Thermalization Of Excitons Research Articles

Luminescence of Excitons Localized by Monolayer Interface Fluctuations in ZnSe/MgS Superlattices Grown by Metalorganic Vapor Phase Epitaxy

Luminescence properties of ZnSe/MgS superlattices (SLs) grown by metalorganic vapor phase epitaxy (MOVPE) were characterized using photoluminescence (PL) and reflection measurements. At low temperature, PL peaks originating from localized excitons which were confined by monolayer fluctuations were clearly observed. It was found that the luminescence efficiency of localized excitons was very high, probably nearly 100%. The thermalization of localized excitons to free excitons with the increase of temperature leads to a blue shift of the PL peak energy and an abrupt decrease of the luminescence intensity. This decrease of the luminescence intensity is attributed to the capture of free excitons by nonradiative centers. A SL which shows luminescence from continuously distributed tailed states exhibited a temperature dependence much different from that of the above SLs which show luminescence from the localized excitons defined by the interface fluctuations.

Hot excitons in ZnSe quantum wells

The optical and acoustic phonon assisted relaxation of free hot excitons in ZnSe quantum wells is reflected in photoluminescence excitation spectra. Slow thermalization of excitons on a 100 ps time scale is directly observed in time-resolved studies of luminescence in the spectral region of the phonon sideband.

Photoluminescence Excitation Study of Lo-Phonon Assisted Excitonic Transitions in Gan

AbstractWe report the results of a photoluminescence excitation (PLE) study on excitonic transitions in GaN. PLE measurements were carried out as a function of temperature to investigate exciton formation, thermalization, and annihilation processes in GaN. The PLE spectra detected at free-exciton resonance were found to exhibit a large number of oscillatory emission structures with an energy spacing of LO-phonon (92 meV), and such emission persisted up to 400K. The observation suggests that a phonon-assisted indirect exciton formation process, which simultaneously generates a free-exciton and an LO-phonon, is very significant and efficient in GaN, and the lifetime of the free-exciton is longer than the relaxation time of LO-phonon emission but much shorter than that of acoustic-phonon emission. The appearance of the broad spectral features at the PLE spectrum detected at free-exciton resonance, similar to those measured at the boundexciton resonance, is attributed to the higher order transition processes which involve acousticphonon scattering and thermal repopulation of free-excitons. In addition, we observed up to the 12th LO-phonon emission in PLE spectrum around 120 K, indicating the existence of the excitons with a large kinetic energy in GaN.

Photoelectric emission from negative-electron-affinity diamond (111) surfaces: Exciton breakup versus conduction-band emission.

We have recently reported that bound electron-hole pairs (Mott-Wannier excitons) are the dominant source of photoelectron emission from specially prepared [``as-polished'' C(111)-(1\ifmmode\times\else\texttimes\fi{}1):H] negative-electron-affinity diamond surfaces for near-band-gap excitation up to 0.5 eV above threshold [C. Bandis and B. B. Pate, Phys. Rev. Lett. 74, 777 (1995)]. It was found that photoexcited excitons transport to the surface, break up, and emit their electron. In this paper, we extend the study of exciton-derived emission to include partial yield (constant final-state) analysis as well as angular distribution measurements of the photoelectric emission. In addition, we find that exciton-derived emission does not always dominate. Photoelectric emission properties of the in situ ``rehydrogenated'' (111)-(1\ifmmode\times\else\texttimes\fi{}1):H diamond surface are characteristically different than emission observed from the as-polished (111)-(1\ifmmode\times\else\texttimes\fi{}1):H surface. The rehydrogenated surface has additional downward band bending as compared to the as-polished surface. In confirmation of the assignment of photoelectric yield to exciton breakup emission, we find a significant enhancement of the total electron yield when the downward band bending of the hydrogenated surface is increased. The functional form of the observed total electron yield demonstrates that, in contrast to the as-polished surface, conduction-band electrons are a significant component of the observed photoelectric yield from the in situ hydrogenated (111)-(1\ifmmode\times\else\texttimes\fi{}1):H surface. Furthermore, electron emission characteristics of the rehydrogenated surface confirms our assignment of a Fan phonon-cascade mechanism for thermalization of excitons.

Exciton thermalization in quantum-well structures.

We show that a unified picture, namely, thermalization within the inhomogeneously broadened exciton band, can be used to explain the excitonic photoluminescence (PL) spectra in quantum wells independently of the presence, or lack thereof, of the low-energy shift of the PL line with respect to the absorption peak (Stokes shift). The Stokes shift itself turns out to be a mere consequence of the thermal equilibrium with a quadratic dependence on the absorption linewidth and a linear dependence on the inverse of the excitonic temperature. The predictions are found to be in excellent agreement with careful measurements in GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As single-quantum-well structures.

