Exciton Decay Time Research Articles

Ultraviolet irradiation effect on the third-order optical nonlinearity of CuCl-microcrystallite-doped glass

We investigate the effect of irradiation by a UV pulse laser on the third-order optical nonlinearity of CuCl-microcrystallite-doped glass as a function of the irradiation’s shot number, energy density per pulse, and wavelength. The response time τ is estimated from the luminescence decay time of the CuCl excitons. The absolute value of the third-order nonlinear susceptibility χ(3) is measured by degenerate four-wave mixing. Values of both τ and χ(3) decrease after UV irradiation. The figure of merit for the optical nonlinearity, χ(3)/ατ, where α is the absorption coefficient, is almost constant before and after UV irradiation and is approximately 500 esu cm s-1 in the case of 8-nm CuCl microcrystallites. This value is 1 order of magnitude larger than that of CdSxSe1-x-microcrystallite-doped glasses. The mechanism by which τ and χ(3) are decreased is discussed, considering defects levels formed at the interface between CuCl microcrystallites and glass matrix.

Optical properties of biexcitons in ZnS

Characteristics of biexcitons in ZnS epitaxial layers with biaxial tensile strain have comprehensively been studied by means of photoluminescence excitation spectroscopy, magnetophotoluminescence spectroscopy, and time-resolved photoluminescence spectroscopy. Photoluminescence excitation measurement of biexcitons allowed us to observe two-photon absorption of biexcitons and to determine the biexciton binding energy of $8.0\ifmmode\pm\else\textpm\fi{}1.2\mathrm{meV}.$ Under a magnetic field, the luminescence line due to the radiative recombination of light-hole free excitons slightly shifted to the lower-energy side (about 0.3 meV at 9 T). On the other hand, the luminescence line due to the radiative recombination of biexcitons split into two components. This splitting (about 1.2 meV at 9 T) originated from the Zeeman splitting of light-hole free excitons in the final state of the optical transition for biexciton luminescence. In addition, the luminescence decay times of both light-hole free excitons and biexcitons were estimated to be about 130 and 80 ps, respectively. The observed temporal behavior was consistent with the expected recombination dynamics between excitons and biexcitons.

Thermalization and Photoluminescence Dynamics in Quantum Well Heterostructures

The time dependence of the photoluminescence signal is calculated for a thermalized population of excitons in quantum well heterostructures. A thermal equilibrium between excitons and free carriers either in the well or in the barrier is assumed. The result of our calculations is compared to experimental data obtained on shallow quantum wells. We show that the decay of their excitonic photoluminescence has very little to do with the true exciton decay time.

Optical Spectroscopy of InGaN/GaN Quantum Wells

We apply photoluminescence, photoluminescence excitation and time-resolved optical spectroscopy for studying a well-characterized set of InGaN/GaN periodic structures. The energy differences between the absorption edge and the photoluminescence peak and the photoluminescence decay times drastically increase with the InGaN quantum well thickness. We quite accurately determined the radiative and non-radiative decay times of excitons in these structures from the measured decay times, the integrated photoluminescence intensity and the photoluminescence intensity immediately after the excitation pulse, at various temperatures. The intrinsic radiative lifetimes, which are inversely proportional to the exciton oscillator strengths, are then calculated from the temperature dependence of the radiative lifetimes. These experimental findings are analyzed using an eight-band k · P model, which quantitatively explains both the Stokes shifts and the intrinsic radiative lifetimes. Their strong dependence on the quantum well width is due to a strong lattice-mismatch strain induced piezoelectric field along the growth axis.

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

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 ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Al}}_{y}\mathrm{N}$ barriers, which depends on their widths and on the electric field in these layers. It is shown in particular that the magnitude of the field 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.

