S-shaped Temperature Dependence Research Articles

Analytic S-Shaped Temperature Dependence of Peak Positions of the Localized-State Ensemble Luminescence and Application in the Analysis of Luminescence in Non- and Semi-Polar InGaN/GaN Quantum-Wells Micro-Array

Two analytic expressions of temperature-dependent peak positions employing the localized-state ensemble (LSE) luminescence model are deduced for the cases of ΔE = E a – E 0 > 0 and < 0, respectively, under the first-order approximation of Taylor’s expansion. Then, the deduced formulas are applied to examine the experimental variable-temperature photoluminescence data of non- and semi-polar InGaN/GaN quantum-wells (QWs) array by jointly considering the monotonic bandgap shrinking described by Pässler’s empirical formula. S-shaped temperature dependence of luminescence peaks of both non- and semi-polar QWs is well reproduced with the analytic formulas. As a result, the localization depths are found to be 31.5 and 32.2 meV, respectively, for non- and semi-polar QWs.

Drastically enhanced cation incorporation in the epitaxy of oxides due to formation and evaporation of suboxides from elemental sources

In the molecular beam epitaxy of oxide films, the cation (Sn, Ga) or dopant (Sn) incorporation does not follow the vapor pressure of the elemental metal sources but is enhanced by several orders of magnitude for low source temperatures. Using line-of-sight quadrupole mass spectrometry, we identify the dominant contribution to the total flux emanating from Sn and Ga sources at these temperatures to be due to the unintentional formation and evaporation of the respective suboxides SnO and Ga2O. We quantitatively describe this phenomenon by using a rate-equation model that takes into account the O2 background pressure, the resulting formation of the suboxides via oxidation of the metal source, and their subsequent thermally activated evaporation. As a result, the total flux composed of the metal and the suboxide fluxes exhibits an S-shaped temperature dependence instead of the expected linear one in an Arrhenius plot, which is in excellent agreement with the available experimental data. Our model reveals that the thermally activated regimes at low and high temperatures are almost exclusively due to suboxide and metal evaporation, respectively, joined by an intermediate plateau-like regime in which the flux is limited by the available amount of O2. An important suboxide contribution is expected for all elemental sources whose suboxide exhibits a higher vapor pressure than that of the element, such as B, Ga, In, La, Si, Ge, Sn, Sb, Mo, Nb, Ru, Ta, V, and W. This contribution can play a decisive role in the molecular beam epitaxy of oxides, including multicomponent or complex oxides, from elemental sources. Finally, our model predicts suboxide-dominated growth in low-pressure chemical vapor deposition of Ga2O3 and In2O3.

Potential energy landscape and thermodynamic transitions of coarse-grained protein models revealed by the multicanonical generalized hybrid Monte Carlo method.

In the present study, thermodynamic properties of coarse-grained protein models have been studied by an extended ensemble method. Two types of protein model were analyzed; one is categorized into a fast folder and the other into a slow folder. Both models exhibit the following thermodynamic transitions: the collapse transition between random coil states and spatially compact, but non-native states and the folding transition between the collapsed states and the folded native states. Caloric curve for the fast folder shows strong statistical ensemble dependence, while almost no ensemble dependence is found for the slow folder. Microcanonical caloric curve for the fast folder exhibits S-shaped temperature dependence on the internal energy around the collapse transition which is reminiscent of the van der Waals loop observed for the first order transition; at the transition temperature, the collapsed and random coil states coexist dynamically. The corresponding microcanonical heat capacity is found to have negative region around the transition. This kind of exotic behaviors could be utilized to distinguish fast folding proteins.

Investigation of carrier localization in InAs/AlSb type-II superlattice material system

We investigate carrier localization in the InAs/AlSb type-II superlattice (T2SL) material system using temperature- and excitation power (Iex)-dependent photoluminescence (PL). Evidence of carrier localization in T2SLs was observed by an S-shaped temperature dependence of the PL peak position. Analysis of the Iex-dependent PL at various temperatures also shows the existence of carrier localization in the T2SLs. The thermal activation energies in T2SLs were extracted to identify the nonradiative recombination mechanisms and the possible origins of localized states. We found that there are two thermal activation energies, E1 = 8.2–1.2 meV and E2 = ∼60 meV at various Iex. We interpret E1 as a thermal activation energy that comes from Anderson localization, associated with roughness due to As2 diffusion into the interfaces. This is because the extracted E1 values are comparable to the exciton binding energy of localization in various quantum structures. Carrier trapping at a state in the InSb interfacial layer (Tamm state) may account for the origin of E2. Based on previous reports, we believe that the 60 meV state might be a Tamm state if we consider thickness variations in the InSb interfacial layer for the T2SLs.

