Showing papers on "Exciton published in 1984"

Electron–electron and electron‐hole interactions in small semiconductor crystallites: The size dependence of the lowest excited electronic state

Abstract: We model, in an elementary way, the excited electronic states of semiconductor crystallites sufficiently small (∼50 A diam) that the electronic properties differ from those of bulk materials. In this limit the excited states and ionization processes assume a molecular‐like character. However, diffraction of bonding electrons by the periodic lattice potential remains of paramount importance in the crystallite electronic structure. Schrodinger’s equation is solved at the same level of approximation as used in the analysis of bulk crystalline electron‐hole states (Wannier excitons). Kinetic energy is treated by the effective mass approximation, and the potential energy is due to high frequency dielectric solvation by atomic core electrons. An approximate formula is given for the lowest excited electronic state energy. This expression is dependent upon bulk electronic properties, and contains no adjustable parameters. The optical f number for absorption and emission is also considered. The same model is applied to the problem of two conduction band electrons in a small crystallite, in order to understand how the redox potential of excess electrons depends upon crystallite size.

Electric field assisted dissociation of charge transfer states as a mechanism of photocarrier production

Abstract: The lowest charge‐transfer excited state (CT1) of electron donor–acceptor crystals or polymers is demonstrated to be a plausible precursor of free charge carriers when such materials are photoexcited Rate constants for the dissociation of charge–transfer states are formulated for two approximate descriptions of CT1: classical ion pair and Wannier exciton The electric field dependence of the dissociation rate constant is postulated to be given by Onsager’s 1934 theory (O‐34) of ion pair dissociation This formulation of CT1 dissociation obviates the need to invoke electron–hole ‘‘thermalization’’ lengths of 2 to 3 nm in order to explain free charge carrier formation in donor–acceptor materials

Room temperature excitonic nonlinear absorption and refraction in GaAs/AlGaAs multiple quantum well structures

Abstract: We present detailed experimental studies and modeling of the nonlinear absorption and refraction of GaAs/AlGaAs multiple quantum well structures (MQWS) in the small signal regime. Nonlinear absorption and degenerate four-wave mixing in the vicinity of the room temperature exciton resonances are observed and analyzed. Spectra of the real and imaginary parts of the nonlinear cross section as a function of wavelength are obtained, and these are in excellent agreement with experimental data. A simple model for excitonic absorption saturation is proposed; it accounts qualitatively for the very low saturation intensities of room temperature excitons in MQWS.

High‐speed optical modulation with GaAs/GaAlAs quantum wells in a p‐i‐n diode structure

Abstract: A new type of high‐speed optical modulator is proposed and demonstrated An electric field is applied perpendicular to GaAs/GaAlAs multiple quantum well layers using a ‘‘p‐i‐n’’ diode doping structure of 4‐μm total thickness The optical absorption edge, which is particularly abrupt because of exciton resonances, shifts to longer wavelengths with increasing field giving almost a factor of 2 reduction in transmission at 857 nm with an 8‐V reverse bias The shifts are ascribed to changes in carrier confinement energies in the wells The observed switching time of 28 ns is attributed to RC time constant and instrumental limitations only, and fundamental limits may be much faster

Energy levels of Wannier excitons in G a A s − Ga 1 − x Al x As quantum-well structures

Abstract: Energy levels of Wannier excitons in a quantum-well structure consisting of a single slab of GaAs sandwiched between two semi-infinite layers of ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Al}}_{x}\mathrm{As}$ are calculated with the use of a variational approach. Owing to lowering of symmetry along the axis of growth of this quantum-well structure and the presence of energy-band discontinuities at the interfaces, the degeneracy of the valence band of GaAs is removed, leading to two exciton systems, namely, the heavy-hole exciton and the light-hole exciton. The values of the binding energies of the ground state and of a few lowlying excited states of these two exciton systems are calculated as a function of the size of the GaAs quantum well for several values of the heights of the potential barriers and their behavior is discussed. The results thus obtained are also compared with the available experimental data. The reliability of the various approximations made in this calculation is discussed.

