S. Mergulhão - Academia.edu (original) (raw)

Papers by S. Mergulhão

Research paper thumbnail of Thermostabilization of Electrical Properties of InAs/GaAs Self-Assembled Quantum Dots Embedded in GaAs/AlAs Superlattices

Japanese Journal of Applied Physics, 2001

The electrical characteristics of the self-assembled quantum dots embbeded in GaAs wells of the G... more 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.

Research paper thumbnail of Fundamentos de Física 1

Research paper thumbnail of Observation of persistent photoconductivity in conducting polyaniline thin films

Applied Physics A, 2005

ABSTRACT Conducting polyaniline thin films have been deposited by a laser-based vaporization tech... more ABSTRACT Conducting polyaniline thin films have been deposited by a laser-based vaporization technique. The films have been characterized by infrared spectroscopy, UV-Vis spectroscopy, four-point conductivity measurements, thermogravimetric analysis, and fluorescence measurements. In addition, the films have been characterized with respect to their photoconductive response to 532-nm laser light. It is found that the films exhibit persistent photoconductivity and it is proposed that defect base sequences may be responsible for the charge localization that results in such a photoconductive response.

Research paper thumbnail of Transient photocurrent measurements in conjungated polymers

Programa e Livro de Resumos, 2004

Research paper thumbnail of Charge carrier mobility investigations in molecularly doped poly(9-vinyl carbazole)

Programa e Livro de Resumos, 2004

Research paper thumbnail of Transporte dispersivo em polianilinas dopadas

Research paper thumbnail of Conductivity of random GaAs/AlGaAs superlattices in regime of controlled disorder

Research paper thumbnail of Synthesis, ferroelectric and optical properties of (Pb,Ca)TiO3 thin films by soft solution processing

Journal of Sol-Gel Science and Technology, 2003

Calcium modified lead titanate sol was synthesized using a soft solution processing, the so-calle... more 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. Pb 0.7 Ca 0.3 TiO 3 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 600 • C for 2 h. The remanent polarization (2P r) and coercive field (E c) were 32 µC/cm 2 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 600 • C, 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.

Research paper thumbnail of Time-of-Flight Technique Limits of Applicability for Thin-Films of Π-Conjugated Polymers

MRS Proceedings, 2012

ABSTRACTTime of flight (ToF) is the most straightforward technique to determine polymeric semicon... more ABSTRACTTime of flight (ToF) is the most straightforward technique to determine polymeric semiconductor mobility for electronic applications. We demonstrate ToF limits of applicability to amorphous PPV derivatives, such as poly[2-methoxy-5-(3’,7’-dimethylloctyloxy)-1-4-phenylene vinylene] (MDMO-PPV) and poly[2-methoxy-5-(2’-ethylhexyloxy)-1-4-phenylene vinylene] (MEH-PPV), and polycrystalline poly(3-hexylthiophene) (P3HT). Hole and electron mobility (μ) in submicrometric films (200 – 500 nm) is overestimated compared to casted layers, due to reduced absorption capability, which is confirmed by Charge Extraction by Linearly Increasing Voltage (CELIV) measurements. Charge transport properties in nanometric films, such as for Field-Effect Transistors (FET), can not be studied by current-mode ToF. Hole mobility of ca. 10-5 cm2/Vs with Poole-Frenkel behavior for PPV derivatives and 10-3 cm2/Vs for P3HT is at least one order of magnitude higher than ToF results.

Research paper thumbnail of Photogenerated carrier profile determination in polymeric light-emitting diodes by steady-state and transient photocurrent measurements

Materials Science and Engineering: B, 2006

Photogenerated charge carrier profiles in poly(2-methoxy-5-(2-ethyl-hexyloxy)-1,4-phenylene vinyl... more 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 −0.6 V 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.

