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Brazilian Journal of Physics, Volume: 29, Número: 4, Publicado: 1999
  • Proceedings of the 9th Brazilian Workshop on Semiconductor Physics

  • "Ab initio" studies of hydrogen-enhanced oxygen diffusion in silicon

    Capaz, R. B.; Assali, L. V. C.; Kimerling, L. C.; Cho, K.; Joannopoulos, J. D.

    Resumo em Inglês:

    A novel microscopic mechanism for hydrogen-enhanced oxygen diffusion in p-doped silicon is proposed. A path for joint diffusion of O and H is obtained from an ab-initio molecular dynamics "kick" simulation. The migration pathway consists of a two-step mechanism, with a maximum energy of 1.46 eV. This path represents a 0.54 eV reduction in the static barrier when compared with the diffusion of isolated O in Si, in excellent agreement with experiments.
  • Erbium in a-Si:H

    Tessler, Leandro R.

    Resumo em Inglês:

    A review of the current status of research on Er 3+ doped hydrogenated amorphous silicon (a-Si:H) is presented. Er has been introduced in a-Si:H and a-SiOx:H by ion implantation, co-sputtering and PECVD. In all cases, the characteristic atomic-like intra-4f 4I13/2 -> 4I15/2 photoluminescence emission at ~ 1.54 µm is observed at room temperature. The Er 3+ luminescence probability is determined by the local neighborhood of the ions. Therefore, local probes like EXAFS and Mössbauer spectroscopy have yielded very important information. A discussion of excitation processes, electroluminescence, and electronic doping effects, is also presented.
  • Metal-insulator transition in two dimensions

    Kravchenko, S. V.

    Resumo em Inglês:

    A metal-insulator transition was recently observed in a variety of two-dimensional electron and hole systems, contrary to the 20 year old belief that it is forbidden in two dimensions. The origin of this transition is still unknown and is being debated. In this paper, I briefly survey the main experimental findings and suggested explanations.
  • Nonlinear spatio-temporal dynamics in semiconductors

    Schöll, Eckehard

    Resumo em Inglês:

    We review recent theoretical advances in the modeling and computer simulation of complex non-linear spatio- temporal dynamics of charge carriers in semiconductors. Among the particularly instructive examples are investigations of current filaments in doped semiconductors in the regime of low temperature impurity breakdown. The nascence of current filaments in thin GaAs films with different contact geometries, including circular samples (Corbino disks) with symmetry breaking instabilities, is investigated, and the role of global couplings induced by the operating load circuit is analysed.
  • Spin depolarization in quantum dots

    Sachrajda, A.S.; Gould, C.; Hawrylak, P.; Feng, Y.; Zawadzki, P.; Wasilewski, Z.

    Resumo em Inglês:

    We have performed single electron spectroscopy experiments on single lateral quantum dots. We demonstrate that the lateral nature of the tunneling as well as the properties of the 2DEG leads can be used to extract new information in the 'spin-flip' regime. Calculations confirm that one needs to go beyond the Hartree-Fock approximation and include correlation effects to explain the experimental observations. The results are found to be consistent with the presence of spin depolarization events at the 'spin-flips'.
  • Ab initio calculation of linear and nonlinear optical properties of semiconductor structures

    Bechstedt, F.; Adolph, B.; Schmidt, W. G.

    Resumo em Inglês:

    The theoretical and numerical approaches are discussed for ab initio calculations of optical properties. The density functional theory (DFT) combined with the local-density approximation (LDA) allows the calculation of the geometry of crystalline solids and their surfaces with a precision of about one percent. The DFT-LDA band structure and single-electron states therefore provide a reasonable starting point for the calculation of linear and nonlinear susceptibilities within the independent-particle approximation. However, this approach has to be improved by taking into account many-body interactions: self-energy effects, local-field corrections, and electron-hole attraction. Three types of optical spectra are studied: the frequency-dependent dielectric function, the second-harmonic generation, and surface reflectance anisotropy spectra. The systems considered are two-atomic semiconductors, their polytypes and their surfaces.
  • Minibands and Wannier-Stark ladders in semiconductor superlattices studied by infrared spectroscopy

    Helm, M.; Hilber, W.; Strasser, G.; DeMeester, R.; Peeters, F. M.

    Resumo em Inglês:

    We review the basic features of interminiband absorption in superlattices, focussing on the joint density of states, the oscillator strength and the associated sum rule. Then we will discuss infrared spectroscopic studies under application of an electric field. With a field in the plane of the layers, a hot-electron distribution can be generated. Using the temperature dependence of the interminiband absorption, energy loss and relaxation rates can be determined. A vertical electric field leads to negative differential resistance, domain formation and the break-up of the minibands into Wannier- Stark ladders. We will present experiments showing the formation of interpenetrating Wannier- Stark ladders in the continuum, which are strongly coupled by Zener resonances. This is the first time to correlate transport and infrared absorption in a biased superlattice.
  • An order-N study of dislocations in homopolar semiconductors

    Nunes, R.W.

    Resumo em Inglês:

    This article surveys the main results of the author's work, in collaboration with David Vanderbilt and John Bennetto, on the application of the order-N density-matrix approach, together with ab initio methods, to investigate the atomic structure of dislocation cores in the homopolar semiconductors silicon, carbon, and germanium. In these systems, the predominant dislocations are the 30° and the 90° partial dislocations. For the three materials, the nature of the reconstruction at the core of the 90°-partial dislocation is considered. Both the traditional single-period and our recently proposed double-period core structures are investigated. The double-period geometry is found to be the ground-state structure in all three cases. For silicon, we have also investigated in detail the structure and dynamics of point excitations (kinks, solitons, and kink-soliton complexes) in the cores of the 30°partial dislocation and the single-period geometry of the 90° partial. Our calculated formation energies and migration barriers for these excitations are in good agreement with available experimental results. Furthermore, we have examined the reactions by which high-energy kinks relax into low-energy ones by soliton emission.
  • Electric field effects on the confinement properties of GaN/Al x Ga1-xN zincblende and wurtzite nonabrupt quantum wells

    Wang, H.; Farias, G. A.; Freire, V. N.

    Resumo em Inglês:

    We investigate the con.nement properties of GaN/Al xGa1-xN zincblende and wurtzite nonabrupt quantum wells (QWs) in an electric field. It is shown that their Stark shifts decrease considerably when the existence of nonabrupt interfaces are considered. Confined excitons in 50 Å wide GaN/Al0.3Ga0.7N zincblende and wurtzite quantum wells (QWs) can exist up to electric field intensities of the order of 500 kV/cm. In all cases, the electric field effects in GaN/Al xGa1-xN wurtzite QWs are stronger than in similar zincblende QWs.
  • Cyclotron mass of electrons in GaN-Al xGa1-xN heterostructures

    Oliveira, Solemar Silva; Souza, Marcio Adriano R.; Borges, Antonio Newton; Osório, Francisco A. Pinto

    Resumo em Inglês:

    The quasi-two-dimensional electron cyclotron mass in GaN-Al xGa1<FONT FACE=Symbol>-</FONT> xN heterostructures is theoretically calculated. The shifting in Landau Levels due to the electron Longitudinal Optical phonon interaction is calculated through the Second Order Improved Wigner Brillouin Perturbation Theory (IWBPT). Experimental results recently obtained for GaN heterojunction are well described by the theory. We have also investigated the e ects of the electronic density on the Cyclotron Resonance frequency.
  • Interband magneto-absorption in narrow-gap semiconductor quantum wells

    López-Richard, V.; Marques, G. E.; Trallero-Giner, C.

    Resumo em Inglês:

    A theoretical study of optical properties in narrow gap semiconductor quantum wells has been developed using an 8x8 Kane Hamiltonian. The interband transition selection rules have been obtained and a detailed description of the interband absorption using Voight and Faraday scattering configurations has been given. Interesting effects produced by the intraband admixture, such as spin-flip transitions, will be discussed. The complementation of measurements using different scattering configurations will be analyzed.
  • Evolution of dynamic localization regimes in coupled minibands

    Rivera, P. H.; Schulz, P. A.

    Resumo em Inglês:

    We consider the behaviour of two coupled symmetric electronic minibands in the presence of an AC intense field. Symmetric coupled minibands are realized as dimerization procedures. The minibands are emulated by a tight-binding finite chain, whereas the interaction with an AC field is described by a time-independent Hamiltonian. We follow the evolution of dressed minibands, non perturbatively, in a wide frequency range. For one dimerization procedure, a transition between both limits can be systematically followed and a dynamic breakdown is unambigously identified. The most striking effect that may occur is a field-induced analog of an insulator-metal transition due to the suppression of a Peierls-like instability.
  • Ambipolar carrier diffusion in In0.53Ga0.47As single quantum wells

    Monte, A.F.G.; Silva, S.W. da; Cruz, J.M.R.; Morais, P.C.; Cox, H.M.

    Resumo em Inglês:

    The microluminescence surface scan technique (MSST) has been used to investigate photocarrier diffusion in undoped In0.53Ga0.47 As - InP single quantum well (QW), in the temperature (T) range from 15 K to 295 K. Narrowing of the photoluminescence (PL) spatial profile is observed as the temperature is lowered, indicating reduction of the photocarrier diffusion length upon cooling. It was found that the width of the PL spatial profile follows a linear function of temperature, but a change in slope by a factor of 2.6 is observed at about 200 K, indicating a change of the dominant carrier scattering mechanism. In the temperature range of 15 K to 200 K, the ambipolar photocarrier diffusion mechanism seems to be correlated to impurity states thermally activated.
  • A quantum formalism for a terahertz acoustic laser

    Camps Rodríguez, Ihosvany; Makler, Sergio Saul; Anda, Enrique Victoriano

    Resumo em Inglês:

    The aim of this work is to improve the study of a phonon laser (saser) proposed by us several years ago[1]. This is a device capable to generate an intense coherent beam of acoustical phonons. Our acoustic laser consists in a double barrier heterostructure tailored such the energy difference between the ground and the first excited state in the well is close to the energy of the LO phonon. The electrons are directly injected into the excited level. Therefore they decay producing a high rate of LO phonons. These phonons are confined inside the well and decay into a pair of phonons[2]: LO -> <img src="http:/img/fbpe/bjp/v29n4/lotil.gif">+ TA. The TA phonons escape the well in the [111] direction constituting an intense coherent beam. Recently were studied (and sometimes realized experimentally) several kinds of phonon lasers. Up to our knowledge our saser is the only that has a very short wavelength (smaller than 25 Å) and a very long range (greater than 1000 mum). Because of that, such beam could have applications to acoustic nanoscopy, acoustic nanolithography and phonoelectronics. In early articles[1, 3, 4, 5, 6] we get the kinetic equations for the averaged electron and phonon populations. Quantum fluctuations were not taken into account. The system Hamiltonian is H = He + Hph + He<FONT FACE=Symbol>-</FONT>ph + Hph<FONT FACE=Symbol>-</FONT>ph + He<FONT FACE=Symbol>-</FONT>e. To solve this Hamiltonian we expand their eigenfunctions in the basis of the eigenstates¦jn1n2n3<FONT FACE=Symbol>ñ</FONT> of the single particle part of it. We obtain a set of coupled equations for the expansion coefficients that can be solved with some approximations. The results are qualitatively similar to those obtained previously.
  • Spin-flip scattering contribution to resonant-tunneling current in semimagnetic semiconductor heterostructures

    Chitta, V. A.; Maialle, M. Z.; Leão, S. A.; Degani, M. H.

    Resumo em Inglês:

    We calculate the characteristic current-voltage curve of a tunneling device based on semimagnetic semiconductor materials. The device is a heterostructure with layers of Cd1<FONT FACE=Symbol>-</FONT> x Mn x Te in which the magnetic ions Mn2+ interact strongly with the conducting electrons via the s-d exchange interaction. Thermal fluctuations of Mn2+ magnetic moments cause spin-dependent electron scattering that modifies the characteristic current-voltage curve. Our calculation shows how this electron-ion scattering is expected to affect the spin dynamics in transport measurements.
  • High magnetic field transport and photoluminescence in doped InGaAs/InP superlattices

    Henriques, A.B.; Hanamoto, L.K.; Oliveira, R.F.; Souza, P.L.; Gonçalves, L.C.D.; Yavich, B.

    Resumo em Inglês:

    Lattice-matched InP/InxGa1<FONT FACE=Symbol>-</FONT> x As short period superlattices (x = 0.53) delta-doped with Si in the middle of the InP barriers were studied. The samples had a high carrier concentration which filled two minibands. In addition to a peak associated with the electrons from the second miniband, E2, the Shubnikov-de Haas spectra showed a well resolved doublet structure that is assigned to E1 electrons of superlattice wave vectors kz = 0 and kz = pi /d. From the lineshape of the Shubnikov-de Haas oscillations, an E1 quantum mobility of 970 cm2/Vs was deduced, which represents an increase of about 40% over the value for periodically delta-doped semiconductors. The photoluminescence ex- hibits a band at photon energies higher than the InGaAs bandgap and whose FWHM approximates the Fermi energy of the confined carriers. Thus the photoluminescence observed is consistent with the recombination of electrons confined by the superlattice potential and photoexcited holes.
  • Magnetotransport in a spatially modulated magnetic field

    Gusev, G.M.; Quivy, A. A.; Leite, J.R.; Bykov, A. A.; Moshegov, N.T.; Kudryashev, V.M.; Toropov, A.I.; Nastaushev, Yu.V.

    Resumo em Inglês:

    We have measured the Shubnikov de Haas (SdH) oscillations of a nonplanar two-dimensional electron gas (2DEG) fabricated by overgrowth of a GaAs/AlGaAs heterojunction on a pre-patterned substrate. When placed in a uniform external magnetic field B, the field normal to the nonplanar 2DEG is spatially modulated, and electrons experience a nonuniform magnetic field. In a tilted magnetic field, the SdH oscillations are much more strongly damped than in a field perpendicular to the substrate. We consider several mechanisms and conclude that the electron scattering by the magnetic field spatial uctuations plays a main role in the transport properties of a nonplanar 2DEG at low magnetic field.
  • Quantum hall effect in a wide parabolic quantum well

    Gusev, G.M.; Leite, J.R.; Olshanetskii, E.B.; Maude, D.K.; Cassé, M.; Portal, J.C.; Moshegov, N.T.; Toropov, A.I.

    Resumo em Inglês:

    We investigate the activated conductivity for Landau levels at lling factors v =1,2 in a parabolic Al xGa1-x As well down to 30 mK. We obtain a very small activation energy ~ 0.02 meV, which is almost 1-2 orders of magnitude smaller than the expected values for energy gaps in a wide quantum well. The resistivity minima vanish completely when a parallel component of magnetic field is applied. The collapse of the energy gaps in the wide parabolic well occurs due to the new electron correlated state, probably the charge density wave state predicted for a uniform wide electron slab in high magnetic field.
  • Spin-dependent resonant tunneling in semiconductor nanostructures

    Silva, Erasmo A. de Andrada e; La Rocca, Giuseppe C.

