S. Mergulhão

Universidade Federal de São Carlos, São Carlos, Estado de Sao Paulo, Brazil

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Publications (22)42.66 Total impact

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    ABSTRACT: Charge transport and shelf-degradation of MEH-PPV thin-films were investigated through stationary (e.g. current versus voltage – JxV) and transient (e.g. Time-of-Flight – ToF, Dark-Injection Space-Charge-Limited Current – DI-SCLC, Charge Extraction by Linearly Increasing Voltage – CELIV) current techniques. Charge carrier mobility in nanometric films was best characterized through JxV and DI-SCLC. It approaches 10-6cm2/Vs under a SCLC regime with deep traps for light-emitting diode applications. ToF measurements performed on micrometric layers (i.e. ~ 3μm) confirmed studies in 100nm-thick films as deposited in OLEDs. All results were comparable to a similar poly(para-phenylene vinylene) derivative, MDMO-PPV. Electrical properties extracted from thin-film transistors demonstrated mobility dependence on carrier concentration in the channel (~ 10-7 – 10-4cm2/Vs). At low accumulated charge levels and reduced free carrier concentration, a perfect agreement to the previously cited techniques was observed. Degradation was verified through mobility reduction and changes in trap distribution of states.
    Journal of Non-Crystalline Solids 02/2012; · 1.72 Impact Factor
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    ABSTRACT: Persistent photoconductivity (PPC) in vanadyl phthalocyanine (VOPc) organic light-emitting diodes was investigated using photoconductive time response, photocurrent–voltage characteristics and charge extraction in linearly increasing voltage (CELIV) measurements. The experiments were performed in phase 1 (amorphous) and in phase 2 (crystalline) samples obtained by the physical vapour deposition (PVD) technique over ITO/glass electrodes with an Al covering electrode. The results indicated a photoconductivity with a long decay time in phase 1 VOPc described by a stretched exponential relaxation. The device showed a rectifying behaviour and the mobility of holes was measured by CELIV, following a dispersive model. In crystalline samples the PPC effect was not observed and the dominant mechanism of transport of holes was hopping in a Gaussian density of states.
    Journal of Physics D Applied Physics 05/2008; 41(12):125107. · 2.53 Impact Factor
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    ABSTRACT: The transport properties (conductivity and mobility) of holes and electrons in poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene) light-emitting diodes were investigated using direct current–voltage curves (I–V) and time of flight measurements (TOF) as a function of electric field and temperature. TOF results revealed that the transport of holes in the bulk follows a non-dispersive behavior at room temperature (300 K), exhibiting a progressive transition to a dispersive behavior as the temperature decreases down to 220 K. The dispersive transport characteristics were interpreted in the framework of carrier hopping in an exponential density of states. On the other hand, the analysis of the negative photocurrent transients indicated that the transport of negative charge carriers (electrons) is strongly dominated by trapping in the entire range of temperature studied. The I–V curves presented remarkable temperature dependence, being analyzed in terms of the classic Fowler–Nordheim tunneling, Richardson–Schottky thermionic emission and trap-controlled transport.
    Thin Solid Films 01/2007; 515:8034-8039. · 1.87 Impact Factor
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    ABSTRACT: Photogenerated charge carrier profiles in poly(2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene) (MEH-PPV) light-emitting diodes were determined from steady-state photocurrent spectra and transient photocurrent decay measurements. The observation that the photocurrent spectra behavior is strongly dependent on the bias polarity and amplitude suggests the existence of an intrinsic electric field, determined by the difference in the work function of the metallic electrodes, as well as a field dependence on the free charge carrier generation rates. The obtained results reveal built-in voltages of +0.2 and -0.6V for Au and Al electrodes, respectively. The photocurrent spectra can be semi-quantitatively explained using a simple model which takes into account the internal built-in electric field of the device structure, the band gap energy and the migration/diffusion of the photogenerated charge carriers through the polymeric film. The drift mobility of carriers was investigated by the time-of-flight technique (TOF). The value for the mobility of holes was obtained from the change of slopes in the double logarithmic plot of the transient current and was independent of the applied field. The TOF transients for electrons showed no change of slopes in the double logarithmic plot and the drift mobility of electrons could not be calculated. (c) 2006 Elsevier B.V. All rights reserved.
