Rodrigo Martins

The effect of substrate temperature (Ts) on the properties of pyrolytically deposited nitrogen (N) doped zinc oxide (ZnO) thin films was investigated. The Ts was varied from 300 °C to 500 °C, with a step of 50 °C. The positive sign of Hall coefficient confirmed the p-type conductivity in the films deposited at 450 °C and 500 °C. X-ray diffraction studies confirmed the ZnO structure with a dominant peak from (1 0 0) crystal plane, irrespective of the variation in Ts. The presence of N in the ZnO structure was evidenced through X-ray photoelectron spectroscopy (XPS) analysis. The obtained high N concentration reveals that the 450 °C is the optimal Ts. Atomic force microscope (AFM) analysis showed that the surface roughness was increased with the increasing Ts until 400 °C but then decreased. It is found that the transmittance of the deposited films is increased with the increasing Ts. The optical band gap calculated from the absorption edge showed that the films deposited with T s of 300 °C and 350 °C possess higher values than those deposited at higher Ts. © 2012 Elsevier B.V.

In this work, the relation between oxidant/monomer ratio and the electrical conductivity of polypyrrole was studied using different ratios. We achieved a maximum value for electrical conductivity of 7.5 S/cm for a ratio of 2:1. We also developed a chemical dip-coating process to produce the cathode layer in tantalum capacitors. We obtained capacitances of about 80 μF after 8 cycles using the sequence Monomer/Oxidant.

The need to improve the efficiency of the European university system is discussed. It is considered possible to increase university funding by letting students pay for their education. It is suggested that European universities raise more money for research from private sources by selling services. It is found appropriate to strive for excellence at the level of specific departments or schools to begin with.

It is shown that the electronic properties of amorphous germanium (a-Ge) prepared by the glow-discharge decomposition of germane can be controlled systematically by substitutional phosphorus doping from the gas phase. Specimens with different doping levels have' been investigated by conductivity, thermoelectric power and Hall effect measurements in a temperature range between 160 and 450 K. The dependence of conductivity on doping level is qualitatively similar to that in a-Si, but the range of control is limited in a-Ge by the smaller mobility gap. Also the larger overall density of gap states in this material reduces the doping sensitivity. The above transport measurements and their temperature dependence can be interpreted in a quantitatively consistent manner- by a two-path model in which conduction takes place in the extended states and in another path through the localized states. As 111 a-Si, the photoconductivity of glow-discharge Ge can be appreciably sensitized by phosphorus doping. The μτ product deduced from such experiments on a-Ge and a-Si are compared for different preparation techniques. The data show that irrespective of the presence of hydrogen the method of deposition remains an important factor in determining the density of gap states. © 1979 Taylor & Francis Ltd.

Experimental results on structure defects in microcrystalline (μc) n- and p-doped μc-S1-x:Cx:H films deposited on alkali-free glass substrates by spatial plasma separation1 and obtained by electron paramagnetic resonance (EPR) are presented. The technique used for subtracting the substrate effect on recorded spectra is also discussed as well as its quantification. The microscopic structure of intrinsic defects and impurity states and their role in transport mechanisms are studied and correlated with the composition of their films. These results are also related to transport properties of deposited films in order to observe the role of dopant centres, located at conduction band tails, in controlling the electrical properties. © 1989.

Where a-Si:H pin devices are concerned, one of the main obstacles regarding improved performance is device stability, usually attributed to adverse behaviour at various interfaces within the device. Several attempts have been made to overcome this problem, such as the use of blocking layers at the interfaces. Although these have led to some improvements in device performance, most of the problems associated with device stability remain. This is mainly due to the defects at the interfaces, since the blocking layers (silicon alloys with carbon, nitrogen or oxygen) usually have a high density of bulk states, in comparison to intrinsic a-Si:H films. In this paper, we present a method that seems to be capable of improving device stability. It consists of performing a controlled removal of oxide interlayers at the interfaces, by an appropriate etching process. This enables the production of highly smoothed interfaces, and reduces possible cross-contamination of the i-layer from the adjacent doped layers. This amounts to a new design of typical pin devices, in which thin absorber layers are placed at the p/i and i/n interfaces. Their purpose is to trap most of the impurity atoms diffused from the doped layers, after which they are removed by appropriate etching. The fabrication of the absorbers (sacrificial layers), the nature of the etching and the tailoring of the defect profile at the interfaces will be discussed, including the performance exhibited by the resulting devices. © 2002 Elsevier Science B.V. All rights reserved.

