Showing papers on "Exciton published in 1985"
TL;DR: Detailed calculations of the shift of exciton peaks are presented including (i) exact solutions for single particles in infinite wells, (ii) tunneling resonance calculations for finite wells, and (iii) variational calculations ofexciton binding energy in a field.
Abstract: We report experiments and theory on the effects of electric fields on the optical absorption near the band edge in GaAs/AlGaAs quantum-well structures. We find distinct physical effects for fields parallel and perpendicular to the quantum-well layers. In both cases, we observe large changes in the absorption near the exciton peaks. In the parallel-field case, the excitons broaden with field, disappearing at fields \ensuremath{\sim}${10}^{4}$ V/cm; this behavior is in qualitative agreement with previous theory and in order-of-magnitude agreement with direct theoretical calculations of field ionization rates reported in this paper. This behavior is also qualitatively similar to that seen with three-dimensional semiconductors. For the perpendicular-field case, we see shifts of the exciton peaks to lower energies by up to 2.5 times the zero-field binding energy with the excitons remaining resolved at up to \ensuremath{\sim}${10}^{5}$ V/cm: This behavior is qualitatively different from that of bulk semiconductors and is explained through a mechanism previously briefly described by us [D. A. B. Miller et al., Phys. Rev. Lett. 53, 2173 (1984)] called the quantum-confined Stark effect. In this mechanism the quantum confinement of carriers inhibits the exciton field ionization. To support this mechanism we present detailed calculations of the shift of exciton peaks including (i) exact solutions for single particles in infinite wells, (ii) tunneling resonance calculations for finite wells, and (iii) variational calculations of exciton binding energy in a field. We also calculate the tunneling lifetimes of particles in the wells to check the inhibition of field ionization. The calculations are performed using both the 85:15 split of band-gap discontinuity between conduction and valence bands and the recently proposed 57:43 split. Although the detailed calculations differ in the two cases, the overall shift of the exciton peaks is not very sensitive to split ratio. We find excellent agreement with experiment with no fitted parameters.
1,731 citations
TL;DR: In this paper, a growth technique of the semiconductor microcrystals in a glassy dielectric matrix has been developed, which permits to vary the size of the grown microcrystal in a controlled manner from some tens to thousands of angstroms.
1,039 citations
TL;DR: In this article, the effects of an exciton gas and an electron-hole plasma on the excitonic optical absorption in a two-dimensional semiconductor and compare these with recent experimental results on absorption saturation in single and multiple-quantum-well structures.
Abstract: We present theoretical results for the effects of an exciton gas and an electron-hole plasma on the excitonic optical absorption in a two-dimensional semiconductor and compare these with recent experimental results on absorption saturation in single- and multiple-quantum-well structures. A simple theoretical description of the nonlinear optical properties of these microstructures is developed for the case of low-density optical excitation near and above the band edge. We argue that the effects of Coulomb screening of excitons by the plasma are relatively weak in these structures but that the consequences of phase-space filling and exchange are significant in each case. We are able to explain the recent unexpected experimental result that ``cold'' excitons are more effective than ``hot'' carriers in saturating the excitonic absorption. Good agreement with the experimental data is obtained without adjustable parameters.
658 citations
TL;DR: Very small ZnS and CdS crystallites were made and stabilized in aqueous and methanolic media without organic surfactants in this paper, and they were characterized by transmission electron microscopy and in situ optical spectroscopy (λ≳200 nm).
Abstract: Very small ZnS and CdS crystallites are made and stabilized in aqueous and methanolic media without organic surfactants. Low temperature (−77 °C) synthesis in methanol produces the smallest crystallites, ≈30 A diameter cubic CdS and <20 A diameter cubic ZnS. The crystallites are characterized by transmission electron microscopy and in situ optical spectroscopy (λ≳200 nm). The crystallites are too small to exhibit bulk band gaps in their optical spectra. In the band gap region, the small crystallites show a higher energy absorption threshold with a resolved spectral feature (quantum size exciton peak), not present in the spectra of larger crystals. The far ultraviolet spectra are unaffected by size at present resolution. These results can be understood in terms of the crystallite molecular orbitals, and an elementary confined electron and hole model.