Temperature dependent photoluminescent properties of InAsxP1−x/InP strained-layer quantum wells

Thermal quenching of photoluminescence from InAsxP1−x/InP strained-layer quantum wells has been investigated over the temperature range of 20–295 K. Structures with compositions of x=0.67 and x=1.0 and quantum well thicknesses of 1–17 monolayers were evaluated using Fourier transform photoluminescence spectroscopy. For InAs/InP heterostructures, the activation energy for thermal quenching depended on well thickness. Luminescence quenching was attributed to thermalization of free excitons from the well, and subsequent nonradiative recombination. Addition of phosphorus to the wells alters the recombination process responsible for luminescence quenching.

Thermalization of excitons in a GaAs/AlGaAs quantum well

The dynamics of excitons is investigated in quantum wells with monolayer fluctuations forming islands of several hundred ångström. The exciton transfer to thicker well regions occurs on a picosecond timescale if the island size is close to the diffusion length of the excitons. The transfer time shortens with rising excess excitation energy of the excitons, due to an increase of the diffusion length. The delayed exciton thermalization causes significant differences between absorption and photo-luminescence excitation spectra.

Spin relaxation and thermalization of excitons in GaAs quantum wells

We present results on the picosecond dynamics of spin relaxation and thermalization of excitons in GaAs quantum wells. The spin relaxation time constant is ≊50 ps for resonant excitation and less than 10 ps for nonresonant excitation, considerably shorter than those for free electrons in the bulk. Photoexcited cold, nonthermal excitons thermalize in ≊50 ps at 10 K and less than 10 ps at 35 K. Exciton-acoustic phonon energy exchange rates are determined from these measurements.

Slow thermalization of singlet excitons in NaNO 2 crystals

We have investigated the luminescence of NaNO 2 crystals and its dependence on the excitation conditions. Using a broad band light source the luminescence lines resulting from transitions into intramolecular vibrational states (“vibronic lines”) are asymmetric. Comparing these line shapes for different states involved we conclude that the asymmetry is caused by the dispersion of the excitons and by that of the vibrational state of the electronic ground state. Exciting the crystal with a narrow band laser we observed at low temperatures a strong dependence of the fluorescence line shapes on the excitation energy. For temperatures above 15 K the “broad band-excitation line shape” was observed independent of the excitation energy. This behaviour leads to the conclusion that in the case of selective excitation the exciton band is not populated uniformly due to the conservation of quasi-momentum and that the decay of excitons is faster than the thermalization within the exciton band at T = 4 K. At temperatures above T ≈ 15 K the thermalization is possibly accelerated by multi-phonon scattering processes and becomes the faster component.

Localization-dependent thermalization of excitons in GaAs/AlxGa1-xAs quantum wells.

Localization-dependent thermalization of excitons in GaAs/AlxGa1-xAs quantum wells.

Thermalization of excitons in Cu 2O

The thermalization of excitons in Cu 2O is investigated by monitoring the lineshape of the phonon-assisted luminescence for different crystal temperatures and excitation conditions. At low temperatures (less than 10 K) the excitons reach thermal equilibrium. At higher temperatures this is the case only for nonresonant excitation, whereas for resonant excitation the exciton distribution is nonthermal. These lineshapes can be explained quantitatively by a theory which takes into account the coupling of excitons to acoustic and optical phonons.

Thermalization of excitons

Thermalization means the achievement of thermal equilibrium. In this article we will be interested in how physical systems achieve thermal equilibrium after optical excitation. Actually we are not always interested in how a system returns to true thermal equilibrium. Often we are interested only in how a part of the system attains thermal equilibrium and this is sometimes known as a quasi-equilibrium. However, for the sake of brevity, I will refer to all processes which enable a system or subsystem to relax towards thermal equilibrium or quasi-equilibrium as thermalization. The word exciton n will also be used in this article in a broad sense. Exciton usually denotes a bound state of an electron and a hole in solids, I will consider mobile electrons and holes not bound to each other as continuum states of an exciton. This more general definition of excitons allows me to discuss the thermalization of electron-hole plasmas in semiconductors also. The study of how systems relax and thermalize has been going on for years. This review article becomes necessary as a result of recent advances in short laser pulse technology. In solids there are typically a large number of electrons. In addition to electron-electron interactions, the electrons interact with lattice vibrations and with electromagnetic radiations. These interactions are often strong in solids and as a result excitons relax and thermalize in times varying from nanoseconds to femtoseconds. Such short time durations made a real time study of thermalization of excitons very difficult if not impossible in the past. Recent advances in short laser pulse technology made it possible for the first time to study these thermalization processes in condensed systems in real time. The purpose of this article is to review some of the recent time-resolved studies of thermalization of excitons in semiconductors. This article is not meant to be exhaustive and only representative examples have been chosen to illustrate the physical principles involved. In particular I want to point out some of the subtleties in the interpretation of the time-resolved photoluminescence experiments. It is hoped that by comparing the results in different systems this article will serve as a guide to understanding time-resolved studies of thermalization in less understood solids. The organization of this article is as follows. In the next section I will survey the present status of experimental techniques available for performing time-resolved studies. Since many articles have already appeared on the generation of short laser pulses, the emphasis of that section will be on matching the detectors to the available light sources. As will be shown, the time resolution of an experiment is often not limited by the light source but rather by the detector. In Section III the theortical background for understanding relaxation processes in solids is reviewed. In the section