The electron transport in n-type polycrystalline zinc oxide, nanocrystalline Zinc-Gallium-Oxygen and amorphous Indium-Zinc-Oxygen systems produced by rf magnetron sputtering at room temperature, under different oxygen partial pressure were investigated. It was found that the carrier transport is not band tail limited, being governed by metal cations irrespective to the film's structure. The highest net room temperature electron mobility was achieved on the amorphous films and noticed that for the single component oxides the mobility decreases as the carrier concentration increases, while the reverse behaviour was observed for the multicomponent oxides, independently of their structure. These behaviours are related to the role that negative charge defects in excess of 1010 cm- 2 generated on multicomponent oxides have on carriers scattering and so on their electronic performances. © 2007 Elsevier B.V. All rights reserved.

The influence of U.V. light on the transport properties of a-Si : H films during its growth in a r.f. double chamber system was investigated by conductivity, optical absorption, I.R. absorption, spectral photoconductivity and X-ray diffraction measurements. It was concluded that the presence of U.V. light during the deposition process controls the way how hydrogen is incorporated in the structure as well as the impurity atoms. The microcrystalline films investigated present sharp peaks in the I.R. spectra. Both boron and phosphorus doped films show conductivities higher than 10 S cm-1 and estimated crystalline sizes of the order of 80 Å. © 1987.

One of the main problems in producing large area amorphous silicon devices concerns films uniformity. In this paper we present data concerning the role of reactor geometry and design and on the film performances as well as the problems related to mechanical mismatches in scaling up the reactor size. © 1991 Elsevier Science Publishers B.V. All rights reserved.

Today there is a strong interest in the scientific and industrial community concerning the use of biopolymers for electronic applications, driven mainly by low-cost and disposable applications. Adding to this interest, we must recognise the importance of the dream of wireless auto-sustained and low-energy-consumption electronics. This dream can be fulfilled by cellulose paper, the lightest and the cheapest known substrate material, as well as the Earth's major biopolymer and of tremendous global economic importance. Most of the paper used up to now is optimised in terms of the required mechanical and physical properties to be used as the support of inks of different origins. In the future, specific electronic heterogeneous paper sheets should be fabricated aiming to get paper fibers with required bulk and surface functionalities, proper water/vapour barrier, size and diameter/thickness of the fibrils and full paper thickness. This will be the function of components/devices to be incorporated/integrated such as thin-film transistors, complementary metal oxide semiconductor devices, passive electronic components (resistances, inductors and capacitors), memory transistors, electrochromics and thin-film paper batteries. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

This paper discusses the electron transport and the optical characteristics of amorphous indium zinc oxide and the role of the oxygen partial pressure on tailoring its properties. The data show that by varying the oxygen partial pressure during the deposition process from 10-3 to 2 × 10-1 Pa, the electrical resistivity varies from about 10-4 to 2 × 101 Ω cm, which corresponds to a variation on the Hall mobility from 60 to 10 cm2 V-1 s-1. The conductivity and mobility analysis show that the transport of carriers is not band tail limited, as happens in conventional disordered semiconductors, but highly dependent on the ionicity and the presence of oxygen vacancies, where mobility is mainly limited by carrier scattering. The optical characteristics inferred from the transmittance data reveal films with optical gaps in the range of 3.68-3.76 eV, very close to the ones observed on crystalline/polycrystalline IZO films (3.7-3.9 eV). © 2006 Elsevier B.V. All rights reserved.

The electrosynthesis of polypyrrole (PPy) has been achieved on aluminium in aqueous medium of tartaric acid by means of cyclic voltammetry, potentiostatic and galvanostatic techniques. Scanning electron microscopy (SEM) and X-ray microanalysis by energy spectroscopy dispersion (EDS) applying on surfaces show that the PPy coating is developed from the metal surface through the cracks of the initial Al2O3 layer. A mechanism involving the participation of the supporting electrolyte and the pyrrole (Py) in distinct active sites was proposed based on the linear sweep voltammetry. It is observed for all applied electrochemical techniques that the pyrrole concentration has to be higher than 0.1 M to allow the polypyrrole electrodeposition in acid medium. Scanning electron microscopy, secondary electrons (SE) and backscattering electrons (BE), shows that the PPy coating obtained in galvanostatic and potentiostatic modes starts with small islands at weak applied potentials or current densities. Moreover, EDS reveals a good homogeneity and compactness of the film achieved in galvanostatic method. The corrosion results in 3% NaCl medium show that the PPy coating decreases the corrosion behaviour of the aluminium. The bilayer Al2O3/PPy shows a capacitor with future applications. © 2006 Elsevier Ltd. All rights reserved.

