Repositório RCAAP

We investigated the effect of adding the field-dependent recombination process, namely field-enhanced trapping, to the generation-recombination processes of charge carriers that model current oscillations in semiconductors. The main new features arising from this modification are identified in bifurcation diagrams with the electric field as the control parameter. The characteristic of the bifurcation diagrams is a function of impurity energy. Thus, we generated a set of bifurcation diagrams for a range of the impurity energy and applied bias. The energy dependence of the bifurcation diagrams is discussed considering the context of the competition between the generation-recombination mechanisms impact ionization and field-enhanced trapping.

Electric transport properties measured by Van der Pauw resistivity experiments of Low-Temperature Molecular Beam Epitaxy (LT-MBE) GaAs samples are used to identify a method to improve the resistivity of GaAs material. We present results on five samples grown at 265, 310, 315, 325, and 345 ºC. The electric measurements were carried out at temperatures ranging from 130 to 300 K. In this temperature range the dominant transport process is identified as variable range hopping. The hopping parameter plotted against the growth temperature is shown to present a maximum. The mechanisms responsible for this behavior are discussed in relation to the compensation ratio.

A theoretical study of the mobility of n-doped III-Nitrides in wurtzite phase is reported. We have determined the nonequilibrium thermodynamic state of the bulk n-InN, n-GaN, and n-AlN systems - driven far away from equilibrium by a strong electric field - in the steady state, which follows after a very fast transient. For this we solve the set of coupled nonlinear integro-differential equations of evolution of the nonequilibrium thermodynamic variables, for the three materials, to obtain their steady state values. The dependence of the mobility (which depends on the nonequilibrium thermodynamic state of the sample) on the electric field strength and the concentration (of electrons and impurities) is derived, which decreases with the increase of the electric field strength and the concentration of carriers, evidencing the influence of the nonlinear transport involved.

We present an experimental study of bifurcation diagrams from low frequency current oscillations (LFO) measurements obtained from semi-insulating GaAs samples grown by low temperature molecular beam Epitaxy (LT-MBE). The considered growth temperatures were 215ºC and 265ºC. LFO are considered to be spontaneously generated oscillations under constant applied bias V. These oscillations were measurement and recorded in the form of time series. The bifurcation diagrams were obtained from the sequence of minima as a function of the applied bias. The standard measurement procedure was described elsewhere. As the control parameter, the bias allows the identification of a bifurcation route to chaos.

We investigated the interaction of As impurities with a 90º partial dislocation in Si. The calculations show that As atoms segregate to the dislocation core. The most stable site for the As atom is at the stacking fault side, which is favorable by 0.26 eV as compared to an As atom in a crystalline position. There is no charge transfer from the As impurity to the dislocation core when the impurity is far from it. The segregation of the impurity can be understood mainly due to structural effects. This result let us conclude that the experimentally observed negatively charged dislocations are due to impurities trapping at the core of the dislocation and not charge transfer from impurities away from the dislocation.

Morphological and microstructure properties of reticulated vitreous carbon (RVC) were analyzed by scanning electron microscopy (SEM), micro-Raman spectroscopy and x-ray diffraction (XRD) techniques as a function of heat treatment temperature (HTT). Samples produced in the HTT range of 1000 to 2400 K have demonstrated a strong dependence of HTT in their structural order mainly attributed to the presence of hydrogen and oxygen (heteroatoms), originated from precursor. In this range, the material is changed from pyropolymer to carbon. The polyfurfuryl alcohol precursor has furanic groups and its cure originates methylenical bonds and sulphur atoms. At HTT higher than 1300 K, these atoms are gradually liberated from the material generating a discrete structure ordering.

We present a theoretical investigation on the properties of interstitial nickel and its related complexes in diamond. The atomic configurations, symmetries, spins, and energetics of interstitial nickel, nickel-boron and nickel-vacancy complexes were investigated by a total energy ab initio methodology. Those results are used to explain certain electrically active centers, commonly found in synthetic diamond.