    Materials Science and Engineering B 01/2006; 135(2):103-107. · 1.85 Impact Factor
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    ABSTRACT: The transport properties of holes and electrons in poly (2-methoxy,5-(2-ethyl-hexyloxi)-1-4 phenylene vinylene) (MEH-PPV) prepared on a p type gallium arsenide (p-GaAs) substrate are investigated by the time of flight technique (TOF). The results are presented in function of applied field, and the laser incident wavelength. Tail broadening parameter has been used to analyze the dispersion of the experimental results.
    Electrets, 2005. ISE-12. 2005 12th International Symposium on; 10/2005
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    ABSTRACT: Calcium modified lead titanate sol was synthesized using a soft solution processing, the so-called polymeric precursor method. In soft chemistry method, soluble precursors such as lead acetate trihydrate, calcium carbonate and titanium isopropoxide, as starting materials, were mixed in aqueous solution. Pb0.7Ca0.3TiO3 thin films were deposited on platinum-coated silicon and quartz substrates by means of the spinning technique. The surface morphology and crystal structure, dielectric and optical properties of the thin films were investigated. The electrical measurements were conducted on metal-ferroelectric-metal (MFM) capacitors. The typical measured small signal dielectric constant and dissipation factor at a frequency of 100 kHz were 299 and 0.065, respectively, for a thin film with 230 nm thickness annealed at 600C for 2 h. The remanent polarization (2Pr) and coercive field (E c) were 32 C/cm2 and 100 kV/cm, respectively. Transmission spectra were recorded and from them, refractive index, extinction coefficient, and band gap energy were calculated. Thin films exhibited good optical transmissivity, and had optical direct transitions. The present study confirms the validity of the DiDomenico model for the interband transition, with a single electronic oscillator at 6.858 eV. The optical dispersion behavior of PCT thin film was found to fit well the Sellmeir dispersion equation. The band gap energy of the thin film, annealed at 600C, was 3.56 eV. The results confirmed that soft solution processing provides an inexpensive and environmentally friendly route for the preparation of PCT thin films.
    Journal of Sol-Gel Science and Technology 01/2003; 27(2):137-147. · 1.66 Impact Factor
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    ABSTRACT: The transport properties of intentionally disordered short-period doped GaAs/AlGaAs superlattices were explored. We found that a redistribution of the electron density in a random potential significantly influences the electron properties of the disordered superlattices resulting in a suppression of the vertical disorder and in the increase of the in-plane mobility of the layers with increasing disorder. © 2002 American Institute of Physics.
    Journal of Applied Physics 09/2002; 92(7):3830-3834. · 2.21 Impact Factor
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    ABSTRACT: Barium strontium titanate ( Ba <sub>0.8</sub> Sr <sub>0.2</sub> TiO <sub>3</sub>) thin films have been prepared on Pt/Ti/SiO <sub>2</sub> /Si substrates using a soft solution processing. X-ray diffraction and also micro-Raman spectroscopy showed that the Ba <sub>0.8</sub> Sr <sub>0.2</sub> TiO <sub>3</sub> thin films exhibited a tetragonal structure at room temperature. The presence of Raman active modes was clearly shown at the 299 and 725 cm <sup>-1</sup> peaks. The tetragonal-to-cubic phase transition in the Ba <sub>0.8</sub> Sr <sub>0.2</sub> TiO <sub>3</sub> thin films is broadened, and suppressed at about 35 °C, with a maximum dielectric constant of 948 (100 kHz). Electrical measurements for the prepared Ba <sub>0.8</sub> Sr <sub>0.2</sub> TiO <sub>3</sub> thin films showed a remnant polarization ( P <sub>r</sub>) of 6.5 μC/cm <sup>2</sup>, a coercive field (E<sub>c</sub>) of 41 kV/cm, and good insulating properties. The dispersion of the refractive index is interpreted in terms of a single electronic oscillator at 6.97 eV. The direct band gap energy (E<sub>g</sub>) and the refractive index (n) are estimated to be 3.3 eV and n = 2.27–2.10, respectively. © 2002 American Institute of Physics.
