Rodrigo Martins

This work analyse mainly the dissociation mechanism of the gas during the hot wire (HW) and hot wire plasma-assisted (HWPA) processes used to produce hydrogenated carbon films and silicon-carbide, using ethylene and silane as gas sources. The results show that ethylene is better decomposed by plasma-enhanced chemical vapour deposition (PECVD) than HW process. The data also show that the HWPA leads to a better carbon © incorporation than HW processes, when silicon carbide alloys are produced and, that the presence of atomic hydrogen (H) is beneficial for all processes. That is, the presence of the plasma and H lead to the formation of higher C radicals such as methylsilane (CH3SiH3), ethylsilane (C2H5SiH3) and silorane (C2H4SiH2), whose contributions are enhanced as the fraction of ethylene (Feth) in the gas mixture increases. © 2001 Published by Elsevier Science B.V.

We have developed large area (up to 80mm×80mm) Thin Film Position Sensitive Detectors (TFPSD) based on hydrogenated amorphous silicon (a-Si:H). Although crystalline silicon PSDs have been realized and applied to optical systems, their detection area is small (less than 10mm×10mm), which implies the need of optical magnification systems for supporting their field of applications towards large area inspection systems, which does not happen by using a-Si:H devices. The key factors for the TFPSDs resolution are the thickness uniformity of the constituting layers, the geometry and the position of the contacts. In this paper we present data on single and dual-axis rectangular TFPSDs correlating, their performances with the different underlying lateral effects. For the single axis-detector, with two opposite extended contacts, the output photocurrent difference to sum ratio is a linear function of the position of a narrow incident light beam, even for low illumination levels (below 20 lux). For the dual-axis detector with extended contacts, at all four sides (except for small gaps at the vertices due to edge effects) an almost linear relation has been found between the incident light spot position along both axis and the corresponding output photocurrents. © 1993.

In this paper a new approach is presented for the simulation of spectroscopic ellipsometry (SE) data to estimate the level of nickel (Ni) contamination in silicon crystallized by metal induced crystallization (MIC). The method employs the addition of Ni as reference for a Bruggemann effective medium approximation (BEMA) to simulate the optical response of the crystallized silicon. This new approach is sensitive to changes in the initial average metal thickness used on the crystallization process to thickness values as low as 0.05 nm. This corresponds to a volume fraction of 0.24%, confirmed by Rutherford backscattering spectrometry (RBS) where it was observed that the Ni volume fraction detected by SE varies linearly with the metal amount inside the crystallized films determined by RBS. © 2008.

In this work metal induced crystallization was studied using aluminium and gold deposited over 150 nm amorphous silicon films grown by LPCVD. Aluminium and gold layers with thickness between 1 and 5 nm were deposited on the silicon films and after that, the samples were annealed at 500°C from 5 up to 30 h. When the crystallization is induced through a gold layer, the Si crystalline fraction is higher than when using aluminium. For samples crystallized for 30 h at 500°C with 2 nm of metal a crystalline fraction of 57.5% was achieved using gold and only 38.7% when using aluminium. © 2005 Springer Science + Business Media, Inc.

Engineering procedures governing the selection or development of printable nanostructured metal oxide nanoparticles for chromic, photovoltaic, photocatalytic, sensing, electrolyte-gated TFTs, and power storage applications are established in this chapter. The main focus is given on how to perform the material selection and formulation of printable dispersion in order to develop functional films for electrochemical applications. This chapter is divided into four main parts. Firstly, a brief introduction on electrochemically active nanocrystalline metal oxide films developed via printing techniques is given. This is followed by the description of the film morphology, structure, and required functionality. A theoretical approach to understand the impact of size and shape of nanoparticles on an ink formulation and electrochemical performance being the subject of the third section provides a greater control over the material selection. We attempt to describe these properties and show that for a given material, geometry and size of the nanoparticles have a major influence on the electrochemical reactivity and response time. This gives the ability to tune the performance of the film simply by varying the morphology of incorporated nanostructures. This section is completed by the recommendations on each major step of an ink formulation, together with imposed critical constraints concerning the fluid control. Finally, the performance of the ink-jetprinted dual-phase electrochromic films is discussed as a case study. By providing such a rather systematic survey, we aim to stress the importance of proper design strategy that would result in both improved physicochemical properties of nanoparticle-loaded inks and enhanced electrochemical performance of printed functional films. © Springer International Publishing Switzerland 2016.

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.

