Rodrigo Martins

The present work focuses on a qualitative analysis of localised I-V characteristics based on the nanostructure morphology of highly dense arrays of p-type NiO nano-pillars (NiO-NPs). Vertically aligned NiO-NPs have been grown on different substrates by using a glancing angle deposition (GLAD) technique. The preferred orientation of as grown NiO-NPs was controlled by the deposition pressure. The NiO-NPs displayed a polar surface with a microscopic dipole moment along the (111) plane (Tasker's type III). Consequently, the crystal plane dependent surface electron accumulation layer and the lattice disorder at the grain boundary interface showed a non-uniform current distribution throughout the sample surface, demonstrated by a conducting AFM technique (c-AFM). The variation in I-V for different points in a single current distribution grain (CD-grain) has been attributed to the variation of Schottky barrier height (SBH) at the metal-semiconductor (M-S) interface. Furthermore, we observed that the strain produced during the NiO-NPs growth can modulate the SBH. Inbound strain acts as an external field to influence the local electric field at the M-S interface causing a variation in SBH with the NPs orientation. This paper shows that vertical arrays of NiO-NPs are potential candidates for nanoscale devices because they have a great impact on the local current transport mechanism due to its nanostructure morphology. © 2013 The Royal Society of Chemistry.

The current transport in metal-amorphous semiconductor barriers is examined by solving the proper Poisson's equation and transport equations within the semiconductor's space charge region taking into account the role of trap shallow states distribution function. The effect of metal is also included through appropriate boundary conditions of the above solutions. Generalized transport equations will be derived either when thermionic drift-diffusion emission process dominates or when the conduction mechanism is mainly due to drift-diffusion emission. Both situations will be analysed with or without neglecting carriers losses during their collision free path, from which a tractable expression for the current-voltage characteristic will be determined.

Curcuma longa L., also known as turmeric, is widely used as a food colorant and has been reported to have antioxidant, anti-inflammatory, anti-mutagenic and anti-cancer properties. The aim of this study was to evaluate the effects of the spray drying on curcuminoid and curcumin contents, antioxidant activity, process yield, the morphology and solubility of the microparticulated solid dispersion containing curcuma extract using a Box Behnken design. The microparticles were spherical in shape, and an increase in outlet temperature from 40 to 80 °C resulted in a significant increase in the yield of microparticles from 16 to 53%. The total curcuminoid content (17.15 to 19.57. mg/g), curcumin content (3.24 to 4.25. mg/g) and antioxidant activity (530.1 to 860.3 μg/mL) were also affected by the spray drying process. The solubility of curcuminoid from C. longa remarkably improved 100-fold in the microparticles, confirming the potential of the ternary solid dispersion technique to improve the dyeing and nutraceutical properties of these compounds. Furthermore, the microparticles were obtained using the spray drying process, can be easily scaled up. © 2011 Elsevier Ltd.

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.

In this work we studied indium zinc oxide (IZO) thin films deposited by r.f. magnetron sputtering at room temperature. The films were annealed at high temperature (1100 K) in vacuum, and the oxygen exodiffusion was monitored in-situ. The results showed three main peaks, one at approximately 600 K, other at approximately 850 K and the last one at 940 K, which are probably from oxygen bonded in the film surface and in the bulk, respectively. The initial amorphous structure becomes microcrystalline, according to the X-ray diffraction. The electrical conductivity of the films decreases (about 3 orders of magnitude), after the annealing treatment. This behavior could be explained by the crystallization of the structure, which affects the transport mechanism. Apart from the changes in the material structure, a small variation was observed on the absorption coefficient. © 2007 Elsevier B.V. All rights reserved.

In this paper an exhaustive analysis is performed on the electrochemical corrosion resistance of Al-doped ZnO (AZO) layers deposited on silicon wafers by a DC pulsed magnetron sputtering deposition technique to test layer durability. Pulse frequency of the sputtering source was varied and a detailed study of the electrochemical corrosion response of samples in the presence of a corrosive chloride media (NaCl 0.06 M) was carried out. Electrochemical impedance spectroscopy measurements were performed after reaching a stable value of the open circuit at 2 h, 192 h and 480 h intervals. Correlation of the corrosion resistance properties with the morphology, and the optical and electrical properties was tested. AZO layers with transmission values higher than 84% and resistivity of 6.54 × 10- 4 â. cm for a deposition process pressure of 3 × 10- 1 Pa, a sputtering power of 2 kW, a pulse frequency of 100 kHz, with optimum corrosion resistance properties, were obtained. © 2015 Elsevier B.V.

