron transfer (ET) from the photoexcited conjugated polymer to the fullerene. The ET dynamics is directly monitored by measuring the transient stimulated emission of the conjugated polymer. The transfer rate depends linearly on the ${\mathrm{C}}_{60}$ concentration and ranges between $(66\mathrm{ps}{)}^{\ensuremath{-}1}$ and $(5\mathrm{ps}{)}^{\ensuremath{-}1}$ for concentrations from 0.5% to 5%. This dependence is in accordance with an exciton diffusion process occurring prior to the ET. The exciton diffusion length in the conjugated polymer is directly determined by measuring the PL quenching in well-defined heterostructures comprising a self-assembled fullerene monolayer and a thin spin-coated polymer layer of variable thickness. From these measurements we infer a value of 14 nm for the exciton diffusion length in ladder-type poly (p-phenylene). Our results are of direct relevance for further optimization of polymer photovoltaic devices.

Exciton-exciton scattering dynamics in a semiconductor microcavity and stimulated scattering into polaritons

Abstract: We study the polariton dynamics in a semiconductor microcavity for small temperatures and zero exciton-cavity detuning, including the exciton-exciton and exciton-phonon scatterings. A bottleneck in the relaxation of excitons into lower polariton is found, which persists up to high densities. We then consider injection of large populations of lower polaritons with an external pump. All scatterings featuring the lower polariton as the final state become stimulated. In particular, scattering of two excitons into both lower and upper polaritons shows direct evidence of stimulation. Within the rate equation approach, we also predict sizable saturation effects due to the stimulated emission.

Fine Structure of Biexciton Emission in Symmetric and Asymmetric CdSe/ZnSe Single Quantum Dots

Abstract: The influence of quantum dot (QD) asymmetry on the emission of single three-dimensionally confined biexcitons in II-VI semiconductor nanostructures has been studied by magnetophotoluminescence spectroscopy. Investigating both the biexciton and the single-exciton transition in the same single QD, we obtain a unified picture of the impact of electron-hole exchange interaction on the fine structure and the polarization properties of optical transitions in QDs. The exchange splitting is demonstrated to have a strong influence on the derivation of the biexciton binding energy, which we determine to be about 17 meV, much less than the separation between exciton and biexciton lines ( $\ensuremath{\approx}24$ meV) in the spectra.

Room Temperature Polariton Emission from Strongly Coupled Organic Semiconductor Microcavities

Abstract: We report a room temperature study of the strong exciton-photon coupling regime in a planar microcavity, containing cyanine dye aggregates with delocalized exciton wave functions ( $J$ aggregates). Giant Rabi splittings of 80 meV between upper and lower polariton branches are observed and cavity polariton emission is detected from the lower polariton branch. The small linewidth and large oscillator strength characteristic of $J$ aggregates make them a favorable physical structure to allow the observation of strong coupling for organic semiconductors.

Many-body pseudopotential theory of excitons in inp and cdse quantum dots

Abstract: We present a pseudopotential approach to the calculation of the excitonic spectrum of semiconductor quantum dots. Starting from a many-body expansion of the exciton wave functions in terms of single-substitution Slater determinants constructed from pseudopotential single-particle wave functions, our method permits an accurate and detailed treatment of the intraconfiguration electron-hole Coulomb and exchange interactions, while correlation effects can be included in a controlled fashion by allowing interconfiguration coupling. We calculate the exciton fine structure of InP and CdSe nanocrystals in the strong-confinement regime. We find a different size dependence for the electron-hole exchange interaction than previously assumed (i.e., ${R}^{\ensuremath{-}2}$ instead of ${R}^{\ensuremath{-}3})$. Our calculated exciton fine structure is compared with recent experimental results obtained by size-selective optical spectroscopies.

