Browsing by Author "Lavareda, G."
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- An indium-oxide electrode with discontinuous Au layers for plasmonic devicesPublication . Vygranenko, Yuri; Lavareda, G.; André, V.; Brogueira, Pedro; Amaral, A.; Fernandes, M.; Fantoni, Alessandro; Vieira, ManuelaIn this contribution we report on a low cost plasmonic electrode for light-sensing applications. The electrode combines a conducting nonstoichiometric indium oxide (InOx) layer with an ultrathin (~5 nm) discontinuous Au layer. The InOx and Au layers were deposited on glass substrates by plasma enhanced reactive thermal evaporation and thermal evaporation, respectively. Several device configurations with one or two Au layer(s) sandwiched between InOx layers were fabricated and characterized. The morphological and structural properties of both Au and InOx layers were analyzed using AFM and XRD techniques. In particular, the effect of thermal annealing (673 K, 15 min) on the surface morphology of Au layers grown on bare glass and InOx-coated substrate was investigated. It has been also found that the oxide film grown above an underlying nanostructured Au layer is amorphous, while a reference InOx film on glass is nanocrystalline with a smooth surface. The electrical properties of InOx grown on the Au surface are worsened due to Au-induced structural disorder. The observed difference in transmission spectra of the glass/InOx/Au and glass/Au/InOx structures indicates the difference in the morphology of the metal layer. Thus, the optical and morphological properties of the InOx electrode can be varied in a wide range by incorporating several Au layers.
- Automated rf-PERTE system for room temperature deposition of TCO coatingsPublication . Fernandes, M.; Vygranenko, Y.; Vieira, M.; Lavareda, G.; Carvalho, C. Nunes de; Amaral, A.In this work we present a fully automated plasma-enhanced reactive thermal evaporation system (rf-PERTE) that can be used for the deposition of transparent metal oxide films without intentional heating of the substrate. The system and developed software enables the full control over critical deposition conditions such as mass flow of oxygen, process pressure, current flowing through crucible and rf-power. These parameters are automatically adjusted during the deposition thus keeping them in a narrow process window. This way, highly transparent and conductive coating can be deposited with a high degree of reproducibility of the optical and electrical characteristics. The resistivity of 9×10-4 Ω-cm and the peak transmittance of 90% in the visible spectral range were achieved for indium oxide films deposited on glass substrates. This technique is also suitable for the deposition of transparent conducting coatings in a wide range of plastic materials for flexible solar cells. In particular, we have successfully deposited indium oxide on PEN (polyethylene naphthalate) sheets with electrical and optical properties approaching the ones for films on glass.
- Colour filtering in a-SiC : H based p-i-n-p-i-n cells: A trade-off between bias polarity and absorption regionsPublication . Vieira, Manuela; Fantoni, Alessandro; Louro, Paula; Fernandes, Miguel; Martins, J.; Schwarz, R.; Lavareda, G.; Carvalho, C. N.A large area colour imager optically addressed is presented. The colour imager consists of a thin wide band gap p-i-n a-SiC:H filtering element deposited on the top of a thick large area a-SiC:H(-p)/a-Si:H(-i)/a-SiC:H(-n) image sensor, which reveals itself an intrinsic colour filter. In order to tune the external applied voltage for full colour discrimination the photocurrent generated by a modulated red light is measured under different optical and electrical bias. Results reveal that the integrated device behaves itself as an imager and a filter giving information not only on the position where the optical image is absorbed but also on it wavelength and intensity. The amplitude and sign of the image signals are electrically tuneable. In a wide range of incident fluxes and under reverse bias, the red and blue image signals are opposite in sign and the green signal is suppressed allowing blue and red colour recognition. The green information is obtained under forward bias, where the blue signal goes down to zero and the red and green remain constant. Combining the information obtained at this two applied voltages a RGB colour image picture can be acquired without the need of the usual colour filters or pixel architecture. A numerical simulation supports the colour filter analysis.