Recombination dynamics of excitons in III-nitride layers and quantum wells

A comparison of the various aspects of current research on exciton dynamics in GaN-based epitaxial layers is proposed. To date, most contributions have been concerned by: (1) micrometric epitaxial layers of GaN, the physics of which is essentially devoted to the identification and control of non-radiative processes; (2) InGaN/GaN multiple quantum wells, for which several hypotheses are currently competing for explaining the very long decay times observed, related to strong localization effects. It has been shown by reviewing recent research that the temporal behaviour of radiative recombinations in these systems is ruled by hardy controlled factors, whereas the physics of low-dimensional excitons in GaN/GaAlN quantum wells is closer to that in the model system GaAs/GaAlAs. Measurements of the decay times of free and localized excitons in GaN/GaAlN quantum wells, versus temperature T, are presented. The radiative lifetime of free excitons increases linearly with T, in agreement with available theories on bidimensional excitonic polaritons. The low-temperature limit of this lifetime is deduced to be much smaller than in GaAs/GaAlAs quantum wells, and consistent with a longitudinal–transverse splitting of 0.6 meV. The theoretical analysis of relative decay times of free excitons and localized states indicates that the latter correspond to electrons and holes having localization radii smaller than the Bohr radius.

Temperature dependence of localized and free exciton lifetime in CdZnSSe/ZnSSe single quantum wells

Time-integrated and time-resolved photoluminescence of Cd0.3Zn0.7S0.06Se0.94/ ZnS0.06Se0.94single quantum wells (QW) have been studied. The radiative recombination process is dominated by localized excitons which have a constant lifetime of ∼ 300 ps when the temperature is less than 130 K. As the temperature increases, localized excitons are thermally activated to form free excitons which are stable up to room temperature because of the large exciton binding energy and strong confinement of electrons and holes in these QWs. Free exciton radiative recombination is thus dominated when temperature is in the 130 K to room temperature (295 K) range. The free exciton decay time of ∼ 300 ps at 130 K increased linearly to ∼ 1ns at 295 K. This linear increase in exciton lifetime with temperature agrees with the theoretical prediction by considering the conservation of momentum requirement for radiative recombination for excitons in QW.

Exciton dynamics in homoepitaxial GaN

Photoluminescence transients have been studied with picosecond resolution in homoepitaxial GaN layers grown with MOCVD. In the studied samples, with doping concentration in the 10 17 cm −3 range, the free exciton (FE) decay time is short, less than 100 ps, mainly due to capture to the shallow impurities, but also to some extent due to nonradiative transitions at defects. The donor bound exciton (DBE) decay time is also short, <140 ps, indicating some excitation transfer to nonradiative defects. The acceptor bound exciton (ABE) has a well-defined decay time of 800 ps below 15 K. The lower decay times for the FE and DBE in the homoepitaxial layers, compared to previously studied heteroepitaxial samples, is tentatively ascribed to the much lower dislocation density in the homoepitaxial layers, which suppresses the gettering of nonradiative point defects.

Tailoring of Si doping layers in GaAs during molecular beam epitaxy

AbstractFor Si δ‐like doping of GaAs lateral ordering processes and segregation in growth direction have been investigated in real time by monitoring longe‐range and short‐range ordering effects using reflection high‐energy electron diffraction (RHEED) and reflectance anisotropy spectroscopy (RAS). When Si is supplied in pulses to GaAs(001) distinct ordering processes occur which are promoted by misorientation steps on the vicinal surface. The incorporation of Si atoms on Ga sites in a single plane can be completed to a high degree. The depth profile of the electron concentration is closely related to Si segregation during GaAs overgrowth. In the case of a doping layer of extremely high coverage it shows a minimum at the centre of the doping spike. For samples grown on vicinal GaAs(001) surfaces under conditions favourable for wire‐like Si incorporation a considerable enhancement of the exciton photoluminescence (PL) intensity and decay time has been found.

Exciton diffusion dynamics in [formula omitted] quantum wells on a V-groove patterned Si substrate

We report on exciton diffusion dynamics in SiGe Si quantum wells (QWLs) on a V-groove patterned Si substrate. The decay time of excitons in the (111) facet QWLs was found to decrease with increasing temperature in contrast to the radiative lifetime of two-dimensional excitons. Reduction of the V-groove period leads to decrease of the decay time of the (111) facet QWL excitons. These behaviors are explained in terms of the competition between the radiative recombination and the exciton diffusion to the other SiGe parts with lower quantized energies. A quantitative analysis of diffusion length is performed by reproducing temporal profiles of the luminescence from the (111) QWLs.

