Search

We studied the excitation-dependent photoluminescence (PL) quantum yield (QY) of monolayer MoS2 (1L-MoS2) with various flake areas grown on SiO2/Si substrates. The PL measurements were carried out by 532, 488, and 325 nm excitations which fulfill the conditions of quasi-resonant excitation of A-exciton, above bandgap, and far above the bandgap excitations, respectively. The PL QY was found to be reduced by decreasing the excitation wavelength, and it is attributed to variation in the thermal energy dissipated to the lattice. PL emission from 1L-MoS2 was observed with 325 nm excitation in large-area flakes (≥532 μm2) because of efficient heat dissipation. In the literature, PL emission of 1L-MoS2 is hardly reported with 325 nm laser excitations. Under 325 nm laser irradiation, 50% of excitation energy is converted to heat, which substantially increases the local temperature. From the temperature-dependent Raman analysis, the rise in the local temperature is approximated to be ∼382 K in the case of a small-area flake, whereas such an effect is alleviated in large-area flakes. Moreover, inter-valley exciton scattering dominates as the excitation wavelength decreases because of a substantial rise in the phonon population for small-area flakes. As a consequence of inter-valley exciton scattering, dark excitons (K-Σ) dominate over the bright excitons (K-K) under the 325 nm excitation. Hence, total suppression of PL emission was observed for small-area flakes because of dark exciton recombination. The noticeable PL emission of large-area flakes is attributed to the improved bright exciton recombination. [ABSTRACT FROM AUTHOR]

As an effective means to endow lead halide perovskite nanocrystals (NCs) with new properties, ion doping has been widely applied. Transition metal Mn-doped CsPbCl3 is of particular interest, as the Mn doping introduces new emission bands, improves photoluminescence quantum yield (PLQY), and even enhances stability. However, it is still insufficient to catch insight of the energy transfer process and luminescence characteristics of these high defect tolerance NCs doped with Mn ion. Here, we systematically studied the energy transfer from exciton to Mn2+ ions in Mn-doped CsPbCl3 NCs by variable temperature steady-state fluorescence spectra and time-resolved spectra. The abnormal attenuation behavior of the exciton and Mn2+ with the change of temperature was observed, from which we predict the existence of shallow traps near the conduction band and these shallow traps will affect the luminescence properties by influencing the energy transfer process. We show a simple physical model invoking only shallow trap states that uniquely describe the temperature-dependent energy transfer between the exciton and the Mn2+ ion. The thermometric performance (30–290 K) was studied in terms of relative thermal sensitivity, in which the calculated relative sensitivity range is ∼0.12%–1% K−1. [ABSTRACT FROM AUTHOR]

Growing green and amber emitting InGaN/GaN quantum wells in the zincblende, rather than the wurtzite, crystal phase has the potential to improve efficiency. However, optimization of the emission efficiency of these heterostructures is still required to compete with more conventional alternatives. Photoluminescence time decays were used to assess how the quantum well width and number of quantum wells affect the recombination rates, and temperature dependent photoluminescence was used to determine the factors affecting recombination efficiency. The radiative recombination lifetime was found to be approximately 600 ps and to increase weakly with well width, consistent with a change in the exciton binding energy. The relative efficiency at room temperature was found to increase by a factor of five when the number of wells was increased from one to five. Furthermore, the efficiency increased by factor 2.2 when the width was increased from 2.5 to 7.5 nm. These results indicate that thermionic emission is the most important process reducing efficiency at temperatures in excess of 100 K. Moreover, the weak dependence of the rate of radiative recombination on well width means that increasing well thickness is an effective way of suppressing thermionic emission and thereby increasing efficiency in zincblende InGaN/GaN quantum wells, in contrast to those grown in the wurtzite phase. [ABSTRACT FROM AUTHOR]

Photoluminescence (PL) spectra of atomically thin 2D lead iodide perovskite films are shown to depend on excited-state density, especially at cryogenic temperatures. At high excited-state densities and low temperatures, free exciton (FE) emission is so suppressed by exciton–exciton annihilation (EEA) that other—normally much weaker—emissions dominate the PL spectrum, such as emission from bound excitons (BEs) or PbI 2 inclusions. In the Ruddlesden–Popper perovskite with phenethylammonium (PEA) ligands (PEA 2 PbI 4 , PEPI), FE emission dominates at all temperatures at the excited-state densities reached with continuous wave excitation. At higher excited state densities reached with femtosecond pulsed excitation, the PL at temperatures under 100 K is dominated by BE emission redshifted from that of FE by 40.3 meV. Weak emission from PbI 2 inclusions 170 meV higher in energy than FE PL is also observable under these conditions. Equilibrium between BE and FE states explains why FE emission first increases with decreasing temperature from 290 until 140 K and then decreases with decreasing temperature as the BEs become stable. A Dion–Jacobson (DJ) material based on 1,4-phenyl-enedimethanammonium (PDMA) supports the reduction of FE emission by EEA at cryogenic temperatures. However, in the PDMA-based DJ material, BE emission is never as pronounced. At low temperatures and high-excited state densities caused by pulsed excitation, a broad emission redshifted by 390 meV from the FE dominates. Based on comparison with temperature-dependent measurements of PbI 2 films, this emission is suggested to arise from PbI 2 inclusions in the material. Possible avenues for improving PL at room temperature are discussed concerning these findings. [ABSTRACT FROM AUTHOR]

The substrate effect is an important issue in the properties of two-dimensional transition metal dichalcogenides (2D TMDs). Quantitatively determining the dependence of the photoluminescence (PL) emission properties and the excitonic behavior of single-layer 2D materials in a specific dielectric environment would provide helpful guidance for the rational design of substrates for high performance 2D TMD PL emission devices. Here, using a WSe2 monolayer on different substrates as a model system, it is demonstrated that the PL emission intensities can drastically change depending on the substrate effect. From the analysis of the excitonic behavior, the results reveal that the spectral weight between the neutral and charged excitons in the PL spectra is significantly modified by the substrate types, and the weight factor is dependent on the laser excitation density. The charged exciton binding energy is obviously negatively related to the substrate dielectric constant. Furthermore, the change trends of the binding energy of the monolayer WSe2 on different substrates are basically the same for the increase of the excitation density. These results suggest that the choice of the substrate plays a significant role in the modulation of the PL properties and exciton states of atomically thin WSe2; hence, substrate engineering should be carefully considered in the design of future 2D devices. [ABSTRACT FROM AUTHOR]

InAs quantum dots (QDs) have been grown on a GaAs (001) substrate in the subcritical region of InAs coverage for transition from a 2-dimensional (2D) to a 3-dimensional growth mode. Evolution of QDs and the corresponding wetting layer (WL) with InAs coverage has been investigated. Under specific growth conditions, quantum dot formation was observed only in samples where InAs coverage is more than 1.48 ML. The QD density increases sharply with InAs deposition initially but slows down with increased coverage. Photoluminescence (PL) shows the existence of a third peak, other than QD and WL peaks, at the low energy side of the WL peak, which is named the precursor peak. Evidence is presented supporting the theory that this peak is due to 2D InAs islands on a monolayer of InAs, which are small enough to localize excitons. Meanwhile, the WL peak is due to larger InAs islands under high compressive strain. During QD formation, the WL peak energy increases with the increase in InAs deposition. This is due to the sudden transfer of material from the bigger size of InAs islands to the QD. Our results show that the QD, WL, and precursor peaks coexist near the onset of QD formation. The power dependence of the three PL peaks is evident, which supports to our conclusion. [ABSTRACT FROM AUTHOR]