    Journal of Applied Physics 06/2002; · 2.21 Impact Factor
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    ABSTRACT: Pb1−xCaxTiO3 (0.10⩽x⩽0.40) thin films on Pt/Ti/SiO2/Si(100) substrates were prepared by the soft solution process and their characteristics were investigated as a function of the calcium content (x). The structural modifications in the films were studied using x-ray diffraction and micro-Raman scattering techniques. Lattice parameters calculated from x-ray data indicate a decrease in lattice tetragonality with the increasing content of calcium in these films. Raman spectra exhibited characteristic features of pure PbTiO3 thin films. Variations in the phonon mode wave numbers, especially those of lower wave numbers, of Pb1−xCaxTiO3 thin films as a function of the composition corroborate the decrease in tetragonality caused by the calcium doping. As the Ca content (x) increases from 0.10 to 0.40, the dielectric constant at room temperature abnormally increased at 1 kHz from 148 to 430. Also calcium substitution decreased the remanent polarization and coercive field from 28.0 to 5.3 μC/cm2 and 124 to 58 kV/cm, respectively. These properties can be explained in terms of variations of phase transition (ferroelectric-paraelectric), resulting from the substitution the lead site of PbTiO3for the nonvolatile calcium. © 2002 American Institute of Physics.
    Journal of Applied Physics 05/2002; 91(10):6650-6655. · 2.21 Impact Factor
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    ABSTRACT: The characteristics of multi-layered InAs/GaAs self assembled quantum dots (SAQDs) annealed after the growth were here studied using a combination of capacitance-voltage (C-V) measurements,Raman scattering and photoluminescence (PL) spectroscopy. The combination of the results obtained with the three techniques, gave evidences that the annealing at 500º C causes the sharpness of the SAQDs interfaces, while the annealing at 600º C eliminated the SAQDs. However, the comparison with the case of single layered SAQDs, revealed a thermal stability of the last system even at an annealing temperature of 700º C, thus confirming the role of the interlayer strain in the low temperature diffusion process.
    Brazilian Journal of Physics 01/2002; · 0.60 Impact Factor
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    ABSTRACT: An electrostatic model was presented for the calculation of the capacitance-voltage characteristics of a semiconductor structure where quantum dots were embedded. The model was based on the linear coupling between the contributions of the quantum dots and the bulk host. We further applied this model to an InAs/GaAs self-assembled quantum dots system. The calculated capacitance was found in good agreement with the experimental curves, providing parameters of the dots ensemble, as the excitation energy of the confined electrons.
    Brazilian Journal of Physics 01/2002; · 0.60 Impact Factor
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    ABSTRACT: The capacitance–voltage characteristics of (GaAs)m/(AlAs)n superlattices and of a GaAs/AlGaAs multiple quantum well system were used as a tool to probe the homogeneity of the studied samples. Evidences of a strong electron localization in the superlattices even with the presence of minibands were found. We interpret this result taking into account the presence of local inhomogeneities in the superlattices which causes the breakdown of the coherence of the miniband transport and therefore, give rise to the electron localization. In order to support this conclusion we numerically calculate the capacitance of the superlattices assuming a localization center near the region where electron confinement takes place and the results were found in good agreement with the measured capacitance.
    Physica E Low-dimensional Systems and Nanostructures 01/2002; 13(1):36-42. · 1.52 Impact Factor
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    ABSTRACT: We present a systematic investigation of the capacitance-voltage measurements and Raman scattering on a multilayer InAs/GaAs self-assembled quantum dots system annealed at different temperatures. We observed a decrease of the electrical coupling of the electrons trapped in the dots located in the different layers. Raman scattering revealed the modifications of the dots morphology which influenced on the observed increase of the localization of electrons in the dots after the annealing process. We also observed that the annealing at 600^\circC altered the plane of the dots into layers of an InxGa1-xAs alloy.
