Rodrigo Martins

Diffraction and other structural measurements on n-type SiC thin films prepared in a TCDDC (two consecutive decomposition and deposition chamber) system indicate the presence of Si microcrystals (without evidence for SiC crystallites). Weakly absorbing, highly conductive layers (σ ≥ 10-1 (Ω-cm)-1) contain up to 20 at.% C and 25 at.% O. The optoelectronic properties of these films can be explained in terms of a sufficient volume fraction (above the percolation threshold) of Si microcrystals surrounded by an a-Si:C:O:H matrix.

In this work, we present a study of the effect of temperature, type and concentration of the dopant on the structural characteristics of ZnO thin films produced by spray pyrolysis; the crystallite size has been determined from profile peak shape analysis. These results are compared to the electrical characterisation performed on these materials. The effect of the dopant on the properties of ZnO thin films depends on its characteristics, mainly its ionic radius. Al, Ga and In have been studied as dopants, the best one being In, since it leads to the lowest resistivity.

Currently, microactuators are being developed using shape memory alloys (SMAs), which allow simple design geometries and provide large work outputs in restricted space. Several techniques have been used to produce NiTi shape memory alloy thin films, but from the practical point of view, only the sputter deposition method has succeeded so far. Vacuum evaporation of NiTi binary alloy entails the potential problem of the evaporation rates of each component not being the same due to differences in vapour pressure. Aiming to study the possible applications of SMAs to microfabrication, NiTi thin films were produced at CENIMAT by sputter and vacuum evaporation using raw materials from different sources. The films were analysed by differential scanning calorimetry (DSC) and X-ray diffraction (XRD) at room temperature, as well as in situ high temperature, in order to characterise the temperature ranges at which the different structural transformations occur. © 2002 Elsevier Science B.V. All rights reserved.

Zinc oxide is getting an enormous attention due to its potential applications in a variety of fields such as optoelectronics, spintronics and sensors. The renewed interest in this wide band gap oxide semiconductor relies on its direct high energy gap (Eg ∼ 3.437 eV at low temperatures) and large exciton binding energy. However to reach the stage of device production the difficulty to produce in a reproducible way p-type doping must be overcome. In this study we discuss the structural and optical properties of ZnO films doped with nitrogen, a potential p-type dopant. The films were deposited by magnetron sputtering using different conditions and substrates. The composition and structural properties of the films were studied combining X-ray diffraction (XRD), Rutherford backscattering (RBS), and heavy ion elastic recoil detection analysis (HI-ERDA). The results show an improvement of the quality of the films deposited on sapphire with increasing radio-frequency (RF) power with a preferentially growth along the c-axis. The ERDA analysis reveals the presence of H in the films and a homogeneous composition over the entire thickness. The photoluminescence of annealed samples evidences an improvement on the optical quality as identified by the well structured near band edge recombination. © 2009 Elsevier Ltd. All rights reserved.

High-quality SrBi2Nb2O9 (SBN) single crystals were grown from a melt using a high-temperature self-flux solution method and Bi2O3 added with B2O3 as a flux. A suitable thermal profile involving slow cooling rates allowed growing large and translucent SBN crystals exhibiting platelet morphology with typical size ∼5 × 5 mm2 and thickness approximately 400 μm. X-ray diffraction revealed a dominant (001)-orientation of the major face of the platelet crystals and edges oriented parallel to the [110] directions. The dielectric properties were evaluated along the ab-plane and in the c-axis direction. The ferro-paraelectric phase transition was observed at TC = 440°C with Curie-Weiss relationship above TC. The anisotropy of dielectric permittivity, i.e., the ratio between permittivity in the ab-plane and along c-axis was about 10 at TC-The obtained results are used to discuss the observed correlations between anisotropy, crystalline orientation, and electrical properties.

Electrospun fibers of poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-bithiophene] (F8T2) with exceptional electro-optical performance are obtained. The I/T characteristics measured in fibers with 7-15 μm diameter and 1 mm length show a semiconductor behavior; their thermal activation energy is 0.5 eV and the dark conductivity at RT is 5 × 10-9 (Ω cm)-1. Besides exhibiting a photosensitivity of about 60 under white light illumination with a light power intensity of 25 mW · cm-2, the fibers also attain RT photoluminescence in the cyan, yellow, and red wavelength range under ultraviolet, blue, and green light excitation, respectively. Optical microscope images of F8T2 reveal homogeneous electrospun fibers, which are in good agreement with the uniformly radial fluorescence observed. The production of electrospun fibers from poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-bithiophene] (F8T2) obtained without a carrier polymer is reported. The obtained fibers are shown to have properties suitable for organic fiber photovoltaic and sensors applications. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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

The aim of this work is to present a model to interpret the static and the dynamic detection and resolution limits of 1D thin film position sensitive detectors based on p-i-n a-Si:H devices. The model can determine the device characteristics that influence the spatial limits and the response time of the device.