- Electrical, optical and photoconductive properties of Sn-doped indium sulfofluoride thin filmsPublication . Vygranenko, Yuri; Fernandes, M.; Vieira, Manuela; Lavareda, G.; Carvalho, C. Nunes De; Brogueira, P.; Amaral, A.; Barradas, N. P.; Alves, E.This work reports on undoped and Sn-doped indium sulfofluoride thin-films deposited by radio-frequency plasma-enhanced reactive thermal evaporation. The deposition was performed evaporating pure indium or indium-tin alloy in SF6 plasma at substrate temperatures ranging from 373 to 423 K. Rutherford backscattering analysis and secondary-ion mass spectrometry were used to determine the chemical composition of the films. The film characterization includes electrical, optical, and photoconductivity measurements. The resistivity of undoped material varies in a wide range of 1 G Omega-cm to 2 T Omega-cm depending on deposition conditions. Sn doping leads to a decrease in the resistance down to 8 M Omega-cm. The films are highly transparent in the visible-infrared region due to an indirect bandgap of 2.7-3 eV. Moreover, the doped material is highly photosensitive in the blue -UV region. Photoconductivity kinetics under various excitation conditions was also studied. The synthesized material is a promising candidate for a buffer layer in chalcogenide-based solar cells.
- Enhanced short wavelength response in laser-scanned-photodiode image sensor using an a-SiC : H/a-Si : H tandem structurePublication . Fantoni, Alessandro; Louro, Paula; Fernandes, Miguel; Vieira, Manuela; Lavareda, G.; Carvalho, C. N. deWe report in this paper the recent advances we obtained in optimizing a color image sensor based on the laser-scanned-photodiode (LSP) technique. A novel device structure based on a a-SiC:H/a-Si:H pin/pin tandem structure has been tested for a proper color separation process that takes advantage on the different filtering properties due to the different light penetration depth at different wavelengths a-SM and a-SiC:H. While the green and the red images give, in comparison with previous tested structures, a weak response, this structure shows a very good recognition of blue color under reverse bias, leaving a good margin for future device optimization in order to achieve a complete and satisfactory RGB image mapping. Experimental results about the spectral collection efficiency are presented and discussed from the point of view of the color sensor applications. The physics behind the device functioning is explained by recurring to a numerical simulation of the internal electrical configuration of the device.
- Field effect and light-assisted a-Si:H sensors for detection of ions in solutionPublication . Costa, João; Fernandes, Miguel; Vieira, Manuela; Lavareda, G.; Carvalho, Carlos N.; Karmali, AminIn this paper we present an amorphous silicon device that can be used in two operation modes to measure the concentration of ions in solution. While crystalline devices present a higher sensitivity, their amorphous counterpart present a much lower fabrication cost, thus enabling the production of cheap disposable sensors for use, for example, in the food industry. The devices were fabricated on glass substrates by the PECVD technique in the top gate configuration, where the metallic gate is replaced by an electrolytic solution with an immersed Ag/AgCl reference electrode. Silicon nitride is used as gate dielectric enhancing the sensitivity and passivation layer used to avoid leakage and electrochemical reactions. In this article we report on the semiconductor unit, showing that the device can be operated in a light-assisted mode, where changes in the pH produce changes on the measured ac photocurrent. In alternative the device can be operated as a conventional ion selective field effect device where changes in the pH induce changes in the transistor's threshold voltage.
- Image and color recognition using amorphous silicon p-i-n photodiodesPublication . Louro, Paula; Vieira, Manuela; Fantoni, Alessandro; Fernandes, Miguel; Carvalho, C. N.; Lavareda, G.Large area hydrogenated amorphous silicon single and stacked p-i-n structures with low conductivity doped layers are proposed as monochrome and color image sensors. The layers of the structures are based on amorphous silicon alloys (a-Si(x)C(1-x):H). The current-voltage characteristics and the spectral sensitivity under different bias conditions are analyzed. The output characteristics are evaluated under different read-out voltages and scanner wavelengths. To extract information on image shape, intensity and color, a modulated light beam scans the sensor active area at three appropriate bias voltages and the photoresponse in each scanning position ("sub-pixel") is recorded. The investigation of the sensor output under different scanner wavelengths and varying electrical bias reveals that the response can be tuned, thus enabling color separation. The operation of the sensor is exemplified and supported by a numerical simulation.