Er3+ emission in the wide bandgap matrix SnO2 is observed either through a direct Er ion excitation process as well as by an indirect process, through energy transfer in samples codoped with Yb3+ ions. Electron-hole generation in the tin dioxide matrix is also used to promote rare-earth ion excitation. Photoluminescence spectra as function of temperature indicate a slight decrease in the emission intensity with temperature increase, yielding low activation energy, about 3.8meV, since the emission even at room temperature is rather considerable.

Mercuric Iodide (HgI2) is a semiconductor candidate for the construction of X- and gamma-ray detectors for digital medical imaging due to its high atomic number (Z Hg = 80, Z I = 53). Also, HgI2 has a wide optical band-gap (2.13 eV) and high photon absorption coefficient for high-energy radiation. Different structures can lead to varying electrical and optical properties of the final material. In this work, HgI2 crystals were produced by the solvent evaporation technique. The solvents used were ethanol (solubility around 20 mg/ml at 25ºC), ether (solubility around 3.5 mg/ml at 25ºC) and acetone (solubility around 24mg/ml at 25ºC). The evaporation conditions were varied in order to produce different final crystals. The Bérnard cells are responsible for crystallites formation due to the Bérnard-Maragoni convection in the liquid. Millimeter-sized crystals can be obtained as seen by Scanning Electron Microscopy.

In this work we report the observation of two different EL2 metastable states in GaAs and the effect of the optical/thermal history of the sample on the behavior of the photoquenching kinetics. In one thermal/optical preparation, the photoquenching curve presented two time constants that have already been interpreted as an indication of two different metastable states. With another preparation, the initial rise in transmittance displayed only one time constant, and we observed that a temperature increase to 83K triggered a second photoquenching process. We associated this new photoquenching with a transition from the first to the second metastable state. The experimental data is explained in terms of a new proposal for the microscopic structure of the EL2 complex.

We report the investigation of Xe clusters in amorphous carbon by x-ray absorption spectroscopy (XAS) to understand the properties of solid xenon. Measurements have been performed on xenon L3 absorption edge at room temperature (300K). Using computational XANES calculation for fcc and hcp structures it was possible to study the XANES fine structure origin and a relation between the x-ray absorption near edge structure and the lattice constant. Comparing those results with our experimental data we determined that the XAS fine near edge structure has a specific behavior for solid xenon and does not have this behavior for gas Xe end Xe diluted in others chemical elements matrices.

We propose the local density approximation (LDA) plus an on-site Coulomb self-interaction-like correction (SIC) potential for describing sp-hybridized bonds in semiconductors and insulators. We motivate the present LDA+U SIC scheme by comparing the exact exchange (EXX) hole with the LDA exchange hole. The LDA+U SIC method yields good band-gap energies Eg and dielectric constants epsilon(omega ~ 0) of Si, Ge, GaAs, and ZnSe. We also show that LDA consistently underestimates the gamma-point effective electron m c and light-hole m lh masses, and the underlying reason for this is a too strong light-hole-electron coupling within LDA. The advantages of the LDA+U SIC approach are a computational time of the same order as the ordinary LDA, the orbital dependent LDA+U SIC exchange-correlation interaction is asymmetric analogously to the EXX potential, and the method can be used for materials and compounds involving localized d- and f-orbitals.

Using a first-principles pseudopotential technique within a generalized gradient approximation of the density functional theory, we have investigated the mechanism of adsorption of molecular oxygen on the CdTe(110) surface. The determination of the more favorables structures and their formation through the activation barrier analysis indicates that the formation can occur in the regimes of low and high temperatures. According to our calculations it was verified that in the regime of low temperatures the oxygen molecule binds exclusively to Cd at the topmost site through Cd-O-O or Cd-O2 bonds. The electronic band structure presents a characteristic state like a double acceptor defect. In the high temperature regime the molecule adsorbs between Cd of the surface and Te of the subsurface, breaking Cd-Te bond and causing an upward dislocation of Cd from the subsurface. In this regime of high temperatures the complex presents Cd-O, Cd-O2, Te-O and O-O bonds and the electronic band structure is almost free of gap of states.