Silicon nitride films were produced on glass and crystalline silicon substrates using r.f. magnetron sputtering to select the best process conditions (substrate temperature, gas pressure and r.f. power) to grow dielectrics for device applications such as low temperature thin film transistors, where special care has to be taken concerning the film's compactness and bulk defects. The films produced were analysed by different techniques such as ultra violet - visible - near infrared spectroscopy Fourier transformed infrared spectroscopy and capacitance measurements, aiming to correlate the films properties with its composition and degree of compactness. The role of the deposition pressure is notorious since films deposited at high pressures are more compact, presenting low oxygen incorporation after deposition. The increase of the substrate temperature up to 373 °K has the same effect, not changing the film's amorphous structure. These data will be discussed aiming to produce films with the required compactness and stoichiometry to grow very thin insulating layers (<10 nm) to be used in MIS structures or devices like thin film transistors.

In this work, we present the structural and electrical properties of HfO 2, HfO 2 +SiO 2, and HfO 2 +Al 2O 3 dielectric composite layers deposited by sputtering without any intentional substrate heating. The films were deposited on glass and 〈100〉 crystalline silicon (c-Si) substrates from ceramic targets by using argon (Ar) and oxygen (O 2) as sputtering and reactive gases, respectively. The incorporation of SiO 2 and Al 2O 3 into hafnia was obtained by co-sputtering and itwas controlled by adjusting the ratio of r.f. power applied between the targets. The HfO 2 films present a microcrystalline structure, when deposited at room temperature (RT). The lowest leakage current in c-Si MIS (Metal-Insulator- Semiconductor) structures (below 10 9A/cm 2 at 10V on films with a thickness around 180 nm) was obtained for an Ar/O 2 ratio of 14:1 sccm, and further increase in O 2 flow does not enhance the electrical characteristics. The codeposition of SiO 2 or Al 2O 3 with hafnia has a strong influence on the structure of the resulting films since they become amorphous. The leakage current in MISstructures incorporating these multi-component dielectrics is reduced at least by a factor of 2, which is accompanied by an increase on the band gap. The dielectric constant is decreased due to the lower values for SiO 2 and Al 2O 3. © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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

Molybdenum doped (0-1 at. %) indium oxide thin films with high near infrared (NIR) transparency and carrier mobility were deposited on Corning-1737 glass substrates at 400 °C by spray pyrolysis experimental technique. Films with mobility as high as ∼149 cm2 /V s were obtained when annealed in vacuum at 550 °C, which also possess carrier concentration of ∼1× 1020 cm-3 and resistivity as low as ∼4.0× 10-4 cm. Further, both the average visible transmittance (500-800 nm) and the average NIR transmittance are >83%. This clearly shows that the transmittance is extended well into the NIR region. © 2009 American Institute of Physics.

The aim of this work is to present a spectroscopic ellipsometry study focused on the annealing time effect on nickel metal induced crystallization of amorphous silicon thin films. For this purpose silicon layers with 80 and 125 nm were used on the top of which a 0.5 nm Ni thick layer was deposited. The ellipsometry simulation using a Bruggemann Effective Medium Approximation shows that films with 80 nm reach a crystalline fraction of 72% after 1 h annealing, appearing to be full crystallized after 2 h. No significant structural improvement is detected for longer annealing times. On the 125 nm samples the crystalline volume fraction after 1 h is only around 7%, requiring 5 h to get a similar crystalline fraction than the one achieved with the thinner film. This means that the time required for full crystallization will be strongly determined by the Si layer thickness. Using a new fitting approach the Ni content within the films was also determined by SE and related to the silicon film thickness. © 2006 Elsevier B.V. All rights reserved.

Co-doped TiO 2 films were characterized by spectroscopic ellipsometry to determine their thickness, deposition rate and optical properties as function of substance temperature and background gas composition. To fit the data we used a combination of a single Tauc-Lorentz oscillator with the Drude free electron model to take in account the free electrons present in the film. The Co doping and addition of H 2 to the gas phase during film growth cause the formation of a titanium oxide which containsfree electrons that absorb the energy of the red part of the spectrum, causing k to increase. The n of the film at 1.5 eV is about 2.3 eV. The fittings also show that the n of films decreases and k increase at the surface. This can be related to a segregation of Co to the surface, which in some cases, of high substrate temperature and high H 2 flow during deposition, can lead to and even higher concentration of free electrons at the surface. © 2008 WILEY-VCH Verlag GmbH & Co. KGaA.

