Rodrigo Martins

Results are presented which are geared towards an understanding of the influence of powder formation during film growth. Plasma chemistry is correlated with the morphology, structure (inferred through infrared spectroscopy, scanning electron microscopy and X-ray diffraction) electro-optical and density of states of intrinsic films deposited under continuous and power modulated operation. Results show that for modulation frequencies where no powder formation occurs and low substrate temperatures T (150°C), silane decomposition gives rise to the growth of inhomogeneous films while in the high modulation frequency regime, at the same temperature, the anions and powder are trapped resulting in films with high deposition rates and low defect density.

The special Professor Walter E. Spear commemoration issue of the Philosophical Magazine, published in October 2009, contains papers which cover the numerous relevant issues, driven by commercial applications, primarily solar energy and displays. Kocka reviews the complex microstructure of crystallites embedded in the amorphous silicon tissue, the transport mechanism is determined by the conductive grains and influenced by the passivation through H at the grain boundaries. Hugger and co-researchers report on transient photocapacitance spectroscopy and drive-level capacitance profiling as a way of elucidating the fundamental electronic properties of hydrogenated ncSi. Tawada recounts the history of a-Si:H pin heterojunction solar cells, emphasizing the role of the p-type silicon carbide layer in the improvement of the device. Schubert and colleagues focus their work on the production of flexible PV modules with many applications in the architectural arena or integrated into clothing.

We present a set of high-efficiency optical sensors for the spectral range from 0.25 to 1.1 μm based on metal oxide-semiconductor heterostructures using different substrates: GaP, GaSe, AlxGa1 - xAs, GaAs and Si. A set of several transparent conductive metal oxide films such as indium, tin and zinc oxides fabricated by the spray pyrolysis method and its doping procedure has been investigated. The results show that heavily doped indium and tin oxide films are preferable as the active transparent conductive electrode in heterojunction surface-barrier structures. The fabricated sensors exhibit several features such as process simplicity, high quantum efficiency, uniformity of sensitivity over the active area and a high response speed. Such sensors can be used for precision measurements in different scientific and technical applications. © 1998 Elsevier Science S.A. All rights reserved.

A new high quantum efficiency gallium phosphide Schottky photodiode has been developed by spray deposition of heavily doped tin oxide films on n-type epitaxial structures, as an alternative to the conventional Schottky photodiodes using a semitransparent gold electrode. It is shown that fluorine-doped tin oxide films are more effective as transparent electrodes than tin-doped indium oxide films. The proposed photodiodes have a typical responsivity near 0.33 A W-1 at 440 nm and an unbiased internal quantum efficiency close to 100%, in the range from 250 to 450 nm. The model used to calculate the internal quantum efficiency (based on the optical constants of tin oxide films and gallium phosphide epitaxial layers) is found to be in good agreement with the experimental results. The data show that the quantum efficiency is strongly dependent on the thickness of the transparent electrode, owing to optical interference effects. The noise equivalent power for 440 nm is 2.7 × 10-15 W Hz-1/2, which indicates that these photodiodes can be used for accurate measurements in the short-wavelength range, even in the presence of stronger infrared background radiation.

We have developed new types of functional and smart optical silicon sensors, based on ITO/multichannel insulator/silicon structures, which are able to execute electronic functions such as amplifying the photocurrent (without avalanche multiplication), transforming the input optical signal into a radio frequency output signal and transforming the analogue input optical signal to a digital output form, without external active electronic components. These new functional optical sensors allow as substantial simplification of the registration of optical signals as well as of the electronic scheme to be used.

This work reports the structure and electro-optical characteristics of different metal oxide films obtained by spray pyrolysis on heated glass substrates, aiming their application in optoelectronic devices. The results show that this technique leads to thin films with properties ranging from dielectric to degenerate semiconductors, offering the following advantages: simplicity, low cost, high productivity and the possibility of covering large areas, highly important for large area device applications.

We present the modelling of a new two-terminal and low-voltage operating optoelectronic device based on MIS silicon structure with multichannel insulator and having as gate a transparent metallic tin-doped indium oxide (ITO) layer deposited by spray pyrolysis technique over the insulator layer. ITO layer has a multiple non-rectifier electrical contact with silicon substrate, in the SiO2 channel's region. Construction details of the process, together with its operating characteristics are given. The devices developed do not require external active electronic components (transistors, microschemes) to execute their functions and to transform analogue input optical signals to digital output form, highly important for a wide range of optoelectronic applications.