In this work we present results of a study performed on MIS diodes with the following structure: substrate (glass) / Cr (2000Å) / a-Si:H n + (400Å) / a-Si:H i (5500Å) / oxide (0-40Å) / Au (100Å) to determine the influence of the oxide passivation layer grown by different techniques on the electrical performance of MIS devices. The results achieved show that the diodes with oxides grown using hydrogen peroxide present higher rectification factor (2×106) and signal to noise (S/N) ratio (1×107 at -1V) than the diodes with oxides obtained by the evaporation of SiO2, or by the chemical deposition of SiO 2 by plasma of HMDSO (hexamethyldisiloxane), but in the case of deposited oxides, the breakdown voltage is higher, 30V instead of 3-10 V for grown oxides. The ideal oxide thickness, determined by spectroscopic ellipsometry, is dependent on the method used to grow the oxide layer and is in the range between 6 and 20 Å. The reason for this variation is related to the degree of compactation of the oxide produced, which is not relevant for applications of the diodes in the range of ± 1V, but is relevant when high breakdown voltages are required.

Hall effect measurements have been used to evaluate the conduction mechanism, exhibited by tin oxide thin film gas sensors deposited by spray pyrolysis. Two experiments have been carried out: (i) Hall measurements in air and (ii) in the presence of methane (first results reported), both as a function of temperature. From the measurements performed it was possible to infer the potential barrier and its dependence with the atmosphere used. The results obtained for the carrier concentration and mobility have been analysed in the light of the oxygen diffusion mechanism at grain boundaries by using the grain boundary-trapping model. In the presence of the methane gas the electrical resistivity decreases due to the lowering of the inter-grain boundary barrier height. © 2001 Published by Elsevier Science Ltd.

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.

In order to understand the kinetics of formation of interface/surface states and its correlation on the final device performance, a preliminary study was performed on MIS structures, before and after surface oxidation/passivation, using different oxidation techniques and oxides: thermal (in air), chemical (in H2O2) and oxygen plasma. The devices used in this work are based on a glass/Cr/a-Si:H(n+)/a-Si:H(i)/SiOx/Pd structures, where the amorphous silicon intrinsic layer (i a-Si:H) with a photosensitivity of 107 was deposited by a modified plasma enhanced chemical vapour deposition (PECVD) triode system. The electrical properties of a-Si:H MIS structures were investigated by measuring their diode current-voltage characteristics in the dark and under illumination as well as the spectral response, as a function of the various oxidation techniques. Infrared spectroscopy and spectroscopic ellipsometry were used as a complementary tool to characterise the oxidised surface.

Wide band gap microcrystalline silicon films have aroused considerable interest since they combine some electro-optical advantages of amorphous and crystalline materials highly important to produce electro-optical devices such as TFTs and solar cells. In this paper we present results concerning the electro-optical characteristics of highly transparent and conductive n-type μc-Si based films. Here, emphasis is given to the production of n-type μc-films with optical gaps of 2.3 eV and dark conductivity's of 6.5 Scm-1.

In order to correlate the MIS devices performance with different surface oxidation methods, AFM, spectroscopic ellipsometry and infrared spectroscopy measurements were performed in a-Si:H films, before and after surface oxidation, using different oxidation techniques and oxides: thermal dry (in air), wet (in H2O2) and by oxygen plasma, while MIS (metal-insulator-semiconductor) devices were characterized by I-V curves, under dark and AM1.5 illumination conditions. The a-Si:H films were grown by the PECVD technique, in a modified triode configuration reactor to allow a precise control of the ion bombardment during the film deposition. We found that the growth of a thin layer of oxide by chemical processes on the top of the a-Si:H surface can cause changes on the surface morphology that are reflected in the electrical behaviour of the devices. The oxygen plasma treatment, cause the rearrangement of the surface atoms leading to a change of their morphology and to the improvement of the electrical properties of the surface for a MIS applications.

The electrosynthesis of polypyrrole (PPy) has been achieved on aluminium in aqueous medium of malic acid by means of cyclic voltammetry, potentiostatic and galvanostatic techniques. Scanning electron microscopy (SEM) and X-ray microanalysis by dispersion energy spectroscopy (EDS) applying on surfaces show that the PPy coating is developed from the metal surface through the cracks of the initial Al2O3 layer. Moreover, the results reveal that the homogeneity of the film achieved increases with the time of electropolymerization. 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 the 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. 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 B.V. All rights reserved.