Ferroelectric and high dielectric permittivity films are currently being investigated in view of their use as gate dielectrics in MIS structures. Along with the suppression of tunnelling currents at small gate thickness, they provide a memory function to MIS structures, which can be used in non-volatile memory applications. In this work we report fabrication and characterization of novel metal-ferroelectric-amorphous silicon structures. The structures consist of glass/ITO substrates coated with PZT 20/80 films (sol-gel) followed by an active layer (i-a-SiC:H, deposited by plasma enhanced chemical vapor deposition (PECVD)). A strong capacitance hysteresis is observed in C-V curves in electron accumulation region (VG > 0), accompanied with a large increase in the capacitance of ferroelectric-semiconductor structures at low frequencies. Threshold voltage for electron accumulation is about 10 V being dependent on the ferroelectric polarization switching. © 2002 Elsevier Science B.V. All rights reserved.

The movement of a micro cantilever was detected via a self constructed portable data acquisition prototype system which integrates a linear array of 32 1D amorphous silicon position sensitive detectors (PSD). The system was mounted on a microscope using a metal structure platform and the movement of the 30 μm wide by 400 μm long cantilever was tracked by analyzing the signals acquired by the 32 sensor array electronic readout system and the relevant data algorithm. The obtained results show a linear behavior of the photocurrent relating X and Y movement, with a non-linearity of about 3%, a spatial resolution of less than 2μm along the lateral dimension of the sensor as well as of less than 3μm along the perpendicular dimension of the sensor, when detecting just the micro-cantilever, and a spatial resolution of less than 1μm when detecting the holding structure. © 2010 by the authors.

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.

A glass was made using bottom ash produced by a Portuguese municipal solid waste (MSW) incinerator. The bottom ash was the single batch material used in the formation of the glass, which was obtained through a conventional melt-quenching method. The glass was then converted to glass-ceramic for further recycling to construction materials. After submitting the glass samples to several heat treatments, between 820 and 1050°C and during different times, it was verified that the optimum heat treatment schedule for the ceramization of the glass was at 1000°C for 10h, as confirmed by microstructural observation and by X-ray diffraction. The major crystalline phases precipitated in the glass-ceramic were wollastonite (CaSiO3) and diopside (Ca(Mg,Al)(Si,Al)2O6). Microstructural analysis of the glass-ceramic revealed that the crystalline phases were present as dendrites and fiber-like structures that were homogeneously distributed in the material. The glassceramic showed good mechanical properties with a hardness of 5.6 MPa and a bending strength of 101 MPa. This material had a density of 2.8 gcm-3 and a thermal expansion coefficient of 9.10-6°C-1. The glass and the glass-ceramic showed an excellent chemical stability against leaching in acidic solution and in alkaline solution. In summary, both the glass and the glass-ceramic have good chemical and mechanical properties and can, therefore, be applied as construction materials.

The microstructural aspects related to crystalline or amorphous structure of as-deposited and annealed films of sol-gel-derived WO 3 are shown in the literature to be critical for electrochromic (EC) performance. In consideration of ion insertion materials, there is a need for developing light and at the same time nanocrystalline structures to improve both coloration efficiency and switching kinetics. By controlling microstructure and morphology, one could design a material with optimal EC performance. This report compares the microstructural and morphological characteristics of standard WO 3 wet deposition techniques versus inkjet printing technology (IPT), correlating these features with their optical and electrochemical performances, emphasizing the importance of the dual-phase a-WO 3/TiO 2/WO X film composition proposed in this work for high-performance EC applications. The effect of the type and content of metal oxide nanoparticles in the precursor sols formulated in various peroxopolytungstic acid (PTA) and oxalic acid (OAD) proportions on film properties is comprehensively studied using multi-factorial design of experiment (DOE). To the authors' knowledge, no other report on sol-gel deposition of inorganic EC materials via the inkjet printing technique exists, in which furthermore the film crystallinity can be controlled under low-temperature process conditions. The proposed method enables development of EC films which irrespective of their composition (a-WO 3, a-WO 3/TiO 2 or a-WO 3/TiO 2/WO X) outperform their amorphous or nanocrystalline analogues presented as the state-of-the-art due to their superior chemical and physical properties. © 2012 The Royal Society of Chemistry.

The present work reports the influence of zinc oxide (ZnO) seed layer annealing temperature on structural, optical and electrical properties of ZnO nanorod arrays, synthesized by hydrothermal method assisted by microwave radiation, to be used as UV sensors. The ZnO seed layer was produced using the spin-coating method and several annealing temperatures, ranging from 100 to 500 °C, have been tested. X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM) and spectrophotometry measurements have been used to investigate the structure, morphology, and optical properties variations of the produced ZnO nanorod arrays regarding the seed layer annealing temperatures employed. After the growth of ZnO nanorod arrays, the whole structure was tested as UV sensors, showing an increase in the sensitivity with the increase of seed layer annealing temperature. The UV sensor response of ZnO nanorod arrays produced with the seed layer annealed temperature of 500 °C was 50 times superior to the ones produced with a seed layer annealed at 100 °C. © 2016 by the authors.

