Acessibilidade / Reportar erro
Brazilian Journal of Physics, Volume: 34, Número: 2b, Publicado: 2004
  • Foreword

    Farias, G. A.
  • STM light emission spectroscopy of individual quantum wells: measurement of transport parameters in real space

    Ushioda, S.; Tsuruoka, T.; Ohizumi, Y.; Hashimoto, H.

    Resumo em Inglês:

    By spectroscopically analyzing the light emitted from the tip-sample gap of the scanning tunneling microscope (STM), we have investigated the carrier transport as well as the luminescence properties of AlGaAs/GaAs quantum wells (QW's). The emission intensity form a target well was measured as a function of the tip position on a cleaved (110) surface of the QW structures. The thermalization length and the diffusion length of the injected electrons were determined in real space.
  • Semiconductor quantum dot nanostructures and their roles in the future of photonics

    Fafard, S.; Hinzer, K.; Allen, C. N.

    Resumo em Inglês:

    This paper discusses the growth and the properties of semiconductor nanostructures based on self-assembled quantum dots (QDs). These QDs confine electrons or excitons in zero-dimension (0D), similar to an artificial atom or to an artificial molecule in the case of coupled QDs with vertical alignment. They are obtained in a simple step during the epitaxy of strained III-V semiconductors such as InAs on GaAs, or InAs on InP. We will elaborate on the unique optical properties and the physics of self-assembled QDs and their applications, including QD lasers.
  • Optical properties of type-I and II quantum dots

    Iikawa, F.; Godoy, M. P. F.; Nakaema, M. K. K.; Brasil, M. J. S. P.; Maialle, M. Z.; Degani, M. A.; Ribeiro, E.; Medeiros-Ribeiro, G.; Carvalho Jr., W.; Brum, J. A.

    Resumo em Inglês:

    This paper focuses on recent results on the optical properties of self-assembled quantum dots involving type-I InGaAs/GaAs and type-II InP/GaAs interfaces. In the first part, we focus on the InGaAs/GaAs quantum dots, that were used to study the influence of a two-dimensional electron gas on the optical emission of single quantum dots. In the second part, we present the results on type-II InP/GaAs quantum dots. In this system, we observed an experimental evidence indicating that the model used to interpret the blue shift of the type-II quantum well optical emission cannot be applied to type-II quantum dots.
  • Manipulation with spin ordering in ferromagnetic semiconductors

    Dietl, Tomasz

    Resumo em Inglês:

    A short overview is given of recent advances in the field of carrier-controlled ferromagnetism in Mn-based III-V and II-VI diluted magnetic semiconductors and their nanostructures. The tailoring of domain structures and magnetic anisotropy by strain engineering and confinement is described. Experiments demonstrating the tunability of T C by light and electric field are presented.
  • Adsorption of C2H2-C2O3 on Si(001)

    Miotto, R.; Ferraz, A. C.; Srivastava, G. P.

    Resumo em Inglês:

    The adsorption of maleic anhydride on the Si(001) surface was investigated using the first-principles pseudopotential formalism. Our results indicate that C2H2-C2O3 adsorbs through a [2+2] cycloaddition of the C-C bond to the dangling bonds of Si-Si dimers, as observed for other small hydrocarbons. According to our calculations, the adsorption of a second maleic anhydride molecule will preferentially occur on first-neighbours Si-Si dimer sites in either the dimer chain our dimer row, i.e. considering a (4×2) reconstruction, C2H2-C2O3 can either form a linear chain or a zig-zag chain, depending on the growth conditions. Although both structures are found to be stable from the energetic point of view, only our STM theoretical images for the linear chain are comparable to available experimental data.
  • TMR effect in a FM-QD-FM system

    Souza, F. M.; Egues, J. C.; Jauho, A. P

    Resumo em Inglês:

    Using the Keldysh nonequilibrium technique, we study current and the tunnelling magnetoresistance (TMR) in a quantum dot coupled to two ferromagnetic leads (FM-dot-FM). The current is calculated for both parallel and antiparallel lead alignments. Coulomb interaction and spin-flip scattering are taken into account within the quantum dot. Interestingly, we find that these interactions play a contrasting role in the TMR: there is a parameter range where spin flip suppresses the TMR, while Coulomb correlations enhance it, due to Coulomb blockade.
  • First-principles materials study for spintronics: MnAs and MnN

    Paiva, R. de; Alves, J. L. A.; Nogueira, R. A.; Leite, J. R.; Scolfaro, L. M. R.

    Resumo em Inglês:

    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.
  • X-ray scattering from self-assembled InAs islands

    Malachias, A.; Neves, B. R. A.; Rodrigues, W. N.; Moreira, M. V. B.; Kycia, S.; Metzger, T. H.; Magalhães-Paniago, R.