Exciton Dynamics in CdTe/CdMnTe Multiquantum Well Structures Grown by Molecular Beam. Epitaxy on GaAs Substrate

The results of picosecond photoluminescence kinetics of four dicherent CdTe/CdMnTe multiquantum well structures grown by MBE on GaAs sub- strates are presented. The experimental results show that excitons in CdTe quantum wells are strongly localized by potential fluctuations. Photolumi- nescence decay times of the localized excitons are considerably shorter (about 120 ps) than those reported for free or quasi-free excitons. An influence of Mn in the barriers on exciton properties is demonstrated. For narrow quan- tum wells as well as for the multiquantum well structure with the highest Mn mole fractioii the excitons migrate during their decay to the states with a lower potential energy. Longer decay times are observed for quasi-localized excitons. We show also that for strongly localized excitons the energy transfer between localized and donor bound excitons is less efficient.

Exciton dynamics in thin [formula omitted] quantum wells grown by MBE

We report high-resolution, picosecond laser spectroscopy measurements of the relaxation of lowest-energy heavy-hole excitons in GaAs quantum wells grown with and without growth interruptions. In both cases excitons relax by losing potential energy in a diffusion motion driven by potential fluctuations in the quantum-well plane. In the quantum wells grown without growth interruptions the low-energy shift of exciton lines is comparable with the width of an inhomogeneously broadened line and shows two-step decay with a slow exponential component characteristic for exciton localization at well interfaces. In the growth-interrupted quantum wells in which the size of islands with constant well thickness is large compared with exciton diameter we observe splitting of the heavy-hole transitions into the multiplets of narrow lines corresponding to one monolayer difference in the well width. The energy shifts of each line in this case amount to only a fraction of the width of individual lines (or there is no shift at all) suggesting the interisland migration of excitons mediated by acoustic phonon scattering as being responsible for exciton relaxation. Again a two-step decay of the luminescence is observed at low temperatures (2 K). Temperature-dependent measurements show that at higher temperatures luminescence decay becomes governed by a single exponential as expected for delocalized excitons (no energy shift is observed during the exciton decay time). This allowed us to study directly intrinsic properties of excitons in quantum wells, i.e. to determine the lifetime of k I = 0 excitons, which is a fundamental parameter of the system and has been assessed by many theories. The experimentally determined, from the present work, values of the radiative lifetime are 24.4 ps for a 13 ML thick well and 21.8 ps for a 17 ML well. These values agree very well with theoretical estimates of Andreani et al. ( Solid State Commun., 77 (1991) 641). The effective lifetimes measured as a time to decay to 1 /e of the value of the maximum of PL intensity are considerably longer than the radiative lifetime, since in thermal equilibrium only a small fraction of excitons occupy the states with k I < k 0 which can decay radiatively.

Cavity-Induced Changes of Spontaneous Emission Lifetime in One-Dimensional Semiconductor Microcavities.

Spontaneous emission lifetime in a one-dimensional AlGaAs distributed Bragg reflector microcavity has been systematically measured as a function of emission wavelength tuned by the quantum-confined Stark effect in order to confirm the cavity-induced alteration of the SE lifetime. The result manifests a substantial cavity-induced fractional change of the exciton decay times, $\ensuremath{\sim}$30% around the on-axis resonant wavelength, despite the relatively large broadening of the excitonic emission being comparable to the cavity resonance width in the low- $Q$ cavity.

Temperature dependent time-resolved exciton luminescence in GaAs/AlGaAs quantum wires and dots

Transient photoluminescence of GaAs/AlGaAs quantum wires and quantum dots formed by strain confinement is studied as a function of temperature. At low temperature, luminescent decay times of the wires and dots correspond to the radiative decay times of localized excitons. The radiative decay time can be either longer or shorter than that of the host quantum well, depending on the size of the wires and dots. For small wires and dots (∼ 100 nm stressor), the exciton radiative recombination rate increases due to lateral confinement. Exciton localization due to the fluctuation of quantum well thickness plays an important role in the temperature dependence of luminescent decay time and exciton transfer in quantum wire and dot structures up to at least ∼ 80 K. Lateral exciton transfer in quantum wire and dot structures formed by laterally patterning quantum wells strongly affects the dynamics of wire and dot luminescence. The relaxation time of hot excitons increases with the depth of strain confinement, but we find no convincing evidence that it is significantly slower in quasi 1-D or 0-D systems than in quantum wells.