This work presents a comparative study between MIS photodiodes produced using high quality amorphous silicon (a-Si:H), deposited by PECVD at 2Å/s, using 13.56 MHz frequency and polymorphous silicon (pm-Si) deposited at 3Å/s using a 27.12 MHz frequency. The results show that the pm-Si:H outperforms the a-Si:H MIS photodiodes by having a rectification ratio of 107, and photosensitivity at AM1.5 conditions of 107, under 1V reverse bias. Apart from that, the pm-Si:H photodiode presents a higher open circuit voltage and better fill factor than a-Si:H MIS photodiode. These results prove that quality devices can be produced at high growth rates by using pm-Si:H. In this work the photodiode performances were correlated to the films properties, aiming to determine the characteristics responsible for the performances exhibited by the pm-Si:H devices.

The devices analyzed in this work present an S-shape J-V characteristic when illuminated. By changing the light flux a non linear dependence of the photocurrent with illumination is observed. Thus a low intensity light beam can be used to probe the local illumination conditions, since a relationship exists between the probe beam photocurrent and the steady state illumination. Numerical simulation studies showed that the origin of this S-shape lies in a reduced electric field across the intrinsic region, which causes an increase in the recombination losses. Based on this, we present a model for the device consisting of a modulated barrier recombination junction in addition to the p-i-n junction. The simulated results are in good agreement with the experimental data. Using the presented model a good estimative of the LSP signal under different illumination conditions can be obtained, thus simplifying the development of applications using the LSP as an image sensor, with advantages over the existing imaging systems in the large area sensor fields with the low cost associated to the amorphous silicon technology. © 2007 Materials Research Society.

Microcrystalline p-i-n silicon devices are a prospective contender for application in large-area optoelectronics. In this paper we analyse the behaviour of a μc-Si:H p-i-n photodevice under non-uniform illumination. The effect of a spatially non-uniform illumination is to create lateral electric fields and current flows inside the structure. We present in this paper a numerical application of a complete bidimensional model describing the transport properties within the structure. The continuity equations forholes and electrons together with Poisson's equation are solved simultaneously along the two directions parallel and perpendicular to the junction. The results of simulating p-i-n μc-Si:H junctions under non-uniform illumination show that the generated lateral effects depend not only in intensity but also in direction on the wavelength of the incident radiation. © 1997 Elsevier Science S.A.

The introduction into a traditional p.i.n structure of two defective buffer layers near the p/i and i/n interfaces can improve the device stability and efficiency through an enhancement of the electric field profile at the interfaces and a reduction of the available recombination bulk centers. The defectous layer ("i′-layer"), grown at a higher power density, present a high density of defects and acts as "gettering centers" able to tailor light induced defects under degradation conditions. If the i-layer density of states remains below 1016 eV-1 cm-3 and assuming a Gaussian distribution of defect states, the gettering center distribution will not affect significantly the carrier population but only its spatial distribution. We report here about a device numerical simulation that allows us to analyse the influence of the "i′-layer" position, thickness and density of states on the a-Si: H solar cells performances. Results of some systematic simulation from the ASCA program (Amorphous Solar Cell Analysis), and for different configurations will be presented. © 1994 Materials Research Society.

Reduction in the accumulation capacitance value was more in Si metal-oxide-semiconductor devices than that of Ge metal-oxide-semiconductor devices after a thermal treatment irrespective of the annealing environment. Relatively, thermal treatment in oxygen environment improves the interface quality of HfO2/Ge stacks considerably, when compared with HfO 2/Si stacks. Whereas, the forming gas annealing at a temperature of 400 °C was not so effective in improving the interface quality at HfO 2/Si stack. The presence of induced negatively charged hydrogen atom in Ge lessens the Fermi level pinning at HfO2 and Ge interface. © 2011 Elsevier B.V. All rights reserved.

Molybdenum doped indium oxide (IMO) thin films rf sputtered at room temperature were studied as a function of oxygen volume percentage (O 2 vol. %) varied between 0 and 17.5. The as-deposited films were amorphous irrespective of O2 vol. %. The minimum transmittance (<10 %) of the films deposited without oxygen has been increased on introducing oxygen (3.5 O2 vol. %) to a maximum of 90 %. The optical band gap has been increased from 3.80 eV (without oxygen) to 3.92 eV (3.5 O 2 vol. %). The films were highly resistive and the hall coefficients were detectable only for the films deposited without oxygen. In order to increase the electrical conductivity, the films were annealed in the range 100-500° C in open-air and N2/H: gas for 1 hour. The annealed films become polycrystalline with enhanced electrical and optical properties. The effect of annealing conditions on these films will be presented and discussed in detail. © 2006 Materials Research Society.