Charge- and energy-transfer processes at polymer/polymer interfaces : a joint experimental and theoretical study

Abstract: When an exciton approaches the interface between two conjugated polymers, either energy or charge transfer can take place. We present a detailed experimental investigation of these processes in various binary polymer systems. The results are interpreted in the context of quantum-chemical calculations that provide estimates of the relative energies of intrachain versus interchain excited states in pairs of various PPV-related chains. In particular, we show that charge-transfer occurs at the interface between MEH-PPV and a cyanosubstituted PPV derivative, whereas energy transfer takes place at the interface of PPV with the same cyanosubstituted polymer. @S0163-1829~99!08031-5#

Exciton localization and the Stokes’ shift in InGaN epilayers

Abstract: We report a comparative study of the emission and absorption spectra of a range of commercial InGaN light-emitting diodes and high-quality epilayers A working definition of the form of the absorption edge for alloys is proposed, which allows a unique definition of the Stokes’ shift A linear dependence of the Stokes’ shift on the emission peak energy is then demonstrated for InGaN using experimental spectra of both diode and epilayer samples, supplemented by data from the literature In addition, the broadening of the absorption edge is shown to increase as the emission peak energy decreases These results are discussed in terms of the localization of excitons at highly indium-rich quantum dots within a phase-segregated alloy

Optical Excitations in Conjugated Polymers

Abstract: We investigate optical absorption spectra and excitons in conjugated polymers from first principles. This is done by calculating the one-particle and the two-particle Green{close_quote}s function, including relevant many-body effects, and evaluating quasiparticle and optical excitations. Trans-polyacetylene and poly-phenylene-vinylene are studied as prototype long chain polymers. The electron-hole interaction gives rise to large exciton binding energies ({approximately}1 thinspthinspeV) and dramatically alters the optical spectra. The calculated exciton wave functions are very extended in real space. {copyright} {ital 1999} {ital The American Physical Society}

Photoluminescence and reflectance spectroscopy of excitonic transitions in high-quality homoepitaxial GaN films

Abstract: We report on highly resolved photoluminescence (PL) and reflectance (RF) spectra of a homoepitaxial GaN layer grown by metal-organic vapor phase epitaxy This sample exhibits narrow linewidths of several PL emission peaks, down to $\ensuremath{\approx}100\ensuremath{\mu}\mathrm{eV}$ full width at half maximum for some bound exciton transitions As a consequence, we have detected new PL features as, eg, a fivefold fine structure of the donor-bound exciton line at $\ensuremath{\approx}3471\mathrm{eV},$ and other known PL transitions could be determined with high precision In RF measurements, the extraordinary quality of the epitaxial layer allowed observation of weakly damped excitonic ground-state transitions and of narrow excited exciton transitions with high signal-to-noise ratio

Enhanced Polar Exciton-LO-Phonon Interaction in Quantum Dots

Abstract: Phonon-assisted exciton transitions in self-organized InAs/GaAs quantum dots (QDs) are investigated by photoluminescence (PL) and PL excitation spectroscopy. An unexpectedly enhanced polar exciton-LO-phonon interaction for such strained low-symmetry QDs is found. Calculations in the adiabatic approximation indicate that the particular quantum confinement and piezoelectric effect together account for the enhanced coupling. The results provide new insights into the long-standing problem of the exciton-phonon interaction in zero-dimensional systems.

A quantitative numerical model of multilayer vapor-deposited organic light emitting diodes

Abstract: A one-dimensional numerical model for the quantitative simulation of multilayer organic light emitting diodes (OLEDs) is presented. It encompasses bipolar charge carrier drift with field-dependent mobilities and space charge effects, charge carrier diffusion, trapping, bulk and interface recombination, singlet exciton diffusion and quenching effects. Using field-dependent mobility data measured on unipolar single layer devices, reported energetic levels of highest occupied and lowest unoccupied molecular orbitals, and realistic assumptions for experimentally not direct accessible parameters, current density and luminance of state-of-the-art undoped vapor-deposited two- and three-layer OLEDs with maximum luminance exceeding 10000 cd/m2 were successfully simulated over 4 orders of magnitude. For an adequate description of these multilayer OLEDs with energetic barriers at interfaces between two adjacent organic layers, the model also includes a simple theory of charge carrier barrier crossing and recombinati...