High quantum efficiency, UV-enhanced monocrystalline zinc sulphide optical sensors for precise radiometric and spectroscopic measurements have been developed by spray deposition of heavy fluorinedoped tin oxide thin films with carrier concentration near 1021 cm-3 onto the surface of zinc sulphide monocrystals as an alternative to the UV-enhanced silicon photodetectors as well as to new detectors based on SiC and GaN. The fabricated sensors have an unbiased internal quantum efficiency that was nearly 100% from 250 to 320 nm, and the typical sensitivity at 290 nm is 0.15 A/W. The sensors were insensitive to solar radiation in earth's conditions and can be used as solar blind photodetectors for precision UV-measurements under direct solar illumination, both terrestrial and space applications.

In this paper we report the success in fabricating FTO/Si surface-barrier photodiodes produced by spray pyrolysis deposition technique, under ambient conditions. Three types of photodetectors for low-voltage-bias operation were developed based on high-resistivity silicon: 1. X-Ray detectors with energy resolution of 16.5% at 661.5 keV (137Cs source), consisting of surface-barrier PIN photodiode with an active area of 50 mm2 operating at 5 V reverse bias, scintillator based on monocrystalline Bi4Ge3O12 and preamplifier (noise of 250 e- RMS.); 2. Fast-response surface-barrier FTO/n–n+ silicon epitaxial photodiodes, operating at 10 V bias with rise times of 2 ns at λ = 0.85 μm; 3. Radiation-resistant drift epitaxial surface-barrier PIN photodiodes for unbiased operating conditions, with an exponential impurity distribution in a 8 μm thick epitaxial layer. A built-in electrical field due to the carrier concentration distribution in the epitaxial layer provides a considerable improvement in the `critical fluence' value (3×1014 cm-2) for neutron irradiation.

UV-enhanced monocrystalline zinc sulphide optical sensors with high quantum efficiency have been developed by spray deposition of heavy fluorine-doped tin oxide (FTO) thin films onto the surface of zinc sulphide monocrystals as an alternative to the UV-enhanced high-efficiency silicon photodetectors commonly used in precise radiometric and spectroscopic measurements as well as to new sensors based on SiC and GaN. The fabricated sensors have an unbiased internal quantum efficiency that is nearly 100% from 250 to 320 nm, and the typical sensitivity at 250 nm is 0.15 A W-1. The sensors are insensitive to solar radiation in conditions on the earth and can be used as solar blind photodetectors for precision UV measurements under direct solar illumination for both terrestrial and space applications.

We present a set of high-efficiency optical sensors for the spectral range from 0.25 to 1.1 μm based on metal oxide-semiconductor heterostructures using different substrates: GaP, GaSe, AlxGa1-xAs, GaAs and Si. A set of several transparent conductive metal oxide films such as indium, tin and zinc oxides fabricated by the spray pyrolysis method and its doping procedure has been investigated. The results show that heavily doped indium and tin oxide films are preferable as the active transparent conductive electrode in heterojunction surface-barrier structures. The fabricated sensors exhibit several features such as process simplicity, high quantum efficiency, uniformity of sensitivity over the active area and a high response speed. Such sensors can be used for precision measurements in different scientific and technical applications.

We have developed new types of functional and smart optical silicon sensors, based on ITO/multichannel insulator/silicon structures, which are able to execute electronic functions such as amplifying the photocurrent (without avalanche multiplication), transforming the input optical signal into a radio frequency output signal and transforming the analogue input optical signal to a digital output form, without external active electronic components. These new functional optical sensors allow a substantial simplification of the registration of optical signals as well as of the electronic scheme to be used. © 1998 Elsevier Science S.A. All rights reserved.

This work's aim is to report for the first time the cubic to hexagonal phase transition in tin-doped In2O3 films with a Sn/In atomic ratio of 0.03, fabricated at low temperature and normal pressure from alcoholic solution of InCl3 and SnCl4. The performed X-ray diffraction measurements show a difference between crystallographic symmetry of thin (100 nm) and thick (400 nm) films prepared in the same conditions: the structure of thick films can be related to high pressure In2O3 hexagonal system with a preferred orientation of c-axis parallel to the substrate surface, while thin films present a cubic symmetry with columnar (400) grain orientation. Phase transition nature is connected with non-axial tensile deformation of indium oxide grid due to insertion of chlorine ions in the position of two diagonally opposite oxygen vacancies in In2O3 network.

We report the properties and optoelectronic applications of transparent and conductive indium and tin oxide films prepared by the spray pyrolysis method and doped with Sn or F, respectively. The film properties have been measured using X-ray diffraction, optical and electrical absorption. As examples of applications we produced a set of selective optical detectors for different spectral regions, covering the wavelength range from 0.25 to 1.1 μm, based on metal oxide-semiconductor heterostructures and using different substrates such as: GaP, GaSe, AlxGa1-xAs, GaAs and Si. The fabricated devices exhibit several features such as: production simplicity, high quantum efficiency, uniform sensitivity over the entire active area and a high response speed. Finally, we present a high quantum efficiency and solar blind monocrystalline zinc sulphide optical sensor fabricated by spray deposition as an alternative to the ultraviolet-enhanced SiC and GaN photodetectors and the performances of a solar cell. © 1998 Elsevier Science B.V. All rights reserved.