Transparent metal oxides thin films are a class of inorganic conductors and semiconductors with significant importance for use in portable electronics, displays, flexible electronics, multi-functional windows and solar cells. Due to the recent development of transparent and flexible electronics, there is a growing interest in depositing metal-oxide thin-film on plastic substrates that can offer flexibility, lighter weight, and potentially lead to cheaper manufacturing by allowing printing and rollto- roll processing. The plastic substrates, however, limit device processing to below 200oC. In this context, the deposition of high-performance semiconductor thin films from dispersions of pre-prepared oxide nanoparticles at temperatures below 200oC represents a potential key route. This paper reports on the preparation of ZnO transparent thin films using solutionprocessed nanoparticles (NPs) precipitated from zinc acetate alcoholic solution with potassium hydroxide. The nanoparticles size distribution, microstructure and crystallinity were measured by dynamic light scattering (DLS), Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD). The thin films were deposited by spin-coating onto soda lima glass substrate, using a dispersion of 1wt% ZnO NPs. The morphology of the films annealed at 120 and 180oC, observed by atomic force microscopy and cross-section scanning electron microscopy, shows columnar grains with diameter ranging between 20 and 70 nm, depending on the conditions of depositions. Optical measurements indicated high transparency, between 85 and 94 %, in the visible range, a direct nature of band-to-band transitions and band gap values between 3,22 and 3,32 eV. The refractive index and extinction coefficient have been calculated from optical transmittance and reflectance spectra.

Results are presented concerning the morphological and structural characteristics exhibited by the Cu-Sn-Cu system to be used in electronic lead-free soldering processes, under different process temperatures and pressures. The results show that the Cu3Sn or Cu6Sn5 phases needed to supply the thermal, mechanical and electrical stability to the joints formed require Sn layers (either electrodeposited or by using preforms) whose thickness depends on the process temperature used. For process temperatures of 533 K the thickness of the Sn layer should be above 20 μm, while for process temperatures of 573 K, the Sn thickness required is reduced to 10 μm. The joints formed support shear stresses above 12 MPa, as required by electronic standards. Apart from that, microcracks start appearing if an excess of Sn is used during the soldering operation. The set of tests performed indicates that this new joint is quite promising to substitute the conventional solder process applied to power diodes.

In this paper we report results of nanocrystalline p-doped silicon films produced by hot wire chemical vapour deposition technique with Ta filaments, using a pre-mixed gas containing silane, diborane, methane, helium and hydrogen. The data obtained show that the films produced exhibit good optoelectronic properties and show a surface morphology dependent on the filament temperature and hydrogen dilution. The increase in the filament temperature, keeping constant the hydrogen dilution (87%), promotes the preferential growth of the crystals in the {220} direction, giving rise to a pyramidal-like surface structure. This behaviour is observed by the SEM micrographs as well as by the micro-Raman and X-ray diffraction analyses. On the other hand, using a constant filament temperature, the increase in the hydrogen dilution contributes to an increase in both {111} and {220} diffraction peaks. Thus, by combining both filament temperature and hydrogen dilution the film surface can be controlled from a smooth to a pyramidal-like structure, without decreasing the crystalline fraction of the films. The structure and morphology is also reflected in the stability of the electrical dark conductivity. We observe that this property depends on the temperature range of the measurements and on the exposition time of films to the atmospheric conditions. © 2002 Elsevier Science Ltd. All rights reserved.

In this work we present sputtered multicomponent dielectrics based on mixtures of HfO 2 and SiO 2. This way it is possible to get stable amorphous structure up to 800°C, that does not happen for pure HfO 2, for instance, that present a polycrystalline structure when deposited without any intentional substrate heating. Besides, also the band gap of the resulting films is increased when compared with pure HfO2 that theoretically is an advantage in getting a suitable band offset with the semiconductor layer on oxide TFTs. Concerning the electrical characterization, the leakage current on c-Si MIS structures is low as 10 -9 Acm -2 at 10 V. The amorphous structure of the films also lead to better dielectric/semiconductor interfaces, as suggested by C-V characteristics on GIZO MIS structures, which do not present strong variation with frequency. On other hand, the dielectric constant decreases due to the incorporation of SiO 2 and Al 2O 3. Further improvement on insulating and interface characteristics is achieved using multilayer stacks and substrate bias during deposition. © 2012 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE).