Photoluminescence Rise Time Research Articles

Photoluminescence dynamics of weakly confined excitons in GaAs thin films

We investigate the dynamics of weakly confined excitons in GaAs thin films measured by time-resolved photoluminescence (PL) technique. When excitation energy was above the resonant energy of the exciton, a long PL rise time of about 200 ps was observed. It is considered that an exciton formation process from excited continuum energy states to discrete energy states of the exciton in the thin film causes the slow PL rise. The observed PL decay time constant was about 14 ns due to high quality fabricated samples. The observed population dynamics can be surely ascribed to the specific features of weakly confined excitons.

Suppression mechanism of optical gain formation in In xGa 1− xN quantum well structures due to localized carriers

We have studied the optical gain and its suppression mechanism of In x Ga 1− x N multiple quantum well structures with different indium compositions. The variable stripe length method and the up-conversion method were employed in this research. It was found that the optical gain spectra became broad and the differential gain decreased with increasing the indium composition. These results indicated that inhomogeneous broadening affected not only spontaneous emission but also stimulated emission. The rise time of photoluminescence under a strong excitation condition was changed from 0.95 ps to 12.4 ps depending on the indium composition. Assuming the rise time reflects the relaxation lifetime from the absorption edge energy to population inversion levels, our calculation suggested that long rise times led to gain suppression in In x Ga 1− x N laser diodes with larger indium compositions.

Temperature dependence of exciton localization dynamics in InxGa1−xN epitaxial films

We have studied the temperature dependence of exciton localization dynamics in InxGa1−xN epitaxial films (x=0.09) by means of optical Kerr-gate time-resolved photoluminescence (PL) spectral measurements. During 30ps after 150fs laser excitation, the PL dynamics is sensitive to the measurement temperature. In the temperature range of 6–50K, the PL rise time decreases and the PL peak energy shifts to higher energy with an increase of temperature. At high temperatures above 80K, the thermal quenching of the PL at shallow localized states occurs. The energy relaxation processes of excitons in localized states of InxGa1−xN films are discussed.

Interplay of exciton and electron-hole plasma recombination on the photoluminescence dynamics in bulk GaAs

We present a systematic study of the exciton/electron-hole plasma photoluminescence dynamics in bulk GaAs for various lattice temperatures and excitation densities. The competition between the exciton and electron-hole pair recombination dominates the onset of the luminescence. We show that the metal-to-insulator transition, induced by temperature and/or excitation density, can be directly monitored by the carrier dynamics and the time-resolved spectral characteristics of the light emission. The dependence on carrier density of the photoluminescence rise time is strongly modified around a lattice temperature of 49 K, corresponding to the exciton binding energy (4.2 meV). In a similar way, the rise-time dependence on lattice temperature undergoes a relatively abrupt change at an excitation density of 120-180x10^15 cm^-3, which is about five times greater than the calculated Mott density in GaAs taking into account many body corrections.

Spectroscopy and carrier dynamics in CdSe self-assembled quantum dots embedded in ZnxCdyMg1−x−ySe

Time-resolved and steady-state photoluminescence, reflectivity, and absorption experiments were performed on CdSe quantum dots in ZnxCdyMg1−x−ySe barriers. Studies of the capture times of the photoexcited carriers into the quantum dots and of electron-hole recombination times inside the dots were performed. Photoluminescence rise time yielded capture times from 20 ps to 30 ps. All samples exhibit fast and slow photoluminescence decays, consistent with observing two independent but energetically overlapping decays. The faster relaxation times for the sample emitting in the blue range is 90 ps, whereas for the two samples emitting in the green it is 345 ps and 480 ps. The slower relaxation times for the sample emitting in blue is 310 ps, whereas for the samples emitting in green is 7.5 ns. These results are explained on the basis of the structural differences among the quantum-dot samples.

Temperature behavior of hot carrier dynamics in InP quantum dots

Photoluminescence (PL) kinetics of the InP self-assembled quantum dots is studied under quasiresonant optical excitation in the temperature range 10--100 K. It is found that the PL rise time abruptly drops with increasing temperature. The model calculations based on the rate equations have shown that this drop cannot result from thermostimulated phonon relaxation of the hot carriers. We developed a model assuming that the main reason for variations in the PL kinetics is related to thermal ejection (``evaporation'') of holes from the quantum dots, with a fraction of the dots acquiring negative charge. The model allowed us to describe quantitatively the PL rise as well as the PL decay over the whole temperature range under study. We identified also a number of effects caused by electron evaporation from the quantum dots at temperatures above 60 K. Comparison of the results of the model calculations with the experimental data has allowed us to determine rate parameters of the ejection processes.