Low-temperature exciton trapping on interface defects in semiconductor quantum wells

Abstract: We present the results of low-temperature photoluminescence experiments performed on GaAs single quantum wells grown by metal organic chemical-vapor deposition. The luminescence line is down shifted by a few milli-electron-volts below the $n=1$ heavy-hole exciton absorption peak. This behavior is interpreted in terms of exciton trapping on interface defects. A simple model provides reasonable values for the exciton binding energy on these defects as well as insights on the lack of thermalization which characterizes the trapped exciton photoluminescence.

Magneto-optical determination of exciton binding energy in GaAs- Ga 1 − x Al x As quantum wells

Abstract: The binding energy of the exciton in GaAs quantum wells confined within Ga-Al-As is determined by the observation of the different behavior of the ground state and the excited states of excitonic transitions of different subbands with excitation spectroscopy in magnetic fields. An increase in the binding energy with decreasing well thickness is found with values higher than theoretically expected. This discrepancy is explained by an experimentally determined higher reduced mass than that used in the theoretical calculations.

Blue Shift of the Exciton Resonance due to Exciton-Exciton Interactions in a Multiple-Quantum-Well Structure

Abstract: Multiple-quantum-well structures (MQWS) are attracting much interest for their potential use as efficient active devices in optoelectronics and nonlinear optical signal processing. Based on their strong excitonic optical nonlinearity, operation of MQWS as optical bistable elements and logical gates has been demonstrated both at low and room temperatures. To obtain information about the intrinsic response time of such devices, we have investigated the dynamics of exciton screening of a MQWS with subpicosecond resolution, at T = 15 K and T = 220 K. We have observed an unexpected shift of the exciton resonance to higher energy as a result of exciton-exciton interactions.

Catalysis of photochemical reactions by colloidal semiconductors

Abstract: Illumination of semiconductor colloids such as ZnS , CdS and Ti02, generates electrons and positive holes which may react with the aqueous solvent or dissolved substances In ZnS and CdS , recombination of the charge carriers is often accompanied by emission of light The absorption spectra of very small CdS or ZnS particles (1 3 nm) show changes which indicate a transition from semiconductor to polymolecular material The conduction band energy is shifted, and the exciton transition band becomes more pronounced The fluorescence spectra of small CdS particles contain a band at the onset of absorption and two bands at longer wavelengths They are attributed to emission from the short-lived exciton state and from longer-lived states of trapped electrons and holes The chemical reactions described include the photo-anodic and the photocathodic dissolution of colloidal sulfides, the reduction of water and carbon dioxide , and the oxidation of alcohols and sulfite anions Two-electron (two-hole) and one-electron (one-hole) transfer mechanisms are discussed Flash photolysis studies of colloidal Ti02 gave the absorption spectra of long-lived electrons and positive holes which are trapped in surface states of different energies Chemical reactions of these charge carriers are also described, including the reduction of tetranitromethane, the oxidation of SCN and OHanions, and of various organic compounds The electrochemical nature of all these photo-reactions is emphasized

Exciton satellites in photoelectron-spectra

Abstract: We propose a new mechanism for satellite peaks in photoelectron spectra. Besides data from the literature new measurements of spectra of early first-row transition-metal compounds are discussed. The observed strong satellites of the metal core peaks of these materials are assigned to the creation of excitons on the anions. In our model, satellite energies can be determined from optical excitation spectra and satellite intensities from anion polarizabilities. It is shown that sharp satellites of this kind are expected if the exciton dispersion is not too large so that exciton states exist over large regions of the Brillouin zone. We also consider competition between charge transfer and exciton satellites. It turns out that only weak charge-transfer satellites are expected for the early $3d$-metal compounds, in agreement with experiment.

Observation of one-monolayer size fluctuations in a GaAs/GaAlAs superlattice

Abstract: The observation of one‐monolayer well size fluctuations in a superlattice is reported. Luminescence experiments show that several exciton peaks occur, each of them corresponding to a discrete well width differing by one monolayer from the next one. This attribution is confirmed by the intentional introduction of a larger well in the structure. It is also checked by comparison with the x‐ray diffraction results that confirm the variations of the superlattice parameters observed in luminescence, and by photoluminescence excitation experiments that also show a splitting.

