Papers by Sergio Mergulhão
Brazilian Journal of Physics, 2002
An electrostatic model was presented for the calculation of the capacitance-voltage characteristi... more 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.
A systematic investigation of the properties of the InAs/GaAs self-assembled quantum dots (SAQDs)... more A systematic investigation of the properties of the InAs/GaAs self-assembled quantum dots (SAQDs) system subjected to a post-growth annealing using capacitance-voltage, Raman scattering and photoluminescence measurements is presented. The application of both electrical and optical methods allowed us to obtain reliable information on the microscopic structural evolution of this system. The single layer and the multilayer quantum dots were found to respond differently to the annealing process, due to the differences in strain that occur in both systems. The diffusion activated by strain provoked the appearance of an InGaAs alloy layer in substitution to the quantum dots layers; this change occurred at the annealing temperature T = 600 °C in the multilayer system. A single dot layer, however, was observed even after the annealing at T = 700 °C. Moreover, the low temperature annealing was found to improve the homogeneity of the multilayer system and to decrease the electrical interlayer coupling.
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.
Polymeric semiconductors as poly[2-methoxy, 5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PP... more Polymeric semiconductors as poly[2-methoxy, 5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) have been widely studied in the last years thanks to their electronic and optical properties suitable for applications on opto-electronic devices such as Light-Emitting Diodes (LED) . These materials are well-suited for the fabrication of rigid and flexible ultra-thin displays, characterized by low power consumption and high-quality images [1]. π-conjugated polymers have been widely studied [2, 3], but despite their importance and all the acquired knowledge in the last years, the charge transport mechanisms are still not fully understood. A detailed description of the states and charge carrier mobility requires many efforts due to difficulties found in conventional techniques such as Time-of-Flight (ToF) and due to the small thickness of the films . In this context, our objective is to apply a new and powerful technique, Charge Extraction in a Linearly Increasing Voltage (CELIV), to determine charge carrier mobility in sandwiched structures where the active layer thickness is in the order of hundreds of nanometers. The validity of the results will be demonstrated by comparing them to more conventional techniques, such as Time-of-flight (ToF), Dark Injection in Space Charge Limited Current (DI-SCLC) and Current density versus Voltage (J vs. V) measurements, reaching the minimum thickness where other techniques fail to work. Results from CELIV measurements applied to PLEDs degradation studies by observing changes in carrier mobility and charge injection on samples exposed to air and kept in vacuum will be shown.
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. 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 600°C for 2 h. The remanent polarization (2Pr) and coercive field (E c) were 32 μC/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 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.
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.
Journal of Non-crystalline Solids
Charge transport and shelf-degradation of MEH-PPV thin-films were investigated through stationary... more 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 − 6 cm 2 /Vs under a SCLC regime with deep traps for light-emitting diode applications. ToF measurements performed on micrometric layers (i.e.~3 μm) confirmed studies in 100 nm-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 − 4 cm 2 /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.
The transport properties of holes and electrons in poly (2-methoxy,5-(2-ethyl-hexyloxi)-1-4 pheny... more 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.
Journal of Physics D-applied Physics, 2008
Whispering Gallery modes in bulk cylindrical Gallium Arsenide and Gallium Phosphide samples have ... more Whispering Gallery modes in bulk cylindrical Gallium Arsenide and Gallium Phosphide samples have been examined both in darkness and under white light at cryogenics temperatures ≤ 50 K.
Materials Science and Engineering B-advanced Functional Solid-state Materials, 2006
Photogenerated charge carrier profiles in poly(2-methoxy-5-(2 -ethyl-hexyloxy)-1,4-phenylene viny... 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.
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.
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.
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 results-including the assumption of depth-dependent density of traps-a 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.
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
Physical Review B, 1997
Evidence of the crucial role of random fluctuations of the well size in vertical transport in dop... more 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.
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
Materials Research-ibero-american Journal of Materials, 2004
A systematic investigation of the properties of the InAs/GaAs self-assembled quantum dots (SAQDs)... more A systematic investigation of the properties of the InAs/GaAs self-assembled quantum dots (SAQDs) system subjected to a post-growth annealing using capacitance-voltage, Raman scattering and photoluminescence measurements is presented. The application of both electrical and optical methods allowed us to obtain reliable information on the microscopic structural evolution of this system. The single layer and the multilayer quantum dots were found to respond differently to the annealing process, due to the differences in strain that occur in both systems. The diffusion activated by strain provoked the appearance of an InGaAs alloy layer in substitution to the quantum dots layers; this change occurred at the annealing temperature T = 600 °C in the multilayer system. A single dot layer, however, was observed even after the annealing at T = 700 °C. Moreover, the low temperature annealing was found to improve the homogeneity of the multilayer system and to decrease the electrical interlayer coupling.
Brazilian Journal of Physics, 2002
ABSTRACT An electrostatic model was presented for the calculation of the capacitance-voltage char... more 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, 2002
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Papers by Sergio Mergulhão