Rodrigo Martins

Thin films of copper oxide were obtained through thermal oxidation (100-450 °C) of evaporated metallic copper (Cu) films on glass substrates. The X-ray diffraction (XRD) studies confirmed the cubic Cu phase of the as-deposited films. The films annealed at 100 °C showed mixed Cu-Cu2O phase, whereas those annealed between 200 and 300 °C showed a single cubic Cu2O phase. A single monoclinic CuO phase was obtained from the films annealed between 350 and 450 °C. The positive sign of the Hall coefficient confirmed the p-type conductivity in the films with Cu2O phase. However, a relatively poor crystallinity of these films limited the p-type characteristics. The films with Cu and CuO phases show n-type conductivity. The surface of the as-deposited is smooth (RMS roughness of 1.47 nm) and comprised of uniformly distributed grains (AFM and SEM analysis). The post-annealing is found to be effective on the distribution of grains and their sizes. The poor transmittance of the as-deposited films (<1%) is increased to a maximum of ∼80% (800 nm) on annealing at 200 °C. The direct allowed band gap is varied between 2.03 and 3.02 eV. © 2008 Elsevier B.V. All rights reserved.

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

The aim of the present work was to study the optical properties of amorphous hydrogenated silicon films produced by capacitive and inductive r.f. glow discharge in a 3%SiH4-Ar gas mixture. The effect of the application of static electric and magnetic fields during the film formation on the photoconductivity, photoactivation energy, recombination mechanisms and optical gap was thoroughly investigated. Films prepared in a capacitively or inductively coupled discharge show bias-dependent photoconductivities, which reach about 10-4 Ω-1 cm-1 for an inductive discharge with a negative bias and about 10-5 Ω-1 cm-1 for a capacitive discharge with a positive bias. The optical gap is of the order of 1.55 eV for capacitive films and is bias dependent for inductive films (1.45-1.85 eV). A superimposed magnetic field (of about 1 kG) increases the photoconductivity by one order of magnitude for both deposition methods. The optical gap is field dependent for inductive films (1.6-1.8 eV) and is about 1.6 eV for capacitive films. The main recombination mechanism at a moderate photon flux (less than 1014 cm-2 s-1) is monomolecular for all deposition conditions. The photoactivation energy lies between 0.1 and 0.2 eV for capacitive films and is about 0.1 eV for inductive films. It was also found that, by using suitable crossed electric and magnetic fields, it was possible to control the density and nature of the defect states in the films. These are correlated with the type of hydrogenated silicon species and with the amount of hydrogen incorporated into the films, which markedly influence the film properties. © 1982.

Thin films of (Ta 2O 5) 0.85(TiO 2) 0.15 were deposited on quartz and p-Si substrates by DC reactive magnetron sputtering at different substrate temperatures (T s) in the range 303 - 873 K. The films deposited at 303 0K were in the amorphous and it transformed to crystalline at substrate temperatures ≥ 573 0K. The crystallite size was increased from 50 nm to 72 nm with the increase of substrate temperature. The surface morphology was significantly influenced with the substrate temperature. After deposition of the (Ta 2O 5) 0.85(TiO 2) 0.15 films on Si, aluminium (Al) electrode was deposited to fabricate metal/oxide/semiconductor (MOS) capacitors with a configuration of Al/(Ta 2O 5) 0.85(TiO 2) 0.15/Si. A low leakage current of 7.7 × 10 -5 A/cm 2 was obtained from the films deposited at 303 K. The leakage current was decreased to 9.3 × 10 -8 A/cm 2 with the increase of substrate temperature owing to structural changes. The conduction mechanism of the Al/(Ta 2O 5) 0.85(TiO 2) 0.15/Si capacitors was analyzed and compared with mechanisms of Poole-Frenkel and Schottky emissions. The optical band gap (E g) was decreased from 4.45 eV to 4.38 eV with the increase in substrate temperature. © Published under licence by IOP Publishing Ltd.

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.

Microcrystalline phosphorus-doped hydrogenated silicon alloy films were deposited in a remote plasma CVD (chemical vapor deposition) system. The film properties were studied as a function of RF power density and hydrogen concentration in the reaction gas mixture. The properties of the deposited films are extremely sensitive to the RF power density in the studied range of 250 mW/cm2 to 625 mW/cm2. Very low values of electrical resistivity were obtained. For an RF power density of 500 mW/cm2, ρ = 3 × 10-2 Ω-cm, while ρ = 1.9 × 103 Ω-cm for 625 mW/cm2, indicating the predominance of the amorphous tissue over the microcrystalline phase. High doping efficiencies which can be correlated to large grain size are indicated by the very low values of the activation energy as low as 30 meV for 500 mW/cm2, that were obtained.

This work presents a study performed on the deposition of pm-Si:H by plasma enhanced chemical vapor deposition using excitation frequencies of 13.56 and 27.12 MHz, where the interest of increasing the excitation frequency relies on higher plasma dissociation and reduced energy of ion bombardment, thus allowing the deposition of superior grade material at higher growth rates. The plasma impedance, which allows the monitoring of particle formation in the plasma, was correlated to the film properties, characterized by spectroscopic ellipsometry and hydrogen exodiffusion experiments. The set of data obtained show that by using the 27.12-MHz excitation frequency the hydrogen dilution and the r.f. power density needed to produce pm-Si:H can be reduced. Growth rates above 3.1 Å/s were obtained, the films being more dense and chemically more stable than those obtained with the standard 13.56 MHz. © 2003 Elsevier B.V. All rights reserved.

It is experimentally known that the linearity and sensitivity of the position sensitive detectors (PSD) are dependent on the resistance of the collecting layer and of the load resistance, mainly if the detection is based on the measurement of the photo-lateral voltage. To determine the value of the load resistance to be used in metal - insulator - semiconductor (MIS) PSDs structures that lead to the maximum value of sensitivity and linearity, we propose an electrical model through which it is able to simulate the proper sensor response and how the load resistance influence the results obtained. This model is valid for PSDs where the resistance of the collecting resistive layer is quite low (≤ 500 Ω), leading to a low output impedance. Under these conditions we conclude that the value of the load resistance should be of about 1 kΩ in order to achieve a good compromise between the linearity and the sensitivity of the PSD. This result is in agreement with the set of experiments performed. © 2005 Materials Research Society.

In this work metal-insulator-semiconductor (MIS) photodiodes with a structure: Cr/a-Si:H(n+)/a-Si:H(i)/oxide/Au were studied, where the main objective was to determine the influence of the oxide layer on the performance of the devices. The results achieved show that their performance is a function of both oxide thickness and oxide density. The a-Si:H oxidation method used was the immersion in H2O2 solution. By knowledge of the oxide growth process it was possible to fabricate photodiodes exhibiting an open circuit voltage of 0.65V and short circuit current density under AM1.5 illumination of 11mA/cm2, with a response times less than 1μs for load resistance <400Ω, and a signal to noise ratio of 1×107. © 2003 Elsevier B.V. All rights reserved.

In this work we present results of a study performed on metal-insulator-semiconductor (MIS) diodes with the following structure: substrate (glass)/Cr (2000 Å)/a-Si:H n+(400 Å)/a-Si:H i (5500 Å)/oxide (0-40 Å)/Au (100 Å) 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 (1×10 7 at -1 V) than the diodes with oxides obtained by the evaporation of SiO2, or by the chemical deposition of SiO2 by plasma of hexamethyldisiloxane. However, in the case of deposited oxides, the breakdown voltage is higher, 30 V 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 ±1 V, but is relevant when high breakdown voltages are required. © 2003 Elsevier B.V. All rights reserved.

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.

Insensitivity to light irradiation is desirable for conventional applications of thin-film transistors, i.e., the active matrices of displays. However, if one produces a device presenting controlled sensitivity to light, many other applications can benefit or can even be created. In this work it is shown the influence of the photon energy on the optoelectronic properties presented by n-type bottom-gate thin-film transistors based on indium zinc oxide. In the dark, the devices present very good electrical properties, working in the enhancement mode, exhibiting on-off ratios higher than 107 and channel mobility above 30 cm2/V s. Remarkable results were achieved when the devices were exposed to light radiation, the most striking one is the possibility to switch between enhancement (in the dark) and depletion (illuminated) modes, with different threshold voltages and on/off ratios, function of the light power density and wavelength used. This type of behavior permits to design circuits where one can have the same transistor working either in the enhancement or depletion modes, function of the light beam and intensity impinging on it, highly important for short wavelength detector applications. © 2006 Elsevier B.V. All rights reserved.

Indium oxide thin films deposited by spray pyrolysis were irradiated by 100 MeV O7+ ions with different fluences of 5 × 1011, 1 × 1012 and 1 × 1013 ions/cm2. X-ray diffraction analysis confirmed the structure of indium oxide with cubic bixbyite. The strongest (2 2 2) orientation observed from the as-deposited films was shifted to (4 0 0) after irradiation. Furthermore, the intensity of the (4 0 0) orientation was decreased with increasing fluence together with an increase in (2 2 2) intensity. Films irradiated with maximum fluence exhibited an amorphous component. The mobility of the as-deposited indium oxide films was decreased from ∼78.9 to 43.0 cm2/V s, following irradiation. Films irradiated with a fluence of 5 × 1011 ions/cm2 showed a better combination of electrical properties, with a resistivity of 4.57 × 10-3 Ω cm, carrier concentration of 2.2 × 1019 cm-3 and mobility of 61.0 cm2/V s. The average transmittance obtained from the as-deposited films decreased from ∼81% to 72%, when irradiated with a fluence of 5 × 1011 ions/cm2. The surface microstructures confirmed that the irregularly shaped grains seen on the surface of the as-deposited films is modified as "radish-like" morphology when irradiated with a fluence of 5 × 1011 ions/cm2. © 2011 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.

In the present work we investigate, the role of zinc oxide (ZnO) thin films passivating layer deposited by rf magnetron sputtering at room temperature on low (18%) and high (80%) porosity porous silicon (PS). The micro-Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR) and atomic force microscopy (AFM) analysis have been carried out to understand the effect of ZnO films coating on PS. A systematic investigation from Raman spectroscopy suggests the formation of a good quality ZnO wurtzite structure on PS. The photoluminescence (PL) measurements on PS and ZnO coated PS shows a red, blue and UV emission bands at around ∼1.8, ∼2.78 and ∼3.2 eV. An enhancement of all PL emission bands have been achieved after ZnO films deposition on high porosity PS. © 2007 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.

Nanocrystalline hydrogenated silicon (nc-Si:H) thin films are generally accepted to be a two phase material-Si crystalline and Si:H amorphous. This work reports the use of impedance spectroscopy to determine the amorphous and crystalline electrical conductivity of a/nc-Si:H films obtained by hot wire chemical vapour deposition. Different relaxation time or time constants are detected, if the film is composed by inhomogeneous material, by measuring ac impedance in a wide range of frequencies. Relating the conduction mechanism of the film to a series of two RC circuits constituted by a resistance and a capacitor in parallel, we may determine distinct ac conductivities and correlate that to the crystalline, amorphous and interface components. The amorphous films analysed exhibit one ac conductivity component while for nanocrystalline films two ac conductivity components are observed. The average value of ac conductivities is in agreement with that of dc conductivity. © 2006.

Copper oxide thin films were obtained by annealing (temperature ranging between 100 and 450 °C) the metallic Cu films deposited on glass substrates by e-beam evaporation. XRD studies confirmed that the cubic Cu phase of the asdeposited films changes into single cubic Cu 2Ophase and single monoclinic CuO phase, depending on the annealing conditions. The crystallite size is varied betweeñ12 and 31 nm. The lattice parameters of cubic Cu and Cu 2Ophases are estimated tõ3.60 and ̃4.26 Å , respectively. The films with Cu 2O phase showed p-type characteristics. The conductivity is decreased linearly with the decreasing temperature (1/T), which has confirmed the semiconductor nature of the deposited films. The calculated activation energy is varied between 0.10 and 0.16 eV. The surface microstructure is changed depending on the variation in the annealing temperature. The poor transmittance of the asdeposited films (<1%) is increased to a maximum of ̃80% (800 nm) on annealing at 200 °C. The estimated direct allowed band gap is varied between 1.73 and 2.89 eV. © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

In this paper ageing effects of the solution used to prepare fluorine-doped ZnO films by the spray pyrolysis technique were investigated, concerning its role on the structure, the electrical and optical properties of films produced. The data reveal that the sheet resistance of the ZnO:F thin film decreases with the age of the solution used, reaching a minimum of 24 Ω/□, after 15 days. On the other hand the optical transmittance increases for films deposited using 6 days aging solution, decreasing afterwards as the aging time increases, being the optical transmittance in the visible range below 55%, for films deposited from solutions 36 days in age. The X-ray diffraction spectra show that the aged films are polycrystalline in nature with a [100] predominant orientation. The data also show that the intensity of (100) peak increases as the time of solution age increases, which is related to an improvement of the film crystallinity. © 2009 Elsevier B.V. All rights reserved.

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.

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.

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.

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 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.

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.