Research paper thumbnail of Transient photocurrents and steady-state conductivities in surface-doped polyanilines

Synthetic Metals, 1999

Steady-state dark currents and transient photocurrents were measured in polyaniline (PANl) and po... more Steady-state dark currents and transient photocurrents were measured in polyaniline (PANl) and poly(o-methoxyaniline) (POG), asymmetrically protonated on one surface with various acids. In both polymers a rectification behaviour was found, indicating that the doped surfaces of the samples act as contacts injecting holes into the undoped bulk. Well defied transit times were measured in POMA: ambient-temperature hole mobility, estimated E-om the experiments, amounts to ca. 3x1u4 cm2Ns. Shapes of transient photocurrent-time dependences in PANl were featureless and decaying monoexponentially to their steady-state values, with the time constants of the order of 1 O-2s.

Research paper thumbnail of One-dimensional character of miniband transport in doped GaAs/AlAs superlattices

Physical Review B, 1997

... These effects will disturb the co-herence between electron wave functions in neighbor quan-tu... more ... These effects will disturb the co-herence between electron wave functions in neighbor quan-tum wells enough to cause disordered spatial localization ... Apparently, such a vertical transport will have 1D char-acter when the lateral width of the conducting channels is smaller than ...

Research paper thumbnail of Electron time-of-flight measurements in sulfur interpreted via an extra surface mobility channel

Physical Review B, 1988

Time-of-flight measurements were carried out in orthorhombic sulfur for various fields, ranging f... more Time-of-flight measurements were carried out in orthorhombic sulfur for various fields, ranging from-2 to-20 kV/cm. No dependence of the mobility with the electric field was found but the current, normalized by the initial current, showed an electric field dependence at small times, decaying faster for larger electric field. After the failure of the usual models in explaining the resultsincluding the assumption of depth-dependent density of trapsa model assuming an extra mobility channel near the surface provided a reasonable set of parameters independent of the electric field. The measurements were carried out at 8.5, 29, 53, 68, and 79'C.

Research paper thumbnail of Transition from dispersive to non-dispersive transport of holes in poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene) light-emitting diodes investigated by time of flight measurements

Thin Solid Films, 2007

The transport properties (conductivity and mobility) of holes and electrons in poly(2-methoxy-5-(... more 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.

Research paper thumbnail of Carrier confinement in an ultrathin barrier GaAs/AlAs superlattice probed by capacitance–voltage measurements

Physica E: Low-dimensional Systems and Nanostructures, 2002

The capacitance–voltage characteristics of (GaAs)m/(AlAs)n superlattices and of a GaAs/AlGaAs mul... more 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.

Research paper thumbnail of Effects of Annealing on Electrical Coupling in a Multilayer InAs/GaAs Quantum Dots System

Japanese Journal of Applied Physics, 2001

We present a systematic investigation of the capacitance–voltage measurements and Raman scatterin... more 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°C altered the plane of the dots into layers of an In x Ga1-x As alloy.

Research paper thumbnail of Capacitance-voltage characteristics of InAs dots: a simple model

Brazilian Journal of Physics, 2002

Research paper thumbnail of Study of charge carriers mobility in poly (9, 9-dioctylfluorenyl-2, 7-diy) capped with N, N-Bis (4-methylphenyl)-4-aniline (PFO)

Research paper thumbnail of Effect of photogenerated holes on capacitance-voltage measurements in InAs/GaAs self-assembled quantum dots

Research paper thumbnail of Transport of holes in uniformly and non-uniformly protonated poly(o-methoxyaniline)

Chemical Physics Letters, 1997

ABSTRACT Steady-state and transient currents were measured in bulk- and surface-protonated poly(o... more ABSTRACT Steady-state and transient currents were measured in bulk- and surface-protonated poly(o-methoxyaniline) emeraldine. The results obtained demonstrate that in surface-doped samples the protonated surface acts as a contact injecting holes into the bulk. The hole mobilities estimated from time-of-flight experiments performed on such samples amounts to 3 × 10−4 cm2/V s at ambient temperature. In uniformly protonated samples, we observed a transient decrease in the currents following the light pulse, recovering exponentially with a time constant equal to ca. 17 ms. Such a behaviour (‘negative photoconductivity’) was attributed to the production of long-lived excited species acting as localization centres.