    Resumo em Inglês:

    The spin-dependent quantum transport of electrons in non magnetic III-V semiconductor nanos-tructures is studied theoretically within the envelope function approximation and the Kane model for the bulk. It is shown that an unpolarized beam of conducting electrons can be strongly polarized in zero magnetic field by resonant tunneling across asymmetric double-barrier structures, as an effect of the spin-orbit interaction. The electron transmission probability is calculated as a function of energy and angle of incidence. Specific results for tunneling across lattice matched politype Ga0.47In0.53As / InP/Ga0.47In0.53As / GaAs0.5Sb0.5 / Ga0.47In0.53 As double barrier heterostructures show sharp spin split resonances, corresponding to resonant tunneling through spin-orbit split quasi-bound electron states. The polarization of the transmitted beam is also calculated and is shown to be over 50%.
  • Temperature dependence of the absorption coefficient in doped quantum wells

    Baldan, M.R.; Serra, R.M.; Emmel, P.D.; Silva, A. Ferreira da

    Resumo em Inglês:

    In this work we present a calculation of the infrared optical absorption coefficient for a Ga1-xAl xAs quantum well (QW) as a function of donor impurity concentration, compensation and temperature. We treat this problem using a Self-Consistent Method (SC) calculation taking into account donors as major impurities. The Variational Method was used to obtain the ground state wave-function of an electron bound to a donor impurity in a QW. The results show a shift of energy in the absorption coefficient.
  • Stochastic dynamics of 2D electrons in antidot lattice in the presence of an in-plane magnetic field

    Choque, N.M. Sotomayor; Santos, M.T. dos; Gusev, G.M.; Leite, J.R.; Kvon, Z.D.

    Resumo em Inglês:

    The classical dynamics of the two-dimensional electron gas in antidot lattices is studied in the presence of an in-plane magnetic field. We find that the distortion of the Fermi contour induced by the in-plane magnetic field changes the Poincaré section of the electron trajectories in the magnetic field. Comparison with the experimental measurements of the commensurability magnetoresistance oscillations is made.
  • Electronic structure in narrow-gap quantum dots

    Prado, S. J.; Marques, G. E.; Trallero-Giner, C.

    Resumo em Inglês:

    In this work we calculated the electronic structure of spherical quantum dots based on zincblend semiconductor compounds. The strong conduction-valence band coupling in this class of semi-conductors induces a strong mixing of the electronic states which requires a theoretical model to properly take into acount these effects. We have used a full 8 x 8 Kane Hamiltonian in order to include the strong admixture and study the set of symetries associated with these electronic states and their angular momentum in this central force problem. As an application, we have calculated the electronic structure in narrow-gap HgCdTe, InSb and CdTe quantum dots.
  • Inelastic coulomb scattering rate of a multisubband Q1D electron gas

    Tavares, M.; Hai, G.-Q.

    Resumo em Inglês:

    We study, the Coulomb scattering lifetimes of electrons in two coupled quantum wires by calculating the quasiparticle self-energy within a multisubband model of quasi-one-dimensional (Q1D) electron system. We consider two strongly coupled quantum wires with two occupied subbands. The intrasubband and intersubband inelastic scattering rates are caculated for electrons in different subbands. Contributions of the intrasubband, intersubband plasmon excitations, as well as the quasiparticle excitations are investigated. Our results show that the plasmon exictations of the first subband are the most important scattering mechanism for electrons in both subbands.
  • Electronic coupling and thermal relaxation in self-assembled InAs quantum dot superlattices

    Petitprez, E.; Moshegov, N.T.; Marega Jr., E.; Basmaji, P.; Mazel, A.; Dorignac, D.; Fourmeaux, R.

    Resumo em Inglês:

    We report optical and structural characterizations of InAs quantum dot superlattices grown by molecular beam epitaxy on GaAs (001). Cross-sectional electron microscopy imaging reveals ~ 20 nm diameter InAs islands well aligned along the growth direction. Low temperature photoluminescence spectra exhibit a clear redshift with decreasing island vertical separation, as a result of electronic coupling between stacked dots, given that the larger shift is obtained for 70 Å spacing. At high excitation power, the photoluminescence spectra show a doublet structure corresponding to coupled and uncoupled quantum dot states. The temperature dependence of the photoluminescence indicates that thermionic emission activation energy is much lower than the estimated barrier height. Such a difference is explained by the presence of non-radiative recombination centers due to strain relaxation and In segregation.
  • Metal-insulator transition at B=0 in an AlGaAs/GaAs two-dimensional electron gas under the influence of InAs self-assembled quantum dots

    Ribeiro, E.; Jäggi, R.; Heinzel, T.; Ensslin, K.; Medeiros-Ribeiro, G.; Petroff, P. M.

    Resumo em Inglês:

    We report the observation of a metal-insulator transition in zero magnetic eld in a two dimensional electron gas under the influence of a plane of InAs self-assembled quantum dots located at 30 Å below the AlGaAs/GaAs heterointerface. The transition is observed as a function of temperature and electric field at B=0. A scaling analysis yields exponents similar to those obtained for Si MOSFET's . We suggest that the disorder introduced by the quantum dots plays a crucial role.
  • Strain and relaxation processes in In1-xGa xAs yP1-y/ InP single quantum wells grown by LP-MOVPE

    Bernussi, A.A.; Carvalho Jr., W.; Furtado, M.T.; Gobbi, A.L.

    Resumo em Inglês:

    Strained and partially relaxed In1-xGa xAs yP1-y /InP single quantum wells (SQWs) with different cap layer thicknesses and biaxial strain values grown by low-pressure metalorganic vapor phase epitaxy (LP-MOVPE) were investigated by double crystal X-ray diffraction, photoluminescence microscopy (PLM) imaging and photoluminescence spectroscopy techniques. Our results indicate a significant improvement of the optical quality of the quaternary wells with increasing values of the cap layer thickness. Tensile and compressive strained In1-xGa xAs yP1-y /InP SQWs grown with the same structure exhibited different relaxation processes, even when the strain magnitude was the same. PLM images of highly compressive quantum wells exhibited a large number of dark lines corresponding to misfit dislocations as a result of the partial relaxation process in the well material. PLM images of similar tensile strained samples revealed only the presence of dark spots with no evidence of misfit dislocations.
  • Influence of laser excitation on raman and photoluminescence spectra and FTIR study of porous silicon layers

    Salcedo, Walter Jaimes; Ramirez Fernandez, Francisco J.; Rubimc, Joel C.

