CALCULATION OF MAGNETODONOR STATES IN InP AND POLARON EFFECTS

Far-infrared transitions in polar semiconductors are known to be affected by the presence of shallow donor impurities, external magnetic fields and the electron-LO-phonon interaction. We calculate the magnetodonor states in indium phosphide by a diagonalization procedure, and introduce the electron-phonon interaction by the Frohlich term. The main effects of this perturbation are calculated by a multi-level version of the Wigner-Brillouin theory. We determine the transition energies, from the ground state to excited states, and find good qualitative agreement with recently reported absorption-spectroscopy measurements in the 100-800 cm(-1) range, with applied magnetic fields up to 30 T. Our calculations suggest that experimental peak splittings in the 400-450 cm(-1) range are due to the electron-phonon interaction.

Mudança organizacional derivada da implantação da norma OHSAS 18.001: uma abordagem fenomenológica

Dissertação apresentada ao Programa de Pós-graduação em Administração da Universidade Municipal de São Caetano do Sul

Thermally activated exchange narrowing of the Gd3+ ESR fine structure in a single crystal of Ce1-xGdxFe4P12 (x approximate to 0.001) skutterudite

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Energia de superfície para nanossuperfícies de TiO2 na direção (001)

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Theoretical study of MgO(001) surfaces: Pure, doped with Fe, Ca, and Al, and with and without adsorbed water

Ab initio calculations of large cluster models have been performed in order to study water adsorption at the five-fold coordinated adsorption site on pure Mg(001) and MgO(001) surfaces doped with Fe, Ca, and Al. The geometric parameters of the adsorbed water molecule have been optimized preparatory to analysis of binding energies, charge transfer, preferential sites of interaction, and bonding distances. We have used Mulliken population analysis methods in order to analyze charge distributions and the direction of charge transfer. We have also investigated energy gaps, HOMO energies, and SCF orbital energies as well as the acid-base properties of our cluster model. Numerical results are compared, where possible, with experiment and interpreted in the framework of various analytical models. (C) 2001 John Wiley & Sons, Inc.

Micro-Raman analysis of cubic GaN layers grown by MBE on (001) GaAs substrate

Cubic GaN layers are grown by molecular beam epitaxy on (001) GaAs substrates. Optical micrographs of the GaN epilayers intentionally grown at Ga excess reveal the existence of surface irregularities such as bright rectangular structures, dark dots surrounded by rectangles and dark dots without rectangles. Micro-Raman spectroscopy is used to study the structural properties of these inclusions and of the epilayers in greater detail. We conclude that the observed irregularities are the result of a melting process due to the existence of a liquid Ga phase on the growing surface.

Photoreflectance studies of optical transitions in cubic GaN grown on GaAs(001) substrates

The optical properties of cubic GaN epitaxial layers were investigated by modulated photoreflectance (PR) and photoluminescence in the temperature interval from 5 to 300 K. The epilayers were grown on GaAs(001) substrates by molecular beam epitaxy using a nitrogen RIF-activated plasma source. The PR spectra show a transition which is well fitted using the third-derivative functional form of the unperturbed dielectric function, which we interpret as band-to-band transition. Our results allow determination of the temperature dependence of the main gap of c-GaN and give insights into the residual strain in the film, as well as allow us to estimate the binding energy of the complex formed by an exciton bound to a neutral acceptor. (C) 2003 Elsevier B.V. B.V. All rights reserved.

Electronic and structural properties of the (001) SrZrO3 surface

The structural and electronic properties of SrZrO3 selected surfaces were investigated by means of density functional theory applied to periodic calculations at B3LYP level. The relaxation effects for two symmetric and asymmetric terminations are analyzed. The electronic and energy band properties are discussed on the basis of band structure as well density of states. There is a more significant rumpling in the SrO as compared to the ZrO2 terminated surfaces. The calculated indirect gap is 4.856, 4.562, 4.637 eV for bulk, ZrO2 and asymmetric terminations, respectively. The gap becomes direct; 4.536 eV; for SrO termination. The contour in the (110) diagonal plane indicates a partial covalent character between Zr and 0 atoms for the SrO terminated surface. (c) 2007 Elsevier B.V. All rights reserved.

Hybrid reflections in InGaP/GaAs(001) by synchrotron radiation multiple diffraction

Hybrid reflections (HRs) involving substrate and layer planes (SL type) [Morelhao et al., Appl. Phys. Len. 73 (15), 2194 (1998)] observed in Chemical Beam Epitaxy (CBE) grown InGaP/GaAs(001) structures were used as a three-dimensional probe to analyze structural properties of epitaxial layers. A set of (002) rocking curves (omega-scan) measured for each 15 degrees in the azimuthal plane was arranged in a pole diagram in phi for two samples with different layer thicknesses (#A -58 nm and #B - 370 nm) and this allowed us to infer the azimuthal epilayer homogeneity in both samples. Also, it was shown the occurrence of (1 (1) over bar3) HR detected even in the thinner layer sample. Mappings of the HR diffraction condition (omega:phi) allowed to observe the crystal truncation rod through the elongation of HR shape along the substrate secondary reflection streak which can indicate in-plane match of layer/substrate lattice parameters. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

CALCULATION of MAGNETODONOR STATES IN InP and POLARON EFFECTS

Far-infrared transitions in polar semiconductors are known to be affected by the presence of shallow donor impurities, external magnetic fields and the electron-LO-phonon interaction. We calculate the magnetodonor states in indium phosphide by a diagonalization procedure, and introduce the electron-phonon interaction by the Frohlich term. The main effects of this perturbation are calculated by a multi-level version of the Wigner-Brillouin theory. We determine the transition energies, from the ground state to excited states, and find good qualitative agreement with recently reported absorption-spectroscopy measurements in the 100-800 cm(-1) range, with applied magnetic fields up to 30 T. Our calculations suggest that experimental peak splittings in the 400-450 cm(-1) range are due to the electron-phonon interaction.

Nucleation and growth evolution of InP dots on InGaP/GaAs

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Carriers escape mechanisms in shallow InGaAs/GaAs quantum wells grown on vicinal (001) GaAs substrates

We used photoluminescence spectroscopy in order to investigate the carriers escape mechanisms in In0.15Ga0.85As/GaAs quantum wells grown on top of nominal (001) and 2°-, 4°-and 6°-off (001) towards (111)A GaAs substrates. We described the escape processes using two models that fit the Arrhenius plot of the integrated PL intensity as a function of the inverse of the sample temperature. In the first model, we considered equal escape probability for electrons and holes. In the second one, we assumed that a single type of carrier can escape from the well. At high temperature, the first model fits the experimental data well, whereas, between 50 K and 100 K, the second model has to be taken into account to describe the data. We observed that the escape activation energy depends on the misorientation angle. An unusual behavior was noted when the full width at half maximum of the photoluminescence main emission was plotted as a function of the sample temperature. We showed that the escape process of the less-confined carriers drives this behavior. © 1999 Academic Press.

Vertical and in-plane electrical transport in InAs/InP semiconductor nanostructures

Vertical and in-plane electrical transport in InAs/InP semiconductors wires and dots have been investigated by conductive atomic force microscopy (C-AFM) and electrical measurements in processed devices. Localized I-V spectroscopy and spatially resolved current images (at constant bias), carried out using C-AFM in a controlled atmosphere at room temperature, show different conductances and threshold voltages for current onset on the two types of nanostructures. The processed devices were used in order to access the in-plane conductance of an assembly with a reduced number of nanostructures. On these devices, signature of two-level random telegraph noise (RTN) in the current behavior with time at constant bias is observed. These levels for electrical current can be associated to electrons removed from the wetting layer and trapped in dots and/or wires. A crossover from conduction through the continuum, associated to the wetting layer, to hopping within the nanostructures is observed with increasing temperature. This transport regime transition is confirmed by a temperature-voltage phase diagram. © 2005 Materials Research Society.

Electrical properties of individual and small ensembles of InAs/InP nanostructures

We investigate electrical properties of InAs/InP semiconductor nanostructures by conductive atomic force microscopy (C-AFM) and current measurements at low temperatures in processed devices. Different conductances and threshold voltages for current onset were observed for each type of nanostructure. In particular, the extremity of the wire could be compared to a dot with similar dimensions. The processed devices were used in order to access the in-plane conductance of an assembly of a reduced number of nanostructures. Here, fluctuations on I-V curves at low temperatures (<40 K) were observed. At these low temperatures and for a suitable range of applied voltages, random telegraph noise (RTN) in the current was observed for devices with dots. These fluctuations can be associated to electrons trapped in dots, as suggested by numerical simulations. A crossover from a semiconductor-like to a metallic transport behavior is also observed for similar parameters. © 2006 WILEY-VCH Verlag GmbH & Co. KGaA.

DOCPAL resúmenes sobre población en América Latina: 112-001

Contiene resúmenes bibliográficos de documentos sobre población y temas relacionados que se producen en América Latina y el Caribe, publicados entre 1984-1987.