Control of unwanted low-energy emission originating due to trap states is crucial for improving the emission efficiency and for maintaining the color purity in blue light-emitting diodes. Here, a truxene-based emitter (T-pyrene) that is a good candidate for the blue emitter is investigated using photophysical studies. Steady-state absorption of the emitter reveals the presence of a charge-transfer state, and the photoluminescence spectrum suggests an occurrence of a strong interchain/exciplex state. Time-resolved photoluminescence (TRPL) and fluence-dependent measurements of the material in the dilute solution show the presence of multiple excitonic processes contributing to the delayed fluorescence. Similar studies on the pristine film suggest that the delayed fluorescence is dominated by the presence of the traps. By blending the oligomer with a higher bandgap polymer matrix, poly(9-vinyl carbazole) (PVK), a type I heterojunction at the T-pyrene:PVK interface is obtained that confines the generated excitons in T-pyrene. With this architecture, device efficiencies with T-pyrene (20 wt. %):PVK show significantly enhanced efficiency than those of the pristine device. We elucidate the relevance of the matrix:T-pyrene system using TRPL and fluence-dependent studies on T-pyrene (20 wt. %):PVK. Time-resolved emission spectra on the T-pyrene (20 wt. %):PVK films do not show red-shifted emission with time delays, which suggests the passivation of the quenching sites due to the dispersion of T-pyrene in PVK to prevent the intermolecular interactions. [ABSTRACT FROM AUTHOR]

In this work, we present a detailed investigation of the low temperature emission properties of ZnSeO alloys by means of photoluminescence, micro-photoluminescence, and time-resolved photoluminescence. We show that the low temperature broad photoluminescence spectrum of ZnSeO attributed to the recombination of localized excitons is composed of sharp lines related to individual trapping states. Based on studies of photoluminescence thermal quenching from individual trapping states and photoluminescence dynamics, the mechanism of nonradiative recombination in ZnSeO alloys is discussed. Moreover, an unexpected decrease of the low temperature Stokes shift with increasing oxygen content is observed in contrast to what has been reported for GaAs based highly mismatched alloys. The possible origin of this effect is proposed. [ABSTRACT FROM AUTHOR]

We report Raman and photoluminescence (PL) emission spectroscopy studies on monolayer MoS 2 /gold micro-disc hybrid structures. The observed frequency shift and broadening of phonon modes of the monolayer MoS 2 are mainly the manifestations of the mechanical strain rather than the plasmonic field induced electron doping. We also observe considerable enhancement of the phonon mode intensities as a consequence of the surface enhanced Raman scattering caused by the plasmonic fields. The interaction of localized surface plasmon resonances (LSPRs) excited near the edge of the micro-discs with excitons in the monolayer MoS 2 enhances A exciton emission, while suppressing that from B excitons and the trions. The changes in the PL emission near the edge are attributed to the modification of the emission rate via the Purcell effect, whereas the PL quenching on a relatively flat disc surface is due to lower density and the non-radiative nature of LSPRs. [ABSTRACT FROM AUTHOR]

Photoluminescence excitation (PLE) spectroscopy was used to investigate the optical properties of the recently discovered 3364.3 meV antimony-related donor bound exciton (D 0 X) in zinc oxide. By suitable control of the growth conditions, we achieved samples in which the Sb-related donor emission was the dominant near-bandgap luminescence feature. Resonant excitation using a tunable UV source enabled the observation of the two electron transitions of the donor bound exciton, enabling the direct determination of the binding energy of the Sb donor at 42.2 ± 0.5 meV, the lowest value yet reported for a donor in this material. The two electron transitions exhibit an unusual blue shift with increasing temperature, which is explained in terms of thermalization of excited states of the D 0 X initial state. The D 0 X excited states were probed by PLE and follow the trends of conventional shallow group III donors such as Al, Ga, and In. [ABSTRACT FROM AUTHOR]

We propose an analysis of the emission properties of anatase and rutile titanium dioxide (TiO2) that emphasizes the role of the strong electron-phonon interaction. We performed measurements of photoluminescence (PL) spectra of bulk monocrystals under continuous wave-laser excitation and of their temperature dependence. We show that in both anatase and rutile, weakly bound self-trapped excitons are actually made out from carrier polarons and give rise to a broad emission band in the visible spectral range. The thermal activation of carrier motion allows their hopping to distant sites that leads to the observed quenching of luminescence. In the specific case of rutile TiO2, the PL spectral shape and its intensity-quenching scenario reveal the presence of dark trap states. Moreover, an additional narrow line structure shows up at low temperatures. The latter is due to localized impurity states that can be attributed to oxygen vacancies and can be fitted with a large Huang-Rhys parameter S = 2.5 within a Franck-Condon model. Both phases show thus a very strong interaction between the photogenerated carriers and the lattice. [ABSTRACT FROM AUTHOR]