In this work, spectroscopic ellipsometry was used to characterise oxide films produced by anodic oxidation of amorphous silicon using an ethylene glycol (0.04 M KNO3) solution. The data obtained show that the growth of the oxide is not only a function of the voltage applied, but also of the current density and of the time process. An empiric model based on a power law is proposed for the growth of the oxide using, as parameters, the voltages and the time process. The oxide produced shows porosity of approximately 12%, which can be reduced down to 6% under well-controlled growth conditions. © 2002 Elsevier Science B.V. All rights reserved.

In this work we report the study of spectral response on large area amorphous silicon solar cells (30×40 cm2), deposited through plasma enhanced chemical vapour deposition technique (PECVD) at excitation frequencies of 27.12MHz. To perform this work, the solar cells were split in units of area of 0.126 cm2, which allows determining the device homogeneity over all the entire solar cell. Emphasis of this work is put the role of thickness and optical band gap of p-doped layer on the collection efficiency, spectral response, current density-voltage curves under standard condition and spectroscopy impedance. The results show that high transparent p-doped layer can be deposited at 42mW/cm2, which allows increasing the collection efficiency in 45%, at the blue region. The spectroscopy impedance performed showed to be efficient in analyzing the device shunt resistance, interfaces role on the device performances and the behaviour of the device depletion region, for the range of frequencies analysed.

This paper reports on a method and a test setup developed to measure the transient dark current and the spectral response characteristics of a-Si:H MIS photosensors. Using this method the segmented-gate/SiNx/a Si:H/n +/ITO structures have been characterized under different biasing conditions. The dependences of the dark and light signals on the refresh pulse amplitude, offset voltage and pulse width were measured and analyzed. It is found that the amplitude of the time-dependent component of the leakage current associated with charge trapping at the insulator-semiconductor interface can be significantly reduced by adjusting the offset voltage. The observed bias dependence of the spectral response characteristics is explained by analyzing the charge carrier transport in the absorption layer at different wavelengths of the incident light. © 2008 WILEY-VCH Verlag GmbH & Co. KGaA.

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 the 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 analyze 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 rom the ASCA program (Amorphous Solar Cell Analysis), and for different configurations will be presented.

Thin films of molybdenum doped indium oxide (IMO) were rf sputtered onto glass substrates at room temperature. The films were studied as a function of oxygen volume percentage (OVP) ranging 1.4 - 10.0% in the sputtering chamber. The thickness of the films found varying between 180 and 260 nm. The X-ray diffraction pattern showed the films are polycrystalline with the peaks corresponding to (222) and (400) planes and one among them showing as a preferential orientation. It is observed that the preferred orientation changes from (222) plane to (400) as the OVP increases from 1.4 to 10.0%. The transmittance spectra were found to be in the range of 77 to 89%. The optical band gap calculated from the absorption coefficient of transmittance spectra was around 3.9 eV. The negative sign of Hall coefficient confirmed the films were n-type conducting. The bulk resistivity increased from 2.26×10 -3 to 4.08×-1 Ωcm for the increase in OVP from 1.4 to 4.1%, and thereafter increased dramatically so as the Hall coefficients were not detectable. From the AFM morphologies it is evaluated that the RMS roughness of the films ranges from 0.9 to 3.2 nm. © 2006 Materials Research Society.

Transparent wide band gap indium molybdenum oxide (IMO) thin films were rf sputtered on glass substrates at room temperature. The films were studied as a function of sputtering power (ranging 40-180 W) and sputtering time (ranging 2.5-20 min). The film thickness was varied in the range 50-400 nm. The as-deposited films were characterized by their structural (XRD), morphological (AFM), electrical (Hall Effect measurements) and optical (visible-NIR spectroscopy) properties. XRD studies revealed that the films are amorphous for the sputtering power ≤ 100 W and the deposition time ≤ 5 min, and the rest are polycrystalline with a strong reflection from (222) plane showing a preferential orientation. A minimum bulk resistivity of 2.65 × 10- 3 Ω cm and a maximum carrier concentration of 4.16 × 1020 cm- 3 are obtained for the crystalline films sputtered at 180 W (10 min). Whereas a maximum mobility (19.5  cm2 V- 1 s- 1) and average visible transmittance (∼ 85%) are obtained for the amorphous films sputtered at 80 W and 100 W respectively for 10 min. A minimum transmittance (∼ 18%) was obtained for the crystalline films sputtered at 180 W (∼ 305 nm thick). The optical band gap was found varying between 3.75 and 3.90 eV for various sputtering parameters. The obtained results are analyzed and corroborated with the structure of the films. © 2007 Elsevier B.V. All rights reserved.