This paper presents data concerning the composition structure and optical characteristics of polymorphous silicon films produced by plasma enhanced chemical vapour deposition at 27.12 MHz and determined respectively by infrared spectrometry, micro Raman, exodiffusion and spectroscopic ellipsometry measurements. When compared to the pm-Si:H films produced at 13.56 MHz, the films produced at 27.12 MHz present hydrogen contents in the range of 21 at%, the sharp peak ascribed to the exodifusion measurements is shifted towards high temperatures and the imaginary part of the dielectric function 〈ε2〉 is larger and shifted to high energies. Apart from that the peaks of the infrared spectra ascribed to the stretching modes shift towards high wave numbers and the half width of the micro Raman peaks shrinks, meaning that the films produced at 27.12 MHz are more compact and dense.

In this work we present results concerning the composition and structure of intrinsic thin film silicon carbide alloys obtained by hot wire and hot wire plasma assisted techniques using ethylene as carbon gas source. The data show that by increasing the percentage of ethylene in the gas mixture from 14% to 60% the optical band gap is enhanced from 1.8 eV to 2.3 eV, for films produced by hot wire technique at a filament temperature of 2123K (1850°C). This is attributed to the increase of carbon incorporation, which was confirmed by the infrared spectra data where an increase is observed in the SiC stretching vibration mode ascribed to the peak located at around 750cm-1. On the other hand, the films produced by combining hot wire and rf plasma show a more efficient carbon incorporation. The SEM photographs of samples produced with hot wire technique reveal an amorphous structure, confirmed by micro-Raman spectroscopy data, while the samples produced with plasma assisting the process show a granular structure with grain sizes in the range of 100-200nm. © 2002 Elsevier Science Ltd. All rights reserved.

In this work, we study the electro-optical behaviour of cellulose/liquid crystal-based composite systems, in particular the influence of the flexible substrates and its conductive layers in the electro-optical behaviour of these kind of cells. Four cells were made using, respectively, two different substrates (a flexible polymer (poly(ethylene terephthalate) (PET)) and a soda lime glass) and two different conductive layers (indium tin oxide (ITO) and aluminium zinc oxide (AZO)). The conductive layer (AZO) was deposited in both, flexible and rigid substrates, for the same conditions, and the same substrates coated with ITO are commercially available. The cells were prepared from solid films of hydroxypropylcellulose (HPC) (30 μm thick) cross linked with 1,4-diisocyanatobutane (BDI) (7% w/w) and the nematic liquid crystal E7 (Merck, UK). The four different substrates were electrically and morphologically characterised. We have analysed all samples by light transmission and determined the maximum transmission, contrast and Von. We show a comparison of the results obtained for both flexible and rigid cells and discuss them in terms of the proposed working mechanism for these systems. © 2002 Elsevier Science B.V. All rights reserved.

A complementary metal oxide semiconductor (CMOS) device is described. The device is based on n-(In-Ga-Zn-O) and p-type (SnO x) active oxide semiconductors and uses a transparent conductive oxide (In-Zn-O) as gate electrode that sits on a flexible, recyclable paper substrate that is simultaneously the substrate and the dielectric. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Using the Flying Spot Technique (FST) we have studied minority carrier transport parallel and perpendicular to the surface of amorphous silicon films (a-Si:H). To reduce slow transients due to charge redistribution in low resistivity regions during the measurement we have applied a strong homogeneously absorbed bias light. The defect density was estimated from CPM measurements. The steady-state photocarrier grating technique (SSPG) is a 1-dimensional approach. However, the modulation depth of the carrier profile is also dependent on film surface properties, like surface recombination velocity. Both methods yield comparable diffusion lengths when applied to a-Si:H.

We demonstrate fully transparent, highly rectifying contacts (TRC) on amorphous GaInZnO and compare them to TRC fabricated on single crystalline bulk ZnO and heteroepitaxial ZnO thin films. The contacts' transparency in the visible spectral range exceeds 70%. From numerical simulations, we conclude that thermionic emission is the dominating transport mechanism, however, for several samples with low net doping density diffusion theory must be applied. The detailed investigation of the rectification properties of the TRC using temperature-dependent current-voltage and capacitance-voltage measurements reveals that barrier inhomogeneities govern the IV-characteristics of all diodes irrespective of the sample crystallinity. Assuming a Gaussian barrier height distribution, the extracted mean barrier heights typically range between 1.1 and 1.3 V. The width of the barrier distribution correlates with the mean barrier height and ranges from 110 to 130 mV. By compiling literature data, we found that this correlation holds also true for Schottky diodes on elemental and III-V semiconductors. © 2013 American Institute of Physics.

