José Leonil Duarte

Temperature dependence of optical transitions in AlGaAs

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. Vina et al. [Phys. Rev. B 30, 1979 (1984)], and R. Passler [Phys. Status Solidi B 200, 155 (1997)] models were used to fit the experimental points. The Passler 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.

Temperature-dependent photoluminescence spectra of GaAsSb/AlGaAs and GaAsSbN/GaAs single quantum wells under different excitation intensities

The mechanism for low-temperature photoluminescence (PL) emissions in GaAsSb/AlGaAs and GaAsSbN/GaAs strained-layer single quantum wells (SQWs), grown by molecular-beam epitaxy, is studied in detail, using PL spectroscopy as a function of temperature and excitation intensity. In all samples, the PL peak energy as well as the full width at half maximum (FWHM), as a function of temperature, present anomalous behaviors, i:e., the PL peak energy shows a successive red/blue/redshift (S-shaped behavior) and the FWHM shows a successive blue/red/blueshift (“inverted S-shaped curve”) with increasing temperature. At sufficiently low excitation intensity and in a narrow temperature interval (50 ‐ 80 K), the nitrogen-containing samples present two clear competitive PL peaks. The low-energy PL mechanism (8 ‐ 80 K) is dominated by localized PL transitions, while the high-energy PL mechanism is dominated by the ground state (e1-hh1) PL transition. Additionally, these PL peaks show different temperature dependence with the low-energy PL peak, showing a stronger redshift than the high-energy PL peak. A competition process between localized and delocalized excitons is used to discuss these PL properties.

Quasi-donor-acceptor pair transitions in GaAsSb and AlGaAsSb on InP

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.

Thermal expansion contribution to the temperature dependence of excitonic transitions in GaAs and AlGaAs

Photoluminescence and photoreflectance measurements have been used to determine excitonic transitions in the ternary AlxGa1�xAs alloy in the temperature range from 2 to 300 K. The effect of the thermal expansion contribution on the temperature dependence of excitonic transitions for different aluminum concentrations in the AlxGa1�xAs alloy is presented. Results from this study have shown that the negative thermal expansion (NTE) in the AlxGa1�xAs alloy, in the low temperature interval, induces a small blueshift in the optical transition energy. In the temperature range from ∼23 to ∼95 K there is a competition between the NTE effect and the electron-phonon interaction. Using the thermal expansion coefficient in the 2 – 300 K temperature range, the thermal expansion contribution to GaAs, at room temperature, represents 21% of the total shift of the excitonic transition energy. After subtracting the thermal expansion contribution from the experimental temperature dependence of the excitonic transitions, in the AlxGa1�xAs alloy, the contribution to the electron-phonon interaction of the longitudinal optical phonon increases, relatively to the longitudinal acoustical phonon, with increasing Al concentration.

Correlation between luminescence properties of AlxGa1−xAs∕GaAs single quantum wells and barrier composition fluctuation

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 (8K⩽T⩽90K) 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 Passler [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 fluctua...

The effect of potential fluctuations on the optical properties of InGaAs∕InAlAs superlattices

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 integr...

Photoluminescence study of interfaces between heavily doped Al0.48In0.52As:Si layers and InP (Fe) substrates

Properties of the interface between the epitaxial layer of heavily doped Al0.48In0.52As: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 Al0.22Ga0.26In0.52As: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) a...

Investigation of optical properties of interfaces between heavily doped Al0.48In0.52As:Si and InP (Fe) substrates by photoreflectance analysis

A set of heavily doped Al0.48In0.52As samples grown by molecular beam epitaxy on InP (Fe) substrates was investigated using the photoreflectance (PR) technique. The spectra at 300 K are characterized by a transition in the vicinity of the InP energy gap, followed by strongly damped Franz–Keldysh oscillations (FKOs) which do not appear when the spectra are obtained at 77 K. The builtin electric field estimated from FKOs shows a small doping dependence but is substantially affected by the inclusion of a thin layer of AlxGayIn1−x−yAs (x≡0.22) at the interface between InP (Fe) and AlInAs:Si. In order to explain these results, a model based on the discontinuity of the energy bands in the InP/AlInAs and InP/AlGaInAs/AlInAs systems and also on the matching of the Fermi levels between the different materials is suggested.

Comparison of some theoretical models for fittings of the temperature dependence of the fundamental energy gap in GaAs

In this work we report on a comparison of some theoretical models usually used to fit the dependence on temperature of the fundamental energy gap of semiconductor materials. We used in our investigations the theoretical models of Vi˜ na, P¨ assler-p and P¨ assler-r to fit several sets of experimental data, available in the literature for the energy gap of GaAs in the temperature range from 12 to 974 K. Performing several fittings for different values of the upper limit of the analyzed temperature range (T max), we were able to follow in a systematic way the evolution of the fitting parameters up to the limit of high temperatures and make a comparison between the zero-point values obtained from the different models by extrapolating the linear dependence of the gaps at high T to T = 0 K and that determined by the dependence of the gap on isotope mass. Using experimental data measured by absorption spectroscopy, we observed the non-linear behavior of Eg(T) of GaAs for T > QD.

Substrate orientation effect on potential fluctuations in multiquantum wells of GaAs∕AlGaAs

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 Passler [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 t...