Rodrigo Martins

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

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

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

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

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

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

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

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

In this paper we report some of the recent advances in transparent thin film oxide semiconductors, specifically zinc oxide (ZnO), produced by rf magnetron sputtering at room temperature with multifunctional properties. By controlling the deposition parameters it is possible to produce undoped material with electronic semiconductor properties or by doping it to get either n-type or p-type semiconductor behavior. In this work we refer our experience in producing n-type doping ZnO as transparent electrode to be used in optoelectronic applications such as solar cells and position sensitive detectors while the undoped ZnO can be used as UV photodetector or ozone gas sensor or even as active layer of fully transparent thin film transistors.

In this work, we investigate the role of amorphous silicon (a-Si:H) thin films deposited by a plasma enhanced chemical vapor deposition (PECVD) technique on porous silicon (PS) to facilitate its water vapor and oxygen gas sensing properties using its electrical response. Overall we notice a rectifying behavior from a-Si:H/PS heterojunction device, where a current enhancement of one and four orders of magnitude was observed in the presence of oxygen gas and water vapor, in comparison with atmospheric air at room temperature, respectively. The photoluminescence (PL) investigation of PS shows a slight blue shift in the PL emission band from 1.72 to 1.77 eV and the intensity of the PL is enhanced by a factor of 5.4 with increase of porosity from 21% to 77%. This PL emission may originate from the O-Si-H related absorbance bands. Alternatively, quenching of the PL intensity was observed after a-Si:H films were deposited on PS specimens. Besides, micro-Raman and atomic force microscopic (AFM) analyse were carried out to understand the structure and morphological features of the PS and a-Si:H/PS specimens. © 2007 Elsevier B.V. All rights reserved.

This chapter gives an overview about GIZO TFTs, comprising an introductory section about generic TFT structure and operation, different semiconductor technologies for TFTs - with special emphasis on AOSs and particularly on GIZO - and then some experimental results obtained for GIZO TFTs fabricated in CENIMAT. Thin-film transistors (TFTs) are important electronic devices which are predominantly used as On/Off switches in active matrix backplanes of flat panel displays (FPDs), namely liquid crystal displays (LCDs) and organic light emitting device (OLED) displays. Even if a-Si:H is still dominating the TFT market in terms of semiconductor technology, oxide semiconductors are emerging as one of the most promising alternatives for the next generation of TFTs, bringing the possibility of having fully transparent devices, low processing temperature, low cost, high performance and electrically stable properties [1, 2]. Amorphous oxide semiconductors (AOS) such as Gallium-Indium-Zinc oxide (GIZO) [3, 4], even if fabricated at temperatures below 150°, are currently capable of providing transistors with field-effect mobility (μFE) exceeding 20 cm2V-1 s-1, threshold voltage (VT) close to 0V, On/Off ratios above 108, subthreshold swing (S) around 0:20V dec-1 and fully recoverable VT shift (ΔVT) lower than 0.5V after 24 h stress with constant drain current of 10 μA. © Springer-Verlag Berlin Heidelberg 2012.

We report in this paper the influence of the rf power on the properties of p-type silicon thin films produced by hot wire plasma assisted chemical vapor deposition (HWPA-CVD) technique, using a gas mixture containing SiH4, B2H6, CH4 and H2. The influence of the rf power in the film morphology, its structure and its composition has been determined by means of scanning electron microscopy (SEM), X-ray diffraction (XRD) and infrared spectroscopy. The electrical dark conductivity, activation energy, optical band gap and growth rate values for the different rf power was also evaluated. The data achieved show that rf power rules the surface morphology, the film structure and its electrical characteristics. © 2001 Elsevier Science B.V. All rights reserved.

In this work we present data concerning the structure, composition and electro-optical performances of nanocrystalline silicon carbide doped films produced at the different filament temperatures and hydrogen dilution ratios. The XRD spectra reveal the presence of the typical Si peaks ascribed to (111) (220) and (311) diffraction planes, where no traces of the carbon peaks were found. The average grain sizes ranges from 10 nm to 30 nm, depending on the temperature of filament and hydrogen dilution used. We observed an enhancement of the peak ascribed to the (220) plane when high H dilution rates are used, meaning that the film starts being textured. The infrared data reveal the typical silicon carbide modes and a hydrogen content that varies from 3% to 1%, with the increase of the filament temperature. Besides that, the IR spectra show the typical SiO2 and SiO modes, associated to the oxide species that are mainly incorporated in the surface of the films and can be removed by proper wet etching. The planar conductivity is enhanced as the temperature of the filament is increased, being the highest conductivity achieved in the range of 0.2 (Ωcm)-1 and almost non activated. © 1998 Elsevier Science Ltd. All rights reserved.

The p/i interface plays a major role in the conversion efficiency of nanocrystalline silicon (nc-Si:H) solar cells. Under plasma-enhanced chemical vapor deposition (PECVD) of the intrinsic (i) nc-Si:H layer, ion bombardment can severely affect the underlying p-doped layer and degrade the solar cell performance. The core of the present work is to investigate the effect of light and heavy ion bombardment on the structural modifications of the p-layer during the p/i interface formation. The properties of the nc-Si:H materials deposited under distinct conditions are analyzed and correlated to the deposition rate and the resulting cell efficiency. To recreate the ion bombardment during the initial stages of the i-layer deposition on the p-layer, hydrogen plasma treatment was performed for 30 s (light ion bombardment), after which a flux of silane was introduced into the deposition chamber in order to initiate the heavy ion bombardment and growth of an ultra-thin (5 nm) i-layer. The structural changes of the p-type nc-Si:H layers were observed by spectroscopic ellipsometry. The obtained results confirm that detrimental structural modifications (e.g. partial amorphization of the sub-surface region and bulk) occur in the p-layer, caused by the ion bombardment. To minimize this effect, a protective buffer layer is investigated able to improve the performance of the solar cells fabricated under increased growth rate conditions. © 2015 Elsevier B.V. All rights reserved.

In this work, the optical and structural properties of high k materials such as tantalum oxide and titanium oxide were studied by spectroscopic ellipsometry, where a Tauc-Lorentz dispersion model based in one (amorphous films) or two oscillators (microcrystalline films) was used. The samples were deposited at room temperature by radio frequency magnetron sputtering and then annealed at temperatures from 100 to 500 °C. Concerning the tantalum oxide films, the increase of the annealing temperature, up to 500 °C does not change the amorphous nature of the films, increasing, however, their density. The same does not happen with the titanium oxide films that are microcrystalline, even when deposited at room temperature. Data concerning the use of a four-layer model based on one and two Tauc-Lorentz dispersions is also discussed, emphasizing its use for the detection of an amorphous incubation layer, normally present on microcrystalline films grown by sputtering. © 2006 Elsevier B.V. All rights reserved.

This paper reports the performance of small area solar cells, 128 linear integrated position sensitive detector arrays and thin film transistors based on nanostructured silicon thin films produced by plasma-enhanced chemical vapour deposition technique, close to the onset of dusty plasma conditions, within the transition region from amorphous to microcrystalline. The small area solar cells, produced in a modified single chamber reactor, exhibited very good electrical characteristics with a conversion efficiency exceeding 9%. The 128 integrated position sensitive detector arrays, based on a similar pin structure, allow real-time 3D object imaging with a resolution higher than 90 l p/mm. The thin film transistors produced exhibited field effect mobility of 2.47 cm 2/V/s, threshold voltage of 2 V, on/off ratio larger than 10 7 and sub-threshold slopes of 0.32 V/decade, which are amongst the best results reported for this type of device. © 2009 Taylor & Francis.

Nanostructured silicon (na-Si:H) thin films were fabricated using plasma enhanced chemical vapour deposition (PECVD) technique under high silane hydrogen dilution and a discharge frequency of 27 MHz, where the substrate was located in the dark region of the plasma, protected by a grounded metal grid. By not exposing the growth surface directly to the plasma we avoid the silicon growth surface to sustain a high ion bombardment leading to a less defective surface and highly compact films. The intrinsic films grown under these conditions were used to produce the channel region of thin film transistors (TFTs) with a bottom gate staggered configuration, integrating different dielectric layers. The devices produced exhibit a field effect mobility close to 1.84 cm 2 V -1S -1, threshold voltage around 2 V, on/off ratio above 10 7 and sub-threshold slope below 0.5 V/decade, depending on the dielectric used. Copyright © 2010 American Scientific Publishers All rights reserved.

The aim of this paper is to present a set of electric data concerning the performances before and after ageing of Cu-Sn-Cu joins used to solder power diodes and to compare the results achieved with the ones obtained in diodes soldered using the conventional technology. The set of results achieved show that the Cu-Sn-Cu joins present even better performances than the ones exhibited by diodes soldered using the conventional technology, without requiring the use of Mo discs to be inserted between the silicon crystal and the metal contacts (stud or finger) to compensate thermal mismatches.

Thin film transistors (TFTs) have been produced by rf magnetron sputtering at room temperature, using non conventional oxide materials like amorphous indium-zinc-oxide (IZO) semiconductor, for the channel as well as for the drain and source regions. The obtained TFTs operate in the enhancement mode with threshold voltages of 2.4 V, saturation mobility of 22.7 cm2/Vs, gate voltage swing of 0.44 V/dec and an ON/OFF current ratio of 7×10 7. The high performances presented by these TFTs associated to a high electron mobility, at least two orders of magnitude higher than that of conventional amorphous silicon TFTs and a low threshold voltage, opens new doors for applications in flexible, wearable, disposable portable electronics as well as battery-powered applications.

Gallium-doped zinc oxide films were prepared by r.f. magnetron sputtering at room temperature as a function of the substrate-target distance. The best results were obtained for a distance of 10 cm, where a resistivity as low as 2. 7 × 10-4 Ω cm, a Hall mobility of 18 cm2/Vs and a carrier concentration of 1.3 × 1021 cm-3 were achieved. The films are polycrystalline presenting a strong crystallographic c-axis orientation (002) perpendicular to the substrate. The films present an overall transmittance in the visible part of the spectra of approximately 85%, on average. © 2003 Elsevier B.V. All rights reserved.

Gallium-doped zinc oxide (GZO) thin films have been deposited onto polyethylene naphthalate (PEN) substrates by r.f. magnetron sputtering at room temperature. The influence of the film thickness (from 70 to 890 nm) on the electrical, structural and morphological properties are presented. The lowest resistivity obtained was 5 × 10-4 Ω cm with a Hall mobility of 13.7 cm2/Vs and a carrier concentration of 8.6 × 1020 cm-3. These values were obtained by passivating the surface of the polymer with a thin silicon dioxide, so preventing the moisture and oxygen permeation inside the film. © 2003 Elsevier B.V. All rights reserved.

A new high quantum efficiency gallium phosphide Schottky photodiode has been developed by spray deposition of heavily doped tin oxide films on n-type epitaxial structures, as an alternative to the conventional Schottky photodiodes using a semitransparent gold electrode. It is shown that fluorine-doped tin oxide films are more effective as transparent electrodes than tin-doped indium oxide films. The proposed photodiodes have a typical responsivity near 0.33 A W-1 at 440 nm and an unbiased internal quantum efficiency close to 100%, in the range from 250 to 450 nm. The model used to calculate the internal quantum efficiency (based on the optical constants of tin oxide films and gallium phosphide epitaxial layers) is found to be in good agreement with the experimental results. The data show that the quantum efficiency is strongly dependent on the thickness of the transparent electrode, owing to optical interference effects. The noise equivalent power for 440 nm is 2.7 × 10-15 W Hz-1/2, which indicates that these photodiodes can be used for accurate measurements in the short-wavelength range, even in the presence of stronger infrared background radiation.

In this paper we present an improved version of large area (5 mm × 80 mm) flexible position sensitive detectors deposited on polyimide (Kapton® VN) substrates with 75 μm thickness, produced by plasma enhanced chemical vapor deposition (PECVD). The structures presented by the sensors are Kapton/ZnO:Al/(pin)a-Si:H/Al and the heterostructure Kapton/Cr/(in)a-Si:H/ZnO:Al. These sensors were characterized by spectral response, photocurrent dependence as a function of light intensity and position detectability measurements. The set of data obtained on one-dimensional position sensitive detectors based on the heterostructure show excellent performances with a maximum spectral response of 0.12 A/W at 500 nm and a non-linearity of ±10%. © 2002 Elsevier Science B.V. All rights reserved.

Large-area thin-film position-sensitive detectors (TFPSDs) using the hydrogenated amorphous silicon (a-Si:H) technology are presented. The detection accuracy of these devices (lengths of about 80 mm) is better than ±0.5% of the value of the full scale of the sensor, the spatial resolution is better than ±20 μm, the non-linearities measured are below ±2% and the frequency response is in the range of a few kilohertz, compatible with the sampling frequency of most electromechanical assembling/control systems. The obtained results are quite promising regarding the application of these sensors to a wide variety of optical inspection systems. © 1998 Elsevier Science S.A. All rights reserved.

Large-area thin-film position-sensitive detectors (TFPSDs) using the hydrogenated amorphous silicon (a-Si:H) technology are presented. The detection accuracy of these devices (lengths of about 80 mm) is better than ±0.5% of the value of the full scale of the sensor, the spatial resolution is better than ±20 μm, the non-linearities measured are below ±2% and the frequency response is in the range of a few kilohertz, compatible with the sampling frequency of most electromechanical assembling/control systems. The obtained results are quite promising regarding the application of these sensors to a wide variety of optical inspection systems.