Controlled spontaneous emission from semiconductor microcavities

AbstractThe advantages of the simultaneous quantum manipulation of electron and photon systems due to quantum‐confined Stark effect and cavity quantum electrodynamics, respectively, in semiconductor microstructures are clarified, manifesting systematic experimental results on high transfer efficiencies of spontaneous emissions and cavity‐induced fractional changes of exciton decay times around on‐axis resonance. The scheme for the simultaneous quantum manipulation is also expected to lead to attractive device concepts for highly efficient, high‐speed spontaneous light emitters, and sub‐Poissonian photon generators in a short time interval.

Thermodynamics and Diffusion of Free Trions in Mixed Type GaAs/AlAs Quantum Wells

AbstractAn emission line which depends on the simultaneous presence of excitons and &lt; 2×1010 cm−2 free photoexcited electrons in a 20 nm quantum well is shown to be due to the recombination of free negatively charged excitons ( “trions”). The binding energy is 1.3 meV and the lifetime is 1/4 that of the exciton. The increase in the radiative decay times of excitons and trions with increasing bath temperature is consistent with that of free particles. More directly, from time- and spatially-resolved measurements, we find that trions are localized at 5 K, and with increasing bath temperature both excitons and trions become mobile. The diffusivity of trions and excitons increases linearly with bath temperature, and at 10 K, the trion diffusivity is found to be factor of two lower than the exciton diffusivity. Our results are the first direct experimental evidence for free trions in GaAs quantum wells.

Radiative lifetimes of excitons in quantum wires.

The excitonic population lifetime and its temperature dependence is measured in nanometer-scale quantum wires. The measurements were performed on two cleaved-edge overgrown GaAs quantum-wire systems, in which the lateral dimensions are comparable to the dimensions of a reference quantum well and an easy comparison between them is straightforward. The excitonic population decay time in the wires is less sensitive to temperature changes than that in the wells. Whereas the latter is linear with temperature, the former is roughly proportional to the square root of the temperature. Consequently, the decay times of the photoluminescence from the wires are longer than those from the wells at low temperature, but shorter at higher temperatures. From the temperature dependence of the lifetimes, which is an unambiguous signature of a one-dimensional system, we find the intrinsic radiative lifetime of excitons in quantum wires. They are an order of magnitude longer than in quantum wells.

Transient photoluminescence of GaAs/AlGaAs quantum wires

We have studied the temperature dependence of exciton decay and rise times in a strain confined quantum wire and its host quantum well. The decay time in the wire increases asT1.7 rather than the predictedT0.5. At high temperature, the radiative decay time in the wire is longer than in the well.

Fluorescent decay times of excitons bound to oxygen traps in ZnTe : O

Decay Times (τ) of the emission of oxygen bound excitons in ZnTe : O were measured from 5 to 300 K. The temperature dependence of τ is determined by the laser excitation energy: for resonant excitation into the oxygen vibrational band, a new and different behaviour appears with respect to the band to band excitation. Interactions which are responsible for the temperature dependence of τ at different excitation energies are discussed.

Exciton and Raman processes of ZnS under tunable picosecond light pulse excitations

The decay time of excitons bound to neutral acceptors (I 1 line) appearing in low temperature ZnS films on GaAs (100) substrates grown by vapor phase epitaxy was measured for the first time with the use of a streak camera. The measured decay time of ≈ 190 ps agreed with a value calculated by the giant oscillator strength model. Under tunable (3.7−3.9 eV) picosecond light pulse excitations, resonant enhancements of Raman scattering intensities were seen for LO-, 2LO-, 3LO-, TO- and 2TO-phonon lines. The dependence of 2LO-phonon Raman line intensity on excitation photon energy showed resonance enhancements with free and bound exciton states.