Rodrigo Martins

In this work, we report results concerning the performances of thin-film transistors (TFTs) where the channel layer is based on microcrystalline zinc tin oxide (ZTO) processed by spray pyrolysis technique. TFTs made with 30 nm thick ZTO channel layer deposited at a substrate temperature of 400 C and 300 Cexhibited, respectively, a saturation mobility of 2.9 cm V s and 1.45 cm V s ; voltage of 0.15 V, and 0.2 V; a sub-threshold swing of 400 mV/dec and 500 mV/dec; ON/OFF ratio at the onset of hard saturation current of 3.5 10 and 6 10 , for a drain to source voltage of 10 V (close to or below the gate to source voltage). This indicates that the substrate temperature is relevant in determining the devices' electronic performances. © 2013 IEEE.

This paper presents a low-cost technology for the realisation of large-area thin film position-sensitive detectors using the a-Si:H technology. The obtained results are quite promising regarding the application of these sensors to a wide variety of optical inspection systems, such as: machine tool alignment and control; angle measuring; rotation monitoring; surface profiling; medical instrumentation; targeting; remote optical alignment; guidance systems; etc., to which automated inspection control is needed.

In this work, we report the electro-optical properties exhibited by ZnO:A1 thin films deposited by r.f. magnetron sputtering. The effect of the deposition parameters on the properties of the films were studied with the aim to determine the most suitable deposition conditions to obtain ZnO:Al thin films with a low resistivity and high transmittance, characteristics required for applications on optoelectronic devices. After annealing, the ZnO:Al thin films present a low resistivity (6.25 × 10-3 Ωcm) and a high transmittance (90%) when produced with a deposition pressure of 1.6 × 10-2 mbar and r.f. power of 150W. © 2002 Elsevier Science Ltd. All rights reserved.

The effect of doping and annealing atmosphere on the performances of zinc oxide thin films prepared by spray pyrolysis have been studied. The results show that the way doping influences the electrical and structural properties depends also on the characteristics of the doping element. Annealing the as-deposited films in an inert atmosphere leads to a substantial reduction in the resistivity of the films deposited and to an increase on the degree of film's crystallinity. © 1999 Elsevier Science S.A. All rights reserved.

Titanium dioxide was synthesized on glass substrates from titanium (IV)isopropoxide and hydrochloride acid aqueous solutions through microwave irradiation using as seed layer either fluorine-doped crystalline tin oxide (SnO2:F) or amorphous tin oxide (a-SnOx). Three routes have been followed with distinct outcome: (i) equimolar hydrochloride acid/water proportions (1HCl:1water) resulted in nanorod arrays for both seed layers; (ii) higher water proportion (1HCl:3water) originated denser films with growth yield dependent on the seed layer employed; while (iii) higher acid proportion (3HCl:1water) hindered the formation of TiO2. X-ray diffraction (XRD) showed that the materials crystallized with the rutile structure, possibly with minute fractions of brookite and/or anatase. XRD peak inversions observed for the materials synthesized on crystalline seeds pointed to preferred crystallographic orientation. Electron diffraction showed that the especially strong XRD peak inversions observed for TiO2 grown from the 1HCl:3water solution on SnO2:F originated from a [001] fiber texture. Transmittance spectrophotometry showed that the materials with finer structure exhibited significantly higher optical band gaps. Photocatalytic activity was assessed from methylene blue degradation, with the 1HCl:3water SnO2:F material showing remarkable degradability performance, attributed to a higher exposure of (001) facets, together with stability and reusability. © 2015 Elsevier B.V.

Hydrogenated amorphous silicon photochemical sensors based on Pd metal/insulator/semiconductor (Pd-MIS) structures were produced by plasma enhanced chemical vapour deposition (PECVD) with two different oxidized surfaces (thermal and chemical oxidation). The behaviour of dark and illuminated current-voltage characteristics in air and in the presence of a hydrogen atmosphere is explained by the changes induced by the gases adsorbed, in the work function of the metal, modifying the electrical properties of the interface. The photochemical sensors produced present more than two orders of magnitude variation on the reverse dark current in the presence of 400 ppm hydrogen. When the sensors are submitted to light it corresponds a decrease of 45% on the open circuit voltage.