    Resumo em Inglês:

    In this work several structural and chemical properties of self-assembled InAs islands grown on GaAs(001) are studied using surface x-ray scattering with synchrotron radiation. The technique of x-ray diffraction under grazing incidence condition was employed to differentiate coherent and incoherent islands. We used a model of a strained pyramidal island to interpret the x-ray results and correlate size and strain-state of these islands. The degree of GaAs interdiffusion in the islands was inferred from the variation of volume of the unit cell. The Poisson's ratio of the two materials involved establishes a limit of tetragonal distortion for this material. Any variation in this distortion is associated with the presence of Ga inside the islands.
  • High-temperature thin-catalytic gate devices for combustion emissions control

    Khan, Shabbir A.; Vasconcelos, Elder A. de; Hasegawa, Y.; Katsube, T.

    Resumo em Inglês:

    4H- and 6H-SiC Schottky diodes responding down to 5 ppm of NO and NO2 gases at temperatures up to 450ºC were fabricated. Upon exposure to gas, the forward current of the devices changes due to variations in the Schottky barrier height. For NO gas, the response follows a simple Langmuir adsorption model. Device parameters were evaluated from linear conductance/current versus conductance plots, with improved accuracy. The linearity of these plots is an indication that the devices are not being affected by interfacial oxide layers or pinning of the Fermi level. 4H-SiC devices showed slightly superior stability and sensitivity for this application.
  • Epitaxial hybrid ferromagnet-semiconductor structures: growth, structural and magnetic properties

    Däweritz, L.

    Resumo em Inglês:

    Ferromagnet-semiconductor heterostructures are promising materials for the integration of magnetic or spin related functions into semiconductor materials and devices. Fe-on-GaAs and MnAs-on-GaAs are canditates for potential room temperature applications. We review our recent results on growth, structural and magnetic properties of these materials, and their use as spin injectors. The most critical issue of Fe/GaAs heterostructures is the delicate interface formation during epitaxy and its possible modification during subsequent processing. The properties of the MnAs/GaAs heterostructures are intimately related to the phase transition between ferromagnetic and paramagnetic phases.
  • Carbon nanotube structures: Y-junctions and nanorings

    Latgé, A.; Muniz, R. B.; Grimm, D.

    Resumo em Inglês:

    The possibility of coupling two Y-junctions of carbon nanotubes, forming a type of nanotube ring is addressed here. Recent experimental evidences of growing carbon nanotube structures forming basically Y-kind junctions give support to this theoretical proposition. A single-band tight-binding Hamiltonian is considered and local electronic properties are investigated via the Green function formalism following renormalization techniques. Modulation on the transport properties of those nanostructured systems may be induced by considering different kind of junctions and different symmetries. Quantum interference phenomena are proven to reduce partial or completely the conductance of such nanoring structures.
  • Vibrational structure of organic semiconductors: the role of conjugation length

    Borges, C. A. M.; Marletta, A.; Faria, R. M.; Guimarães, F. E. G.

    Resumo em Inglês:

    Site-selective luminescence spectroscopy (SSL) was employed to resolve the vibracional modes in self-assembled poly(p-phenylene vinylene), PPV. Using Franck-Condon analysis, the PPV spectra can be well described by three effective vibrational modes at 330, 1160 and 1550 cm-1. As the temperature increases from 4 to 180 K the electron-phonon coupling (S factor) remains constant for the 1550 cm-1 mode while for other lower energy modes the coupling increases monotonically. This trend is consistent with the decrease of the conjugation length due to the increase of thermal disorder. We find that the temperature dependence of vibrational progression in PPV films is determined mainly by low frequency torsional modes.
  • Phase separation and ordering in group-III nitride alloys

    Teles, L. K.; Marques, M.; Scolfaro, L. M. R.; Leite, J. R.; Ferreira, L. G.

    Resumo em Inglês:

    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
  • Magnetic nanostructures for lateral spin-transport devices

    Grundler, Dirk; Hengstmann, Thomas M.; Rolff, Haiko

    Resumo em Inglês:

    We review our investigations on nanostructured ferromagnets fabricated on semiconductor heterostructures. We show that ballistic electrons in the semiconductor are in particular sensitive to local stray fields. The reversal of the ferromagnets give rise to characteristic magnetoresistance traces in the ferromagnet/semiconductor hybrid structures. We use these stray-field effects for detailed studies on the magnetic properties of rectangular micromagnets, nanodisks and nanorings. The nanostructures might be favorable candidates for ferromagnetic contacts in future spin-injection experiments in lateral feromagnet/semiconductor hybrid structures.
  • Titanium impurities in silicon, diamond, and silicon carbide

    Assali, L. V. C.; Machado, W. V. M.; Justo, J. F.