The aim of this study was to investigate the influence of ultrasound and high-shear mixing on the properties of microparticles obtained by spray congealing. Dispersions containing 10% carbamazepine and 90% carrier Gelucire® 50/13 (w/w) were prepared using magnetic stirring, high-shear mixing, or ultrasound. Each preparation was made using hot-melt mixing spray congealing to obtain the microparticles. All microparticles presented a spherical shape with high encapsulation efficiency (>99%). High-shear mixing and ultrasound promoted a decrease in the size of microparticles (D90) to 62.8 ± 4.1 μm and 64.9 ± 3.3 μm, respectively, while magnetic stirring produced microparticles with a size of 84.1 ± 1.4 μm. The use of ultrasound led to microparticles with increased moisture content as identified through sorption isotherm studies. In addition, rheograms showed distinct rheological behaviour among different dispersion preparations. Therefore, the technique used to prepare dispersions for spray congealing can affect specific characteristics of the microparticles and should be controlled during the preparation. © 2013 Informa UK Ltd. All rights reserved.

This paper deals with the engineering aspects related to the rf energy coupling in Plasma Enhanced Chemical Vapour Deposition (PECVD) processes, in a diode-type unit in which an extra grid is used. The main emphasis is given in the determination of the real power delivered to the gas and comparing it with the total power losses, besides determining the best way to control the powder formed during the process. © 1994.

In this paper, we report the environmental, optical, and gate bias stress stability of amorphous zinc-tin-oxide (ZTO) thin-film transistors (TFTs) fabricated by sol-gel spin-coating method. The ZTO TFTs showed excellent environmental and optical stability. The threshold voltage stability of ZTO TFTs was sensitive to both positive and negative gate bias stress. Maximum threshold voltage shifting of +1.9 and -3.2 V was observed under a gate bias stress of +10 and -10 V, respectively, with no significant change to subthreshold swing value. © 2006 IEEE.

In this work we studied the influence of the dopant elements and concentration on the properties of ZnO thin film deposited by spray pyrolysis. The results show that the doping affects the thin films properties mainly the electrical ones, function of dopant concentration and nature. The most important changes were observed for films doped with 1at% of indium which exhibit a resistivity of 1.9 × 10-1 Ωcm associated with a transmitance of 90%. After the annealing treatment, the resistivity of the film decreases to 5.9 × 10-3 Ωcm without significative changes in the optical properties. The films were also used to produce amorphous silicon solar cells where the best results were obtained for ZnO:In. © 2002 Elsevier Science Ltd. All rights reserved.

The influence of doping on the properties of zinc oxide thin films deposited by spray pyrolysis has been studied. The results show that the doping affects the properties of the films, mainly the electrical ones, function of its concentration and nature. The most significative improvement were observed for films doped with 1 at.% of indium exhibiting a resistivity of 1.9 × 10-1 Ω m associated to a transmittance of 86%, characteristics required for applications on the optoelectronic devices. © 2001 Elsevier Science Ltd. All rights reserved.

In this work we present the results of a study concerning the role of the properties of zinc oxide thin films deposited by spray pyrolysis on its sensitivity to methane or ethane. It was found that using highly resistive thin layers leads to an increase in the film sensitivity. The variation of the operating temperature of the film leads to a significant change in the sensitivity of the sensor and the ideal operating temperature dependence of the gas used. The sensitivity of the ZnO thin films changes linear with the increase of the gas concentration. However these films seem to be more appropriated for the detection of methane than for ethane since the value of sensitivity obtained are higher and its variation with the gas concentration more pronounced. After the annealing treatment performed the films sensitivity decreases.

The influence of doping on the morphology, physical and electrical properties of zinc oxide produced by the sol-gel method was examined. Undoped zinc oxide was observed to form relatively porous films. Addition of an Al dopant influenced the sheet resistance, but did not result in a change in morphology, examined by atomic force microscopy when compared to undoped films. In the case of electrical measurements, undoped ZnO films were extremely resistive. A minimum dopant concentration of 2 at.%. Al was required to produce materials which were more conductive, as observed by sheet resistance measurements, which were shown to vary with annealing temperature. The versatile nature of sol-gel processing was demonstrated by selective ink-jet deposition of sol-gel droplets which were annealed to form oxide materials. © 2011 Elsevier B.V. All rights reserved.