An advanced compact and analytical drain current model for the amorphous gallium indium zinc oxide (GIZO) thin film transistors (TFTs) is proposed. Its output saturation behavior is improved by introducing a new asymptotic function. All model parameters were extracted using an adapted version of the Universal Method and Extraction Procedure (UMEM) applied for the first time for GIZO devices in a simple and direct form. We demonstrate the correct behavior of the model for negative VDS, a necessity for a complete compact model. In this way we prove the symmetry of source and drain electrodes and extend the range of applications to both signs of VDS. The model, in Verilog-A code, is implemented in Electronic Design Automation (EDA) tools, such as Smart Spice, and compared with measurements of TFTs. It describes accurately the experimental characteristics in the whole range of GIZO TFTs operation, making the model suitable for the design of circuits using these types of devices. © 2016 Elsevier Ltd

Amorphous undoped a-Si:H films have been produced by hot wire plasma assisted chemical vapour deposition (HWPA-CVD), which combines the hot wire chemical vapour deposition (HW-CVD) and plasma enhanced chemical vapour deposition techniques. In this work we analyse the dissociation mechanism of the gas during the film growth in both processes with a quadrupole mass spectrometer. Besides that, the energy delivered to the gas dissociation is determined and correlated with the films properties. Thus, based on the results of the dissociated species for each deposition condition and process, we explain why the growth rate is enhanced when the filament temperature rises in HW-CVD process and why it decreases as r.f. power is enhanced in HWPA-CVD process. © 2002 Elsevier Science B.V. All rights reserved.

Aluminum Zinc Oxide has been extensively investigated as a cheap alternative to transparent conducting tin oxide films for electronic and optoelectronic applications. Thin films of Aluminum Zinc Oxide have been developed successfully through a combination of solution combustion synthesis and solution synthesis. Zn(NO3)3·6H2O as metal source was dissolved in 2-methoxyethanol as solvent through combustion synthesis with Urea as fuel while dopant source of AlCl3·6H2O was mixed separately in solvent to avoid aluminum oxide formation in the films. Precursor solutions were obtained mixing Zn & Al separate solutions in 9:1, 8:2, and 7:3 ratios respectively with oxide, fuel and dopant concentrations of 0.5, 0.25, 0.1, and 0.05 M. The film stacks have been prepared through spin-coating with heating at 400°C for 10 minutes after each deposition to remove residuals and evaporate solvents. Thermal annealing in oven at 600°C for 1 hour followed by rapid thermal annealing at 500°C & 600°C first in vacuum and then in N2-5%H2 environment respectively for 10 minutes each reduced the resistivity of film stacks. Film stack with 10 layers for an average thickness of 0.5μm gave the best Hall Effect resistivity of 3.2 × 10-2 -cm in the case of 0.5M solution with Zn:Al mixing ratio of 9:1 for RTA annealings at 600°C with an average total transparency of 80 % in the wavelength range of 400-1200 nm. The results show a clear trend that increasing the amount of ingredients resistivity could further be decreased. © 2015 IEEE.

The color sensing ability of a data acquisition prototype system integrating a 32 linear array of 1D amorphous silicon position sensitive detectors (PSD) was analyzed. Besides being used to reproduce a 3D profile of highly reflective surfaces, here we show that it can also differentiate primary red, green, blue (RGB) and derived colors. This was realized by using an incident beam with a RGB color combination and adequate integration times taking into account that a color surface mostly reflects its corresponding color. A mean colorimetric error of 25.7 was obtained. Overall, we show that color detection is possible via the use of this sensor array system, composed by a simpler amorphous silicon pin junction. © 2013 Elsevier B.V. All rights reserved.

N, (N+Ga) and (N+Al) doped ZnO films were deposited on c-plane sapphire substrates by RF magnetron sputtering at room temperature. The samples were characterized by their structural, surface morphological, compositional and optical properties. The x-ray diffraction studies confirmed the co-doping of (N+Ga) and (N+Al) besides showing improvement in the crystallinity when compared with the single Ndoping. The surface of the films becomes rougher after co-doping. The x-ray photoelectron spectroscopy and Rutherford back-scattering analysis indicate that the co-doping changes the chemical states and varies the amount of nitrogen (N) in ZnO. The amount of 'N' has been greatly increased for (N+Ga) co-doping, indicating that it is the best co-doping pair for p-type ZnO. Additionally, co-doping has increased the average visible transmittance (40-650nm) and the optical band gap is shifted towards shorter wavelength. In the case of (N+Al) co-doping, the band gap becomes wider than that of undoped ZnO. © IOP Publishing Ltd.