Optical properties of n- and p-type modulation doped GaAsBi/AlGaAs quantum well structures

In this work, optical properties of n- and p-type modulation doped GaAsBi/AlGaAs single quantum well (QW) heterostructures are investigated via temperature- and excitation power-dependent photoluminescence (PL) and integrated photoluminescence (IPL) studies and the results are compared to the n- and p-type GaAs/AlGaAs QW structures to determine the influence of Bi and doping type on the optical properties. The results of the temperature dependent PL peak energy reveal that, as an effect of doping type, the temperature dependence of the PL peak energy exhibit different characteristic for n- and p-type samples. The temperature dependence of the PL peak energy reveals S-shaped trend for the n-type GaAsBi/AlGaAs QW sample. On the other hand, the characteristic follows Varshni law for the p-type GaAsBi/AlGaAs QW sample. The observed S-shaped behaviour for the n-type sample is explained by considering the contribution of the Bi-induced states above valence band (VB) to the PL. As for p-type sample, the localised states-related contribution to the PL signal is drastically diminished, resulting in an almost S-shape free temperature dependence of the optical transition energy, which can be explained by the compensation of acceptor-like states. The observed PL spectra of n- and p-type samples successfully reconstructed by two Gaussian peaks, which are assigned to optical transitions due to recombination of free and localised excitons. The localised exciton-related peak is observed to be very weak in p-type sample compared to that in n-type one. The allowed transitions in GaAsBi QW is calculated by self-consistently solving the Schrödinger-Poisson equation to identify the origin of the observed transitions. A comparison of the PL results of the Bi-containing samples with the results of the Bi-free ones is exhibited approximately 80 meV/Bi% decrease in the fundamental optical transition. Using the excitation-dependent IPL measurements, it is found that at low temperatures and under low excitations, recombination process is under the effect of Shockley-Read-Hall (SRH) non-radiative process. When the excitation power density increases, radiative recombination becomes dominant. At higher-temperatures and in the high-intensity regime, we observed Auger effect in recombination process for n- and p-type GaAsBi samples, but the effect of Auger loss is observed to be much less in the p-type GaAsBi sample due to enhanced spin orbit split-off energy as a result of incorporation of bismuth. Furthermore, when we compare the result for Bi-free and Bi-containing p-type samples, again, Auger recombination is found to be less effective for the Bi-containing sample.

Optical Characterization of Quaternary AlInGaN Multiquantum Wells

Quaternary AlInGaN/AlInGaN multiquantum-well (MQW) structures were grown by metal-organic vapour-phase epitaxy (MOVPE). The influence of QW pairs and QW barrier width on the optical properties of the samples is investigated by means of temperature and incident-power dependent photoluminescence (PL) measurements. The PL emission peaks redshift and the PL peak intensities increase as the number of QW pairs increases. On the other side, a blueshift of the PL peak energy and reduction of the PL peak intensity are observed for the sample with thinner barrier width. These results are explained by carrier localization effect. The S-shaped temperature-dependence of the PL peak energies indicates the existence of localized states induced by the potential fluctuations. The estimated degree of localization effect is found to be enhanced for the sample with more QW pairs and thicker barrier layers. Observing the incident-power dependent PL spectra, the estimated amounts of localization effect of the samples are consistent with the incident-power induced blueshifts of PL peak energies and the slope exponents of power-law form of integrated PL intensity.

Carrier localization in In0.21Ga0.79As/GaAs multiple quantum wells: A modified Pässler model for the S-shaped temperature dependence of photoluminescence energy

The optical properties of In0.21Ga0.79As/GaAs MQWs, with triple unequal layer thickness NW (3 nm), MW (6 nm) and WW (9 nm) grown on (001) and (113) GaAs substrates, is studied by using continuous wave photoluminescence (cw-PL) spectroscopy. A comparative study has been performed to demonstrate the influence of electric field and QW thickness on the exciton localization. An S-shaped form in temperature-dependent PL peak energy has been observed in polar middle QW (MW (113)) but not seen in non-polar ones (MW (001)). This behavior is linked to carrier localization in triangular potential and polarity. We have observed also this atypical evolution in non-polar wide QW (WW (001)) but not in non-polar middle QW (MW (001)), which is attributed to potential fluctuation in larger ones. With the aid of modified Pässler model for including the effect of localized states, we can persuasively reproduce the S-shaped temperature dependence of PL band gap energy and contribute to the estimated value of exciton localization energy. The values of σ are obtained from adjustment of experimental points, which indicate the degree of localization in QW layer.

Stimulated emission via electron-hole plasma recombination in fully strained single InGaN/GaN heterostructures

The optical properties of fully coherently grown single InGaN/GaN heterostructures for 12&amp;lt;In%&amp;lt;17 were investigated under low and high density excitations. At lower density, S-shape temperature dependence of the main emission peak, associated with localized exciton recombination was observed. The activation energy of the localized excitons remains invariable in the given range of In-contents. Most interestingly, under high density pulse-excitations, stimulated emission by electron-hole plasma recombination was observed for temperatures up to 295 K.

A Model of Radiative Recombination in (In,Al,Ga)N/GaN Structures with Significant Potential Fluctuations

The potential fluctuations in III-nitride quantum wells lead to many effects like emission broadening and S-shape energy vs. temperature dependence. The best description of the energy dependence comes from calculations based on Gaussian density of states. However, in most of the published reports, changes of carrier lifetime with energy and temperature are not taken into account. Since experimental evidence shows that lifetime significantly depends on energy and temperature, here we propose a model that describes two basic parameters of luminescence: lifetime of carries and emission energy as a function of temperature in the case of quantum wells and layers that are characterized by potential fluctuations. Comparison of the measured energy and lifetime dependences on temperature in specially grown InGaN/GaN quantum wells and InAlGaN layer shows very good agreement with the proposed theoretical approach.

A comparative photoluminescence study on Mn-Free GaAs/AlAs and Mn-containing Ga1-xMnxAs/AlAs quantum wells (QWs) grown on various orientations by MBE

Optical properties of diluted magnetic semiconductor Ga0.999Mn0.001As/AlAs quantum well structures grown on (1 0 0 ), (1 1 0), (3 1 1)B and (4 1 1)B by molecular beam epitaxy are reported. Temperature-dependent spectral photoluminescence (PL) measurement was performed at temperatures between 15 and 300 K. The PL measurements showed that band gap of the alloy decreases with increasing lattice temperature regardless the growth orientations. S-shaped temperature dependence has been observed in the samples grown on (1 0 0), (3 1 1)B, (4 1 1)B orientations. PL emission energy is fitted with Varshni and Bose–Einstein Approximation to determine Debye temperature (β), (ΘE) and thermal expansion coefficient (α), the exciton–phonon coupling strength (aB).

Temperature-dependent photoluminescence spectra of GaN epitaxial layer grown on Si (111) substrate**Project supported by the National Natural Science Foundation of China (Grant Nos. 61474110, 61377020, 61376089, 61223005, and 61176126) and the National Science Fund for Distinguished Young Scholars of China (Grant No. 60925017).

In this paper, the temperature-dependent photoluminescence (PL) properties of GaN grown on Si (111) substrate are studied. The main emission peaks of GaN films grown on Si (111) are investigated and compared with those grown on sapphire substrates. The positions of free and bound exciton luminescence peaks, i.e., FXA and D0X peaks, of GaN films grown on Si (111) substrates undergo red shifts compared with those grown on sapphire. This is attributed to the fact that the GaN films grown on sapphire are under the action of compressive stress, while those grown on Si (111) substrate are subjected to tensile stress. Furthermore, the positions of these peaks may be additionally shifted due to different stress conditions in the real sample growth. The emission peaks due to stacking faults are found in GaN films grown on Si (111) and an S-shaped temperature dependence of PL spectra can be observed, owing to the influence of the quantum well (QW) emission by the localized states near the conduction band gap edge and the temperature-dependent distribution of the photo-generated carriers.

Spatially indirect radiative recombination in InAlAsSb grown lattice-matched to InP by molecular beam epitaxy

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.

Alloy inhomogeneity and carrier localization in AlGaN sections and AlGaN/AlN nanodisks in nanowires with 240–350 nm emission

We present structural and optical studies of AlGaN sections and AlGaN/AlN nanodisks (NDs) in nanowires grown by plasma-assisted molecular beam epitaxy. The Al-Ga intermixing at Al(Ga)N/GaN interfaces and the chemical inhomogeneity in AlGaN NDs evidenced by scanning transmission electron microscopy are attributed to the strain relaxation process. This interpretation is supported by the three-dimensional strain distribution calculated by minimizing the elastic energy in the structure. The alloy inhomogeneity increases with the Al content, leading to enhanced carrier localization signatures in the luminescence characteristics, i.e., red shift of the emission, s-shaped temperature dependence, and linewidth broadening. Despite these effects, the emission energy of AlGaN/AlN NDs can be tuned in the 240–350 nm range with internal quantum efficiencies around 30%.

Bismuth-induced effects on optical, lattice vibrational, and structural properties of bulk GaAsBi alloys

Bulk GaAs1 - xBix/GaAs alloys with various bismuth compositions are studied using power- and temperature-dependent photoluminescence (PL), Raman scattering, and atomic force microscopy (AFM). PL measurements exhibit that the bandgap of the alloy decreases with increasing bismuth composition. Moreover, PL peak energy and PL characteristic are found to be excitation intensity dependent. The PL signal is detectable below 150 K at low excitation intensities, but quenches at higher temperatures. As excitation intensity is increased, PL can be observable at room temperature and PL peak energy blueshifts. The quenching temperature of the PL signal tends to shift to higher temperatures with increasing bismuth composition, giving rise to an increase in Bi-related localization energy of disorders. The composition dependence of the PL is also found to be power dependent, changing from about 63 to 87 meV/Bi% as excitation intensity is increased. In addition, S-shaped temperature dependence at low excitation intensities is observed, a well-known signature of localized levels above valence band. Applying Varshni’s law to the temperature dependence of the PL peak energy, the concentration dependence of Debye temperature (β) and thermal expansion coefficient (α) are determined. AFM observations show that bismuth islands are randomly distributed on the surface and the diameter of the islands tends to increase with increasing bismuth composition. Raman scattering spectra show that incorporation of Bi into GaAs causes a new feature at around 185 cm-1 with slightly increasing Raman intensity as the Bi concentration increases. A broad feature located between 210 and 250 cm-1 is also observed and its intensity increases with increasing Bi content. Furthermore, the forbidden transverse optical (TO) mode becomes more pronounced for the samples with higher bismuth composition, which can be attributed to the effect of Bi-induced disorders on crystal symmetry.PACS78.55Cr 78.55-m 78.20-e 78.30-j

A versatile phenomenological model for the S-shaped temperature dependence of photoluminescence energy for an accurate determination of the exciton localization energy in bulk and quantum well structures

Temperature dependence of the photoluminescence (PL) peak energy of bulk and quantum well (QW) structures is studied by using a new phenomenological model for including the effect of localized states. In general an anomalous S-shaped temperature dependence of the PL peak energy is observed for many materials which is usually associated with the localization of excitons in band-tail states that are formed due to potential fluctuations. Under such conditions, the conventional models of Varshni, Viña and Passler fail to replicate the S-shaped temperature dependence of the PL peak energy and provide inconsistent and unrealistic values of the fitting parameters. The proposed formalism persuasively reproduces the S-shaped temperature dependence of the PL peak energy and provides an accurate determination of the exciton localization energy in bulk and QW structures along with the appropriate values of material parameters. An example of a strained InAs0.38P0.62/InP QW is presented by performing detailed temperature and excitation intensity dependent PL measurements and subsequent in-depth analysis using the proposed model. Versatility of the new formalism is tested on a few other semiconductor materials, e.g. GaN, nanotextured GaN, AlGaN and InGaN, which are known to have a significant contribution from the localized states. A quantitative evaluation of the fractional contribution of the localized states is essential for understanding the temperature dependence of the PL peak energy of bulk and QW well structures having a large contribution of the band-tail states.

Optical investigation of strong exciton localization in high Al composition AlxGa1-xN alloys

The exciton localization in wurtzite AlxGa₁-xN alloys with x varying from 0.41 to 0.63 has been studied by deep-ultraviolet photoluminescence (PL) spectroscopy and picosecond time-resolved PL spectroscopy. Obvious S-shape temperature dependence was observed indicating that the strong exciton localization can be formed in high Al composition AlxGa₁-xN alloys. It was also found that the Al composition dependence of exciton localization energy of AlGaN alloys is inconsistent with that of the excitonic linewidth. We contribute the inconsistency to the strong zero-dimensional exciton localization.

Investigation of localization effect in GaN-rich InGaN alloys and modified band-tail model

The temperature-dependent PL properties of GaN-rich In x Ga1 − x N alloys is investigated and S-shaped temperature dependence is observed in all InGaN samples. It is found that the origin of localization effect in samples A and B are different from that in sample C. For samples A and B, In content fluctuations should be the origin of localization effect, while the localization effect can be attributed to In-rich clusters and metallic indium inclusions for sample C. In addition, the band-tail model is modified and the modified band-tail model is used to investigate the degree of localization effect in the three samples.

Carrier Density Dependent Localization and Consequences for Efficiency Droop in InGaN/GaN Quantum Well Structures

We report on the observation of a reduction in the depth of the S-shape in the temperature dependence of the photoluminescence peak energy with increasing excitation power density. Over the range of excitation power density where the depth of the S-shape is reduced, we also observe a reduction in the integrated photoluminescence intensity per unit excitation power, i.e., efficiency droop. Hence, the onset of efficiency droop occurs at the same carrier density as the onset of carrier delocalization. We correlate these experimental results with the predictions of a theoretical model of the effects of carrier localization due to local variations in the concentration of the randomly distributed In atoms on the optical properties of InGaN/GaN quantum wells. On the basis of this comparison of theory with experiment we attribute the reduction in the S-shape temperature dependence to the saturation of the available localized states. We propose that this saturation of the localized states is a contributory factor to efficiency droop whereby nonlocalized carriers recombine non-radiatively.

Luminescence studies in InxGa1−xN epitaxial layers with different indium contents

The optical properties of InxGa1−xN epitaxial layers (x=0.02, 0.04, 0.11, 0.15, 0.30 and 0.33) grown by metalorganic chemical vapor deposition (MOCVD) have been investigated by temperature-dependent photoluminescence (PL) measurement. The surface morphologies of InGaN samples are studied by scanning electron microscopy (SEM) images. The PL feature at 12K has shown an increase in full-width at half-maximum (FWHM) with increasing In content. An anomalous S-shaped temperature dependence of the PL peak energy exhibited by InGaN films with higher In content enabled the evaluation of the exciton localization energy. The broadened FWHM and S-shaped emission shift are attributed to larger compositional fluctuation due to compositional inhomogeneity of In. Additionally, the luminescence mechanism relating to the phase separation has to be considered for the much larger FWHM value and the pronounced S-shaped behavior for the InGaN samples with In content of 0.30 and 0.33.

Study of Ultraviolet Emission Enhancement in AlxInyGa1–x–yN Quaternary Alloy Film

The effect of indium cluster on enhanced ultraviolet luminescence in AlxInyGa1–x–yN quaternary alloys with increasing Al composition was investigated. X-ray diffraction (XRD), temperature-dependent photoluminescence (PL), transmittance spectra, and Raman scattering measurements have been employed to study the AlxInyGa1–x–yN quaternary alloy. Abnormal S-shaped temperature dependence of PL emissions in high Al content sample originated from carrier localization induced by indium nanoclusters, which is in good agreement with the pure InN mode measured by Raman scattering. A large Stokes shift between the emission peak and the absorption edge is also found in the high Al content sample. All these observations suggested that the enhancement of carrier localization by indium nanoclusters is responsible for the increase in radiative recombination probability with increasing Al composition. Our results demonstrated that AlxInyGa1–x–yN quaternary alloy can exhibit a larger band gap than that of GaN and enhanced ul...