S. Mergulhão

Influence of Ca concentration on the electric, morphological, and structural properties of (Pb,Ca)TiO3 thin films

Pb1−xCaxTiO3 (0.10⩽x⩽0.40) thin films on Pt/Ti/SiO2/Si(100) substrates were prepared by the soft solution process and their characteristics were investigated as a function of the calcium content (x). The structural modifications in the films were studied using x-ray diffraction and micro-Raman scattering techniques. Lattice parameters calculated from x-ray data indicate a decrease in lattice tetragonality with the increasing content of calcium in these films. Raman spectra exhibited characteristic features of pure PbTiO3 thin films. Variations in the phonon mode wave numbers, especially those of lower wave numbers, of Pb1−xCaxTiO3 thin films as a function of the composition corroborate the decrease in tetragonality caused by the calcium doping. As the Ca content (x) increases from 0.10 to 0.40, the dielectric constant at room temperature abnormally increased at 1 kHz from 148 to 430. Also calcium substitution decreased the remanent polarization and coercive field from 28.0 to 5.3 μC/cm2 and 124 to 58 kV/c...

Capacitance spectroscopy of InAs self-assembled quantum dots embedded in a GaAs/AlAs superlattice

The characteristics of the InAs self-assembled quantum dots embedded both in a GaAs bulk matrix and in a GaAs/AlAs superlattice were investigated. Evidences of electrons confinement inside the InAs quantum dots were obtained using both capacitance–voltage measurements and Raman spectroscopy. A much stronger electron localization was detected for the quantum dots embedded in the superlattice in comparison with those embedded in bulk GaAs. As a consequence, the electrical characteristics of the structures with quantum dots grown in superlattices were found to be significantly thermo-stabilized. The origins of these effects are discussed in connection with the differences between the electronic features of the two kinds of structures.

Parallel conductivity of random GaAs/AlGaAs superlattices in regime of controlled vertical disorder

The transport properties of intentionally disordered short-period doped GaAs/AlGaAs superlattices were explored. We found that a redistribution of the electron density in a random potential significantly influences the electron properties of the disordered superlattices resulting in a suppression of the vertical disorder and in the increase of the in-plane mobility of the layers with increasing disorder.

Carrier confinement in an ultrathin barrier GaAs/AlAs superlattice probed by capacitance–voltage measurements

Abstract The capacitance–voltage characteristics of (GaAs)m/(AlAs)n superlattices and of a GaAs/AlGaAs multiple quantum well system were used as a tool to probe the homogeneity of the studied samples. Evidences of a strong electron localization in the superlattices even with the presence of minibands were found. We interpret this result taking into account the presence of local inhomogeneities in the superlattices which causes the breakdown of the coherence of the miniband transport and therefore, give rise to the electron localization. In order to support this conclusion we numerically calculate the capacitance of the superlattices assuming a localization center near the region where electron confinement takes place and the results were found in good agreement with the measured capacitance.

Effects of annealing on electrical and optical properties of a multilayer InAs/GaAs quantum dots system

A systematic investigation of the properties of the InAs/GaAs self-assembled quantum dots (SAQDs) system subjected to a post-growth annealing using capacitance-voltage, Raman scattering and photoluminescence measurements is presented. The application of both electrical and optical methods allowed us to obtain reliable information on the microscopic structural evolution of this system. The single layer and the multilayer quantum dots were found to respond differently to the annealing process, due to the differences in strain that occur in both systems. The diffusion activated by strain provoked the appearance of an InGaAs alloy layer in substitution to the quantum dots layers; this change occurred at the annealing temperature T = 600 oC in the multilayer system. A single dot layer, however, was observed even after the annealing at T = 700 oC. Moreover, the low temperature annealing was found to improve the homogeneity of the multilayer system and to decrease the electrical interlayer coupling.

Thermostabilization of Electrical Properties of InAs/GaAs Self-Assembled Quantum Dots Embedded in GaAs/AlAs Superlattices

The electrical characteristics of the self-assembled quantum dots embbeded in GaAs wells of the GaAs/AlAs superlattices were studied by capacitance spectroscopy and were compared with results obtained for the dots embbeded in bulk GaAs. A much stronger electron localization was detected for the quantum dots embedded in the superlattice in comparison with those embedded in bulk GaAs. As a consequence, the electrical characteristics of the structures with quantum dots grown in superlattices were found to be significantly thermo-stabilized. It was shown that these structures present different strengths of the localization of electrons caused by the effective increase of the heights of the barriers when the dots were grown in the superlattices.

Investigation of the InAs/GaAs self-assembled quantum dots using the relationship between the capacitance and the density of states

Abstract A negative differential capacitance observed in the InAs/GaAs self-assembled quantum dots is shown to be an essential feature of the zero-dimensional electron system. A direct link of the capacitance to the electron density of states is presented. As a result, the density of the dots, the excitation energy of electrons and its dispersion were obtained and found to be in good agreement with the photoluminescence data.

Effects of Annealing on Electrical Coupling in a Multilayer InAs/GaAs Quantum Dots System

We present a systematic investigation of the capacitance–voltage measurements and Raman scattering on a multilayer InAs/GaAs self-assembled quantum dots system annealed at different temperatures. We observed a decrease of the electrical coupling of the electrons trapped in the dots located in the different layers. Raman scattering revealed the modifications of the dots morphology which influenced on the observed increase of the localization of electrons in the dots after the annealing process. We also observed that the annealing at 600°C altered the plane of the dots into layers of an InxGa1-xAs alloy.

Influence of annealing on the optical and electrical properties of multilayered InAs/GaAs quantum dots

The characteristics of multi-layered InAs/GaAs self assembled quantum dots (SAQDs) annealed after the growth were here studied using a combination of capacitance-voltage (C-V) measurements,Raman scattering and photoluminescence (PL) spectroscopy. The combination of the results obtained with the three techniques, gave evidences that the annealing at 500o C causes the sharpness of the SAQDs interfaces, while the annealing at 600o C eliminated the SAQDs. However, the comparison with the case of single layered SAQDs, revealed a thermal stability of the last system even at an annealing temperature of 700o C, thus confirming the role of the interlayer strain in the low temperature diffusion process.

Capacitance-voltage characteristics of InAs dots: a simple model

An electrostatic model was presented for the calculation of the capacitance-voltage characteristics of a semiconductor structure where quantum dots were embedded. The model was based on the linear coupling between the contributions of the quantum dots and the bulk host. We further applied this model to an InAs/GaAs self-assembled quantum dots system. The calculated capacitance was found in good agreement with the experimental curves, providing parameters of the dots ensemble, as the excitation energy of the confined electrons.