Rodrigo Martins

Undoped and doped hydrogenated amorphous silicon semiconductors (a-Si:H) have been produced by a two consecutive decomposition deposition chamber (TCDDC) system assisted by electromagnetic static fields. Through this technique, a spatial separation is achieved between the plasma chemistry and that of the deposition to avoid ion and electron (with high energies) bombardment on the growing surface. Besides this, the use of a static magnetic field perpendicular to the substrate will promote plasma confinement, so avoiding its contamination by residual gases adsorbed on the reactor walls. On the other hand, the use of two grids dc biased in the deposition chamber, will allow control of the main film precursors, responsible for the electro-optical and structural properties of deposited films. In this paper we shall discuss the deposition method used as well as the transport, structural and morphological properties presented by deposited films and its dependence on deposition parameters used. © 1989.

A wide bandgap and highly conductive p-type hydrogenated nanocrystalline silicon (nc-Si:H) window layer was prepared with a conventional RF-PECVD system under large H dilution condition, moderate power density, high pressure and low substrate temperature. The optoelectrical and structural properties of this novel material have been investigated by Raman and UV-VIS transmission spectroscopy measurements indicating that these films are composed of nanocrystallites embedded in amorphous SiHx matrix and with a widened bandgap. The observed downshift of the optical phonon Raman spectra (514.4 cm-1) from crystalline Si peak (521 cm-1) and the widening of the bandgap indicate a quantum confinement effect from the Si nanocrystallites. By using this kind of p-layer, a-Si:H solar cells on bare stainless steel foil in nip sequence have been successfully prepared with a Voc of 0.90 V, a fill factor of 0.70 and an efficiency of 9.0%, respectively. © 2006 Elsevier B.V. All rights reserved.

Solution-processed p-type oxide semiconductors have recently attracted increasing interests for the applications in low-cost optoelectronic devices and low-power consumption complementary metal-oxide-semiconductor circuits. In this work, p-type nickel oxide (NiOx) thin films were prepared using low-temperature solution process and integrated as the channel layer in thin-film transistors (TFTs). The electrical properties of NiOx TFTs, together with the characteristics of NiOx thin films, were systematically investigated as a function of annealing temperature. By introducing aqueous high-k aluminum oxide (Al2O3) gate dielectric, the electrical performance of NiOx TFT was improved significantly compared with those based on SiO2 dielectric. Particularly, the hole mobility was found to be 60 times enhancement, quantitatively from 0.07 to 4.4 cm2/V s, which is mainly beneficial from the high areal capacitance of the Al2O3 dielectric and high-quality NiOx/Al2O3 interface. This simple solution-based method for producing p-type oxide TFTs is promising for next-generation oxide-based electronic applications. © 2016 Author(s).

The aim of this paper is to present data on the dependence of the electro-optical characteristics and structure of n-type microcrystalline silicon films on the r.f. power used during the deposition of films produced by the plasma-enhanced chemical vapour deposition technique. The interest of these films arise from the fact that they combine some electro-optical advantages of amorphous (wide optical gap) and crystalline materials (electronic behaviour), highly interesting in the production of a wide variety of optoelectronic devices such as solar cells and thin film transistors. In this paper, microcrystalline n-type films presenting simultaneously optical gaps of about 2.3 eV, dark conductivity of 6.5 S cm-1 and Hall mobility of about 0.86 cm2 V-1 s-1 will be reported, the highest combined values for n-type microcrystalline silicon films, as far as we know. © 1997 Elsevier Science S.A.

Molybdenum-doped indium oxide (IMO) thin films were deposited at 450 °C for varying molybdenum concentrations in the range of 0.5-2 at% by the spray pyrolysis technique. These films confirmed the cubic bixbyite structure of polycrystalline In2O3. The preferred growth orientation along the (2 2 2) plane shifts to (4 0 0) on higher Mo doping levels. The films doped with 0.5 at% Mo showed high mobility of 76.9 cm2/(V s). The high visible transmittance extends well into the near-infrared region. A possibility of using the produced IMO films in nanocrystalline (nc) silicon solar cell applications is discussed in this article. The morphological studies showed a change in the microstructure, which is consistent with the change in crystallographic orientation. © 2008 Elsevier B.V. All rights reserved.

Molybdenum doped indium oxide (IMO) thin films were deposited on the glass substrates preheated to 450 °C by spray pyrolysis technique. The Mo doping was varied between 0 and 2.0 at.%. The films were characterized by their structural, electrical and optical properties. The films are confirmed to be cubic bixbyite In2O3 with a strongest orientation along (222) plane, which is shifted to (400) plane for the increase in Mo doping to 1.25 and 2 at.%. The film deposited with 0.5 at.% Mo doping shows high mobility of 76.9 cm2V- 1s- 1 , resistivity of 1.8 × 10- 3 Ω-cm and high carrier concentration of 4.6 × 1019 cm- 3 with 81.3% transmittance in the visible range between 500 and 800 nm. Further, the transparency extents well into the near-IR range. © 2008 Elsevier B.V. All rights reserved.

Molybdenum (0-1 at. %) doped indium oxide thin films with high near-infrared (NIR) transparency and high carrier mobility were deposited on Corning-1737 glass substrates at 400 °C by a spray pyrolysis experimental technique. X-ray diffraction (XRD) analysis confirmed the cubic bixbyite structure of indium oxide. The preferred growth orientation along the (222) plane for the low Mo doping level (0.5 at. %) shifts to (400) for higher Mo doping levels (<0.6 at. %). The crystallite size extracted from the XRD data corroborates the changes in full width at half maximum due to the variation in Mo doping. A scanning electron microscopy study illustrated the evolution in the surface microstructure as a function of Mo doping. The negative sign of the Hall coefficient confirmed the n -type conductivity. A high carrier mobility of ∼122.4 cm2 /V s, a carrier concentration of ∼9.5× 1019 cm-3, a resistivity of ∼5.3× 10-4cm, and a high figure of merit of ∼4.2× 10-2 -1 are observed for the films deposited with 0.5 at. % Mo. The obtained high average transparency of ∼83% in the wavelengths ranging from 400 to 2500 nm confirmed the extension of transmittance well into the NIR region. © 2009 American Institute of Physics.

This paper deals with high transparent and high conductive oxides based on polycrystalline titanium (Ti) doped (0.5-3 at.%) indium oxide (IO) thin films produced on glass substrates at 400 °C by spray pyrolysis technique. X-ray diffraction analysis confirmed the cubic bixbyite structure of indium oxide. A high mobility of ∼ 97 cm2 V- 1 s- 1, a carrier concentration of ∼ 1.55 × 1020 cm- 3 and a resistivity of ∼ 4.11 × 10- 4 Ω-cm with ∼ 83% of transmittance in the wavelength ranging between 400 and 2500 nm were obtained for 2 at.% Ti doping films, rivalling so to the best known transparent conducting oxide based on indium tin oxide. Moreover, the transmittance in the broad wavelength ranging between 400 and 2500 nm is over 83%, leading so to an increasing carrier generation towards the near infrared region of the spectrum, as required for applications such as solar cells. We also notice that increasing the doping concentration widened the optical band gap and caused a small Burstein-Moss shift, due to mobility decrease, as expected. © 2012 Published by Elsevier B.V.

In this work the electrical and structural properties of two high k materials as hafnium oxide (HfO2) and tantalum oxide (Ta2O5) produced at room temperature are exploited. Aiming low temperature processing two techniques were employed: r.f. sputtering and electron beam evaporation. The sputtered HfO2 films present a nanocrystalline structure when deposited at room temperature. The same does not happen for the evaporated films, which are essentially amorphous. The density and the electrical performance of both sputtered and evaporated films are improved after annealing them at 200 °C. On the other hand, the Ta2O5 samples deposited at room temperature are always amorphous, independently of the technique used. The density and electrical performance are not so sensitive to the annealing process. The set of data obtained show that these dielectrics processed at temperatures below 200 °C present promising properties aiming to produce devices at low temperature with improved interface properties and reduced leakage currents. © 2007 Elsevier B.V. All rights reserved.