    Resumo em Inglês:

    We carried a theoretical investigation on the electronic and structural properties of titanium impurities in silicon, diamond, and silicon carbide. The calculations were performed using the spin-polarized full-potential linearized augmented plane wave method in the supercell approach. The atomic configurations and transition and formation energies of isolated Ti impurities were computed.
  • Monte Carlo studies of miniband conduction in extreme type-II superlattices

    Henriques, A. B.; Deacon, R.; Nicholas, R. J.

    Resumo em Inglês:

    Miniband charge transport was investigated by Monte Carlo simulations of electronic motion in short period superlattices of type-II energy band alignment (InAs/GaSb composition). The strong decrease of the miniband energy width when the electronic in-plane energy increases, characteristic of type-II superlattices, leads to a conductivity that is very sensitive to a magnetic field applied parallel to the axis of the superlattice. For structures with a miniband energy width greater than the optic phonon energy, the differential conductance can be changed from positive to negative by the magnetic field, due to the suppression of optic phonon emission.
  • Initial growth of GaN and InN over GaAs (110) substrates

    Paiva, A. de; Alves, J. L. A.; Alves, H. W. Leite

    Resumo em Inglês:

    We present, a systematic theoretical study of the adsorption of Ga, In and N over GaAs (110) surfaces based on parameter-free, self-consistent total energy and force calculations using the density functional theory. We analyzed the changes in the bond-lengths and in the bond-angles before and after deposition, as well as the total energy behaviour with the adsorbate chemical potential variation. Our results show that the GaN growth over GaAs (110) is energetically more favorable than the InN growth. However, the resulting InN film can be cubic only if we apply small strains on the substrate, while the GaN one is hexagonal.
  • Electronic structure of the n-type doped AgInO2 and CuAlO2 delafossites: similarities and differences

    Lalic, M. V.; Mestnik-Filho, J.; Carbonari, A. W.; Saxena, R. N.

    Resumo em Inglês:

    We performed the first-principles band-structure calculations for a pure and a Cd-doped AgInO2 delafossite compound. The results are carefully analyzed and compared with the results obtained for a pure and a Cd-doped CuAlO2 compound, calculated previously by our group. The electronic structures of both systems are found to be similar in many details, being characterized by the same hybridization scheme that occurs at both Cu and Ag positions. Introduction of Cd impurity into the Cu site produces a shallow band within the CuAlO2 gap, while the Cd presence at the Ag site creates a wide band pinned at the Fermi level in the AgInO2 spectrum. In both cases, however, the n-type conductivities are predicted.
  • Shubnikov-de Haas oscillations in Digital Magnetic Heterostructures

    Freire, Henrique J. P.; Egues, J. Carlos

    Resumo em Inglês:

    In this paper we theoretically investigate the magnetic-field and temperature dependence of the Shubnikov-de Haas oscillations in group II-VI modulation-doped Digital Magnetic Heterostructures. We self-consistently solve the effective-mass Schrödinger equation within the Hartree approximation and calculate the electronic structure and the magneto-transport properties. Our results show i) a shift of the Shubnikov-de Haas minima to lower magnetic fields with increasing temperature, and ii) an anomalous oscillation which develops when two opposite Landau levels cross near the Fermi energy. Both of these are consistent with recent magneto-transport measurements in such heterostructures.
  • Ab initio calculation of the (100) and (110) surface phonon dispersion of GaAs and GaN

    Alves, H. W. Leite; Alves, J. L. A.; Santos, A. M.; Scolfaro, L. M. R.; Leite, J. R.

    Resumo em Inglês:

    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.
  • Optical and structural properties of GaAs/GaInP quantum wells grown by Chemical Beam Epitaxy

    Martins, M. R.; Oliveira, J. B. B.; Tabata, A.; Laureto, E.; Bettini, J.; Meneses, E. A.; Carvalho, M. M. G.