519 citations
TL;DR: In this article, a comprehensive discussion of optical transitions in these microstructures is given, including excitonic effects and the specific features of room-temperature exciton resonances.
Abstract: We review the nonlinear-optical effects observed at room temperature in semiconductor quantum-well structures photoexcited near the band gap. A comprehensive discussion of optical transitions in these microstructures is given, including excitonic effects and the specific features of room-temperature exciton resonances. Experimental investigations using continuous-wave, picosecond-, and femtosecond-laser sources are presented. They show extremely efficient nonlinear processes. In the case of excitations that are long compared with the exciton-ionization time, the induced changes in absorption and refraction do not depend on the wavelength or on the duration of excitation. These changes depend only on the density of absorbed photons and are interpreted in terms of electron–hole plasma screening and band filling. In contrast, for ultrashort excitation, nonlinear processes depend critically on the excitation wavelength. The selective generation of excitons is found to produce effects larger than a plasma of the same density. This unexpected result is shown to arise from the low temperature of the exciton gas before it interacts with the lattice and from the decrease of screening that is the reduced dimensionality of quantum-well structures.
321 citations
TL;DR: The observation of interface phonons by Raman scattering from GaAs-AlAs superlattices are reported, which have frequencies close to the optical phonons of bulk GaAs and AlAs and resonate strongly for laser energies near the confined exciton levels of the GaAs quantum wells.
Abstract: We report the observation of interface phonons by Raman scattering from GaAs-AlAs superlattices. These modes have frequencies close to the optical phonons of bulk GaAs and AlAs and resonate strongly for laser energies near the confined exciton levels of the GaAs quantum wells. The results are analyzed in terms of an electrostatic continuum model. In the long-wavelength limit this theory predicts the phonons of the layered media proposed by Merlin et al.
276 citations
TL;DR: In this article, the authors present results obtained for excitons, polaritons and biexcitons in semiconductors with conduction band minimum and valence band maximum at the same point of the Brillouin zone (direct-gap materials) and a band to band transition which is dipole allowed.
186 citations
TL;DR: In this article, the authors summarize some of the salient properties of excitons in GaAs quantum wells and in doing so it will emphasize work at AT&T Bell Labs with which the authors have been associated.
173 citations
IBM1
TL;DR: A well-resolved core exciton at the bulk diamond C-$1s$ absorption edge has been observed using high-resolution partial-yield spectroscopy with synchrotron radiation, and the obtained excitonic binding energy agrees well with a first-principles effective-mass approximation (EMA).
Abstract: A well-resolved core exciton at the bulk diamond C-$1s$ absorption edge has been observed using high-resolution partial-yield spectroscopy with synchrotron radiation. The obtained excitonic binding energy, 0.19\ifmmode\pm\else\textpm\fi{}0.015 eV, agrees well with a first-principles effective-mass approximation (EMA). This is in sharp contrast to other semiconductors (Si, Ge, and GaAs) where reported excitonic shifts far exceed EMA estimates. In light of these results, one must question whether previous measurements overestimate the core-hole interaction or if they indicate a breakdown of the EMA for core excitons.
173 citations
TL;DR: In this article, the quantum-confined Stark effect on the photoluminescence of the lowest free exciton for electric fields perpendicular to the quantum well layers is investigated.
Abstract: We report on picosecond luminescence studies of GaAs/AlGaAs quantum wells in the regime of the quantum-confined Stark effect. A drastic increase of the recombination lifetime is accompanied by a Stark shift of the photoluminescence of the lowest free exciton for electric fields perpendicular to the quantum-well layers. A consistent picture of the quantum-confined Stark effect is presented.