    Japanese Journal of Applied Physics 01/2001; 40. · 1.07 Impact Factor
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    ABSTRACT: The electrical characteristics of the self-assembled quantum dots embbeded in GaAs wells of the GaAs/AlAs superlattices were studied by capacitance spectroscopy and were compared with results obtained for the dots embbeded in bulk GaAs. A much stronger electron localization was detected for the quantum dots embedded in the superlattice in comparison with those embedded in bulk GaAs. As a consequence, the electrical characteristics of the structures with quantum dots grown in superlattices were found to be significantly thermo-stabilized. It was shown that these structures present different strengths of the localization of electrons caused by the effective increase of the heights of the barriers when the dots were grown in the superlattices.
    Japanese Journal of Applied Physics 01/2001; 40. · 1.07 Impact Factor
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    ABSTRACT: A negative differential capacitance observed in the InAs/GaAs self-assembled quantum dots is shown to be an essential feature of the zero-dimensional electron system. A direct link of the capacitance to the electron density of states is presented. As a result, the density of the dots, the excitation energy of electrons and its dispersion were obtained and found to be in good agreement with the photoluminescence data.
    Physica E Low-dimensional Systems and Nanostructures 01/2001; · 1.86 Impact Factor
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    ABSTRACT: The characteristics of the InAs self-assembled quantum dots embedded both in a GaAs bulk matrix and in a GaAs/AlAs superlattice were investigated. Evidences of electrons confinement inside the InAs quantum dots were obtained using both capacitance–voltage measurements and Raman spectroscopy. A much stronger electron localization was detected for the quantum dots embedded in the superlattice in comparison with those embedded in bulk GaAs. As a consequence, the electrical characteristics of the structures with quantum dots grown in superlattices were found to be significantly thermo-stabilized. The origins of these effects are discussed in connection with the differences between the electronic features of the two kinds of structures. © 2000 American Institute of Physics.
    Journal of Applied Physics 08/2000; 88(4):1987-1991. · 2.21 Impact Factor
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    ABSTRACT: In this paper, we investigate the effects of the illumination on the capacitance-voltage measurements of a semiconductor structure where three planes containing InAs/GaAs self-assembled quantum dots were embedded. Nonexpected features of the capacitance attributed to the photogenerated holes accumulation in the quantum dots were observed. Contrary to the capacitance caused by electrons, the one associated with holes was found to be strongly dependent on the frequency, showing slower response of holes.
    Physical Review B 02/2000; 61(7):4481-4484. · 3.66 Impact Factor
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    ABSTRACT: The application of the usual expressions to calculate the capacitance-voltage (CV) profiles in samples with quantum dots gives erroneous results, mainly due to the presence of the characteristic negative differential capacitance of a system with dimensionality lower than 2. We developed a simple electrostatic model to calculate the CV profiles in these systems, and we applied it to a sample with an InAs self-assembled quantum dots system in order to obtain informations about the structure of the dots. As a result, the local distribution of electrons in the quantum dots (CV profile) was obtained.
    Physical review. B, Condensed matter 02/2000; 61(8):5499-5504. · 3.77 Impact Factor
  • Synthetic Metals 01/1999; 101(1). · 2.11 Impact Factor
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    ABSTRACT: Evidence of the crucial role of random fluctuations of the well size in vertical transport in doped GaAs/AlAs superlattices with broad minibands has been obtained by both Fourier-transform reflection spectroscopy and C-V measurements. It turned out that even monolayer fluctuations of the periodicity, or random fluctuations of the impurity potentials, which are unavoidable, can cause a partial localization of electrons providing one-dimensional conducting channels where the periodicity is conserved, and through which the electron transport across the superlattice would occur. This was found to be the reason why, instead of the constant vertical conductivity (independent of the electron density) predicted by the theory to occur when the Fermi energy exceeds the miniband width, a drop of the conductivity giving a metal-to-dielectric phase transition was observed.
    Physical Review B 08/1997; 56(7). · 3.66 Impact Factor