Direct imaging of the columnar structure of GaAs quantum wells

The columnar structure of GaAs/AlGaAs quantum wells grown by MBE with interruptions of the growth of ≈2 min at the interfaces is directly visualized for the first time. The columns have an approximate height of integer multiples of a monolayer and are caused by growth islands differing by ≈2.8 Å (one monolayer) height at the GaAs AlGaAs heterointerfaces. The method used to obtain these images is scanning cathodoluminescence. The dependence of the lateral extension of these islands on growth conditions is investigated. For low growth temperature and rate the lateral extension of such islands can be as large as 6–8 μm, larger than the carrier diffusion length. Under these conditions inter-island thermalization of excitons is largely suppressed. For fixed growth rate r s ≈ 0.5 monolayer/s the mean island size decreases to 2 μm upon an increase of growth temperature from T s = 600°C to 660°C. The growth process changes from a planar to a three-dimensional one. Time resolved cathodoluminescence images directly visualize the lateral diffusion of excitons. Their lateral diffusion velocity is 10 5 cm/s.

Density operator description of excitons in molecular aggregates: Optical absorption and motion. I. The dimer problem

A compact formalism is proposed for a rigorous calculation of the optical absorption and of the subsequent exciton motion and thermalization in molecular aggregates. The formalism is based on superoperators technique which is used to derive a macroscopic evolution operator from Liouville's equation written in Lie's algebra, the basis of which is formed with eigenoperators of the constants of motion. The compactness advantages of such an algebra are displayed in actual calculations of observables, where the evolution equation is decoupled in a number of independent equations of easier handling, each one corresponding to a specific physical process. Comparison is made with the technique used by Haken et al. to derive the same density-operator equation. The formalism allows simple derivation of optical line shapes, for different rates of coherent to incoherent couplings ($\frac{V}{{\ensuremath{\gamma}}_{0}}$). Actual calculations are presented for a simple case of a statistical ensemble of pairs of anthracene molecules, in their crystal-cell mutual orientation. In that case, the line-shape formula reduces to that obtained by Haken and Reineker. The formalism is also used to derive a time-dependent exciton density operator $\mathcal{E}$, which describes rigorously the time-dependent nonstationary state of an ensemble of molecular aggregates excited optically. The initial state, the motion of the excitation, and its thermalization are discussed in terms of time-dependent first and second moments and coherence of the excitation, calculated with the kernel of $\mathcal{E}$ and using the stochastic exciton Hamiltonian proposed by the precited authors. The results are paralleled with those provided by models of motion such as coherent and incoherent hopping excitons; cases where models might provide artificial motions are discussed. The two moments, taken as position and volume, lead to a consistent and pictorial description of the exciton statistical motion towards its thermalized state. Recipe formulas and tables are provided to use the superoperators technique, which is shown to be very flexible to allow exact solutions for "one-dimension exciton" crystals, for clusters or larger aggregates observed as traps in optical and ESR experiments in mixed crystals.

Hot excitons and exciton excitation spectra

Excitation Spectra (E.S.) of free exciton emission in CdS crystals are measured at 4.2 and 77°K both for zero-phonon emission lines ( n = 1 A, n = 1 B and n = 2 A) and for line of exciton annihilation with simultaneous creation of LO phonon ( A 1 - LO). Distinctly pronounced oscillating structure with energy period of LO phonon can be observed in the E.S. of all exciton emission lines. This structure arises due to kinetic energy relaxation processes in exciton bands, the appearance of oscilations is conditioned by incomplete thermalization of excitons during exciton life-time. Observation of discrete structure in the E.S. permits one to draw some conclusions concerning the mechanism of exciton formation under optical excitation. Creation of excitons with kinetic energies up to 0,2 eV, considerably exceeding the binding energy 0.03 eV, was observed. Non-equilibrium character of zerophonon free exciton emission is briefly discussed.