The effect of the magnetic field on radiative and non-radiative mechanisms of charge carriers in GaAs/AlGaAs quantum wells (QWs) is investigated via quasi-simultaneous magneto-photoluminescence (PL) and magneto-surface photo-voltage (SPV) spectroscopy. At low-temperature, the luminescence intensity of ultra-low disordered GaAs/AlGaAs QWs generally increases under strong magnetic perturbation. Even at relatively high-temperature (100K), the magnetic field driven enhancement of PL intensity is observed for thick QWs. On the other hand, it is found that the PL intensity of narrow QWs gradually decreases under a strong magnetic field at 100 K. The magnetic field driven enhancement (suppression) of radiative recombination efficiency for wide (narrow) QWs is investigated by considering the oscillator strength, thermal effects, and carrier re-distribution in energy states. Also, the charge carriers which escape from narrow QWs or are captured by interface defects are probed via magneto-SPV measurements. Radiative recombination and thermionic emission of charge carriers, investigated by magneto-PL and magneto-SPV spectroscopy, provide a clear guideline of the critical QW width which would be essential for magnetic field driven hightemperature operation of advanced emission based-devices. [ABSTRACT FROM AUTHOR]

Here, we report on the strong temperature dependence of PL properties in CdSe/ZnS core/shell quantum dots, within a broad temperature range of 80–500 K. Our results demonstrate different PL quenching mechanisms in high and low temperature ranges. In the low temperature range of below 320 K, PL quenching is proved to be dominated by thermally activated carrier escape. While in the high temperature range of above 320 K, PL undergoes additional quenching because of surface trapping by the thermally created surface defects, which is calculated and proved by the rapidly decreasing PL lifetime. Our calculating result proves that the model of thermally activated carrier escape is also responsible for PL quenching in the high temperature range. However, the red-shifted PL peak energy and the broadened FWHM with increasing temperature follow the same mechanisms in the whole temperature range. The former is due to the temperature-dependent band-gap shrinkage and the latter is due to the exciton scattering with acoustic and longitudinal-optical phonons. [ABSTRACT FROM AUTHOR]

We investigate the excited states of a neutral donor bound exciton (D0X) in bulk GaN by means of high-resolution, polychromatic photoluminescence excitation (PLE) spectroscopy. The optically most prominent donor in our sample is silicon accompanied by only a minor contribution of oxygen—the key for an unambiguous assignment of excited states. Consequently, we can observe a multitude of Si0X-related excitation channels with linewidths down to 200 μeV. Two groups of excitation channels are identified, belonging either to rotational-vibrational or electronic excited states of the hole in the Si0X complex. Such identification is achieved by modeling the excited states based on the equations of motion for a Kratzer potential, taking into account the particularly large anisotropy of effective hole masses in GaN. Furthermore, several ground- and excited states of the exciton-polaritons and the dominant bound exciton are observed in the photoluminescence (PL) and PLE spectra, facilitating an estimate of the associated complex binding energies. Our data clearly show that great care must be taken if only PL spectra of D0X centers in GaN are analyzed. Every PL feature we observe at higher emission energies with regard to the Si0X ground state corresponds to an excited state. Hence, any unambiguous peak identification renders PLE spectra highly valuable, as important spectral features are obscured in common PL spectra. Here, GaN represents a particular case among the wide-bandgap, wurtzite semiconductors, as comparably low localization energies for common D0X centers are usually paired with large emission linewidths and the prominent optical signature of exciton-polaritons, making the sole analysis of PL spectra a challenging task. [ABSTRACT FROM AUTHOR]