UV-enhanced monocrystalline zinc sulphide optical sensors with high quantum efficiency have been developed by spray deposition of heavy fluorine-doped tin oxide (FTO) thin films onto the surface of zinc sulphide monocrystals as an alternative to the UV-enhanced high-efficiency silicon photodetectors commonly used in precise radiometric and spectroscopic measurements as well as to new sensors based on SiC and GaN. The fabricated sensors have an unbiased internal quantum efficiency that is nearly 100% from 250 to 320 nm, and the typical sensitivity at 250 nm is 0.15 A W-1. The sensors are insensitive to solar radiation in conditions on the earth and can be used as solar blind photodetectors for precision UV measurements under direct solar illumination for both terrestrial and space applications. © 1998 Elsevier Science S.A. All rights reserved.

Electrochemically active bacteria (EAB) have the capability to transfer electrons to cell exterior, a feature that is currently explored for important applications in bioremediation and biotechnology fields. However, the number of isolated and characterized EAB species is still very limited regarding their abundance in nature. Colorimetric detection has emerged recently as an attractive mean for fast identification and characterization of analytes based on the use of electrochromic materials. In this work, WO 3 nanoparticles were synthesized by microwave assisted hydrothermal synthesis and used to impregnate non-treated regular office paper substrates. This allowed the production of a paper-based colorimetric sensor able to detect EAB in a simple, rapid, reliable, inexpensive and eco-friendly method. The developed platform was then tested with Geobacter sulfurreducens, as a proof of concept. G. sulfurreducens cells were detected at latent phase with an RGB ratio of 1.10 ± 0.04, and a response time of two hours.

Here we report the performance of a selective floating gate (V GS) n-type non-volatile memory paper field-effect transistor. The paper dielectric exhibits a spontaneous polarization of about 1 mCm-2 and GIZO and IZO amorphous oxides are used respectively as the channel and the gate layers. The drain and source regions are based in continuous conductive thin films that promote the integration of fibres coated with the active semiconductor. The floating memory transistor writes, reads and erases the stored information with retention times above 14500 h, and is selective (for VGS > 5 ± 0.1 V). That is, to erase stored information a symmetric pulse to the one used to write must be utilized, allowing to store in the same space different information. © 2009 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.

This paper deals with the determination of plasma impedance and ion density in r.f. plasmas using different mixtures of silane with methane or ethylene and r.f. powers. The aim is to correlate these parameters with carbon and hydrogen contents of the films produced. The data achieved show that the best carbon incorporation is achieved using ethylene gas mixtures, under low gas mixture concentration, where the substrate also sustains a low ion bombardment. The data also show that particulates in the plasma can be more easily formed in the ethylene-based processes. © 2001 Published by Elsevier Science B.V.

The aim of this work is to present a model able to explain the role of the lateral drift current on the experimental behaviour exhibited by p-i-n amorphous silicon solar cells (J-V characteristics, responsivity and the apparent device degradation behaviour), when the ratio between the covered and uncovered metal collected areas of the device is higher than one or recrystallization occurs in the edges of the p-i-n junction.

The aim of this work is to provide the basis for the interpretation of the lateral photoeffect in p-i-n a-Si:H one-dimensional thin-film position-sensitive detectors (1D TFPSDs) under steady state, through an analytical model. The experimental data recorded in 1D TFPSD devices with different characteristics are compared with the predicted curves and the obtained correlations are discussed. © 1996.

This new production technique is based on the growth of a-Si films on a reactor where gas decomposition promoted by a capacitively coupled r. f. power system takes place in a chamber separated from that where amorphous films are deposited under the action of an electromagnetic static field. Using this method, we shall reduce films contamination caused by the residual gas desorbed from reactor walls. At the same time, there is a reduction plasma ion and electron damages on the deposited films. The main species impinging upon our substrates will be mainly composed of long life radicals with high mobilities and high diffusion rates, which will give origin to a random silicon network free of long poly-silane chains.

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.

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.

In this paper we present results concerning the performance exhibited by an integrated array of 32 one-dimensional amorphous silicon thin film position sensitive detectors based on nip and hetero amorphous silicon structures, with a total active area size below 1 cm2 linearity, its spatial resolution and response time, that make it one of the most interesting analog detector to be used in unmanned optical inspection control systems where a continuous detection process is required. This opens a wide range of applications for amorphous silicon devices in the area of image processing. © 2002 Elsevier Science B.V. All rights reserved.