The high visible-near infrared transparent and high carrier mobility (μ) Mo doped (0.5 at%) indium oxide (IMO) films were deposited by the spray pyrolysis technique. The deposited films were irradiated by 50MeV Li 3+ ions with different fluences of 1×1011, 1×1012 and 1×1013 ions cm-2. X-ray diffraction analysis confirmed the cubic bixbyite structure of indium oxide. A fascinating feature is that the ion irradiation process has introduced a fraction of the molybdenum oxide phase. The μ of as-deposited IMO films is decreased from ̃122.4 to 93.3 cm2 V-1 s-1, following the ion irradiation. The theoretically calculated μ and carrier density values were correlated with those measured experimentally. The transport mechanism has been analysed based on the ionized and neutral impurity scattering centres. The average transmittance (400-2500 nm) of the as-deposited IMO films is decreased from 83% to 60% following irradiation. © 2011 IOP Publishing Ltd.

Shape memory alloys (SMA) represents a class of metallic materials that has the capability of recovering a previously defined initial shape when subject to an adequate thermomechanical treatment. The present work aims to study the influence of thermal cycles on the transition temperatures of a Ni-Ti alloy. In this system, small variations around the equiatomic composition give rise to significant transformation temperature variations ranging from 173 to 373 K. SMA usually presents the shape memory effect after an annealing treatment at ca. 973 K. The optimisation of the thermomechanical treatment will allow to "tune" the material to different transformation temperature ranges from the same starting material, just by changing the processing conditions. Differential scanning calorimeter (DSC) and in situ high-temperature X-ray diffraction (XRD) have been used to identify the transformation temperatures and the phases that are present after different thermal cycles. The results concerning a series of thermal cycles with different heating and cooling rates (from 1.67×10-2 to 1.25×10-1 K/s) and different holding temperatures (from 473 to 1033 K) are presented. © 2004 Elsevier B.V. All rights reserved.

In this work, HfO2 was deposited by r.f. sputtering at room temperature and then annealed for different times at 200°C in a forming gas atmosphere. After annealing for 2 hours the HfO2 layers present a reduction on the flat band voltage of about 1 V, relatively to the as deposited film, decreasing from -2.23V down to -1.28 V. This means an improvement of the interface properties and a reduction on the oxide charge density from 1.33×1012 cm-2 to 7.62×1011 cm -2. The dielectric constant reaches a maximum of 18.3 after 5h annealing due to film's densification. When annealing for longer times such as 10h a small degradation of the electrical properties is observed. After 10h annealing the dielectric constant, flat band voltage and fixed charge density are respectively, 14.9, -2.96 V and 1.64×1012 cm-2 and the leakage current also increases due to film's crystallization.

Doped and undoped a-Si:H and a-SiC:H films were deposited by R. F. decomposition of silane and silane/methane mixtures respectively, in a two consecutive (decomposition and deposition) chambers glow discharge capacitively coupled system. Their electro-optical properties were extensively investigated through dark conductivity, photoconductivity, spectral response, optical absorption, R. F. transmission spectra, electron spin ressonance and CxV MOS measurements.

Abstract: The present work reports the synthesis of zinc oxide (ZnO) nanoparticles with hexagonal wurtzite structure considering a solvothermal method assisted by microwave radiation and using different solvents: water (H2O), 2-ethoxyethanol (ET) and ethylene glycol (EG). The structural characterization of the produced ZnO nanoparticles has been accessed by scanning electron microscopy, X-ray diffraction, room-temperature photoluminescence and Raman spectroscopies. Different morphologies have been obtained with the solvents tested. Both H2O and ET resulted in rods with high aspect ratio, while EG leads to flower-like structure. The UV absorption spectra showed peaks with an orange shift for synthesis with H2O and ET and blue shift for synthesis with EG. The different synthesized nanostructures were tested for photocatalyst applications, revealing that the ZnO nanoparticles produced with ET degrade faster the molecule used as model dye pollutant, i.e. methylene blue. Graphical Abstract: [Figure not available: see fulltext.] © 2015 Springer Science+Business Media New York

We present the results obtained with an extended-gate ISFET totally based on amorphous oxides (GIZO as the semiconductor, +{\hbox{Ta}}-{2}{\hbox{O}}-{5}{\ hbox{:SiO}}-inf2-inf as the dielectric and +{\hbox{Ta}}-{2}{\hbox{O}}-inf5-inf as the sensitive layer). A full characterization of the device was performed with constant ionic strength pH buffer solutions, revealing a sensitivity of 40 mV/pH with small hysteresis, and good linearity in the pH 4-pH 10 range buffer solutions. These results clearly show that it is possible to produce room-temperature disposable and low cost bio-sensors. © 2005-2012 IEEE.