Electrochromic coatings are used in a wide range of technical fields. This paper refers to the properties of amorphous Indium-tin-oxide (ITO) and WO 3 thin films obtained by dc-magnetron reactive sputtering in different oxygen partial pressures (PO2) at room temperature (RT). SEM, XRD and UV-Vis-NIR spectroscopy characterized the films prepared in terms of morphology, structure, and optical and electrical properties. The maximum of the optical transmittance in the visible range for the ITO and WO3 films was above 97% and 98%, respectively. These films were incorporated into ECDs (electrochromic devices) whose performance was assessed by optical methods.

This work aims to study the role of the r.f. electrode configuration on the plasma characteristics of a PECVD asymmetric reactor. The configurations used are the usual diode configuration, the triode configuration and a new configuration that we named short-circuited grid electrode (SGE). The plasma generated was characterized with the use of a Langmuir probe and an impedance probe. We demonstrate that the plasma parameters are highly dependent on the reactor geometry. The results achieved show that by changing the r.f. electrode configuration the DC self-bias varies from about 100 to close to 0 V. This variation causes changes in the ion bombardment of the reactor surfaces, which can affect the growing of the films deposited. We also demonstrate that for the SGE configuration the area seen by the plasma does not correspond to the exposed physical area of the electrode, and we suggest a model to explain this phenomenon.

The effect of post-deposition annealing temperature on the pH sensitivity of room temperature RF sputtered +{\hbox{Ta}}-{2}{\hbox{O}}5 was investigated. Structural and morphological features of these films were analyzed before and after annealing at various temperatures. The deposited films are amorphous up to 600 +^{\circ}{\hbox{C}}+ and crystallize at 700 +^{\circ}{\hbox{C}}+ in an orthorhombic phase. Electrolyte-insulator- semiconductor (EIS) field effect based sensors with an amorphous +{\hbox{Ta}}-{2}{\hbox{O}}5 sensing layer showed pH sensitivity above 50 mV/pH. For sensors annealed above 200 +^{\circ}{\hbox{C}}+ pH sensitivity decreased with increasing temperature. Stabilized sensor response and maximum pH sensitivity was achieved after low temperature annealing at 200 +^{\circ}{\hbox{C}}+ , which is compatible with the use of polymeric substrates and application as sensitive layer in oxides TFT-based sensors. © 2005-2012 IEEE.

In this work metal induced crystallization (MIC) using nickel (Ni) was employed to obtain poly-Si by crystallization of amorphous films for application as active layer in TFTs. Ni layers with thicknesses of 0.5 nm, 1 nm and 2 nm were used to crystallize the silicon. The TFTs were produced with a bottom gate configuration using a multi-layer Al2O3/TiO2 insulator produced by atomic layer deposition (ALD) as gate dielectric. The best performances of the TFT produced were obtained when using very thin Ni layers for the crystallization. This is attributed to a lower metal contamination and to the enhancement of grain size, as a result of the lower nucleation density achieved, when using the thinnest Ni layer. Devices that exhibit effective mobility of 45.5 cm2V-1s-1 and an on/off ratio of 5.55×104 were produced using a 0.5 nm Ni layer to crystallize the active channel area.

In this paper we present results of intrinsic/non-doped zinc oxide deposited at room temperature by radio frequency magnetron sputtering able to be used as a semiconductor material on electronic devices, like for example ozone gas sensors, ultra-violet detectors and thin film transistors. These films present a resistivity as high as 2.5×108 Ω cm with an optical transmittance of 90%. Concerning the structural properties, these films are polycrystalline presenting a uniform and very smooth surface. © 2005 Elsevier B.V. All rights reserved.

The aim of this paper is to study the role of gold (Au) induced crystallization on amorphous silicon (a-Si) films produced by low pressure chemical vapor deposition (LPCVD) at low process temperatures (550 °C) to allow the use of glass substrates. Concerning the crystallization process Au was deposited by e-beam thermal evaporation over the silicon (Si), using different metal thickness, from 5 to 100 Å. The samples were then annealed at 450, 500 and 550 °C and the crystallization time was changed from 5 up to 30 h. The structure of the films was analyzed by X-ray diffraction (XRD) and spectroscopic ellipsometry (SE) while electrical conductivity measurements were performed to obtain the electrical properties of the films produced, namely the activation energy (EA) and how it changes with the Au thickness used. The data achieved show that the increase of the metal layer thickness decreases the time needed to get full crystallization. However this leads to lower conduction activation energy (EA) meaning that there is also an increase of Au incorporation that leads to the production of doped films. © 2004 Elsevier B.V. All rights reserved.