170 citations
TL;DR: In this paper, the compositional dependence of the band gap energy was determined and the free and bound exciton lines for x = 0 and 1 and only broadened bound excititon peaks for other compositions were quantitatively explained based on compositional fluctuations of the cations.
Abstract: Photoluminescence (PL) experiments were carried out at 300 and 12 K to investigate the electro‐optical properties of Cd1−xZnxTe grown by molecular beam epitaxy on GaAs substrates. The compositional dependence of the band‐gap energy was determined. It has a quadratic dependence on x. The near band edge PL spectra at 12 K show free and bound exciton lines for x=0 and 1 and only broadened bound exciton peaks for other compositions. The bound exciton broadenings are quantitatively explained based on the compositional fluctuations of the cations. The PL line shapes give indications of the high quality of the layers.
TL;DR: In this paper, the effect of the electron-electron correlation on inter-metal sites has been investigated in detail in the visible or infrared energy regions, and the dependence of the CT excitation energies on the species M and X is shown to be consistent with the prediction by the Peierls-Hubbard model.
Abstract: Polarized reflection and luminescence have been measured for the single crystals of [MA 2 ][MX 2 A 2 ](ClO 4 ) 4 (M=Pt, Pd, X=Cl, Br, I and A=ethylenediamine, cyclohexanediamine). The strong absorption bands due to the charge-transfer (CT) exciton transitions between the mixed-valent metal ions have been investigated in detail in the visible or infrared energy regions. The dependence of the CT excitation energies on the species M and X is shown to be consistent with the prediction by the Peierls-Hubbard model which incorporates the effect of the electron-electron correlation on inter-metal sites. The oscillator strength of the CT excitons are observed to be enhanced by substituting heavier halogen ions. This enhancement is interpreted by a halogen-linked super-transfer mechanism. The unusually large values of the oscillator strength can be qualitatively explained in terms of the trimer CT model.
TL;DR: Theoretical results are in qualitative agreement with recent absorption data and the effect of a constant electric field on the energy position of the exciton ground state in semiconductor single-quantum wells is calculated.
Abstract: We have calculated the effect of a constant electric field on the energy position of the exciton ground state in semiconductor single-quantum wells. We discuss only the case of quantum wells where electron and hole wave functions are mostly localized within the same layer at zero electric field (e.g., GaAs-${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Al}}_{\mathrm{x}}$As structures). In such quantum wells an electric field applied parallel to the growth axis polarizes free-electron and -hole wave functions in opposite directions and therefore weakens the excitonic binding. Our theoretical results are in qualitative agreement with recent absorption data.
TL;DR: In this article, a cubic rock salt structure with a lattice constant unchanged, within experimental error, from the bulk value was examined in the regime where bulk properties are not fully formed.
Abstract: Tiny single PbS crystals of ∼25 A diameter are synthesized and studied optically in low‐temperature colloidal solutions. Electron microscopic examination shows a simple cubic rock salt structure with a lattice constant unchanged, within experimental error, from the bulk value. These crystallites lack the near infrared electronic absorption characteristic of bulk PbS. The small crystallite absorbance in the visible rises more steeply than does the bulk absorbance. These results reflect electron and hole localization if one considers the variation in effective mass across the band structure. A simple discussion of localization anywhere in the Brillouin zone is given. For the first time, crystallite syntheses are carried out in solvent mixtures that form transparent glasses upon cooling. The PbS spectra are independent of temperature (at current experimental resolution) down to 130 K, in contrast to earlier results for quantum size exciton peaks in ∼20 A ZnS crystallites. Previously published observations of size dependence in the excited state electronic properties of AgI and AgBr are explained as consequences of electron and hole localization in the small crystallites. AgBr appears to be the first indirect gap semiconductor to be examined in the regime where bulk properties are not fully formed.