Thermal emission and band-filling effects on the photoluminescence rise time of InGaAs/InAs/GaAs quantum dots

In this paper, we present an experimental study of the thermal emission and band-filling effects on the rise time in high-quality InGaAs/InAs/GaAs quantum dots emitting at 1.2 μm at 10 K. At excitation density of 104 W cm −2, the photoluminescence rise time, which is dominated by the relaxation time, of the ground and first excited states are independent of temperature (40 ps). At excitation density of 5600 W cm −2 the ground (first excited) state photoluminescence rise time varies from 90 ps (110 ps) at 10 K to 40 ps (70 ps) at 160 K. This behavior is attributed to the band-filling and the thermal emission effects on the efficiency of the relaxation.

Thermalization and photoluminescence dynamics of indirect excitons at low bathtemperatures

The thermalization and photoluminescence (PL) dynamics of indirect excitons inGaAs/AlGaAs coupled quantum wells (QWs) at very low bath temperature,Tb<1 K, are analysed theoretically and modelled numerically in order to clarify the origin of therecently observed sharp increase of the PL signal (PL-jump) right after a high-intensityexcitation pulse. We conclude that the PL-jump effect is mainly due to classicalcooling of indirect excitons after the pump pulse, which heats the exciton system.Thus the effect cannot unambiguously be attributed to bosonic stimulation ofthe scattering processes into the ground-state mode . It is argued that the narrowing effect, i.e. an effective screening of in-plane QW disorderby dipole–dipole interacting indirect excitons, naturally explains the observed stronglynonlinear dependence of the PL rise time on the pump intensity. We also discuss and analysean evaporative optical heating/cooling effect: the change of the effective temperatureT of indirect excitons, due to their resonant optical decay.

Effective excitation cross section of erbium in amorphous hydrogenated silicon under optical pumping

The effective excitation cross section of erbium embedded in an amorphous silicon matrix and the total lifetime of erbium ions in the excited state are determined by measuring the photoluminescence rise time of erbium ions under pulsed excitation of erbium-doped amorphous hydrogenated silicon. An analysis of the rate equations describing the excitation and deexcitation of erbium ions in a semiconducting matrix sheds light on the physical meaning of the effective excitation cross section. It is shown that measurement of the effective excitation cross section permits evaluation of the concentration of optically active erbium ions in the amorphous silicon matrix.

Carrier capture and relaxation in Stranski-KrastanowInxGa1−xAs/GaAs(311)Bquantum dots

We have investigated the structure and optical properties of ${\mathrm{In}}_{0.6}{\mathrm{Ga}}_{0.4}\mathrm{As}/\mathrm{GaAs}(311)B$ quantum dots (QD's) formed by the Stranski-Krastanow growth mode during metal-organic chemical-vapor deposition. We find that $(311)B$ QD structures display a higher energy QD luminescence emission and a stronger wetting-layer emission than (100) QD's of similar diameter and density. Temperature-dependent photoluminescence (PL) measurements reveal shallow QD confinement energies and strong interaction between neighboring quantum dots. Longer PL rise times of the ground-state emission of $(311)B$ QD's compared to (100) QD's are ascribed to the effect of differing numbers, energies, and level spacings of QD confined states on intersublevel relaxation mechanisms at low-carrier excitation densities.

Exciton dynamics inGaAs/Ga1−xAlxAsheterojunctions and GaAs epilayers

We present a study of the exciton dynamics in modulation doped ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{G}\mathrm{a}}_{1\ensuremath{-}x}{\mathrm{Al}}_{x}\mathrm{As}$ heterojunctions and GaAs epilayers. The comparison permits to identify the features characteristic for the heterojunctions. In particular, we analyze the rise time of the transient photoluminescence (PL) intensity. In general, we find a longer PL rise time for the lower-energy excitons indicating that the time required for the energy relaxation process increases with increasing binding energy of the excitons. Moreover, the rise time of the free excitons turns out to be conspicuously longer in heterojunctions than in epilayers although the time integrated PL spectra of the two systems are similar. From our analysis we conclude that the long rise time observed in the heterojunctions is closely connected with the vertical drift of the photoexcited carriers driven by the interface potential. In fact, we find that the carriers (i.e., holes in n-doped samples) photoexcited in the heterojunctions drift vertically from the heterointerface to the flat band region, where they finally recombine after forming excitons (bimolecular formation) with oppositely charged and locally excited carriers (i.e., electrons in n-doped samples).

Carrier energy relaxation by means of Auger processes in InAs/GaAs self-assembled quantum dots

Carrier relaxation processes are investigated in self-assembled InAs/GaAs quantum dots using time-resolved photoluminescence spectroscopy. The quantum-dot photoluminescence rise time has been measured as functions of carrier excitation density and excitation wavelengths. The measured relaxation time is about 32 ps at low excitation density and decreases by 1 over the excitation density from about 3 W/cm2, under nonresonant laser excitation. The threshold of this density-dependent regime occurs at a slightly higher density as the excitation wavelength increases and it disappears when the photon pumping energy is below the wetting layer barrier energy. These results clearly establish the regime where Auger processes become the dominant carrier relaxation mechanism in these self-assembled quantum dots.