    Resumo em Inglês:

    A porous silicon lm (PS) was investigated by FTIR, Raman and photoluminescence (PL) spectroscopies. The Raman and PL spectra were obtained using four different laser excitations: 488, 514, 633 and 782 nm. The analysis of the first order and second order Raman scattering lines permits to identify the band energy structure of the crystallites inside the PS film. The analysis of PL spectra shows that the intensity and full width at half-maximum values of PL emission depends on intensity and energy of laser excitation. The linear polarization degree (LPD) of the PL spectra also presents a dependence of laser excitation. The observed dependence of Raman and PL spectra due to laser excitation energy cannot be explained within the quantum confinement alone. We propose a mechanism for PL emission in PS layers, in which the radiative recombination occurs in localized centers at pore/crystallite interface. These quasi-molecular centers are Jahn-Teller active.
  • Environment of Er in a-Si:H: co-sputtering versus ion implantation

    Piamonteze, Cínthia; Tessler, Leandro R.; Alves, M. C. Martins; Tolentino, H.

    Resumo em Inglês:

    We report a comparative Extended X-Ray Fine Structure (EXAFS) study of Er in a-Si:H prepared by Er implantation in a-Si:H and by co-sputtering undergoing the same cumulative annealing processes. It was found that the Er environment in as-implanted samples is formed by Si atoms, which are replaced by oxygen under annealing. In the co-sputtered samples, the initial low coordination oxygen environment evolves under thermal treatment to an Er2O3 -like neighborhood.
  • Electrochemical characterization on semiconductors p-type CVD diamond electrodes

    Ferreira, N. G.; Silva, L. L. G.; Corat, E. J.; Trava-Airoldi, V. J.; Iha, K.

    Resumo em Inglês:

    Semiconductor boron-doped CVD diamond films were prepared on Si substrates by the hot filament technique. The surface morphology analysis by SEM presented continuous and well faceted films. The samples were grown with different boron concentrations by controlling the B/C ratio in the feeding gas. Raman results showed a drastic change of diamond films for different doping levels. The characteristic line at 1332 cm-1decreases and shifts to lower energy as a function of the film resistivity. It was also observed a broad peak around 1220 cm-1 caused by the incorporation of boron in the diamond lattice. Photocurrent-voltage behaviour of undoped and boron-doped diamond electrodes was investigated in dark and UV visible irradiation. The voltammograms showed that doped electrodes illuminated with a xenon lamp exhibited currents significatively higher because of the increased conductivity. For undoped electrodes it was observed a small photocurrent for anodic and cathodic polarization in the order ofmuA for the potential range of +1.0 V and -1.0 V (Ag/AgCl), for 1.0 M KCl. Mott-Schottky plots studied the interfacial processes at diamond-electrolyte junction. The flatband potential Uf b was found between 0.6 and 0.8 V (Ag/AgCl) which varies with the presence of sp² -type carbon as an impurity. From the curve slope the acceptor concentrations were found in the range of 10(18) and 10(21) cm-3 .These values agree with the estimated concentration obtained by Raman measurements.
  • Size effects on the growth mode and roughness of sub-micron structures grown by selective area epitaxy

    Gutiérrez, H.R.; Cotta, M.A.; Nakaema, W.M.; Carvalho, M.M.G.de; Gobbi, A.L.

    Resumo em Inglês:

    We have observed changes in the morphology of InP films grown in epi-ready substrates and on patterns created by photo and plow-mode AFM lithography. These changes are attributed to different levels of contamination of the surface, which favor nucleation of clusters over a two-dimensional film for the lithography-processed surface. The area of nucleation is smaller than the dimensions of the patterns created by lithography, so no size effects due to the presence of the pattern could be observed. The film morphologies exhibited well defined exponents for system sizes smaller than 0.2 mum but no defined growth exponent.
  • Plasma ion implantation of nitrogen into silicon: high resolution x-ray diffraction

    Beloto, A. F.; Abramof, E.; Ueda, M.; Berni, L. A.; Gomes, G. F.

    Resumo em Inglês:

    In the present study we use x-ray diffraction methods to characterize the surface of Si wafers irradiated with nitrogen by Plasma Immersion Ion Implantation. This is a non-line-of-sight ion implantation method, which allows three-dimensional treatment of materials including semiconductors, metals and dielectrics. The atomic concentration profiles in the implanted Si wafers were measured by Auger electron spectroscopy. The (004) Si rocking curve ( omega-scan) was measured in a high resolution x-ray diffractometer equipped with a Ge (220) four-crystal monochromator before and after implantation. A distortion of the Si (004)-rocking curve was clearly observed for the as-implanted sample. This rocking curve was successfully simulated by dynamical theory of x-ray diffraction, assuming a Gaussian strain profile through the implanted region. The analysis made by x-ray difraction and Auger electron spectroscopy revealed successful implantation of ions with ac- cumulated nitrogen dose of 1.5 x <FONT FACE=Symbol>10(17) </FONT>cm-3 .The Si wafers can be used as high sensitivity monitors in the Plasma Immersion Ion Implantation process, especially at the low dose range.
  • Band crossing evidence in PbSnTe observed by optical transmission measurements

    Ferreira, S. O.; Abramof, E.; Motisuke, P.; Rappl, P. H. O.; Closs, H.; Ueta, A. Y.; Boschetti, C.; Bandeira, I. N.