Research paper thumbnail of Thermostabilization of Electrical Properties of InAs/GaAs Self-Assembled Quantum Dots Embedded in GaAs/AlAs Superlattices

Japanese Journal of Applied Physics, 2001

The electrical characteristics of the self-assembled quantum dots embbeded in GaAs wells of the G... more 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.

Research paper thumbnail of Fundamentos de Física 1

Research paper thumbnail of Observation of persistent photoconductivity in conducting polyaniline thin films

Applied Physics A, 2005

ABSTRACT Conducting polyaniline thin films have been deposited by a laser-based vaporization tech... more ABSTRACT Conducting polyaniline thin films have been deposited by a laser-based vaporization technique. The films have been characterized by infrared spectroscopy, UV-Vis spectroscopy, four-point conductivity measurements, thermogravimetric analysis, and fluorescence measurements. In addition, the films have been characterized with respect to their photoconductive response to 532-nm laser light. It is found that the films exhibit persistent photoconductivity and it is proposed that defect base sequences may be responsible for the charge localization that results in such a photoconductive response.

Research paper thumbnail of Transient photocurrent measurements in conjungated polymers

Programa e Livro de Resumos, 2004

Research paper thumbnail of Charge carrier mobility investigations in molecularly doped poly(9-vinyl carbazole)

Programa e Livro de Resumos, 2004

Research paper thumbnail of Transporte dispersivo em polianilinas dopadas

Research paper thumbnail of Conductivity of random GaAs/AlGaAs superlattices in regime of controlled disorder

Research paper thumbnail of Synthesis, ferroelectric and optical properties of (Pb,Ca)TiO3 thin films by soft solution processing

Journal of Sol-Gel Science and Technology, 2003

Calcium modified lead titanate sol was synthesized using a soft solution processing, the so-calle... more 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. Pb 0.7 Ca 0.3 TiO 3 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 600 • C for 2 h. The remanent polarization (2P r) and coercive field (E c) were 32 µC/cm 2 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 600 • C, 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.

Research paper thumbnail of Time-of-Flight Technique Limits of Applicability for Thin-Films of Π-Conjugated Polymers

MRS Proceedings, 2012

ABSTRACTTime of flight (ToF) is the most straightforward technique to determine polymeric semicon... more ABSTRACTTime of flight (ToF) is the most straightforward technique to determine polymeric semiconductor mobility for electronic applications. We demonstrate ToF limits of applicability to amorphous PPV derivatives, such as poly[2-methoxy-5-(3’,7’-dimethylloctyloxy)-1-4-phenylene vinylene] (MDMO-PPV) and poly[2-methoxy-5-(2’-ethylhexyloxy)-1-4-phenylene vinylene] (MEH-PPV), and polycrystalline poly(3-hexylthiophene) (P3HT). Hole and electron mobility (μ) in submicrometric films (200 – 500 nm) is overestimated compared to casted layers, due to reduced absorption capability, which is confirmed by Charge Extraction by Linearly Increasing Voltage (CELIV) measurements. Charge transport properties in nanometric films, such as for Field-Effect Transistors (FET), can not be studied by current-mode ToF. Hole mobility of ca. 10-5 cm2/Vs with Poole-Frenkel behavior for PPV derivatives and 10-3 cm2/Vs for P3HT is at least one order of magnitude higher than ToF results.

Research paper thumbnail of Photogenerated carrier profile determination in polymeric light-emitting diodes by steady-state and transient photocurrent measurements

Materials Science and Engineering: B, 2006

Photogenerated charge carrier profiles in poly(2-methoxy-5-(2-ethyl-hexyloxy)-1,4-phenylene vinyl... more 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 −0.6 V 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.