The excitonic optical properties of an Al0.60Ga0.40N/Al0.70Ga0.30N multiple quantum well structure were studied by photoluminescence (PL) spectroscopy at various temperatures. An analysis of the temperature dependence of PL peak energy revealed the stronger localization of excitons than biexcitons, consistent with the trend predicted from the difference between the spatial extents of excitons and biexcitons. The PL linewidth of the excitons increased linearly with increasing temperature up to 300 K and then more rapidly above 300 K. This result indicated that the exciton-longitudinal optical phonon interaction became prominent as the broadening mechanism of the exciton linewidth above 300 K. The luminescence of both excitons and biexcitons exhibited low thermal quenching, with their intensities at 300 K remaining at 37% and 47%, respectively, of their intensities at 4 K. These observations reflected the high thermal stability of the excitons and biexcitons in this multiple quantum well structure. In addition, the ratio of the PL intensity of the exciton-biexciton scattering to the product of the exciton and biexciton luminescence intensities was independent of temperature up to 200 K and increased rapidly with increasing temperature above 250 K. The temperature-independent behavior of this ratio up to 200 K originated from the localization of excitons and biexcitons, and the rapid increase in this ratio with temperature above 250 K reflected an increase in the frequency of interactions between the excitons and biexcitons due to the gradual delocalization of the excitons and biexcitons with increasing temperature. [ABSTRACT FROM AUTHOR]

It is assessed in detail both experimentally and theoretically how the interlayer coupling of transition metal dichalcogenides controls the electronic properties of the respective devices. Gated transition metal dichalcogenide structures show electrons and holes to either localize in individual monolayers, or delocalize beyond multiple layers--depending on the balance between spin-orbit interaction and interlayer hopping. This balance depends on the layer thickness, momentum space symmetry points, and applied gate fields. The design range of this balance, the effective Fermi levels, and all relevant effective masses is analyzed in great detail. A good quantitative agreement of predictions and measurements of the quantum confined Stark effect in gated MoS2 systems unveils intralayer excitons as the major source for the observed photoluminescence. [ABSTRACT FROM AUTHOR]

Knowledge of acceptors is critical to the understanding of the p-type doping mechanism of ZnO. Here, we report the influence of annealing temperature and polarity on the acceptor formation in Na-implanted a-plane ZnO bulk crystals, which has been studied by low temperature photoluminescence spectroscopy. The formation of a Na acceptor is evidenced by the acceptor bound exciton emission around 3.35 eV, which depends strongly on the annealing temperature. Unlike the reported results in ZnO thin films, the conditions for Na acceptor formation are harsher in nonpolar ZnO than in the polar one. The acceptors gradually transform into donors when the annealing temperature is elevated or when the crystals are aged, thus calling for effective control over the stability. [ABSTRACT FROM AUTHOR]

We investigate optical transitions and carrier dynamics in hybrid structures containing type-I GaAs/AlGaAs quantum wells (QWs) and type-II GaSb/AlGaAs quantum dots (QDs). We show that the optical recombination of photocreated electrons confined in the QWs with holes in the QDs and wetting layer can be modified according to the QW/QD spatial separation. In particular, for low spacer thicknesses, the QW optical emission can be suppressed due to the transference of holes from the QW to the GaSb layer, favoring the optical recombination of spatially separated carriers, which can be useful for optical memory and solar cell applications. Time-resolved photoluminescence (PL) measurements reveal non-exponential recombination dynamics. We demonstrate that the PL transients can only be quantitatively described by considering both linear and quadratic terms of the carrier density in the bimolecular recombination approximation for type-II semiconductor nanostructures. We extract long exciton lifetimes from 700 ns to 5 μs for QDs depending on the spacer layer thickness. [ABSTRACT FROM AUTHOR]

The processes of recombination of the photoexcited electron-hole pairs were studied in GaAs/AlGaAs weakly coupled multiple quantum wells, where the photoluminescence emission was composed of the contributions from the Γ - Γ and Γ - X conduction band minibands. Remarkable enhancement of the recombination time was observed when the magnetic field caused depopulation of the higher energy Γ - X miniband. The observed effect is attributed to the magnetic field induced variation of the electron density of states. [ABSTRACT FROM AUTHOR]

Linearly polarized photoluminescence is observed for type-II ZnTe/ZnSe submonolayer quantum dots (QDs). The comparison of spectral dependence of the degree of linear polarization (DLP) among four samples indicates that the optical anisotropy is mostly related to the elongation of ZnTe QDs. Numerical calculations based on the occupation probabilities of holes in px and py orbitals are performed to estimate the lateral aspect ratio of the QDs, and it is shown that it varies between 1.1 and 1.4. The value of anisotropic exchange splitting for bright excitonic states is found to be ~200 μeV from the measurement of the degree of circular polarization as a function of the magnetic field. The results also show that heavy-light hole mixing ratio is about 0.16. [ABSTRACT FROM AUTHOR]

We performed the studies of optical, photoelectric, and transport properties of a hydrothermal bulk n-type ZnO crystal by using the contactless optical techniques: photoluminescence, light-induced transient grating, and differential reflectivity. Optical studies revealed bound exciton and defectrelated transitions between the donor states (at ∼60 meV and ∼240 meV below the conduction band) and the deep acceptor states (at 0.52 eV above the valence band). The acceptor state was ascribed to VZn, and its thermal activation energy of 0.43 eV was determined. A low value of carrier diffusion coefficient (∼0.1 cm2/s) at low excitations and temperatures up to 800K was attributed to impact the recharged deep acceptors. Electron and hole mobilities of 140 and ∼80 cm2/Vs, correspondently, were determined at room temperature. The decrease of carrier lifetime with excitation was ascribed to increasing rate of radiative recombination at low temperatures and nonradiative recombination above the room temperature. [ABSTRACT FROM AUTHOR]

We analyze exciton spin dynamics in GaSe under nonresonant circularly polarized optical pumping with an exciton spin-flip rate-equation model. The model reproduces polarized time-dependent photoluminescence measurements in which the initial circular polarization approaches unity even when pumping with 0.15 eV excess energy. At T=10 K, the exciton spin relaxation exhibits a biexponential decay with sub-20 ps and >500 ps time constants, which are also reproduced by the rate-equation model assuming distinct spin-relaxation rates for hot (nonequilibrium) and cold band-edge excitons. [ABSTRACT FROM AUTHOR]

We performed temperature-dependent magnetic measurements and infrared photoluminescence (PL) measurements in various geometries on a series of p-type Hg1-xMnxTe single crystals (0.20 ≤ x ≤ 0.26). Evolution of PL features was observed, and zero-field spin splitting was identified for the acceptor-bound magnetic polaron (A0BMP). The results show direct evidence for local spontaneous magnetization of the A0BMP. Comparison with the Ditel-Spałek model indicates that besides the fluctuation and collective regimes, the A0BMP exhibits a new regime at low temperatures owing to the formation of the spin-glass state in Hg1-xMnxTe. The dissociation energy of the exciton bound to the A0BMP ((A0,X)BMP) varied rapidly with temperature, and the ratio of the dissociation energy of the (A0,X)BMP to the binding energy of the A0BMP was larger than the classical value of the A0X and no longer a constant, which breaks the Haynes rule. The free exciton localization process helps enhance the local magnetization of the (A0,X)BMP by transferring energy from the carrier system to the Mn-spin system, and it may lead to a photoinduced configuration of non-interacting ferromagnetic domains or the photoinduced magnetization effect. [ABSTRACT FROM AUTHOR]