Solution-processed field-effect transistors are strategic building blocks when considering low-cost sustainable flexible electronics. Nevertheless, some challenges (e.g., processing temperature, reliability, reproducibility in large areas, and cost effectiveness) are requirements that must be surpassed in order to achieve high-performance transistors. The present work reports electrolyte-gated transistors using as channel layer gallium-indium-zinc-oxide nanoparticles produced by solvothermal synthesis combined with a solid-state electrolyte based on aqueous dispersions of vinyl acetate stabilized with cellulose derivatives, acrylic acid ester in styrene and lithium perchlorate. The devices fabricated using this approach display a ION/IOFF up to 1 × 106, threshold voltage (VTh) of 0.3-1.9 V, and mobility up to 1 cm2/(V s), as a function of gallium-indium-zinc-oxide ink formulation and two different annealing temperatures. These results validates the usage of electrolyte-gated transistors as a viable and promising alternative for nanoparticle based semiconductor devices as the electrolyte improves the interface and promotes a more efficient step coverage of the channel layer, reducing the operating voltage when compared with conventional dielectrics gating. Moreover, it is shown that by controlling the applied gate potential, the operation mechanism of the electrolyte-gated transistors can be modified from electric double layer to electrochemical doping. © 2014 American Chemical Society.

Solution-processed metal-oxide thin films with high dielectric constants (k) have been extensively studied for low-cost and high-performance thin-film transistors (TFTs). In this report, MgO dielectric films were fabricated using the spin-coating method. The MgO dielectric films annealed at various temperatures (300, 400, 500, and 600 °C) were characterized by using thermogravimetric analysis, optical spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and atomic-force microscopy. The electrical measurements indicate that the insulating properties of MgO thin films are improved with an increase in annealing temperature. In order to clarify the potential application of MgO thin films as gate dielectrics in TFTs, solution-derived In2O3 channel layers were separately fabricated on various MgO dielectric layers. The optimized In2O3/MgO TFT exhibited an electron mobility of 5.48 cm2/V s, an on/off current ratio of 107, and a subthreshold swing of 0.33 V/dec at a low operation voltage of 6 V. This work represents a great step toward the development of portable and low-power consumption electronics. © 2016 Author(s).

High-k alkaline lithium oxide (LiOx) thin films are fabricated by spin-coating method. The LiOx thin films are annealed at different temperatures and characterized by various techniques. An optimized LiOx dielectric is achieved at an annealing temperature of 300 °C and exhibits wide bandgap of ≈5.5 eV, smooth surface, relatively permittivity of ≈6.7, and low leakage current density. The as-fabricated LiOx thin films are integrated, as gate dielectrics, in both n-channel indium oxide (In2O3) and p-channel cupric oxide (CuO) transistors. The optimized In2O3/LiOx thin-film transistor (TFT) exhibits high performance and high stability, such as Ion/Ioff of 107, electron mobility of 5.69 cm2 V−1 s−1, subthreshold swing of 70 mV dec−1, negligible hysteresis, and threshold voltage shift of 0.1 V under bias stress for 1.5 h. Meanwhile, the p-channel CuO TFT based on LiOx dielectric shows high Ion/Ioff of 105 and hole mobility of 1.72 cm2 V−1 s−1. All the electrical performances are achieved at an ultra-low operating voltage of 2 V. Considering the simple procedure, the moderate annealing temperature, and the low power consumption merits, these outstanding characteristics represent a significant advance toward the development of battery compatible and portable electronics. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

A commercial sheet of paper based on natural cellulose fibers acting as permeable membrane with thin film metal cathode (Cu) and anode (Al) layers in each face was used to produce paper batteries that could be interconnected in series and rechargeable using water as electrolyte. Their electrical characteristics and the set of electrochemical reactions that support the experimental behavior observed are described in this paper. A series of integrated batteries able to supply a voltage of about 3 V and a current ranging from 0.7 μA to 25 μA in cells with sizes of 1.2 cm × 3.0 cm for a relative humidity in the range of 50-65% were produced in a single sheet of paper, and successfully applied to control the ON/OFF gate state of paper transistors. © 2010 Elsevier Ltd. All rights reserved.

In this paper, the role of electrode architecture (conventional and interdigital), device structure (vertical or planar), and tungsten oxide (WO3) channel thickness on the electro-optical performances of room temperature sputtered electrochromic transistors (EC-Ts) is reported. A larger number of electro-reducible tungsten sites in thicker WO3 films provide improved optical density and coloration efficiency. However, overall transistor performance is found to suffer in planar EC-Ts with the conventional electrode architecture, where the step to planar interdigital electrodes leaves the devices to be almost insensitive to WO3 thickness. Vertical structures result in improved device properties and stability, given to the shorter distance between gate electrode and semiconductor and to the encapsulation effect provided by such structures. These devices show an On–Off ratio of 5 × 106 and a transconductance (g m) of 3.59 mS, for gate voltages (V G) between −2 and 2 V, which to the authors' knowledge are the best values ever reported for electrochemical transistors. The simple and low-cost processing together with the electrical/optical performances well supported into a comprehensive analysis of device physics opens doors for a wide range of new applications in display technologies, biosensors, fuel cells, or electrochemical logic circuits. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim