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We present anisotropic nonparabolic position-dependent effective-mass calculations of the bound energy levels of electrons confined in lens-shaped InxGa1-xAs quantum dots embedded in a GaAs matrix. The strain and In gradient inside the quantum dots and their wetting layer (due to the strong In segregation effect present in the InxGa1-xAs/GaAs system) were taken into account. The bound eigenstates and eigenenergies of electrons in a finite 3D confinement potential were determined by the full numerical diagonalization of the Hamiltonian. The quantum dots and their wetting layer were sliced into a finite number of monolayers parallel to the substrate surface, each one with a specific In concentration, in order to be able to reproduce any composition profile along the growth direction. A comparison between the eigenenergies of the 'pure' InAs quantum dots and the quantum dots with an inhomogeneous In content indicates that In segregation dramatically affects their electronic structure and must be taken into account if one wishes to accurately simulate the real optoelectronic properties of such nanostructures. [ABSTRACT FROM AUTHOR]

The photoluminescence (PL) technique as a function of temperature and excitation intensity was used to study the optical properties of multiquantum wells (MQWs) of GaAs/AlxGa1-xAs grown by molecular beam epitaxy on GaAs substrates oriented in the [100], [311]A, and [311]B directions. The asymmetry presented by the PL spectra of the MQWs with an apparent exponential tail in the lower-energy side and the unusual behavior of the PL peak energy versus temperature (blueshift) at low temperatures are explained by the exciton localization in the confinement potential fluctuations of the heterostructures. The PL peak energy dependence with temperature was fitted by the expression proposed by Pässler [Phys. Status Solidi B 200, 155 (1997)] by subtracting the term σE2/kBT, which considers the presence of potential fluctuations. It can be verified from the PL line shape, the full width at half maximum of PL spectra, the σE values obtained from the adjustment of experimental points, and the blueshift maximum values that the samples grown in the [311]A/B directions have higher potential fluctuation amplitude than the sample grown in the [100] direction. This indicates a higher degree of the superficial corrugations for the MQWs grown in the [311] direction. [ABSTRACT FROM AUTHOR]

The temperature dependence of the photoluminescence properties of a thin film of poly[2-methoxy-5-(2′-ethylhexyloxy)-p-phenylene-vinylene], MEH-PPV, fabricated by spin coating, is analyzed. The evolution with temperature of the peak energy of the purely electronic transition, of the first vibronic band, of the effective conjugation length, and of the Huang–Rhys factors are discussed. The asymmetric character of the pure electronic transition peak and the contribution of the individual vibrational modes to the first vibronic band line shape are considered by a model developed by Cury et al. [J. Chem. Phys. 121, 3836 (2004)]. The temperature dependence of the Huang–Rhys factors of the main vibrational modes pertaining to the first vibronic band allows us to identify two competing vibrational modes. These results show that the electron coupling to different vibrational modes depends on temperature via reduction of thermal disorder. [ABSTRACT FROM AUTHOR]

The luminescence mechanism at low temperatures in AlxGa1-xAs/GaAs single quantum wells grown by molecular-beam epitaxy with different aluminum concentrations in the barrier has been studied in detail using the photoluminescence spectroscopy (PL) as function of temperature (8 K≤T≤90 K) combined with the excitation intensity. The asymmetry presented by the PL spectra at the low-energy side, the blueshift behavior of the PL peak energy, and the PL line broadening with increasing temperature are explained through the exciton localization in confinement potential fluctuations. The exciton localization effects on the PL spectra are progressively reinforced with the increase of the Al concentration in the barrier constituent material. The PL peak energy dependence on temperature has been fitted through the expression proposed by Pässler [Phys. Status Solidi B 200, 155 (1997)] adapted to systems with potential fluctuations, by subtracting the term σE2/kBT, where σE is the standard deviation of the potential fluctuations. It was verified that σE increases systematically with the Al concentration in the barrier, according to the AlGaAs alloy compositional disorder theory. [ABSTRACT FROM AUTHOR]

A study was conducted on potential fluctuations using the photoluminescence (PL) technique with temperature and excitation intensity variations, in a sample containing InGaAs/InGaAlAs coupled double quantum wells, with spikes of InGaAlAs of different thicknesses, and a single quantum well as the control. With the inclusion of a spike at the center of the well, an increase in the magnitude of the potential fluctuation occurs, in relation to the control well, due to an increase in the contribution from both the interface roughness and the compositional disorders to the potential fluctuations. The enhancement of the effects of the potential fluctuations, such as the PL linewidth and the blueshift of the PL peak energy with the temperature, as the spike thickness increases, was attributed to the increase in the contribution from the alloy composition fluctuation to the potential fluctuation, caused by an increase in the interaction of the wave function with the central and lateral barriers. This study also discusses the strong influence of the excitation intensity on the variation of the emission energy with temperature. [ABSTRACT FROM AUTHOR]

We identify quasi-donor-acceptor pair transitions in the photoluminescence spectra of GaAsSb and AlGaAsSb layers, lattice matched to InP, and grown by molecular-beam epitaxy. These alloys show compositional inhomogeneity due to phase separation resulting from miscibility gaps. The presence of Al in the quaternary alloy increases the fluctuation of the electrostatic potential in the epitaxial layer, increasing the variation of recombination energy as a function of intensity excitation in the range of low temperatures. [ABSTRACT FROM AUTHOR]

Photoluminescence (PL) measurements as a function of temperature and excitation intensity were carried out in a sample containing two InGaAs/InAlAs superlattices, grown on the same InP substrate, with quantum wells and barriers of different widths. The fluctuations in the confinement potential for excitons in both structures are investigated by following the blueshift of the PL peaks with increasing temperature as well as with rising excitation intensity, at low temperatures. A decrease in the full width at half maximum of the PL peaks with increasing excitation power was also observed. The change in the PL linewidth with excitation power is interpreted in terms of the variation of the relative contribution of the excitons localized at the excitonic band tail (due to the potential fluctuations) and of the nonlocalized excitons. Moreover, the activation energies of the nonradiative channels responsible for the thermal quenching of the photoluminescence peaks are deduced from an Arrhenius plot of the integrated PL intensity. [ABSTRACT FROM AUTHOR]

GaAsSbN/GaAs strained-layer single quantum wells grown on a GaAs substrate by molecular-beam epitaxy with different N concentrations were studied using the photoluminescence (PL) technique in the temperature range from 9 to 296 K. A strong redshift in optical transition energies induced by a small increase in N concentration has been observed in the PL spectra. This effect can be explained by the interaction between a narrow resonant band formed by the N-localized states and the conduction band of the host semiconductor. Excitonic transitions in the quantum wells show a successive red/blue/redshift with increasing temperature in the 2-100 K range. The activation energies of nonradiative channels responsible for a strong thermal quenching are deduced from an Arrhenius plot of the integrated PL intensity. [ABSTRACT FROM AUTHOR]

Properties of the interface between the epitaxial layer of heavily doped Al[sub 0.48]In[sub 0.52]As:Si and the InP(Fe) substrate are investigated by photoluminescence in AlInAs:Si/InP(Fe) heteroestructures grown by molecular beam epitaxy. The effect on heterostructure optical properties of including a thin Al[sub 0.22]Ga[sub 0.26]In[sub 0.52]As:Si layer at the interface is investigated as well. To explain the different interface emission energies observed, the results are analyzed by using the mixed-type I-II interface model, which considers in the type II interface a narrow InAs well, with variable width, between AlInAs and InP. The observation of the interface emission at energies as high as 1.36 eV, at low excitation intensity, is explained taking into account the high doping level of the samples. The observed interface transition luminescence thermal quenching is tentatively explained by analyzing the spatial distribution of electrons in the triangular quantum well formed at the type II interface (or at the mixed I-II interface) as a function of the temperature. [ABSTRACT FROM AUTHOR]

AlGaAs structures with different aluminum concentration (x=0.0, 0.17, 0.30, and 0.40) were characterized by photoluminescence and photoreflectance techniques. The temperature dependence of optical transitions in the temperature ranging from 2 to 300 K were investigated. Y. P. Varshni [Physica (Utrecht) 34, 194 (1967)], L. Vin˜a et al. [Phys. Rev. B 30, 1979 (1984)], and R. Pa¨ssler [Phys. Status Solidi B 200, 155 (1997)] models were used to fit the experimental points. The Pa¨ssler model gave the best adjustment to the experimental points. The tree models showed that the empirical parameters obtained through the adjustment of the experimental data in the three different models are aluminum composition dependent in the ternary alloy. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Wet thermal oxidation of AlAsSb was investigated. The oxidation kinetics was studied as a function of temperature and oxidation duration. An expression to allow accurate determination of the oxide depth for any temperature and time was established. Secondary ion mass spectrometry profiles and x-ray diffraction were used to demonstrate that an interfacial layer, composed of pure Sb and As and textured on InP, is formed during the oxidation process. [ABSTRACT FROM AUTHOR]

In this work we studied the optical properties of poly(p-phenylene vinylene) (PPV) produced by the thermal conversion of a precursor polymer blended with a synthetic dye (Reactive Black 5). The production of PPV by this method decreases the overall time and cost of the process. We observed that the introduction of the dye resulted in an additional absorbance band near 550-700 nm, which can be beneficial to the photon harvesting capacity of the polymer if it is used as the donor material in a photovoltaic device. We studied how the optical and structures properties of this blend change when different quantities of TiO2 nanoparticles are introduced. For that, thin films were produced by the cast deposition of pre- PPV:dye:TiO2. The scanning electronic microscopic images showed that the inorganic semiconductor form large agglomerates of approximately 200 nm, indicating a very rough surface where the dye can be adsorbed. The analysis of photoluminescence and Raman peaks indicated a reduction of the mean conjugation length of the polymer chains in the presence of TiO2 nanoparticles. [ABSTRACT FROM AUTHOR]

AlGaAsSb/AlAsSb Bragg mirrors lattice matched on InP with six pairs of layers were grown by molecular beam epitaxy. The effect of Te doping on the electrical and optical properties of the Bragg mirrors, and the presence of digital alloy gradient layers between the ternary and quaternary layers are analysed. The presence of digital alloy layers at the interfaces reduces the electrical resistance through the perpendicular direction of the Bragg mirror, without significantly affecting the reflectivity. [Copyright &y& Elsevier]

We present effective-mass calculations of the bound-state energy levels of electrons confined inside lens-shaped InxGa1−xAs quantum dots (QDs) embedded in a GaAs matrix, taking into account the strain as well as the In gradient inside the QDs due to the strong In segregation and In-Ga intermixing present in the InxGa1−xAs/GaAs system. In order to perform the calculations, we used a continuum model for the strain, and the QDs and wetting layer were divided into their constituting monolayers, each one with a different In concentration, to be able to produce a specific composition profile. Our results clearly show that the introduction of such effects is very important if one desires to correctly reproduce or predict the optoelectronic properties of these nanostructures. [ABSTRACT FROM AUTHOR]