A photoluminescence (PL) spectroscopy study of the bulk quaternary alloy InAlAsSb is presented. Samples were grown lattice-matched to InP by molecular beam epitaxy and two different growth temperatures of 450 °C and 325 °C were compared. Interpolated bandgap energies suggest that the development of this alloy would extend the range of available direct bandgaps attainable in materials lattice-matched to InP to energies as high as 1.81 eV. However, the peak energy of the observed PL emission is anomalously low for samples grown at both temperatures, with the 450 °C sample showing larger deviation from the expected bandgap. A fit of the integrated PL intensity (I) to an I ∞ Pk dependence, where P is the incident power density, yields characteristic coefficients k=1.05 and 1.18 for the 450 °C and 325 °C samples, respectively. This indicates that the PL from both samples is dominated by excitonic recombination. A blue-shift in the peak emission energy as a function of P, along with an S-shaped temperature dependence, is observed. These trends are characteristic of spatially-indirect recombination associated with compositional variations. The energy depth of the confining potential, as derived from the thermal quenching of the photoluminescence, is 0.14 eV for the 325 °C sample, which is consistent with the red-shift of the PL emission peak relative to the expected bandgap energy. This suggests that compositional variation is the primary cause of the anomalously low PL emission peak energy. The higher energy PL emission of the 325 °C sample, relative to the 450 °C sample, is consistent with a reduction of the compositional fluctuations. The lower growth temperature is therefore considered more favorable for further growth optimization. [ABSTRACT FROM AUTHOR]

Excitation correlation photoluminescence (ECPL) measurements are often analyzed in the approximation of a cross correlation of charge carrier populations generated by the two delayed pulses. In semiconductors, this approach is valid for a linear non-radiative recombination path, but not for a non-linear recombination rate as in the general Shockley-Read-Hall recombination scenario. Here, the evolution of the ECPL signal was studied for deep trap recombination following Shockley-Read-Hall statistics. Analytic solutions can be obtained for a fast minority trapping regime and steady state recombination. For the steady state case, our results show that the quadratic radiative term plays only a minor role, and that the shape of the measured signal is mostly determined by the non-linearity of the recombination itself. We find that measurements with unbalanced intense pump and probe pulses can directly provide information about the dominant non-radiative recombination mechanism. The signal traces follow the charge carrier concentrations, despite the complex origins of the signal, thus showing that ECPL can be applied to study charge carrier dynamics in semiconductors without requiring elaborate calculations. The model is compared with measurements on a reference sample with alternating layers of InGaAs/InAlAs that were additionally cross-checked with time resolved optical pump terahertz probe measurements and found to be in excellent agreement. [ABSTRACT FROM AUTHOR]

Low-temperature photoluminescence of undoped and moderately δ-doped Si1-xGex/Si (x<0.1) quantum wells has been studied. The influence of boron δ-layer on the excitonic luminescence and the luminescence caused by a dense electron plasma was demonstrated. The conditions under which the luminescence spectra of quantum wells are dominated by impurity-bound excitons (BE) have been established. Some unusual properties of these BE are explained in terms of type II bandoffset in Si1-xGex/Si (x<0.1) quantum wells, which favors a spatial separation of electrons and holes. It is shown that the temperature dependence of an excitonic emission in the quantum wells allows to calculate the BE-related density of states and, thus, can be used for contactless estimation of the impurity concentration in quantum wells. [ABSTRACT FROM AUTHOR]

We present a photoreflectance (PR) study of multi-layer InAs quantum dot (QD) photodetector structures, incorporating InGaAs overgrown layers and positioned asymmetrically within GaAs/AlAs quantum wells (QWs). The influence of the back-surface reflections on the QD PR spectra is explained and a temperature-dependent photomodulation mechanism is discussed. The optical interband transitions originating from the QD/QW ground- and excited-states are revealed and their temperature behaviour in the range of 3-300K is established. In particular, we estimated the activation energy (∼320 meV) of exciton thermal escape from QD to QW bound-states at high temperatures. Furthermore, from the obtained Varshni parameters, a strain-driven partial decomposition of the InGaAs cap layer is determined. [ABSTRACT FROM AUTHOR]

We have systematically investigated the photoluminescence (PL) dynamics of free excitons in GaAs/Al0.3Ga0.7As single quantum wells, focusing on the energy relaxation process due to exciton-acoustic-phonon scattering under non-resonant and weak excitation conditions as a function of GaAs-layer thickness from 3.6 to 12.0 nm and temperature from 30 to 50 K. The free exciton characteristics were confirmed by observation that the PL decay time has a linear dependence with temperature. We found that the free exciton PL rise rate, which is the reciprocal of the rise time, is inversely linear with the GaAs-layer thickness and linear with temperature. This is consistent with a reported theoretical study of the exciton-acoustic-phonon scattering rate in the energy relaxation process in quantum wells. Consequently, it is conclusively verified that the PL rise rate is dominated by the exciton-acoustic-phonon scattering rate. In addition, from quantitative analysis of the GaAs-layer thickness and temperature dependences, we suggest that the PL rise rate reflects the number of exciton-acoustic-phonon scattering events. [ABSTRACT FROM AUTHOR]

We model the dynamic behaviour observed for impurity-trapped excitons in SrF2:Yb2+ using transient photoluminescence enhancement induced via a two-frequency, sequential excitation process employing an UV optical parametric amplifier synchronized to an IR free electron laser (FEL). We observe sharp transitions interpreted as a change of state of the localized hole and broad bands interpreted as a change of state of the delocalized electron. Our modeling indicates that the 4f crystal-field interaction is 25% smaller than in CaF2. The photoluminescence enhancement transients are analyzed across a range of excitation frequencies using a system of rate equations. The temporal behavior is explained in terms of intra-excitonic relaxation, local lattice heating by the FEL, and liberation of electrons from trap states. [ABSTRACT FROM AUTHOR]

We have studied Be-acceptors in a p-type GaInNAs epilayer using magneto-luminescence spectroscopy. The band edge photoluminescence (PL) spectra at T=7K contain two features: the first is associated with the free exciton while the second with the conduction band to acceptor (CB → A) transition. The intensity of the latter decreases with increasing temperature while the excitonic feature survives up to T=250 K. From the energies of the two PL features, as well as the exciton binding energy in GaInNAs, we determined the Be-acceptor binding energy to be equal to 42 meV. The energy of the CB → A feature varies linearly with magnetic field B and has a slope of 5:5 × 10-4 eV/T. [ABSTRACT FROM AUTHOR]

High quality GaInAsSb/AlGaAsSb quantum wells (QWs) have been grown by molecular beam epitaxy using proper interface treatments. By controlling the group-V elements at interfaces, we obtained excellent optical quality QWs, which were free from undesired localized trap states, which may otherwise severely affect the exciton recombination. Strong and highly efficient exciton emissions up to room temperature with a wavelength of 2.2 lm were observed. A comprehensive investigation on the QW quality was carried out using temperature dependent and power dependent photoluminescence (PL) measurements. The PL emission intensity remains nearly constant at low temperatures and is free from the PL quenching from the defect induced localized states. The temperature dependent emission energy had a bulk-like behavior, indicating high quality well/barrier interfaces. Because of the uniformity of the QWs and smooth interfaces, the low temperature limit of inhomogeneous line width broadening is as small as 5 meV. [ABSTRACT FROM AUTHOR]

Optical properties of GaN/Al0.2Ga0.8N multiple quantum wells grown with semi-polar (10-11) orientation on patterned 7°-off Si (001) substrates have been investigated. Studies performed at 8K reveal the in-plane anisotropic behavior of the QW photoluminescence (PL) intensity for this semi- polar orientation. The time resolved PL measurements were carried out in the temperature range from 8 to 295K to deduce the effective recombination decay times, with respective radiative and non-radiative contributions. The non-radiative component remains relatively weak with increasing temperature, indicative of high crystalline quality. The radiative decay time is a consequence of contribution from both localized and free excitons. We report an effective density of interfacial defects of 2.3 x 1012 cm-2 and a radiative recombination time of τloc = 355 ps for the localized excitons. This latter value is significantly larger than those reported for the non-polar structures, which we attribute to the presence of a weak residual electric field in the semi-polar QW layers. [ABSTRACT FROM AUTHOR]