    Resumo em Inglês:

    In this work we investigated the optical and structural properties of GaAs/GaInP quantum wells (QW) grown by Chemical Beam Epitaxy (CBE). The samples were characterized by photoluminescence (PL), photoluminescence excitation (PLE) and transmission electron microscopy (TEM). Simulations of the quantum well potential profiles, using the Van De Walle-Martin model, supplemented by our experimental results, allowed us to associate the interface properties with the growth procedures. We concluded that a thin GaP layer grown at the interface improves its quality and also that the observed broad emission band in the PL spectrum is related to quaternary Ga1-xIn xAs1-yPy.
  • Numerical simulation of the optical properties of SiC/SiO2 quantum dots

    Sousa, J. S. de; Freire, V. N.; Farias, G. A.; Silva Jr, E. F. da

    Resumo em Inglês:

    We perform a theoretical investigation of the absorption and emission properties of quantum confined SiC/SiO2 spherical quantum dots, focusing on the the size-dependent emission and absorption spectra of an independent single dot. We observed strong oscillations in the absorption lines as a consequence of the interband transitions rules, and a blue-shift of the emission spectra lines with the decrease of the quantum dot diameter.
  • Hall effect in InAs/GaAs superlattices with quantum dots: identifying the presence of deep level defects

    Rubinger, R.M.; Ribeiro, G.M.; Oliveira, A.G. de; Albuquerque, H.A.; Silva, R.L. da; Rodrigues, W.N.; Moreira, M.V.B.

    Resumo em Inglês:

    We have carried out van der Pauw resistivity and Hall effect measurements on a series of Molecular Beam Epitaxy InAs/GaAs superlattice samples containing InAs quantum dots. Three growth parameters were varied, the InAs coverage, the number of repetitions of the InAs/GaAs layers, and the GaAs spacer thickness. The results can be grouped in two sets, those samples presenting low and high resistivity. The group presenting low resistivity is composed by the samples with GaAs spacer of 30 monolayers (ML) and InAs coverage of 1.9 monolayers. The group presenting high resistivity is composed of samples with GaAs spacer of 40 ML. We claim that the high resistivity characteristic is due to the presence of deep level. Increasing the spacer from 30 to 40 ML decouples the InAs planes favouring the deep level formation.
  • STM images and energetics of the bi-covered (<FONT FACE=Symbol>Ö</font>3 × <FONT FACE=Symbol>Ö</FONT>3) reconstructed Si(111) surface

    Schmidt, T. M.; Miwa, R. H.; Srivastava, G. P.

    Resumo em Inglês:

    Bi-covered (<FONT FACE=Symbol>Ö</font>3× <FONT FACE=Symbol>Ö</font>3) reconstructed Si(111) surface has been studied by first principles calculations. Three different Bi coverages have been considered: 1 monolayer (ML), 1/3 ML and 2/3 ML, leading to the milkstool, T4 and the honeycomb structural models, respectively. Our total energy calculations show that the milkstool model is the energetically most stable structure for high Bi coverages followed by the T4 model for low Bi coverages, without going through a stable structure for the honeycomb model. We performed theoretical STM simulations for the three structures. For 1 ML coverage we observe the formation of Bi-trimers for occupied states, and a honeycomb image for empty states. It is suggested that the experimentally obtained STM image of a honeycomb structure does not correspond to a Bi-coverage of 2/3 ML, but it could represent a STM image of empty states localized in the T4 sites aside the Bi-trimers of the milkstool model.
  • Spin-dependent conductance in nonmagnetic InGaAs asymmetric double barrier devices

    Araujo, C. Moyses; Ferreira da Silva, A.; Persson, C.; Ahuja, R.; Silva, E. A. de Andrada e

    Resumo em Inglês:

    The spin dependence of the conductance of an asymmetric double-barrier InGaAs device is studied within the multiband k·p and envelope function approximations. The spin-dependent transmission probability for electrons across the structure is obtained using transfer matrices and the low bias conductance per unit area is calculated as a function of the Fermi energy (or doping) in the contacts. The possibility to obtain spin polarized currents in such devices is demonstrated, however, the resulting degree of polarization is rather small (a few percent) in the specific InGaAs structures considered here.
  • Atomic structure and energetic stability of the Bi-covered InAs(110) surface

    Miwa, R. H.; Takahashi, E. K.

    Resumo em Inglês:

    We have performed ab initio calculations of the atomic structure and energetic stability of the Bi-covered InAs(110) surface. The calculations were performed within the density-functional theory, using norm-conserving fully separable ab initio pseudopotentials. Two experimentally proposed structural models have been considered: (1×1)-ECLS and (1×2). Our total energy calculations indicate that the formation of the (1×2) model is energetically more favourable than the (1×1) model by 41 meV/(1×2). The calculated equilibrium geometries, for the InAs(110)-Bi(1×1) and -Bi(1×2) surfaces, show in general a good agreement with the experimental x-ray measurements carried out by Betti et al.
  • Evolution from commensurability to size-effects structures in three-dimensional billiards

    Sotomayor, N. M.; Gusev, G. M.; Leite, J. R.