TL;DR: Determination experimentale des coefficients absolus d'absorption optique et des intensites d'oscillateurs d'excitons dans des super-reseaux AlGaAs−GaAs a 4,2 K trouve que l'amplitude du coefficient d' absorption de bande a bande soit pratiquement independante de la largeur du puits quantique.
Abstract: We have experimentally determined the absolute optical-absorption coefficients and exciton oscillator strengths in AlGaAs-GaAs superlattices at liquid-helium temperatures. We find that although the magnitude of the band-to-band absorption coefficient is essentially independent of the quantum-well width, the absorption coefficient due to exciton absorption is greatly enhanced as the well width becomes smaller. The absorption energies and exciton oscillator strengths are compared to theoretical calculations and shown to be in very good agreement. We find that the oscillator strength per unit volume is proportional to 1/${L}^{2}$ in narrow wells. The effects due to coupling between wells was also investigated by the comparison of structures with barriers of 100 A\r{} to those with barriers of only 25 A\r{}. The subband energies in the coupled wells decreased from their values in uncoupled wells in quantitative agreement with our calculations while the oscillator strength also decreased.
TL;DR: The first observation of strong and well-resolved exciton peaks in the room-temperature absorption spectra of infrared band-gap multiple quantum well structures (MQW) was reported in this article.
Abstract: The first observation of strong and well‐resolved exciton peaks in the room‐temperature absorption spectra of infrared band‐gap multiple quantum well structures (MQW’s) is reported. Assignment of the optical resonances in the absorption spectra of GaInAs/AlInAs MQW’s yields the material parameters of this new heterojunction. The discontinuities of the conduction and valence bands are found to be ΔEc=0.44 eV and ΔEv=0.29 eV, respectively.
TL;DR: In this article, a generalized transformation is applied to a model Hamiltonian incorporating both local and non-local exciton-phonon coupling, and the difference between the transformed coupling and its thermal average over the free phonon ensemble must be small.
Abstract: A generalized transformation is applied to a model Hamiltonian incorporating both local and nonlocal exciton–phonon coupling. For the application of the usual transport theory, the difference between the transformed coupling and its thermal average over the free phonon ensemble must be small. This is achieved by a temperature‐dependent set of transformation coefficients defined by a transcendental equation which is soluble numerically after some approximation. Nonlocal coupling increases both the exciton binding energy and the usual band narrowing, but it also changes the band shape in a way which may introduce new minima and band broadening. The exciton velocities are then no longer related to the exciton hopping and scattering rates in the same way as in local coupling.
TL;DR: In this article, the theoretical description of electron-hole pairs in three-and quasi-two-dimensional semiconductors is reviewed, and the band gap shrinks and the Coulomb forces are strongly reduced.
Abstract: Semiconductors are known to show in the band-gap region relative large changes of their optical properties with increasing light intensity. These changes are due to the creation of electron–hole pairs, which modify by band filling the intraband and interband contributions to the complex optical dielectric function. Most important, with increasing concentration of electron–hole pairs, the band gap shrinks, and the Coulomb forces are strongly reduced so that excitonic effects disappear. The theoretical description of these phenomena in three- and quasi-two-dimensional semiconductors is reviewed.
TL;DR: Time-resolved photoluminescence measurements at room temperature of the n=1 heavy hole transition in a GaAs/AlxGa1−xAs multiple quantum well structure reveal a single-exponential decay with τ≊1 ns over a wide range of excitation densities.
Abstract: Time‐resolved photoluminescence measurements at room temperature of the n=1 heavy hole transition in a GaAs/AlxGa1−xAs multiple quantum well structure reveal a single‐exponential decay with τ≊1 ns over a wide range of excitation densities. Time‐integrated photoluminescence increases as the square of excitation energy density. These data indicate that the observed decay rate is due to nonradiative recombination. Free carriers, not excitons, govern radiative recombination in this sample.