This paper describes the development of a novel microfluidic platform for multifactorial analysis integrating four label-free detection methods: electrical impedance, refractometry, optical absorption and fluorescence. We present the rationale for the design and the details of the microfabrication of this multifactorial hybrid microfluidic chip. The structure of the platform consists of a three-dimensionally patterned polydimethylsiloxane top part attached to a bottom SU-8 epoxy-based negative photoresist part, where microelectrodes and optical fibers are incorporated to enable impedance and optical analysis. As a proof of concept, the chip functions have been tested and explored, enabling a diversity of applications: (i) impedance-based identification of the size of micro beads, as well as counting and distinguishing of erythrocytes by their volume or membrane properties; (ii) simultaneous determination of the refractive index and optical absorption properties of solutions; and (iii) fluorescence-based bead counting. © 2016 by the authors.

A double-chamber system was used to deposit large-area hydrogenated amorphous silicon films for photovoltaic applications. The electro-optical characterisation of films of area 400 cm2 deposited on glass substrates is described in this paper. The deposition rate of the films is dependent on the r.f. power delivered, the substrate bias and the partial pressure of the reactive gas. The film thickness was observed to have a uniformity of better than 0.5%. The best film quality was obtained for a deposition rate of about 1.5 Å s-1. The optical gap, activation energy, photosensitivity, density of gap states and hydrogen content were determined. © 1985.

In the past we have developed a transient technique, called the Flying Spot Technique (FST). FST allows, not only to infer the ambipolar diffusion length but also the effective lifetime of the photogenerated carriers once the light spot velocity and geometry of the structure were known. In this paper, we propose to apply this technique backwards in order to detect the path and velocity of an object that is moving in the direction of a light source. The light reflected back from the object is analyzed through a p.i.n structure being the transient transverse photovoltage dependent on the movement of the object (position and velocity). Assuming that the transport properties of the material and the geometry of the device are known and using a triangulation method we show that it is possible to map the movement of the object. Details concerning material characterization, simulation and device geometry are presented.

PIN devices based on hydrogenated amorphous silicon (a-Si:H) became fundamental elements of many different types sensors, based on either the transverse or the lateral photovoltaic effect. In the past we have developed a transient technique, called the Flying Spot Technique (FST), based on the lateral photoeffect. FST allows, not only to infer the ambipolar diffusion length but also the effective lifetime of the photogenerated carriers once the light spot velocity and geometry of the structure were known. In this paper we propose to apply this technique backwards in order to detect the path and velocity of an object that is moving in a light source direction. The light reflected back from the object is analyzed through p.i.n. structure being the transient transverse photovoltage dependent on the object movement (position and velocity). Assuming known the transport properties of the material and the geometry of the device and using a triangulation method we show that it is possible to map the object movement. Details concerning material characterization, simulation and device geometry are presented.

Hydrogenated nanocrystalline silicon (nc-Si:H) n-layers have been used to prepare heterojunction solar cells on flat p-type crystalline silicon (c-Si) wafers. The nc-Si:H n-layers were deposited by radio-frequency (RF) plasma enhanced chemical vapor deposition (PECVD), and characterized using Raman spectroscopy, optical transmittance and activation energy of dark-conductivity. The nc-Si:H n-layers obtained comprise fine grained nanocrystallites embedded in amorphous matrix, which have a wider bandgap and a smaller activation energy. Heterojunction solar cells incorporated with the nc-Si n-layer were fabricated using configuration of Ag (100 nm)/lT0 (80 nm)/n-nc-Si:H (15 nm)/buffer a-Si:H/p-c-Si (300 μm)/Al (200 nm), where a very thin intrinsic a-Si:H buffer layer was used to passivate the p-c-Si surface, followed by a hydrogen plasma treatment prior to the deposition of the thin nanocrystalline layer. The results show that heterojunction solar cells subjected to these surface treatments exhibit a remarkable increase in the efficiency, up to 14.1% on an area of 2.43 cm2. © 2006 Elsevier B.V. All rights reserved.