Photoluminescence of AlGaAs/GaAs quantum wells grown by metalorganic chemical vapor deposition

Abstract: Photoluminescence of AlxGa1−xAs/GaAs(x=0.54) single quantum wells grown by metal organic chemical vapor deposition has been investigated at both 75 and 4.2 K. AlGaAs/GaAs heterojunction abruptness was estimated to be within a few atomic layers by comparing the peak energy of the quantum well photoluminescence with the values calculated on the assumption that the radiative transition takes place between the n=1 electron subband and the n=1 heavy‐hole subband in the finite square potential well at 75 K. At 4.2 K, however, the peak energy shifts by several meV below the calculated energy, the cause of which may be formation of a two‐dimensional free exciton. The sharp photoluminescence line of the narrower well indicates that the fluctuation in thickness is less than one half the lattice constant. The emission peak shift to lower energy with the increase of excitation intensity may be the result of exchange interaction among carriers.

Delocalized excitons in semiconductor heterostructures

Abstract: A delocalized exciton state is revealed by resonant Raman scattering in GaAs-AlGaAs multiple-quantum-well heterostructures where only lower energy, confined quasi-two-dimensional excitons had previously been observed. In spite of its extension across the abrupt GaAs-AlGaAs interface, the delocalized exciton remains a well-defined state inhomogeneously broadened to widths between 6 and 14 meV. We estimate an exciton binding energy greater than 2 meV. Scattering of the delocalized state by the exciton-optical phonon interaction results in transitions to both delocalized and quasi-two-dimensional localized excitons.

Theory of non-equilibrium diffusive transport in disordered materials

Abstract: A new method has been developed to study non-equilibrium transport and energy relaxation phenomena in disordered organic and inorganic semiconductors. Given a distribution of localized states, the mean-square displacement and average energy of the excitation may be evaluated as functions of initial energy and time. The theory has a wide range of possible applications, for example to exciton and carrier transport in disordered organic and inorganic semiconductors. In disordered organic semiconductors, the density of excited states is usually well described by a Gaussian model. Current and energy relaxation and temperature-dependent mobilities have been studied for this Gaussian model using Monte-Carlo simulations. The theoretical results are found to be in excellent agreement with those of the simulations.

Light and heavy valence subband reversal in GaSb-AlSb superlattices

Abstract: : We report optical-absorption measurements in high-quality GaSb-AlSb superlattices. The spectra exhibit the two-dimensional density of states and free-exciton peaks. The effect of strain induced by the lattice mismatch is manifested by the anomalous energies of the absorption edges and the reversal of the heavy-and light-hole exciton positions. The data are in good agreement with a simple effective-mass theory. originator-supplied key words include: GaSb/AlSb superlattices; light/heavy hole subbands; lattice mismatch induced strain; X-ray diffraction; and optical absorption.

ODMR of recombination centres in crystalline quartz

Abstract: The well known 2.8 eV luminescence band in quartz has been studied using optically detected magnetic resonance (ODMR). A triplet state with a very large fine-structure splitting is found to contribute to the emission. The principle axes of the triplet are identified. The possibility that the luminescence originates in the decay of a self-trapped exciton involving a transient oxygen Frenkel pair is discussed.

Luminescence studies of individual dislocations in II-VI (ZnSe) and III-V (InP) semiconductors

Abstract: Results are presented of high-resolution luminescence studies from individual dislocations and related defects in ZnSe and InP performed in a transmission electron microscope. In the case of ZnSe unusual luminescence bands (Y at 2.60 eV and S at 2.52 eV) originally observed in photoluminescence studies are attributed to dislocations. In some instances, complete quenching of the excitonic transitions was observed to correlate with the presence of Y emission from complex dislocation tangles. In the case of individual screw dislocations this quenching of the exciton luminescence was found to be variable; for example reduction of the exciton signal was not always observed. For InP, donor-exciton-related transitions were quenched at individual screw dislocations. Donor-acceptor pair/free-to-bound and deep level (band C) transitions were unaffected. For the case of InP, unlike ZnSe, no dislocation-related luminescence was observed within the system detection limit (0.7-4.0 eV).

Extended Peierls-Hubbard Model for One-Dimensional N-Sites N-Electrons System. III. Lattice Relaxation after Optical Excitation in CDW

Abstract: The self-trapping of exciton in the CDW phase of one-dimensional many-electron system with short-ranged electron-phonon(e-p) and electron-electron(e-e) interactions is studied by a variational method for exciton and in the adiabatic limit for phonons, so as to clarify its lattice relaxation after the light excitation. The adiabatic potential surface describing the relaxation process is calculated in a wide region of four parameters characterizing the system: the transfer energy of electron T , the e-p coupling energy S , intra- and inter-site e-e repulsive energies U and V . It is shown that in the strong coupling case S ≫2 T , the exciton relaxes to metastable ground states such as an island of SDW phase, and only in the inter-mediate case S ∼2 T it relaxes to a luminescent state localized in the middle of the energy-gap. These results can well explain the luminescence of Wolffram's red salt.

Luminescence of Free and Self-Trapped Excitons in α- and β-Perylene Crystals

Abstract: Luminescence spectra and decaytimes have been measured over a wide temperature range. Thermal equilibrium population between free and self-trapped excitons has been confirmed in both dimeric (α) and monomeric (β) forms of perylene crystals. Two-center type self-trapped exciton in α-perylene, and one-center type and two-center type self-trapped excitons in β-perylene are stable and play prominent roles in exciton radiative decay. The decaytimes of free and self-trapped exciton luminescence are discussed in terms of exciton-lattice interaction. The exciton band width is briefly discussed.

Picosecond time-resolved study of excitons in GaAs-A1As multi-quantum-well structures

Abstract: Energy- and time-resolved measurements of luminescence of $1s$ excitons ($n=1,e\ensuremath{-}hh$) in GaAs-A1As multi-quantum-well structures have been carried out for the first time in the picosecond time domain. Dynamical population changes of excitons are directly visualized in the energy-time coordinates. Results indicate that excitons lose their energy in the exciton band at a rate of 1.0\ifmmode\times\else\texttimes\fi{}${10}^{6}$ eV/s. This rate is much slower than the calculated kinetic-energy-loss rate and is ascribed to the random nature of the well.

Polarons and Excitons in Polar Semiconductors and Ionic Crystals

Abstract: Studies of the Free and Bound Magneto-Polaron and Associated Transport Experiments in n-InSb and Other Semiconductors- Experimental Study of Hot Electrons in Silver Halides at Crossed Electric and Magnetic Fields- Dynamical and Nonlinear Profiles of Polarons and Excitons in Pure and Ultrapure AgCl, AgBr, and AgClxBr1-x- Some Recent Developments in the Theory of Polarons- Two Applications of Polaron Theory: Cyclotron Resonance and Relaxation of Hot Charge Carriers- Coupled Plasmon-Polar Phonon Modes in Semiconductors: Spatial Dispersion and Other Properties- General Aspects of the Functional-Integral Approach to the Polaron and Related Systems- to the Theory of Excitons- Excitons and Exciton Relaxation in Silver Halides- Electron-Hole Liquid Condensation in Semiconductors- Transient Luminescence, Transport and Photoconductivity in Chalcogenide Glasses- Surface State Electrons above a Liquid Helium Film and the Surface Polaron Problem- Author Index- Material Index

Effects of carrier confinement in graded AlGaAs/GaAs heterojunctions

Abstract: An unusual new line has been observed in the 1.4‐K photoluminescence spectrum of GaAs/AlGaAs epilayers grown by liquid phase epitaxy. A very strong, broad, and asymmetric line is seen, with peak energy ranging from the bound exciton to the shallowest acceptor, increasing roughly linearly with the logarithm of the excitation power. By employing a novel step‐etching technique, this transition is shown to originate from the GaAs/AlGaAs heterojunction. A qualitative model is proposed to explain the observed data.

Coulomb Interaction and Excitons in a Superlattice

Abstract: An exact expression is found for the potential of a point charge in a periodic layered system, consisting of alternating layers with two different values of the dielectric constant. The most interesting limiting cases for this potential are considered. Energy spectrum, bound energy, and the ground state radius of the exciton, which is formed by an electron and a hole, belonging to the same layer of the superlattice, are found. [Russian Text Ignored].