The aim of this paper is to study the role of different metals (aluminium, molybdenum, nickel and titanium) in inducing crystallization in films produced by LPCVD at high and low temperature processes and to compare the structural, morphological, optical and electrical properties of the various films produced. This work envisages the use of the most suitable conditions that lead to the production of films for optoelectronic applications such as solar cells. © 2003 Elsevier B.V. All rights reserved.

The authors report on the performance of polymer-based light-emitting diodes, LEDs, using amorphous zinc oxide-doped indium oxide, IZO, as anode. In particular, LEDs with poly[(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] as electroluminescent layer and aluminium cathodes, show higher efficiency with this IZO anode (0.015 cd/A) than with indium-tin oxide (ITO) (0.010 cd/A). Inspite of the higher resistance of this IZO electrode, compared with ITO, the fact that it can be processed at lower temperatures and allows similar or even higher efficiency values for polymer LEDs make this material a good candidate for display and other optoelectronic applications. © 2009 Elsevier B.V. All rights reserved.

Despite of a growing interest in this material, until now the studies on polymorphous silicon (pm-Si:H) have been performed on small laboratory reactors working at 13.56 MHz. Envisaging an industrial application of pm-Si:H, the technology was transferred to a large area plasma enhanced chemical vapour deposition reactor (25 × 40 cm2) working at excitation frequencies of 13.56 and 27.12 MHz. The plasma was characterized by impedance probe measurements and the films were characterized by spectroscopic ellipsometry, infrared spectroscopy and hydrogen evolution experiments, which are techniques that allow a rapid and reliable identification of pm-Si:H structure. Conductivity measurements were also performed to determine their transport properties. The results show that scaling up using the 13.56 MHz was successfully done and pm-Si:H films were deposited at a growth rate of ≈ 12 nm/min. Moreover, by using the 27.12 MHz excitation frequency the growth rate was even further increased to above 18 nm/min, as desired for industrial production. © 2002 Elsevier Science B.V. All rights reserved.

This paper describes, for the first time, a method of producing polymorphous silicon (pm-Si:H) films by plasma-enhanced (PE) CVD, using an excitation frequency of 27.12 MHz. The aim is to produce, at high growth rates, nanostructured films that are more stable than the conventional amorphous or polymorphous silicon films grown by PECVD at 13.56 MHz. The processing data show that, at 27.12 MHz, the pm-Si:H films are produced close to the transition region from amorphous to microcrystalline silicon films, at a growth rate of about 0.3 nm s-1, using pressures above 160 Pa. Apart from that, the analysis of the exodiffusion, spectroscopic ellipsometry (SE), and micro Raman data reveal that these films are more dense and compact than the polymorphous films grown at 13.56 MHz.

This paper describes, for the first time, a method of producing polymorphous silicon (pm-Si:H) films by plasma-enhanced (PE) CVD, using an excitation frequency of 27.12 MHz. The aim is to produce, at high growth rates, nanostructured films that are more stable than the conventional amorphous or polymorphous silicon films grown by PECVD at 13.56 MHz. The processing data show that, at 27.12 MHz, the pm-Si:H films are produced close to the transition region from amorphous to microcrystalline silicon films, at a growth rate of about 0.3 nms-1, using pressures above 160 Pa. Apart from that, the analysis of the exodiffusion, spectroscopic ellipsometry (SE), and micro Raman data reveal that these films are more dense and compact than the polymorphous films grown at 13.56 MHz.

This work reports the use of undoped porous amorphous/nanocrystalline hydrogenated silicon (a/nc-Si:H) thin films produced by hot wire chemical vapour deposition (HW-CVD) as ethanol detector above 50ppm and as a primary fuel cell where a power of 4μW/cm2 was obtained in structures of the type glass/ITO/i-a-nc-Si:H/Al. The porous silicon looks like a sponge constituted by grains and cluster of grains that determines the type of surface morphology and the behaviour of the structure under the presence of vapour moisture. Apart from that, the detector/device performances will also depend on the type of interlayer and interfaces with the metal contacts. The sponge like structure adsorbs the OH groups in uncompensated bonds, which behave as donor-like carriers, leading to an increase in the current flowing through the material, directly dependent on the ethanol vapour pressure. The corresponding role of the components of the microstructure on this detector was investigated by spectroscopic impedance. The response time of the current of the sensor and its recovery time are in the range of 10-50s at room temperature. © 2004 Elsevier B.V. All rights reserved.

In this paper we report on the fabrication and performance of a portable and low cost optoelectronic platform integrating a double color tuned light emitting diode as light source, an amorphous/nanocrystalline silicon photodetector with a flat spectral response in the wavelength range from 520. nm to 630. nm and integrated electronic for signal acquisition and conditioning constituted by current to voltage converter, a filter and an amplification stage, followed by an analog to digital converter, with appropriate software for full automation to minimize human error. Incorporation of the double color tuned light emitting diode provides for a simple yet innovative solution to signal acquisition independently from the light intensity and/or solution concentration, while considerably decreasing production costs. Detection based on Au-nanoprobes constitutes the biorecognition step and allowed identification of specific sequences of Mycobacterium tuberculosis complex, namely Mycobacterium bovis and M. tuberculosis in biological samples. © 2010 Elsevier B.V.

Thin films of molybdenum-doped indium oxide (IMO) were prepared by a 3-source, cylindrical radio-frequency magnetron sputtering at room temperature. The films were post-annealed and were characterized by their structural (X-ray diffraction) and optical (UV-VIS-NIR spectrophotometer) properties. The films were studied as a function of oxygen volume percentage (O2 vol.%) ranging from 3.5 to 17.5. The structural studies revealed that the as-deposited amorphous films become crystalline on annealing. In most cases, the (222) reflection emerged as high intensive peak. The poor visible transmittance of the films as-deposited without oxygen was increased from ∼ 12% to over 80% on introducing oxygen (3.5 O2 vol.%). For the films annealed in open air, the average visible transmittance in the wavelength ranging 400-800 nm was varied between 77 and 84%. The films annealed at high temperatures (> 300 °C) decreased the transmittance to as low as < 1%. The optical band gap of the as-deposited films increased from the range 3.83-3.90 to 3.85-3.98 eV on annealing at different conditions. © 2007 Elsevier B.V. All rights reserved.

In this work Laponite was combined with a modified abundant natural polymer, (caboxymethyl cellulose), acrylic sodium salt polymer and lithium perchlorate aiming to produce inexpensive and sustainable nanocomposite electrolytes for functional electrochemical devices. Optical, electrochemical, structural, morphological and rheological characterization was performed in order to evaluate their properties and potential advantages as electrolyte. It was verified that the addition of Laponite led to an ionic conductivity at room temperature (25 C) in the range of 6 to 9 × 10- 5 Scm - 1 this value being then determined by the composition of the nanocomposite. These electrolytes were applied to electrochromic devices using evaporated nickel oxide thin film as the electrochromic layer. The devices exhibited a significant transmittance modulation that exceeds 45 % at 600 nm. © 2013 Elsevier B.V.

Flexible large area thin film position sensitive detectors based on amorphous silicon technology were prepared on polyimide substrates using the conventional plasma enhanced chemical vapor deposition technique. The sensors were characterized by spectral response, illuminated I-V characteristics position detectability measurements and atomic force microscopy. The obtained one-dimensional position sensors, 5-mm wide and 60-mm long, presented a maximum spectral response at 600 nm, an open circuit voltage of 0.6 V and a position detectability with a correlation of 0.9989 associated to a S.D. of 1×10-2, comparable to those produced on glass substrates.

The room temperature ozone sensing properties of polycrystalline undoped zinc oxide (ZnO) thin films have been investigated. ZnO thin films have been produced by the d.c. and r.f. magnetron sputtering technique as well as with spray pyrolysis with a variety of parameters. The as-grown films were brought to a high conducting state through a reversible photoreduction process by UV light exposure and were subsequently exposed to ozone resulting in a strong resistivity increase caused by re-oxidation. The magnitude of the effect was largest for the sputtered films, which exhibited resistivity changes of more than 8 orders of magnitude, whereas films deposited by spray pyrolysis showed changes of less than 3 orders of magnitude. XRD and AFM analysis of the films revealed that all films were microcrystalline. The film texture, however, was strongly related to the growth technique and the parameters used. Best results were achieved with r.f.-sputtered films, which have been deposited at high total pressures. These films exhibited a sensor response of 1.2 × 108. © 2002 Elsevier Science B.V. All rights reserved.