- Membrane selectivity versus sensor response in hydrogenated amorphous silicon CHEMFETs using a semi-empirical modelPublication . Costa, João; Fernandes, Miguel; Vieira, Manuela; Lavareda, G.; Karmali, AminToxic amides, such as acrylamide, are potentially harmful to Human health, so there is great interest in the fabrication of compact and economical devices to measure their concentration in food products and effluents. The CHEmically Modified Field Effect Transistor (CHEMFET) based onamorphous silicon technology is a candidate for this type of application due to its low fabrication cost. In this article we have used a semi-empirical modelof the device to predict its performance in a solution of interfering ions. The actual semiconductor unit of the sensor was fabricated by the PECVD technique in the top gate configuration. The CHEMFET simulation was performed based on the experimental current voltage curves of the semiconductor unit and on an empirical model of the polymeric membrane. Results presented here are useful for selection and design of CHEMFET membranes and provide an idea of the limitations of the amorphous CHEMFET device. In addition to the economical advantage, the small size of this prototype means it is appropriate for in situ operation and integration in a sensor array.
- Optical and photoconductive properties of indium sulfide fluoride thin filmsPublication . Vygranenko, Yuri; Vieira, Manuela; Lavareda, G.; Carvalho, C. Nunes de; Brogueira, Pedro; Amaral, A.; Pessoa Barradas, Nuno; Alves, E.This work reports on transparent semiconducting indium sulfide fluoride (ISF) thin-films exhibiting high sensitivity to ultraviolet radiation. The films were deposited on fused silica and silicon substrates using a radiofrequency plasma-enhanced reactive thermal evaporation system. The deposition was performed evaporating pure indium in SF6 plasma at a substrate temperature of 423 K. Rutherford backscattering measurements were used to determine the chemical composition of the films deposited on silicon substrates. The surface morphology was studied using scanning electron microscopy technique. The film characterization includes electrical, optical, and photoconductivity measurements. The synthesized compound is highly-resistive (similar to 700 M Omega-cm at 300 K) and exhibits an evident semiconducting behavior. The activation energy of 0.88 eV is deduced from the temperature dependence of electrical resistivity. The indirect band energy gap of 2.8 eV is determined from transmittance spectra of the ISF films. The photoconductivity band is centered at 345 nm wavelength. The photoconductivity spectrum also shows the Urbach tail with a characteristic energy of 166 meV. ISF is a promising candidate for a buffer layer in chalcogenide-based solar cells.
- Structural, electrical and magnetic studies of Co:SnO2 and (Co,Mo):SnO2 films prepared by pulsed laser depositionPublication . Dalui, S.; Rout, S.; Silvestre, António Jorge; Lavareda, G.; Pereira, L. C. J.; Brogueira, P.; Conde, O.Here we report on the structural, optical, electrical and magnetic properties of Co-doped and (Co,Mo)-codoped SnO2 thin films deposited on r-cut sapphire substrates by pulsed laser deposition. Substrate temperature during deposition was kept at 500 degrees C. X-ray diffraction analysis showed that the undoped and doped films are crystalline with predominant orientation along the [1 0 1] direction regardless of the doping concentration and doping element. Optical studies revealed that the presence of Mo reverts the blue shift trend observed for the Co-doped films. For the Co and Mo doping concentrations studied, the incorporation of Mo did not contribute to increase the conductivity of the films or to enhance the ferromagnetic order of the Co-doped films. (C) 2012 Elsevier B.V. All rights reserved.