H.W.L. Alves

Theoretical study of the AlxGa1−xN alloys

Abstract In this work we use a first-principles method based on the density functional theory, the full-potential linear augmented plane-wave method (FPLAPW), in order to calculate the electronic structures of the Al x Ga 1− x N alloys in the cubic modification. We adopt a model which allows the simulation of the composition x =0.0, 0.25, 0.50, 0.75 and 1.0. We obtain the equilibrium lattice parameters, the bulk moduli, the formation energies, the miscibility curves and the effective masses of the conduction and valence bands in the [100], [111] and [110] directions. The results can be used in the parameterization of theories based on effective hamiltonians. To our knowledge, this is the first time such a systematic ab initio study of effective masses of these semiconductor alloys is accomplished.

Dynamical and thermodynamic properties of III-nitrides

In this work, we have calculated ab initio the equation of state, the principal phonon modes, the effective charges and the temperature dependence of the specific heat for the III-Nitrides, by using the density-functional theory within the local density approximation, plane wave expansions and the pseudopotential method. A good agreement with the experiment and other calculations is obtained, whenever a comparison is possible. From our results, we speculate whether these systems undergo, by a second-order phase transition, from the wurtzite structure to the zincblende one.

Study of the oxygen vacancy influence on magnetic properties of Fe- and Co-doped SnO2 diluted alloys

Transition-metal (TM)-doped diluted magnetic oxides (DMOs) have attracted attention from both experimental and theoretical points of view due to their potential use in spintronics towards new nanostructured devices and new technologies. In the present work, we study the magnetic properties of Sn0.96TM0.04O2 and Sn0.96TM0.04O1.98(VO)0.02, where TM = Fe and Co, focusing in particular in the role played by the presence of O vacancies nearby the TM. The calculated total energy as a function of the total magnetic moment per cell shows a magnetic metastability, corresponding to a ground state, respectively, with 2 and 1 μB/cell, for Fe and Co. Two metastable states, with 0 and 4 μB/cell were found for Fe, and a single value, 3 μB/cell, for Co. The spin-crossover energies (ES) were calculated. The values are ES0/2 = 107 meV and ES4/2 = 25 meV for Fe. For Co, ES3/1 = 36 meV. By creating O vacancies close to the TM site, we show that the metastablity and ES change. For iron, a new state appears, and the state with zero magnetic moment disappears. The ground state is 4 μB/cell instead of 2 μB/cell, and the energy ES2/4 is 30 meV. For cobalt, the ground state is then found with 3 μB/cell and the metastable state with 1 μB/cell. The spin-crossover energy ES1/3 is 21 meV. Our results suggest that these materials may be used in devices for spintronic applications that require different magnetization states.

Electronic and magnetic properties of SnO2/CrO2 thin superlattices.

In this article, using first-principles electronic structure calculations within the spin density functional theory, alternated magnetic and non-magnetic layers of rutile-CrO2 and rutile-SnO2 respectively, in a (CrO2)n(SnO2)nsuperlattice (SL) configuration, with n being the number of monolayers which are considered equal to 1, 2, ..., 10 are studied. A half-metallic behavior is observed for the (CrO2)n(SnO2)nSLs for all values of n. The ground state is found to be FM with a magnetic moment of 2 μB per chromium atom, and this result does not depend on the number of monolayers n. As the FM rutile-CrO2 is unstable at ambient temperature, and known to be stabilized when on top of SnO2, the authors suggest that (CrO2)n(SnO2)nSLs may be applied to spintronic technologies since they provide efficient spin-polarized carriers.

Theoretical studies of poly(para-phenylene vinylene) (PPV) and poly(para-phenylene) (PPP)

The electroluminescence from PPV and the blue light emission from PPP constitute exciting subjects of study. The band gap of these semiconducting polymers can be engineered in a wide range from red to ultraviolet by structural changes made on them. In the present work, we present a theoretical approach based on semiempirical and ab initio total energy and force calculations of PPV and PPP. We perform a conformational analysis in order to investigate the connection between their structural, optical and electronic properties. We use the large cell approach, in connection with the semiempirical quantum method Extended Huckel (BICON-CEDiT code) and the density functional theory (DFT) within the full-potential linearized augmented-plane-wave method (FPLAPW) as implemented in the computational code WIEN2k. Our results are compared to other calculations and to optical absorption measurements.

Theoretical optical parameters for III-nitride semiconductors

Abstract The III-nitride compounds (GaN, AlN, BN, and InN) are semiconductor materials which are promising for application in optoelectronics. They find applications in light emitting diodes, laser diodes and luminescent alloys. In the present work we calculate by means of an ab initio method the optical response functions for these compounds in their cubic phase (zinc-blend). We obtain the absorption coefficients, α ( E ), the dielectric constant, ϵ ( E ), the reflectance, R ( E ), and the index of refraction, n ( E ) from the calculated energy band structures of the semiconductors. The values are compared to the available values in the literature.

Lattice dynamics of Ga1−xMnxN and Ga1−xMnxAs by first-principle calculations

In this work, we present theoretical results, using first-principle methods associated to the virtual crystal approximation model, for the vibrational mode frequencies of both the Ga1−xMnxN (in both cubic and hexagonal structures) and the Ga1−xMnxAs alloys, with the Mn contents in the range of 0% to 20%. The dependence of the calculated phonon frequencies with the Mn content was analyzed, and the results indicate that the phonon frequencies decrease with the increasing of Mn composition, leading to the false impression that they obey the Vegard rule in some cases. Moreover, the hexagonal Ga1−xMnxN alloys are elastically unstable for Mn concentrations at the order of 20%, which explains in part the experimentally observed deterioration of these alloys. These findings can be used in future technologies as a guide for the synthesis of spintronic nanostructured devices, such as nanowires, based on these materials.

Ab initio Results for the Structural and Electronic Properties of Intrinsic Defects in PbTe

In this work, we have performed spin-polarized calculations for the structural and electronic properties of vacancies and anti-site defects in the rocksalt PbTe. Our obtained results have shown that both the Pb and Te antisites are the favorable defects in Pb and Te rich conditions, respectively. Moreover, in the perfect stoichiometry condition, the antisites, as well as the Te vacancy, are equally probable to find. Considering the charge injection in the system, all the defects change from the 2+ charge state to the 2one within 14 meV, at around 80 meV from the valence band edge within the bandgap. This feature makes it difficult to experimentally characterize these defects in PbTe.

Atomic and electronic structures of InxGa1−xN quantum dots

Abstract We have calculated the atomic and electronic structures of the In x Ga 1− x N random alloys (0≤ x ≤1) based on 950 atom clusters. A valence-force-field method with a Keating-type potential is used for the strain energy calculation and, therefore, the equilibrium geometries. We analyzed the bond-length and bond-angle distribution in the alloy due to the random fluctuations of the atom positions. The change in the average Ga–N and In–N lengths is calculated as a function of the composition x and compared to recent experimental data. Once we have the equilibrium geometries we calculate the electronic structures of the alloys by means of the semi-empirical quantum method Extended Hueckel.

Vibrational spectra of adsorbed hydrogen on GaN(001) surfaces

Abstract The infrared spectroscopy (IR) of adsorbed hydrogen on a surface is an experimental technique used to analyze semiconductor surfaces. The hydrogen atoms are bonded to the surface dangling-bonds and the vibrational frequencies of the M–H bonds are very sensitive to the local chemical character at the surface. Usually the analysis of the results is based on the comparison with the IR spectra of molecules in the gaseous phase. This comparison is difficult when structures at the surface have no exact analogous in the gas phase. One alternative way is to base the comparison on theoretical results obtained for molecular clusters simulating the complex adsorbed at the surface. The calculated frequencies of the ‘stretching’ mode of the M–H bonds corresponding to the various ways the hydrogen binds to the surface may be associated to the experimental IR spectra. In the present work we carry out ab initio calculations of Ga x N y H z clusters in order to analyze the adsorption models of hydrogen by Ga or N dimers occurring on the reconstructed GaN(001) surfaces. We use the Gaussian 94 code of the quantum chemistry in order to calculate the molecular structures; the clusters are allowed to relax in order to minimize the total energy. We obtain the vibrational spectra and the associated normal modes.