In this work, by using the density-functional theory within the local density approximation, plane wave expansions and the pseudopotential method, we present our preliminary results for the strain-induced shifts of the zone center phonon modes and of the effective charges on the III-Nitrides. Our results are in good agreement with the experiment as well as with other calculations, whenever a comparison is possible.

The electronic, structural, and thermodynamic properties of cubic (zinc blende) group-III nitride ternary In xGa1-xN and quaternary Al xIn yGa1-x-yN alloys are investigated by combining first-principles total energy calculations and cluster expansion methods. External biaxial strain on the alloys are taken into account in the calculations. While alloy fluctuations and strain effects play a minor role in the physical properties of AlGaN, due to the small lattice-mismatch, in the InGaN alloys we found a remarkable influence of strain on the phase separation. The effect of biaxial strain on the formation of ordered phases has been also investigated, and the results are used to clarify the origin of the light emission process in In xGa1-xN based optoelectronic devices. The dependence of the lattice constant and energy bandgap in quaternary Al xIn yGa1-x-yN alloys was obtained as function of the compositions x and y. PACS: 61.66.Dk, 64.75.+g, 71.20.Nr

The utilization of the quantum properties of the electron spin wave function will allow the development of a new class of devices. The problem is still controversial and unsettled, even qualitatively, especially for concentrated spin systems such as 3d metals and their alloys. The variety of crystal structures of 3d metals makes difficult the direct comparison between the experimental results and the theoretical conclusions. On the account of this difficulty, substitutional alloys with the same crystal structure, especially face-centered cubic alloys, have been investigated extensively. In this work the properties of diluted Ga1-xMn xN (x = 0.0630; 0.0315) alloys are calculated in the zinc-blende phase, within the framework of the density functional theory, using the fullpotential linearized augmented plane wave (FLAPW) method and the local density approximation (LDA). The alloys are simulated using 32-atom and 64-atom large unit cells, containing one substitutional Mn atom for a Ga atom. The calculations are spin-polarized and we analyze band structures, density of states and total magnetizations. A half-metallic state is predicted at a0 ~ 4:45Å. The majority-spin band has a rather sharp peak, characteristic of a narrow band, while the minority-spin has a gap. The total magnetization of the cell is 4.00muB which does not change with the Mn concentration. The valence band is ferromagnetically coupled with the Mn atoms, and the spin splitting is not linearly dependent on the Mn concentration.

We report ab-initio all electrons density- functional calculations for the electronic structure of the compounds MnAs and MnN, in the zinc-blende phase. They are potential materials for use in fabrication of new functional semiconductors taking advantage of the spin degree of freedom. The aim is the establishing of the semiconductor spin electronics (spintronics) as a practical technology [H. Ohno, Semiconductor Science and Technology 17, 4 (2002).]. We compare results obtained using the theoretical approaches LDA (Local Density Approximation) and GGA (Generalized Gradient Approximation). The calculations are spin-polarized and we follow the evolution of the band structures as a function of lattice parameter. We compare also the evolution of the density of states of the majority-spins and of the minority-spins. We conclude that, depending on the lattice parameter, both materials may be half-metallic, therefore showing conduction by charge carriers of one spin direction exclusively: the majority- spin band is continuous, while the minority-spin has a gap. Both materials reach a total magnetization of the order of 4 mB. MnN changes from paramagnetic to ferromagnetic with the increase of the lattice parameter.

In this work, we have calculated ab initio the phonon dynamics of the Ga-rich GaAs (100) (1 × 1), GaAs (100) (2 × 1), GaN (100) (1 × 1), GaAs (110) and GaN (110) surfaces. Our results for the (100) surfaces show that, while the (2 × 1) reconstruction is characterized by dimer vibrations, the (1 × 1) reconstruction presents interesting features which is closely related to the reduced mass of the compound. All the studied cases show the presence of a Rayleigh mode. The calculated properties for the (110) surfaces agree very well with the available experimental data from HEELS and inelastic He-atom scattering as well as with other theoretical calculations.

In this work, using Density Functional, Hartree-Fock and Extended Hückel Theories together with the supercell and cluster model approaches, we present our preliminary results for the simulation of the adsorption of Si and C atoms over the (111) SiC surfaces as well as the torsion of SiC molecules at that surfaces. Our results shown that, before and after the adsorption, the cubic structure are the most stable one, when compared with the hexagonal ones. However, the torsional energy barrier between the cubic and the hexagonal structures is small enough that, at the conditions of SiC growth, it facilitates the SiC polytype formation.

Using the density-functional theory within the Full Potential Linear Augmented Plane-Wave (FP-LAPW) method, we have calculated, in this work, the equation of state, the effective charges and the phonon dispersion along [100] and [111] directions for the cubic III-Nitrides(c-BN, c-AlN, c-GaN and c-InN), including hydrostatic strains dependence. A good agreement with the micro-Raman scattering experiment is obtained for the phonon modes at <FONT FACE=Symbol>G.</FONT> Our results show that, for c-InN case, the apparent discrepancy between the known experimental results is a consequence of hydrostatic effects on the sample due to differences of the used Raman methods.

First-principles calculations of electronic band structures of the ordered cubic alloys Al xGa1-xN and Cd xZn1-xTe are carried out. The band structures are used to provide e ective masses and Luttinger parameters which are useful in the parametrization of theories based on e ective hamiltonians.

A theoretical semi-classical method to calculate the modulated field profile in a semiconductor is presented. The behavior of quasi-Fermi levels and chemical potentials for the majority and minority carriers is investigated. Systematic studies of the non-thermodynamic equilibrium electric potential profile as a function of the modulation light intensity are performed.