    Resumo em Inglês:

    Using high quality epitaxial layers, we have obtained direct evidence of the band inversion in the Pb1-xSn xTe system .The samples, covering the whole composition range, were grown by molecular beam epitaxy on (111)BaF2 substrates. A minimumin the resistivity as a function of temperature was observed for all samples with Sn composition 0.35 <= x <= 0.70. In the same samples and at the same temperature, temperature dependent optical transmission measurements have revealed a change in signal of the energy gap temperature derivative, a direct evidence of the band inversion. However, the temperature for which the inversion occurs is not the one expected by the band inversion model. This discrepancy is supposed to be due to the Burstein-Moss shift caused by the relatively high hole concentration observed in these samples.
  • Quasi-fermi levels, chemical and electric potential profiles of a semiconductor under illumination

    Santos, A. M. dos; Beliaev, D.; Scolfaro, L. M. R.; Leite, J. R.

    Resumo em Inglês:

    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.
  • A tight-binding study of acceptor levels in semiconductors

    Menchero, J. G.

    Resumo em Inglês:

    Acceptor binding energies in zinc-blende semiconductors are determined within the tight-binding formalism. The importance of fitting the valence-band masses in the (100) as well as (111) directions is discussed, and parametrizations that specifically fit the valence-band anisotropy are used to calculate Ge acceptor levels in Al xGa1-x As alloys. The sensitivity of the calculated energies to the parameters that determine bulk masses is investigated, as well as the effect of varying the on-site energy of the impurity. A comparison is made between first-neighbor and second-neighbor hopping models. For shallow levels, both approaches give the same results. For deeper levels, however, important differences arise. Experimental evidence suggests that first-neighbor models are better suited for describing intermediate to deep levels.
  • High temperature behavior of subpicosecond electron transport transient in 3C- and 6H-SiC

    Bezerra, E. F.; Caetano, E. W. S.; Freire, V. N.; Silva Jr., E. F. da; Costa, J. A. P. da

    Resumo em Inglês:

    A study of the subpicosecond high-field electron transport transient in 3C- and 6H-SiC polytypes at high lattice temperatures is performed. Electric field intensities up to 1000 kV/cm and lattice temperatures of 300, 673, and 1073 K are considered. It is shown that the transient regime behavior depends on the electric field intensity as well as on the lattice temperature, but it is always shorter or of the order of 0.3 ps. For the lowest lattice temperature, an electron velocity overshoot can always occur when the electric field intensity is higher than 300 kV/cm, but it decreases and can even disappear when the lattice temperature is raised.
  • SXS and XPS study of the adsorption and desorption of Te on GaAs (100)

    González, J.C.; Rodrigues, W.N.; Silva, C.M.; Moreira, M.V.B.; Oliveira, A.G. de; Abbate, M.; Vicentin, F.C.

    Resumo em Inglês:

    The adsorption and desorption of Te on GaAs (100) has been investigated using Soft X-ray Spectroscopy (SXS) and X-ray Photoelectron Spectroscopy (XPS). The SXS measurements have been done at the Brazilian synchrotron facility LNLS, and the XPS study was performed using laboratory sources. The samples were MBE GaAs (100) layers covered in-situ by Te from an effusion cell. Two families of samples were studied: samples analyzed in-situ, and samples that were exposed to air for at least one week. The desorption of Te of the non-oxidized samples indicates that traces of Te remains in the surfaces for temperatures as high as 680 °C. However, for the case of the oxidized samples, the Te film sublimates easily at around of 250 °C, leaving a clean GaAs surface. It seems that the oxidized Te film works as a suitable protection for epitaxial GaAs layers.
  • Impurity breakdown in GaAs samples grown by molecular beam Epitaxy

    Rubinger, R.M.; Oliveira, A.G. de; Ribeiro, G.M.; Bezerra, J.C.; Silva, C.M.; Rodrigues, W.N.; Moreira, M.V.B.

    Resumo em Inglês:

    In this work we present studies for the behavior of impurity breakdown of two GaAs samples grown by Molecular Beam Epitaxy at 200°C and 300°C. We vary the temperature and the illumination intensity. For the sample grown at 200°C, the transport mechanism after the breakdown is of the same type of the main free carriers at low electric fields. Below 100K, a clear dependence of the threshold electric field with temperature was observed. The sample grown at 300°C shows a breakdown due to an acceptor level at 41meV from the valence band. The threshold electric field increases with illumination due to the generation of free electrons by deep levels. A similar behavior was also observed for a Cr doped GaAs sample.
  • Observation of low frequency oscillations in GaAs samples grown by molecular beam epitaxy at low temperatures

    Rubinger, R. M.; Oliveira, A. G. de; Ribeiro, G. M.; Soares, D. A. W.; Moreira, M. V. B.