    Resumo em Inglês:

    We calculated the classical electron dynamics of three-dimensional electrons in a billiard system in tilted magnetic field and analyzed the evolution of trajectories in phase space by means of Poincaré space of sections. The low field magnetoresistance rhoxx was calculated through linear response theory and found that nonlinear resonances between the cyclotron radius Rc, the antidot lattice period a, and the well width W, are reflected in the observed magnetoresistance peaks.
  • Characterization of PbTe p - n+ junction grown by molecular beam epitaxy

    Barros, A.S.; Abramof, E.; Rappl, P.H.O.; Ueta, A.Y.; Closs, H.

    Resumo em Inglês:

    In this work we investigate the electrical properties of PbTe p - n+ junction. Mesa diodes were fabricated from p - n+ PbTe layers grown on (111) BaF2 substrates by molecular beam epitaxy. From the analysis of the current versus voltage characteristic measured at 80K, the incremental differential resistance and the series resistance were determined. The capacitance versus voltage curves were measured at a frequency of 1MHz. The one-sided abrupt junction was checked through the 1/C²xV plot. From the linear fit, the hole concentration and the depletion layer width in the p-side were obtained. The high detectivity values measured for the p - n+ PbTe diode confirm that it is very suitable for infrared detection.
  • Electric and magnetic field effects on electronic structure of straight and toroidal carbon nanotubes

    Rocha, C. G.; Pacheco, M.; Barticevic, Z.; Latgé, A.

    Resumo em Inglês:

    Nanotubes have been proved as promising candidates for many technological applications in the nanoscale word and different physical properties have been studied and measured along the few recent years. Here we investigate the role played by external magnetic and electric fields on the electronic properties of toroidal and cylindrical straight carbon nanotubes. A single-p band tight-binding Hamiltonian is used and two types of model-calculations are adopted: real-space renormalization techniques, based on Green function formalism, and straight diagonalization calculation. Both electric and magnetic fields may be properly applied, in different configurations, to modify the energy spectra and transport properties, providing metal-insulator transitions for particular tube geometries.
  • Diluted magnetic Ga1-xMn xN alloys: a first-principles study

    Paiva, R. de; Alves, J. L. A.; Nogueira, R. A.; Leite, J. R.; Scolfaro, L. M. R.

    Resumo em Inglês:

    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.
  • Luminescence from miniband states in heavily doped superlattices

    Oliveira, R.F.; Henriques, A.B.; Lamas, T.E.; Quivy, A.A.; Pires, M.P.; Souza, P.L.; Yavich, B.; Abramof, E.

    Resumo em Inglês:

    We have studied doped superlattices of GaAs/AlGaAs composition. When the doping atoms are introduced into the barriers surrounding the superlattice, as well as to the inner ones, but with half of the concentration, the photoluminescence due to interband transitions from extended superlattice states is detected. This is demonstrated by a study of the sample´s photoluminescence in a magnetic field, whose intensity oscillates in concomitance with the SdH spectrum of electrons confined in the miniband.
  • Electrical properties of bi-doped PbTe layers grown by molecular beam epitaxy on BaF2 substrates

    Anjos, A.M.P. dos; Abramof, E.; Rappl, P.H.O.; Ueta, A.Y.; Closs, H.

    Resumo em Inglês:

    Resisistivity and Hall measurements were performed at temperatures from 10 to 320K on Bi-doped PbTe layers grown on (111) BaF2 by molecular beam epitaxy. Samples with electron concentration varying from 1x10(17) to 4x10(19)cm-3 were obtained. Results indicated that all offered Bi atoms in the vapor phase were effectively incorporated in the PbTe as active donors. No thermal activation in the whole doping range was observed, indicating that the Bi donor level lies resonant with the conduction band. The mobility curve showed that the PbTe layers tend to a metallic behavior as the electron concentration increases. A value around 1x10(19) cm-3 is suggested for n+ PbTe contact layers in device application.
  • A new route for preparing CdS thin films by chemical bath deposition using EDTA as ligand

    Feitosa, A. V.; Miranda, M. A. R.; Sasaki, J. M.; Araújo-Silva, M. A.

    Resumo em Inglês:

    Progress is reported towards the development of a new route to obtain cadmium sulfide (CdS) thin films by using ethylene-diamine-tetra-acetic acid (EDTA) ligand on the chemical bath deposition (CBD) method. Different chemical baths are used to study changes in the structures of the CdS films for different EDTA concentrations. X-ray diffraction (XRD) is used to prove the structural characterization of the samples, and to obtain the grain size with the Scherrer´s equation. The pH variations during the growth also affect the structural film quality, and it was verified by using ammonia chloride as an auxiliar-buffer plus Triton-X100.
  • Photophysics of poly(p-phenylene vinylene) films

    Anni, M.; Gigli, G.; Cingolani, R.; Gobato, Y. Galvão; Vercik, A.; Marletta, A.; Guimarães, F.G. E.; Faria, R. M.

    Resumo em Inglês:

    We report on time resolved photoluminescence (PLRT) measurements in poly(p-phenylene vinylene) (PPV) films irradiated by laser in the presence of air. We observe a PL intensity enhancement and a biexponential decay dynamics of PL signal for all irradiated films. These results can be understood in terms of a chain shortening process due to carbonyl incorporation and formation of an energy profile that extends and migrates into the film and enables efficient spectral diffusion of excited carriers to a non-degraded PPV segments by Förster energy transfer.
  • Franz-Keldysh effect in semiconductor T-wire in applied magnetic field

    Madureira, Justino R.; Maialle, Marcelo Z.; Degani, Marcos H.

    Resumo em Inglês:

    We have calculated the optical absorption spectra of magnetoexcitons in T-wire semiconductor. It is calculated using the semiconductor Bloch equations in time and real-space domains. The peak and the linewitdth of the fundamental exciton transition are investigated as a function of the applied magnetic and static electric field. The latter is applied along the wire axis and induces a broadening in the absorption spectra line, energy shift, and the characterisitic Franz-Keldysh oscillations. The exciton binding energy enhances increasing the magnetic field strength.
  • Electronic properties of coupled quantum rings in the presence of a magnetic field

    Fuster, Gonzalo; Pacheco, Mónica; Barticevic, Zdenka

    Resumo em Inglês:

    We have studied the energy spectrum of a system formed by two concentric, coupled, semiconductor quantum rings. We have investigated the effect of a uniform magnetic field applied along the rings axis, on the energy spectrum of the system. We found that the spectrum of the coupled rings with similar confinement length corresponds roughly to the superposition of the spectra of two separate rings, although modified by the anticrossings produced among different states of the individual rings with same angular momentum. The Aharonov-Bohm oscillations do not have a definite period.
  • Electronic properties of isolated nickel in diamond

    Larico, R.; Justo, J. F.; Machado, W. V. M.; Assali, L. V. C.

    Resumo em Inglês:

    Isolated nickel impurities in diamond have been investigated using the spin-polarized full-potential linearized augmented plane wave total energy method. The electronic and atomic structures, symmetries, transition energies, and formation energies of substitutional and interstitial Ni impurities in diamond were computed. The results were discussed in the context of the electrically active centers in synthetic diamond.
  • MBE growth and characterization of Sn1-xEu xTe

    Ueta, A. Y.; Rappl, P. H. O.; Closs, H.; Motisuke, P.; Abramof, E.; Anjos, V. R. dos; Chitta, V. A.; Coaquira, J. A.; Oliveira Jr., N. F.; Bauer, G.

    Resumo em Inglês:

    Epilayers of Sn1-xEu xTe (0 < x < 0:03) were grown by molecular beam epitaxy on freshly cleaved BaF2(111) substrates and their structural, electrical and optical properties were investigated. The thicknesses of epilayers were about 1.5 mum and deposition was carried out at growth temperatures of 300 ºC. The structural properties were investigated by high resolution X-ray diffraction and a sharp film degradation could be observed with increasing europium content. Electrical measurements with temperature varying from 300 to 10K indicated that the epilayers present carrier concentration ranging between 3 x 10(20) and 6 x 10(20)cm-3 and a low resistivity from 6.3 x 10-5 to 1.2 x 10-4 omega.cm. From optical measurements it could be seen that spectra present a low energy edge corresponding to the beginning of intra band excitations and the high energy edge due to inter band excitations.
  • Stark effect in CdTe/Cd1-xMn xTe strained double quantum wells

    Freire, J. A. K.; Farias, G. A.; Silva Jr., E. F. da

    Resumo em Inglês:

    We report a detailed analysis of how the existence of interface roughness can change exciton localization in CdTe/Cd1-xMn xTe strained double quantum wells (DQW´s) taking into account magnetic and electric field effects. We consider that the potential, effective mass, and the intrinsic magnetization are dependent on the profile of the Mn molar fraction at the interfacial region. Results obtained with CdTe/Cd0.68Mn0.32Te DQW´s show that the energy band tailoring generated by the strain and the Zeeman effect is responsible by a displacement of the exciton energy peaks and that small changes in the interface width (10 Å) can be responsible for 30 meV exciton energy broadening.
  • Strain-induced shifts of the zone-center phonons of III-Nitrides

    Alves, H. W. Leite; Alves, J. L. A.; Santos, A. M.; Scolfaro, L. M. R.; Leite, J. R.

    Resumo em Inglês:

    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.
  • A first principles investigation of mercuric iodide: bulk properties and intrinsic defects

    Ayres, F.; Assali, L. V. C.; Machado, W. V. M.; Justo, J. F.

    Resumo em Inglês:

    We carried a theoretical investigation on the properties of mercuric iodide in the red tetragonal crystalline phase, and its respective intrinsic defects. Our calculations were performed using the ab initio spin-polarized full-potential linearized augmented plane wave method, and taking into account full atomic relaxation. The results on the structural, electronic, and optical properties were compared to available experimental data.
  • Contribution of the charge image potential to carrier confinement in graded Si-based quantum wells

    Pereira, T. A. S.; Caetano, E. W. S.; Freire, J. A. K.; Freire, V. N.; Farias, G. A.

    Resumo em Inglês:

    In order to analyze the charging effects on electronic spectrum of Silicon (Si) based quantum wells (QW's), we use a method based on the calculation of the image charge potential by solving Poisson equation in cylindrical coordinates. The numerical results shows that the confined electron ground state energy level can be shifted by more than 50 meV due to the contribution of the image charges to the confinement potential of the graded quantum well.
  • Magnetic field induced absorption in Pb xEu1-xTe magnetic semiconductors

    Hanamoto, L. K.; Henriques, A. B.; Rappl, P. H. de Oliveira; Oliveira, N. F.; Ueta, A. Y.; Abramof, E.

    Resumo em Inglês:

    We report an investigation of the optical absorption spectrum, using non-polarized light, in Pb xEu1-xTe, x=0 and x=0.095, epitaxial thick layers grown by molecular beam epitaxy (MBE). The absorption edge is described by a broad band, due to the electronic transitions from the 4f7 of Eu2+ to the states in 4f6 5d configuration, as seen previously in bulk Eu chalcogenides. When a magnetic field is applied, a narrow absorption band (full width ~50 meV) emerges from the broad one. The energy of this absorption peak red shifts when the magnetic field increases, and reaches saturation when the Eu2+ attain ferromagnetic arrangement. This behaviour can be described by a localized excitation model with d - f exchange interaction.
  • CH3CN on Si(001): adsorption geometries and electronic structure

    Miotto, R.; Oliveira, M. C.; Pinto, M. M.; León-Pérez, F. de; Ferraz, A. C.

    Resumo em Inglês:

    In this work we employ the state of the art pseudopotential method, within a generalized gradient approximation to the density functional theory, to investigate the adsorption process of acetonitrile on the silicon surface. Our first-principles calculations indicate that CH3CN adsorbs via a [2+2] cycloaddition reaction through the C<FONT FACE=Symbol>º</FONT>N group with an adsorption energy around 35 kcal/mol, close to the 30 kcal/mol estimated by Tao and co-workers. The electronic structure and the surface states calculated for the adsorbed system are also discussed.
  • Some key issues on the optimization of multiple quantum well structures for amplitude modulation

    Tribuzy, C.V-B.; Pires, M. P.; Yavich, B.; Souza, P. L.

    Resumo em Inglês:

    Main parameters of InGaAs/InAlAs multiple quantum well amplitude modulators, such as contrast ratio, insertion loss and chirp parameter, were calculated in order to find a quantum well structure which optimizes them. The parameters were estimated from the theoretical absorption curves, which were calculated for different values of applied reverse bias and were compared with experimental data. A study of the device parameters as a function of the Ga content and operation voltage was performed. The study showed that optimum values for the modulator parameters cannot be obtained simultaneously. The influence of the residual doping level and the applied pre-bias are emphasized.
  • Influence of a magnetic field on the energy fow of surface polaritons propagating in semiconductor cylinders

    Farias, G. A; Nobre, E. F.; Moretzsohn, R. S.; Almeida, N. S.

    Resumo em Inglês:

    This work reports the influence of a dc external magnetic field on the polaritons propagating in cylindrical systems. We present numerical results for the dispersion relation and energy flow for surface polaritons modes propagating in hollow and massive cylinders made of GaAs and InSb.
  • Photoluminescence of MEH-PPV/PS blends

    Marletta, A.; Gonçalves, V.; Balogh, Débora T.

    Resumo em Inglês:

    In this work we studied the optical properties of poly[2-methoxy-5-(2'-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV) using polymeric blends with polystyrene (PS). The photoluminescence (PL) spectrum of MEHPPV present three characteristic peaks correlating to aggregate electronic transition at 608, 674 and 742 nm at 10K. An addition of PS in MEH-PPV films induces a red shift in the absorption spectrum and the PL spectrum presents a new peak at high energy centred at about 572 nm at 10K. It is attributed to intra band transition between localized intraband states.
  • Study of the RPA pair-correlation function in GaAs-AlGaAs parabolic quantum well wires

    Cunha, J. B. B. da; Cunha, J. F. R. da; Machado, P. C. M.; Osório, F. A. P.; Borges, A. N.

    Resumo em Inglês:

    The ground state intrasubband pair-correlation function for a quasi-one-dimensional electron gas confined in a GaAs-Al xGa1-xAs parabolic quantum well wire within the Random-Phase Approximation (RPA) is calculated. We have considered two wires with subband energies separation homega = 2:0 meV and homega = 2:5 meV. The dependence of the pair-correlation function on the electronic density was studied and the regions where the RPA approach cannot be used were precisely determined.
  • Exciton confinement in InGaN/GaN cylindrical quantum wires

    Caetano, E. W. S.; Freire, V. N.; Farias, G. A.; Silva Jr., E. F. da

    Resumo em Inglês:

    We investigate the confined exciton properties in InGaN/GaN cylindrical quantum wires. We have solved numerically Schrödinger-like equations in the effective mass approximation and have found the energy levels and wavefunctions for confined electrons and holes, using these results to calculate variationally the confined exciton energy. A comparison was made between graded and abrupt quantum wires, showing that the existence of smooth interfaces enhances the confined exciton energy if the wire radius is smaller than 50 Å. We conclude that the existence of graded interfaces must be considered for a better description of the exciton based optical properties of InGaN/GaN quantum wires.
  • Electron spin-phonon relaxation in quantum dots

    Alcalde, A. M.; Marques, G. E.

    Resumo em Inglês:

    We calculate the spin relaxation rates in parabolic quantum dots due to the phonon modulation of the spin-orbit interaction in presence of an external magnetic field. Both, deformation potential (DP) and piezoelectric (PE) electron-phonon couplings are included in the Pavlov-Firsov spin-phonon Hamiltonian. We demonstrate that the spin relaxation rates are particularly sensitive with the Landé g-factor, and that for InAs dots the DP can be necessarily considered in the spin relaxation analysis. Low-temperature (T ~ 0) relaxation rates are found to be small and to depend strongly on size, g-factor, and on magnetic field.
  • A theoretical study of acrylonitrile adsorption on Si(001)

    León-Pérez, F. de; Miotto, R.; Ferraz, A. C.

    Resumo em Inglês:

    The present work is a comparative study of possible adsorption structures of the conjugated molecule acrylonitrile on Si(001) employing the state of the art pseudopotential method, within a generalized gradient approximation to the density functional theory. In the recent literature it is proposed the interaction of acrylonitrile with Si(001) through a cycloaddition reaction of the cyano group, in competition with the bounding of the two outer atoms of the molecule skeleton with the Si dimer in cross-dimer and cross-trench geometries; between other geometries like which correspond the reaction of the C = C bond with a Si dimer. Starting from a large number of configurations our calculations favor the planar cycloaddition through the terminal N and C atoms on the Si dimer. In this way we explain the electronic and vibrational features obtained experimentally.
  • AC electrical conductivity of Cr-doped polyaniline/poly(vinyl alcohol) blends

    Soares, D.A.W.; Souza, P. H. O. de; Rubinger, R. M.; Queiroz, A.A.A. de; Higa, O.Z.; Souza, L.R. de

    Resumo em Inglês:

    Polyaniline (PANI) and their charge transfer complexes are engineering materials that continue to attract considerable attention from engineers due to their unique electrochemical behavior, environmental stability and high degree of processability. Extensive works on the preparation of electrically conductive PANI blends containing a polymeric matrix having desirable physical and good mechanical characteristics have pointed as a promising route to overcome the PANI intractabilities. This paper reports the electrical properties of the dichromated PANI/poly(vinyl alcohol) (PANI:Cr3+/PVA) blends. Doped polyaniline powder in the emeraldine base form was synthesized by the classical chemical oxidation of aniline with ammonium persulfate as the oxidant in the presence of chromium (Cr3+). The SEM (Phillips XL 30) was used to understand the changes occurring in the conductivity to a microstructure level. The distribution of PANI:Cr3+ (50% w/w) in the host polymer (PVA) show some tendency to occur in "flocculated structures". The samples were cut into circular shapes to form films about 500 microns thick. Electrical contacts were made by sputtering gold over sample surfaces. We have carried out impedance spectroscopy measurements in the range of 20Hz to 1MHz. Measurements carried at 100K and 250K indicates that thermally activated carrier process between PANI:Cr3+ grains in the PANI:Cr3+/PVA blend sample are responsible for its high temperature conductivity.
Sociedade Brasileira de Física Caixa Postal 66328, 05315-970 São Paulo SP - Brazil, Tel.: +55 11 3091-6922, Fax: (55 11) 3816-2063 - São Paulo - SP - Brazil
E-mail: sbfisica@sbfisica.org.br