TL;DR: In this article, a model for the nonlinear excitonic absorption and refraction was proposed and the renormalized single-particle energies and the exciton binding energy were calculated as functions of the carrier density and temperature.
TL;DR: In this paper, extensive characterizations of nominally undoped, Ga-doped, and P-Doped ZnSe grown on (001)GaAs substrate have been made, and the free exciton emission line split into two lines due to the residual strain.
TL;DR: In this paper, a new one-dimensional quantum well structure is proposed, which is created by converting one side of the potential barrier of an asymmetric quantum well into a periodically indented potential.
Abstract: A new one‐dimensional quantum well structure is proposed. The structure is created by converting one side of the potential barrier of an asymmetric quantum well into a periodically indented potential. Both the electron and hole states are confined in the one‐dimensional channel adjacent to the indented region of the side potential barrier, making it possible to achieve enhanced exciton binding energies and oscillator strengths for optical properties.
TL;DR: In this paper, the authors studied the oscillator strength of the lowest 1s heavy excitons in GaAs-AlAs quantum wells as a function of well-layer thickness by means of optical absorption.
Abstract: We have studied the oscillator strength of the lowest 1s heavy excitons in GaAs-AlAs quantum wells as a function of well-layer thickness by means of optical absorption. The oscillator strength of the lowest 1s heavy excitons is largely enhanced with the decrease of well-layer thickness. The result is the first full experimental observation of two-dimensional shrinkage of the exciton wave function in quantum wells.
TL;DR: In this article, time-resolved photoluminescence measurements of excitons in GaAs−Ga1−xAlxAs quantum wells subject to an electric field perpendicular to the well plane have been made.
Abstract: Time‐resolved photoluminescence measurements of excitons in GaAs‐Ga1−xAlxAs quantum wells subject to an electric field perpendicular to the well plane have been made. With increasing field, both integrated luminescence (as previously reported) and luminescence lifetime decrease. Thus the electric field increases the exciton nonradiative decay rate. Estimates of several possible mechanisms suggest that Fowler–Nordheim tunneling is responsible for the quenching. With increasing pump laser intensities, larger electric fields are required to quench the lifetime because of exciton screening of the field.
TL;DR: In this paper, an unusual new line has been observed in the low-temperature photoluminescence spectrum of GaAs/AlGaAs epilayers grown by liquid phase epitaxy, which represents a unique probe of the heterointerface.
Abstract: An unusual new line has been observed in the low‐temperature photoluminescence spectrum of GaAs/AlGaAs epilayers grown by liquid‐phase epitaxy, which represents a unique probe of the heterointerface. A very strong, broad and asymmetric line is seen, with peak energy ranging from that of the bound exciton to that of the shallowest acceptor, increasing roughly linearly with the logarithm of the excitation power. The intensity is seen to exhibit a very strong temperature dependence as the temperature is varied from 1.4 K up to about 20 K where the line falls below the background luminescence intensity. Intensity and energy dependence of the luminescence as a function of the Al mole fraction on one side of the junction is also investigated. 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.
TL;DR: In this article, optical properties of single and multiple quantum well structures grown by gas source molecular beam epitaxy were described, and it was estimated that over 50% of the InGaAs/InP band discontinuity resides in the conduction band.
Abstract: We describe optical properties of single and multiple quantum well structures grown by gas source molecular beam epitaxy. Absorption and photoluminescence were used in conjunction with x‐ray and transmission electron microscopy techniques to determine the confined particle energy levels and well thicknesses. Well defined exciton transitions were observed in the ternary and quaternary well superlattices even above room temperature. In single well structures energy shifts as large as 260 and 370 meV were observed for GaInAs and GaInAsP wells, respectively. On the basis of these results we estimated that over 50% of the InGaAs/InP band discontinuity resides in the conduction band.
TL;DR: In this article, the EELS of a number of alkali metal fluorides, chlorides and bromides have been measured with primary electron beams in the range 50 eV to 1.0 keV.