In the last few years intense efforts have been devoted to the growth and characterization of semiconductor nanostrucutures. In particular, quantum dots of CdTe grown by hot wall epitaxy on Si(111) substrates constitute a very interesting example. CdTe/Si systems follow the Volmer-Weber growth mode with nucleation of 3D CdTe islands on the Si substrate even for less than one monolayer of evaporated material. In the present work, we proposed a simple one-dimension model to reproduce a very peculiar behavior observed in the quantum dot height and size distributions at distinct temperatures. The model, which includes CdTe deposition, diffusion, and revaporization, qualitatively reproduces these distributions. Moreover, the width distributions suggest a transition from Stranski-Krastanow growth mode at lower temperatures to Volmer-Weber growth mode at higher temperatures.

In this work, we present our preliminary ab initio results for the structural and electronic properties of both Si- and C-terminated SiC (100) surfaces in (2×1) and c(2×2) reconstruction patterns. Based on our results, we found that the Si-terminated surfaces are dominated by weak bonded Si-dimers, which are stabilized only at Si-rich conditions, leading to (3×2) or more complex reconstruction patterns, as verified experimentally. Also, our results show that the C-terminated surfaces is characterized by strong triply-bonded C-dimers, in a c(2×2) reconstruction pattern, which consists of C2 pairs over Si bridge sites, in agreement with experimental results.

In this work, we presented our preliminary ab initio results for the vibrational modes and the phonon frequencies of the SiC (100) surfaces. Our results are in good agreement with the available experimental data whenever this comparison is possible. For the accepted models of the C-terminated surfaces in the c(2×2) reconstruction, while in the bridge-dimer model there is an acetylene-like vibrational A1 mode at 2031 cm-1, which is infrared active, in the staggered-dimer model, there is a Füchs-Kliewer (FK) mode at 1328 cm-1, which is experimentally detected. For Si-terminated surfaces in the p(2×1) reconstruction, instead, no FK was obtained, in contradiction with the HREELS experimental results for the Si-terminated surfaces, but they are in consonance with the fact that this surface should be described by a (3×2) or more complex models.

We present, our preliminary results of a systematic theoretical study of the adsorption of N over As-terminated GaAs (100) (2×1) surfaces. We analyzed the changes in the bond-lengths, bond-angles and the energetics involved before and after deposition. Our results show that the N-atoms will prefer the unoccupied sites of the surface, close to the As dimer. The presence of N pushes the As dimer out of the surface, leading to the anion exchange between N and As atoms. Based on our results, we discussed the kinetics of the N islands formation during epitaxial growth of the III-Nitrides.

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 dissociative adsorption process of glycine on the silicon surface. Our total energy calculations indicate that the chemisorption of the molecule is as follow. The gas phase NH2-C2H2-OOH adsorbs molecularly to the electrophilic surface Si atom and then dissociates into NH2-C2H2-OO and H, bonded to the electrophilic and nucleophilic surface silicon dimer atoms respectively, with an energy barrier corresponding to a thermal activation that is smaller than the usual growth temperature, indicating that glycine molecules will be observed in their dissociated states at room temperature. This picture is further support by our calculated vibrational modes for the considered adsorbed species.

Hot-filament chemical vapour deposition (HFCVD) is a common method employed for diamond deposition. Typically in this method a dilute mixture of carbon containing gas such as methane in hydrogen is thermally activated at sub atmospheric pressures by a hot filament. Due to the filament-substrate proximity, large temperature variation across the substrate is possible. In this work we investigate the role of fluid flow and heat transfer from the filament to substrate in determining the quality of diamond growth. The commercial software CFX was used to calculate velocity field, temperature distribution and fluid flow. A vortex was identified on the substrate.

In this work we have investigated the properties of CdTe thin films grown on glass substrates by Hot Wall Epitaxy. Its most important feature is the growth at very low temperatures, which would allow the growth even on polymer substrates. Our samples were grown at temperatures between 150 and 250 ºC at a growth rate between 0.2 and 2 µm/h. The CdTe films were characterized by x-ray diffraction, scanning electron microscopy and optical transmission. The x-ray teta-2teta scans revealed films with cubic structure and a very high degree of preferential orientation. In fact, for films thicker than 1 µm, no other reflections have been observed on the spectra besides the (111), (333) and (444). The scanning electron micrographs showed a grain size about 0.3 µm and the optical transmission indicated a very good optical quality of the interfaces, showing pronounced interference fringes.