Research paper thumbnail of Transient photocurrents and steady-state conductivities in surface-doped polyanilines

Synthetic Metals, 1999

Steady-state dark currents and transient photocurrents were measured in polyaniline (PANl) and po... more Steady-state dark currents and transient photocurrents were measured in polyaniline (PANl) and poly(o-methoxyaniline) (POG), asymmetrically protonated on one surface with various acids. In both polymers a rectification behaviour was found, indicating that the doped surfaces of the samples act as contacts injecting holes into the undoped bulk. Well defied transit times were measured in POMA: ambient-temperature hole mobility, estimated E-om the experiments, amounts to ca. 3x1u4 cm2Ns. Shapes of transient photocurrent-time dependences in PANl were featureless and decaying monoexponentially to their steady-state values, with the time constants of the order of 1 O-2s.

Research paper thumbnail of One-dimensional character of miniband transport in doped GaAs/AlAs superlattices

Physical Review B, 1997

... These effects will disturb the co-herence between electron wave functions in neighbor quan-tu... more ... These effects will disturb the co-herence between electron wave functions in neighbor quan-tum wells enough to cause disordered spatial localization ... Apparently, such a vertical transport will have 1D char-acter when the lateral width of the conducting channels is smaller than ...

Research paper thumbnail of Electron time-of-flight measurements in sulfur interpreted via an extra surface mobility channel

Physical Review B, 1988

Time-of-flight measurements were carried out in orthorhombic sulfur for various fields, ranging f... more Time-of-flight measurements were carried out in orthorhombic sulfur for various fields, ranging from-2 to-20 kV/cm. No dependence of the mobility with the electric field was found but the current, normalized by the initial current, showed an electric field dependence at small times, decaying faster for larger electric field. After the failure of the usual models in explaining the resultsincluding the assumption of depth-dependent density of trapsa model assuming an extra mobility channel near the surface provided a reasonable set of parameters independent of the electric field. The measurements were carried out at 8.5, 29, 53, 68, and 79'C.

Research paper thumbnail of Transition from dispersive to non-dispersive transport of holes in poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene) light-emitting diodes investigated by time of flight measurements

Thin Solid Films, 2007

The transport properties (conductivity and mobility) of holes and electrons in poly(2-methoxy-5-(... more 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.

Research paper thumbnail of Carrier confinement in an ultrathin barrier GaAs/AlAs superlattice probed by capacitance–voltage measurements

Physica E: Low-dimensional Systems and Nanostructures, 2002

The capacitance–voltage characteristics of (GaAs)m/(AlAs)n superlattices and of a GaAs/AlGaAs mul... more 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.

Research paper thumbnail of Effects of Annealing on Electrical Coupling in a Multilayer InAs/GaAs Quantum Dots System

Japanese Journal of Applied Physics, 2001

We present a systematic investigation of the capacitance–voltage measurements and Raman scatterin... more 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°C altered the plane of the dots into layers of an In x Ga1-x As alloy.

Research paper thumbnail of Capacitance-voltage characteristics of InAs dots: a simple model

Brazilian Journal of Physics, 2002

Research paper thumbnail of Study of charge carriers mobility in poly (9, 9-dioctylfluorenyl-2, 7-diy) capped with N, N-Bis (4-methylphenyl)-4-aniline (PFO)

Research paper thumbnail of Effect of photogenerated holes on capacitance-voltage measurements in InAs/GaAs self-assembled quantum dots

Research paper thumbnail of Transport of holes in uniformly and non-uniformly protonated poly(o-methoxyaniline)

Chemical Physics Letters, 1997

ABSTRACT Steady-state and transient currents were measured in bulk- and surface-protonated poly(o... more ABSTRACT Steady-state and transient currents were measured in bulk- and surface-protonated poly(o-methoxyaniline) emeraldine. The results obtained demonstrate that in surface-doped samples the protonated surface acts as a contact injecting holes into the bulk. The hole mobilities estimated from time-of-flight experiments performed on such samples amounts to 3 × 10−4 cm2/V s at ambient temperature. In uniformly protonated samples, we observed a transient decrease in the currents following the light pulse, recovering exponentially with a time constant equal to ca. 17 ms. Such a behaviour (‘negative photoconductivity’) was attributed to the production of long-lived excited species acting as localization centres.