    Resumo em Inglês:

    We have observed Low Frequency Oscillations (LFO) in a GaAs sample grown by molecular beam epitaxy (MBE) at 300 °C as a function of applied electric field, temperature and illumination intensity. Time series of the oscillations for different values of the applied electric field at room temperature were obtained. We have also obtained the jxE characteristics as a function of temperature and light intensity. We were able to control the LFO using three experimental parameters, the electrical field, which controls the oscillation frequency, and the temperature and illumination intensity, which control the carrier density and field-enhanced trapping. We were able to identify the LFO dependence on the carrier density and on the field-enhanced trapping.
  • Atomic geometry and energetics of native defects in cubic boron nitride

    Orellana, W.; Chacham, H.

    Resumo em Inglês:

    We use first-principles calculations to investigate the electronic structure, atomic geometries and formation energies of point vacancies (V B , V N ), antisites (B N , N B ) and interstitials (Bi , Ni ) in cubic boron nitride (c-BN). We find that nitrogen and boron vacancies exhibit the lowest formation energies in nonstoichiometric c-BN for p-type and n-type conditions, respectively, showing intrinsic donor(V B ) and acceptor (V N ) characters. The equilibrium geometries show large outward breathing relaxations for both vacancies and for B N , with slight Jahn-Teller distortions from Td symmetry. For N B we nd an off-center distortion inducing a negative-U behavior in this center. For both interstitial centers we find stable confi- gurations in which an atomic pair occupies a nitrogen site in the lattice, N-N for Ni and N-B for Bi . These systems are stable for the different charge states investigated.
  • Experimental evidence for the distinction between metastability and persistence in optical and electronic properties of bulk GaAs and AlGaAs

    Pinheiro, M.V.B; Krambroc, K.

    Resumo em Inglês:

    In this paper we present experimental evidence supporting that persistent behavior in the electronic and optical properties of bulk GaAs and AlGaAs can have different origins as: the metastability of a defect, the persistent charge transfer from a defect to another defect, and the transfer from a defect to the conduction band. In particular, we show three different cases in which defects are either directly or indirectly related to persistent changes of the optical absorption, the magnetic circular dichroism of the absorption (MCDA) and the photoconductivity (PC). The three cases are as follows: the transfer of EL2 to the metastable state EL2(0*) in semi-insulating (SI) GaAs causing persistent quenching of the absorption and photoconductivity (PPCQ); the persistent transfer of electrons from stable EL2(0) to arsenic vacancies in lightly n-type GaAs; and the correlated appearance of persistent MCDA and photoconductivity in AlGaAs due to the photoionization of DX-centers.
  • Theoretical study of surfactant action of Te on Si(001)/Ge surfaces

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

    Resumo em Inglês:

    We have performed first-principles total energy calculations to investigate the adsorption process of Ge on Si(001)/Te, and the surfactant action of Te during the epitaxial deposition of Ge on Si(001)/Te, based on an atomic exchange process, Ge <-> Te. The total energy calculation indicates that the adsorption of Ge atom in "vacancy" site is energetically more favourable than its adsorption in "bridge" site. The adsorption of Ge in "on-top" and "anti-bridge" sites can be ruled out. The segregation process of Te atoms, based on an atomic exchange mechanism, reduces the total energy of the surface, confirming the surfactant effect of Te atoms.
  • Theoretical study of Si(001)/Te - (1×1), (2×1) and (3×1) surfaces

    Miwa, R. H.; Ferraz, A. C.

    Resumo em Inglês:

    The atomic geometry, chemical bonding and energetics for full -, 2/3, 1/2 and 1/3 of a monolayer of Te coverages on the Si(001)/Te -(1 x 1), (2 x 1), and (3 x 1) surfaces have been studied employing first - principles total-energy calculations. For one monolayer coverage we nd that the (1 x 1) structure is energetically favourable against (2 x 1) structure, in agreement with experimental results, indicating that the dimerization of Te atoms is not an exothermic process. For a coverage of 2/3 of a monolayer, the formation of Te-Si-Te mixed trimers, in a (3 x 1) - reconstruction, is an energetically stable configuration. At 1/3 coverage, we have obtained the formation of Si-Si dimers with a single Te atom, at the surface in a (3 x 1) - reconstruction. In this structure there is a very weak interaction between the dimer and the single Te atom.
  • Ab initio theoretical studies of atomic and electronic structures of III-nitride (110) surfaces

    Alves, H. W. Leite; Alves, J. L. A.; Nogueira, R. A.; Leite, J. R.

    Resumo em Inglês:

    We present a systematic theoretical study of several III-nitride (110) surfaces based on accurate, parameter-free, self-consistent total energy and force calculations using the density functional theory, the generalized gradient approximation(GGA) for the exchange-correlation term, and the Full Potential Linear Augmented Plane Wave (FPLAPW) approach associated with the slab supercell model. We studied AlN, BN, GaN and InN and analyzed the theoretical trends for the equilibrium atomic structures and surface band structures. We used supercells built up of 7 atomic layers and a vacuum region equivalent of 5 atomic layers. For the Ga and In species, the 3d and 4d electrons were treated properly as valence electrons. In connection with the atomic structures, we are concerned with the LEED parameters delta1<FONT FACE=Symbol>^</FONT>, delta1x, delta2<FONT FACE=Symbol>^</FONT> , d12<FONT FACE=Symbol>^</FONT>, d12x and omega for the (110) surface. We analyzed the changes in the bond-lengths and in the bond-angles at the anion and cation sites. We conclude that similarly to the III-arsenide (110) and III-phosphide (110) surfaces, the III-nitride (110) surfaces relax such that the cation-surface atom moves inward and the N-surface atom moves outward. The large Coulomb energy of the III-nitrides as compared with the other III-V compounds re ects in the smaller value of the tilt angle omega and in the small value of delta1<FONT FACE=Symbol>^</FONT>. To our knowledge, this is the rst time the FPLAPW method is used for such a systematic study of III-nitrides, and we compare our results with recent results obtained with other approaches as reported in the literature.
  • Tellurium - modified surface states of GaAs(001) and InAs(001)

    Silva, R. Claudino da; Ferraz, A. C.