Luminescence Properties of Thiol-Stabilized CdTe Nanocrystals

Abstract: Thiol-capped CdTe nanocrystals with cubic zinc blende structure are synthesized in aqueous solution. Their steady-state and time-resolved luminescence characteristics are studied at room and liquid nitrogen temperatures. A strong exciton luminescence peak at 2.3 eV dominates the emission spectrum of CdTe nanocrystals at room temperature, whereas the trap band centered at 2.0 eV undergoes substantial temperature quenching. Luminescence excitation spectra reveal different channels leading to radiative recombination via either excitons or traps. The mean luminescence decay time of CdTe nanocrystals at room temperature decreases from 120 ns at 1.94 eV to 20 ns at 2.43 eV. Luminescence decay kinetics of CdTe nanocrystals are strongly nonexponential and are described by extremely broad lifetime distributions lying within the range from a few hundred picoseconds to a few hundred nanoseconds.

Biexciton versus Exciton Lifetime in a Single Semiconductor Quantum Dot

Abstract: The decay characteristics of excitons and biexcitons in one single semiconductor quantum dot (QD) are directly monitored using time- and spatially resolved photoluminescence spectroscopy. The experiments are performed on a CdSe/ZnSe QD, occupied by either one or two excitons at a time, allowing a direct comparison between the radiative lifetime of a biexciton and an exciton confined in the same QD. The rather surprising result of comparable recombination rates for both states is related to the spatial wave function distribution and the spin structure of the particles and their coupling to the photon field, i.e., the superradiance effect.

Optical properties of InGaN quantum wells

Abstract: The emission mechanisms of strained InGaN quantum wells (QWs) were shown to vary depending on the well thickness L and InN molar fraction x . The QW resonance energy was shifted to lower energy by the quantum confined Stark effect (QCSE) due to the internal piezoelectric field, F PZ . The absorption spectrum was modulated by QCSE and quantum-confined Franz–Keldysh effect (QCFK) for the wells, in which, for the first approximation, the product of F PZ and L (potential drop across the well) exceeds the valence band discontinuity, Δ E V . In this case, dressed holes are confined in the triangular potential well formed at one side of the well. This produces apparent Stokes-like shift (vertical component). The QCFK further modulated the absorption energy for the wells with L greater than the three dimensional free exciton Bohr radius, a B . For the wells having high InN content ( F PZ × L >Δ E V , Δ E C ), electron and hole confined levels drop into the triangular potential wells formed at opposite sides of the wells, which reduces the wavefunction overlap. Doping of Si in the barriers partially screens F PZ resulting in a smaller Stokes-like shift, shorter recombination decay time, and higher emission efficiency. Si-doping was found to improve the interface quality and surface morphology, resulting in an efficient carrier transfer from high to low bandgap energy portions of the well. Effective in-plane localization of carriers in quantum disk size potential minima, which are produced by nonrandom alloy potential fluctuations enhanced by the large bowing parameter and F PZ , produces confined e–h pair whose wavefunctions are still overlapped. Their excitonic features are pronounced provided that L a B and F PZ × L E V (quantized exciton). Several cw laser wafers exhibit stimulated emission from these energy tail states even at room temperature.

Excitons and Optical Spectrum of the Si ( 111 ) − ( 2 × 1 ) Surface

Abstract: We investigate excitons at the $\mathrm{Si}(111)\ensuremath{-}(2\ifmmode\times\else\texttimes\fi{}1)$ surface and their optical spectrum from first principles. This is done by solving the Bethe-Salpeter equation for the two-particle Green's function, including the electron-hole interaction. The optical spectrum of the surface is dominated by a surface exciton formed from the $\ensuremath{\pi}$-bonded surface states. The excitonic binding energy is more than 1 order of magnitude larger than in bulk Si. The two-particle wave function of the exciton state is strongly localized at the surface and exhibits distinct anisotropy due to the surface reconstruction.

Resonant hole localization and anomalous optical bowing in InGaN alloys

Abstract: Using large supercell empirical pseudopotential calculations, we show that alloying of GaN with In induces localization in the hole wave function, resonating within the valence band. This occurs even with perfectly homogeneous In distribution (i.e., no clustering). This unusual effect can explain simultaneously exciton localization and a large, composition-dependent band gap bowing coefficient in InGaN alloys. This is in contrast to conventional alloys such as InGaAs that show a small and nearly composition-independent bowing coefficient. We further predict that (i) the hole wave function localization dramatically affects the photoluminescence intensity in InGaN alloys and (ii) the optical properties of InGaN alloys depend strongly on the microscopic arrangement of In atoms.

Direct observation of excitonic rabi oscillations in semiconductors

Abstract: We observe multiple excitonic optical Rabi oscillations in a semiconductor quantum well. Up to eight oscillation periods of the heavy-hole exciton density on a subpicosecond time scale are observed. An approximate linear dependence of the oscillation frequency on the light field amplitude is established. The experiment is based on a two-color detection scheme which allows for the observation of the heavy-hole exciton density via transmission changes at the light-hole exciton. The observations are in good agreement with theoretical computations based on multiband semiconductor Bloch equations.

Exciton dissociation charge transport, and recombination in ultra thin, conjugated polymer-TiO2 nanocrystal intermixed composites

Abstract: A detailed study of the optoelectronic processes occurring in ultrathin ${\mathrm{TiO}}_{2}$ nanocrystal-conjugated polymer, poly(p-phenylene vinylene) (PPV) composites is presented Composites of ultrathin films (about 100 nm) are studied spectroscopically and as the active medium in photovoltaic devices of the structure (Al/composite/indium tin oxide) By varying the weight ratio of the nanocrystals and using results of photoluminescence efficiency, photocurrent, and photovoltaic measurements and time-resolved microwave conductivity, we are able to construct a well-defined picture of the relevant processes in the composite: including exciton dissociation, charge transport, and recombination We combine the experimental results with a hopping model for charge transport in a nanocrystal lattice (random walk or biased random walk) to determine the probability of electron collection as a function of distance from a collecting contact in a nanocrystal lattice The combined results indicate that most photogenerated excitons lead to charge separation at the interface between the polymer and the nanocrystals above 20-wt % ${\mathrm{TiO}}_{2}$ nanocrystals, but the electron collection efficiency in photovoltaic devices is limited by fast recombination The transport model indicates that even for a relatively long recombination time and a well-ordered nanocrystal lattice, most of the collected charge will originate from the first several nanocrystal layers at the electrode rather than from sites throughout the film, due to the recombination process We also argue that the existence of a photocurrent in these and related devices is not necessarily evidence of charge transport through a network of the nanocrystals (or other component), as the quantum yields can be accounted for by interfacial charge transfer at the contact alone Quantum yields for collecting charge following direct band-gap excitation of the ${\mathrm{TiO}}_{2}$ are more than a factor of 10 larger than for excitation into the polymer, suggesting that either hole transfer to the polymer, or some preceding process, is rate limiting and much slower than the corresponding process following polymer excitation We also examine the key differences between the mechanisms underlying conjugated polymer:nanocrystal devices and conventional, silicon pn devices

Quantum-Confined Stark Effect in an AlGaN/GaN/AlGaN Single Quantum Well Structure

Abstract: Excitonic absorption was observed in a transmittance spectrum of AlGaN/GaN/AlGaN single quantum well structure with a well width of 5 nm at room temperature. The total internal electric field strength in the well was about 0.73 MV/cm, which was estimated from the absorption peak position based on a simple calculation, neglecting excitons. The observation is clearly due to the quantum-confined Stark effect. While excitonic absorption was clearly observed even in such a high internal field, no light emission was detected under a He-Cd laser excitation at temperatures ranging from room temperature to T = 10 K. Light emission accompanied by a blue shift of the emission peak and an increase of emission intensity was observed under higher excitation power density. The obvious conclusion in the present case is that the presence of a high internal electric field in the well is a disadvantage for light emission.

Time-resolved photoluminescence as a probe of internal electric fields in GaN-(GaAl)N quantum wells

Abstract: Very strong coefficients for spontaneous and piezoelectric polarizations have recently been predicted for III-V nitride semiconductors with natural wurtzite symmetry. Such polarizations influence significantly the mechanisms of radiative emissions in quantum-confinement heterostructures based on these materials. The photoluminescence decay time of excitons is used as a probe of internal electric fields in GaN-(Ga, Al)N quantum wells in various configurations of strain, well widths, and barrier widths. The measured decays are not only controlled by radiative lifetimes, which depend on the fields inside GaN wells but also on the nonradiative escape of carriers through Ga1 - xAlyN barriers, which depends on their widths and on the electric field in these layers. It is shown in particular that the magnitude of the held in the wells is not a simple function of the strain of these layers via the only piezoelectric effect, but rather the result of the interplay of spontaneous and piezoelectric polarizations in both well and barrier materials. [S0163-1829(99)02923-9].

Exciton Effects of Optical Transitions in Single-Wall Carbon Nanotubes.

Abstract: Absorption spectra have been measured in the near-infrared region for a thin-film sample of single-wall carbon nanotubes of 1.2–1.5 nm diameter. Two absorption peaks are observed at 0.7 and 1.2 eV. The observed spectrum has been compared with the calculation based on the tight-binding model taking into account the distribution of diameter and the helical arrangement of tubes. We have found that a one-dimensional exciton effect plays an important role in the fundamental optical transition of semiconducting single-wall carbon nanotubes.

Synthesis of cadmium sulfide Q particles in water-in-CO2 microemulsions

Abstract: Semiconductor nanoparticles of cadmium sulfide were synthesized in ammonium perfluoropolyether (PFPE-NH{sub 4}) stabilized water-in-CO{sub 2} microemulsions. The particle size was tuned by varying the water-to-surfactant molar ratio ({omega}{sub 0}): {omega}{sub 0} ratios of 5 and 10 yielded nanocrystals with exciton energies of 3.86 and 3.09 eV, corresponding to mean particle radii of 0.9 and 1.8 nm, respectively. These exciton energies are significantly higher than the bulk band gap energy for CdS (2.45 eV) due to quantum confinement effects. Effectively, {omega}{sub 0} controls the size of the compartmentalized water droplets in which the particles grow.

The rate of radiative recombination in the nitride semiconductors and alloys

Abstract: The radiative recombination rates of free carriers and lifetimes of free excitons have been calculated in the wide band gap semiconductors GaN, InN, and AlN of the hexagonal wurtzite structure, and in their solid solutions GaxAl1−xN, InxAl1−xN and GaxIn1−xN on the base of existing data on the energy band structure and optical absorption in these materials. We determined the interband matrix elements for the direct optical transitions between the conduction and valence bands, using the experimental photon energy dependence of absorption coefficient near the band edge. In our calculations we assumed that the material parameters of the solid solutions (the interband matrix element, carrier effective masses, and so on) could be obtained by a linear interpolation between their values in the alloy components. The temperature dependence of the energy gap was taken in the form proposed by Varshni [Physica 34, 149 (1967)]. The calculations of the radiative recombination rates were performed in a wide range of temp...

Coulomb memory signatures in the excitonic optical stark effect

Abstract: Differential absorption spectra of high-quality InGaAs quantum wells are presented for various pump detunings and polarization configurations. For low intensity pump pulses tuned well below the exciton a redshift is observed for opposite circularly polarized probe pulses. Microscopic calculations show that this redshift originates from memory effects in the Coulomb-induced excitonic correlations.

Vibronic structure of PTCDA stacks: the exciton–phonon-charge-transfer dimer

Abstract: Perylenetetracarboxylic acid dianhydride (PTCDA) stacks face-to-face in crystals and multiple quantum wells (MQWs). Excitations of PTCDA stacks are mixed molecular (Frenkel) and charge-transfer (CT) states coupled to a molecular vibration. Eclipsed stacks and molecular conjugation imply strong Frenkel–CT mixing in absorption and electroabsorption, with k=0 at the top of the exciton band, and negligible mixing at k=π for emission from the bottom. The exciton–phonon-CT dimer developed for k=0 processes is a nonadiabatic approximation for narrow CT bands. Quantitative dimer spectra are obtained in the vibronic basis of displaced harmonic oscillators for excited PTCDA and radical ions. We present a joint analysis of absorption and emission in PTCDA stacks and MQWs using parameters from solution, molecular calculations, and related conjugated systems. Polarized single-crystal absorption decisively relates the entire 2–3 eV system to molecular π–π* transitions, while electroabsorption with field along the stack implicates adjacent ions in the stack. The simple structure and extensive PTCDA spectra make possible detailed modelling of mixed Frenkel–CT vibronics that were far less accessible in previous organic molecular crystals. Since the coupled mode is closely related to polyenes and conjugated polymers, PTCDA provides a bridge between molecular insulators and extended systems capable of charge transport.