Exciton relaxation in Ga1 − xInxAs/GaAs self-organized quantum dots

The exciton dynamics in Ga1−xInxAs/GaAs self-organized quantum dots grown on GaAs (111)B substrates are studied by the time-resolved photoluminescence (PL). We have found the intra-dot exciton relaxation by the reduction of the linewidth and peak energy and also by the energy-dependent PL rise time in the transient PL spectra. Compared with the energy relaxation in the reference quantum wells, we have confirmed that the exciton relaxation in three-dimensionally confined quantum dots is slower than in the quantum wells.

Carrier capture into InGaAs/GaAs quantum wells via impurity mediated resonant tunneling

We have investigated the photoexcited carrier dynamics in In1−xGaxAs/GaAs quantum wells using the photoluminescence up-conversion technique. We found a unique capture process which was exceptional both in terms of the capture time and its temperature dependence. In the case of a specific quantum well with wide barriers, the photoluminescence rise time, a parameter which includes the overall capture time and the exciton formation time, was less than 600 fs instead of the expected few hundred picoseconds. We show in this work that this unusually rapid process is the result of the capture of the photoexcited carriers (or excitons) by impurities in the GaAs barriers, from where they resonantly tunnel into the quantum well.

Influence of the kinetic energy of electrons on the formation of excitons in a shallow InxGa1-xAs/GaAs quantum well.

Time-resolved investigations were performed with systematic variation of the excitation energy in order to study the formation of excitons. In a proper designed shallow ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As/GaAs single quantum well heterostructure an electron system with well-defined kinetic excess energy relative to the excitonic ground state of the quantum well was prepared by photoexcitation and subsequent relaxation via cascaded LO-phonon scattering. Oscillations of the photoluminescence rise time and, in counter phase, a modulation of the decay transient are observed as function of the excitation energy. While the decay time reflects the electron transfer process into the ground state due to different phonon relaxation mechanisms, the rise time reveals an exciton formation process, which depends on the kinetic energy of the electrons. \textcopyright{} 1996 The American Physical Society.

Rapid carrier relaxation in self-assembled InxGa1-xAs/GaAs quantum dots.

Carrier capture and relaxation processes in self-assembled 15-nm ${\mathrm{In}}_{0.5}$${\mathrm{Ga}}_{0.5}$As/GaAs quantum dots are investigated by means of time-resolved photoluminescence spectroscopy. In a systematic study of photoluminescence rise times and barrier decay times (variation of temperature, excitation energy, and excitation density) we aim to identify the physical mechanisms responsible for fast carrier capture and relaxation in quantum dots. Both processes are separated by using appropriate excitation energies. Carrier capture and relaxation are shown to proceed with rates as high as \ensuremath{\sim}2\ifmmode\times\else\texttimes\fi{}${10}^{10}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ at low temperature even if less than one electron-hole pair per dot and excitation pulse is created. We interpret our results in terms of multiphonon processes at low excitation densities and in terms of Auger processes at high excitation densities. \textcopyright{} 1996 The American Physical Society.

Barrier-confinement-controlled carrier transport into quantum wires

By using subpicosecond laser light pulses with a polarization perpendicular to deep etched ${\mathrm{In}}_{0.53}$${\mathrm{Ga}}_{0.47}$As/InP quantum wires, initial spatial electron-hole distributions have been prepared that are predominantly located in the unpatterned InP buffer layer. Under these excitation conditions the photoluminescence rise time increases significantly with decreasing wire width from about 25 ps for a two-dimensional unetched reference structure to about 150 ps for 34-nm-wide wires. Temperature-dependent studies of the rise times indicate the existence of an effective barrier in the InP regions of the quantum wires. From detailed calculations of the electronic states, which extend from the unpatterned InP buffer into the quantum wires, we show that confinement effects in the InP barriers control the carrier transport from the buffer into the ${\mathrm{In}}_{0.53}$${\mathrm{Ga}}_{0.47}$As wire.

Dynamics of exciton relaxation in GaAs/AlxGa1−xAs quantum wells

We present an experimental study of the photoluminescence (PL) rise time of a single GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As quantum well. The PL rise time is investigated as a function of the excitation energy for different temperatures at low excitation densities (\ensuremath{\approxeq}${10}^{\mathrm{\ensuremath{-}}8}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$). A significant slowing down of the relaxation process is observed at low temperature when exciting the quantum well at the light-hole exciton energy; the slowing down disappears as either the temperature or the excitation power is increased.