Advanced Materials Forum III

Paper Title Page

Authors: Simone C. Trippe, Joana Catarina Madaleno, Luiz Pereira
Abstract: In this work fluorinated Diamond Like Carbon (DLC) films have been grown with different CF4 concentrations and have been studied by electrical DC measurements in a temperature range from 30 to 300 K. It was found that the samples grown with lower CF4 concentration show a small rectification, with a potential barrier lower than 0.3 V. The bulk conduction shows a trapcontrolled Space Charge Limited Current (SCLC), with characteristic trap energy between 0.08 and 0.13 eV, confirmed by the differential conductivity analysis. The activation energy (ranging from 50 to 140 meV) is also dependent on the sample fluorine concentration, decreasing with the fluorine concentration increase.
Authors: Luís Pereira, Pedro Barquinha, Elvira Fortunato, Rodrigo Martins
Abstract: 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.
Authors: Leandro Raniero, Alexandra Gonçalves, Ana Pimentel, Shibin Zhang, Isabel Ferreira, Paula M. Vilarinho, Elvira Fortunato, Rodrigo Martins
Abstract: In this work we studied the influence of the power density of hydrogen plasma on electrical and optical properties (Hall mobility, free carrier concentration, sheet resistance, optical transmittance and a.c. impedance) of indium zinc oxide films, aiming to determine their chemical stability. This is an important factor for the optimization of amorphous/nanocrystalline p-i-n hydrogenated silicon (a/nc-Si:H) solar cells, since they should remain chemically highly stable during the p layer deposition. To perform this work the transparent conductive oxide was exposed to hydrogen plasma at substrate temperature of 473 K, 87 Pa of pressure and 20 sccm of hydrogen flow. The results achieved show that IZO films were reduced for all plasma conditions used, which leads mainly to a decrease on films transmittance. For the lowest power density used in the first minute of plasma exposition the transmittance of the IZO films decreases about 29%.
Authors: Pedro Barquinha, Elvira Fortunato, Alexandra Gonçalves, Ana Pimentel, António Marques, Luís Pereira, Rodrigo Martins
Abstract: The purpose of this work is to present in-depth electrical characterization on transparent TFTs, using zinc oxide produced at room temperature as the semiconductor material. Some of the studied aspects were the relation between the output conductance in the post-pinch-off regime and width-to-length ratios, the gate leakage current, the semiconductor/insulator interface traps density and its relation with threshold voltage. The main point of the analysis was focused on channel mobility. Values extracted using different methodologies, like effective, saturation and average mobility, are presented and discussed regarding their significance and validity. The evolution of the different types of mobility with the applied gate voltage was investigated and the obtained results are somehow in disagreement with the typical behavior found on classical silicon based MOSFETs, which is mainly attributed to the completely different structures of the semiconductor materials used in the two situations: while in MOSFETS we have monocrystalline silicon, our transparent TFTs use poly/nanocrystalline zinc oxide with grain sizes of about 10 nm.
Authors: Viorica Muşat, Paula M. Vilarinho, Regina da Conceição Corredeira Monteiro, Elvira Fortunato, E. Segal
Abstract: The thermoreactivity of a zinc acetate non-alkoxide solution used for the preparation of ZnO-based thin films was investigated in the temperature range 20-600°C by TG-DTA, XRD and SEM data. We found that the formation in air of ZnO crystallites from the sol-gel precursor occurs above 150°C simultaneously with the decomposition of an intermediary compound, most probably carbonate hydroxide (sclarite and/or hydrozincite). At 200 °C, the crystalline structure is well defined in terms of ZnO hexagonal lattice parameters, although residual organic compounds and water were not yet fully removed and an amorphous phase coexists. A kinetic investigation on the thermal decomposition of sol-gel precursor from DTA data using Kissinger differential equation is also presented. Apparent activation energy values of about. 100 kJ mol-1 corresponding to the nonisothermal decomposition of solid precursors in the temperature range 170-250oC have been found.
Authors: V.M. Silva, Manuel P.F. Graça, Joana Catarina Madaleno, Luís Cadillon Costa, Luiz Pereira
Abstract: In the present work, the nature of the electrical mechanism for carrier transport in Alq3 is studied by current-voltage measurements and broadband dielectric spectroscopy. The d.c. currentvoltage characteristics at low applied electrical field exhibits a classical “N” shape due to interfacial states located at metal-organic interface, but tend to disappear when successive higher forward bias is applied. Using dielectric spectroscopy it is possible to observe that the main relaxation peak shifts to a higher frequency with the increase of the applied d.c. voltage (from approximately 100 Hz with 0 V d.c. bias to approximately 400 Hz with 6 V d.c. bias) indicating a semiconductor structure change. The logarithmic Z’’ vs. Z’ plot has a slope about 0.7 that decreases to 0.5 with the increasing applied d.c. voltage, reaching a classic Debye relaxation. An attempt to correlate with some structural changes is made.
Authors: Alexandra Gonçalves, Gonçalo Gonçalves, Elvira Fortunato, António Marques, Ana Pimentel, Rodrigo Martins, Manuela A. Silva, Michael J. Smith, João Bela, João P. Borges
Abstract: Electrochromic materials have attracted considerable attention during the last two decades as a consequence of their potential application in several different types of optical devices. Examples of these devices include intelligent windows and time labels. In this paper the authors describe results obtained with thin tungsten oxide films produced at room temperature by rf magnetron sputtering under an argon and oxygen atmosphere on transparent conductive oxide coated glass substrates. To protect the surface of the electrochromic film, prevent water absorption and obtain a good memory effect under open circuit voltages, a layer of Ta2O5 was deposited over the WO3 films. In this study, the effect of different electrolyte compositions on the open circuit memory of optical devices has been characterized. The best results were obtained for electrochromic devices with polymer gel p(TMC)3LiClO4 and p(TMC)8LiClO4 electrolytes. These prototype devices present an overall transmittance of ~75% in their bleached state and after coloration 40.5 and 52.5% respectively. These devices also show memory effect and an optical density considered satisfactory for some electrochromic applications.
Authors: Sergey K. Poznyak, Vladislav V. Kharton, Jorge R. Frade, Mário G.S. Ferreira
Abstract: Several alkaline baths based on different complexing agents were examined for iron electroplating. The resultant films were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). It was shown that adherent and smooth iron coatings with uniform microstructure can be obtained using alkaline Fe (II) baths containing pyrophosphate and tartrate ions as complexing agents. The average grain size can be substantially decreased by glycine additions in the pyrophosphate bath. The faradaic efficiency in these electrolytes may achieve up to 40-50%. The tartrate-containing baths are characterized with a higher throwing power and an increased buffer capacity with respect to the pyrophosphate-based electrolytes. The resultant Fe coatings are single-phase, whilst substantial broadening of the XRD peaks indicates nano-scale grain size. The alkaline baths based on EDTA complexes of iron (III) give black dull iron deposits and are characterized by rather low cathodic current efficiencies, especially at low current densities.
Authors: António F. da Cunha, F. Kurdzesau, Pedro M.P. Salomé
Abstract: The potential of RF-magnetron sputtering to achieve high quality Cu(In,Ga)Se2 (CIGS) thin films and efficient solar cells with the goal of using a single technique for all solar cell processing steps is explored. The end point detection method was adapted to RF-magnetron deposition of CIGS in two- or three stages sputtering process. It allows the control of the final composition of the deposited layers in a reproducible way. The influence of substrate temperature and Ar pressure during the deposition on the surface and crossectional morphology of CIGS films was studied for two and three-stage sputtering process sequence. The solar cells prepared with films deposited by two-stage sputtering nave showed a better performance with maximum efficiency above 8 %.
Authors: M. Manuela M. Raposo, Ana M. R. C. Sousa, António Maurício C. Fonseca, Gilbert Kirsch
Abstract: New tricyanovinyl- derivatives 1 of 1-(alkyl)aryl-2-(2´-thienyl)pyrroles 2 have been synthesized and characterized. Compounds 1 display dramatic reductions in both their optical and electrochemical band gaps in comparison to thienylpyrroles 2. The solvatochomic behavior of tricyanovinyl- derivatives 1 was investigated in a variety of solvents. In agreement with the solvatochromic and the electrochemical studies for push-pull derivatives 1 the new compounds prepared, can find application for manufacturing new materials with notable non-linear optical properties.

Showing 11 to 20 of 346 Paper Titles