    Resumo em Inglês:

    We investigate the stability and electronic structure of 1×2 and 2×2 GaAs(001):Te and InAs(001):Te surfaces with different degrees of tellurium coverage (thetaTe = <img src="http:/img/fbpe/bjp/v29n4/a4414.gif" alt="a4414.gif (106 bytes)" align="absmiddle">, <img src="http:/img/fbpe/bjp/v29n4/a4412.gif" alt="a4412.gif (100 bytes)" align="absmiddle">, <img src="http:/img/fbpe/bjp/v29n4/a4434.gif" alt="a4434.gif (104 bytes)" align="absmiddle">, 1), by means of rst-principles pseudopotential calculations within density-functional theory. The adsorption stability decreases as the tellurium coverage increases. The adsorption on InAs(001) is more stable than on GaAs(001). As-Ga (or As-In) bonds for the uppermost As atoms are s²p² -like (nearly planar), while the Te bonds at the surfaces are sp³-like. The hetero-dimers in thetaTe = <img src="http:/img/fbpe/bjp/v29n4/a4412.gif" alt="a4412.gif (100 bytes)" align="absmiddle"> modify the character of GaAs(001):Te and InAs(001):Te surfaces resulting in a semiconductor structure.
  • Vacancy diffusion in silicon: analysis of transition state theory

    Gattass, R. R.; Koiller, Belita; Capaz, R. B.

    Resumo em Inglês:

    Transition state theory (TST) is the most widely used formalism for theoretical calculations of diffusion coeffcients of defects in solids. In this work, we test its validity for the case of vacancy diffusion in silicon. The diffusion coefficient directly obtained from molecular-dynamics simulations with a classical (Stillinger-Weber) potential is compared with TST predictions. Our results confirm the validity of TST for this system.
  • Dynamics of internal electric and phonon fields in n-GaAs pumped with ultrashort pulses

    Souza, Fabrício M.; Egues, J. Carlos

    Resumo em Inglês:

    We investigate the ultrafast dynamics of an electron-hole plasma coupled to phonons in bulk GaAs excited with femtosecond laser pulses. Our approach is based on balance equations directly derived from the Boltzmann equation within the relaxation-time approximation. Poisson's equation together with a phenomenological driven-harmonic-oscillator equation supplements our description by accounting for time-dependent electric and vibrational fields. Our calculated internal fields show oscillations at frequencies characteristic of those of coupled plasmon-phonon modes. Our results are consistent with recent experimental data.
  • Quantum well to quantum wire crossover in AlAs/GaAs/AlAs heterostructures induced by interface roughness increase

    Dargam, T. G.; Capaz, R. B.; Koiller, Belita

    Resumo em Inglês:

    Adopting a real-space tight-binding supercell approach, we investigate interface roughness effects in semiconductor heterostructures. AlAs/GaAs/AlAs (001) QWs of average width W are considered, in which one of the interfaces is planar and the other has a shape defined by periodic steps with amplitude A and wavelength lambda. The oscillator strength f of the fundamental transition in the well describes the optical nature of the heterostructures. By investigating the wavefunctions as a function of the interface parameter A, we conclude that the f behavior with A is an optical signature of the quantum well to quantum wire crossover in the heterostructures. Recently, photoluminescence experiments showed that hydrostatic pressure produces an increase in the optical eficiency of heterostructures in which the interfaces present a high degree of roughness. In order to investigate this optical behavior, we discuss hydrostatic pressure effects on rough-interface heterostructures.
  • Morphological, optical and structural properties of zero-net-strained InGaAsP/InP structures grown by LP-MOVPE for 1.55mum laser applications

    Carvalho Jr, Wilson de; Bernussi, Ayrton André; Furtado, Mario Tosi; Gobbi, Angelo Luiz; Cotta, Mônica

    Resumo em Inglês:

    Zero-Net-Strained (ZNS) InGaAsP/InGaAsP/InP Multi Quantum Wells (MQW) structures grown by Low Pressure Metalorganic Vapor Phase Epitaxy for 1.55mum laser applications were investigated using atomic force microscopy, photoluminescence spectroscopy and X-ray diffraction. The morphology exhibits a strong anisotropic and modulated behavior. The photoluminescence spectrum shows a broad emission band below the fundamental quantum well transition. The results indicate a strong influence of the growth rate, growth temperature and barrier composition on the surface morphology, and on the optical and structural properties of the ZNS structures. Ridge wave-guide ZNS-MQW laser structures grown at optimized conditions exhibited excellent electro-optic characteristics with low threshold current and high efficiency.
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