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A
Amaral, A., P. Brogueira, G. Lavareda, and C. Nunes de Carvalho. "On the Role of Tin Doping in InOx Thin Films Deposited by Radio Frequency-Plasma Enhanced Reactive Thermal Evaporation." JOURNAL OF NANOSCIENCE AND NANOTECHNOLOGY 10 (2010): 2713-2716. Abstract

In view of the increasing need for larger-area display devices with improved image quality it becomes increasingly important to decrease resistivity while maintaining transparency in transparent conducting oxides (TCOs). Accomplishing the goal of increased conductivity and transparency will require a deeper understanding of the relationships between the structure and the electro-optical properties of these materials. In this work we study the role of tin doping in InOx thin films. Undoped indium oxide (InOx) and indium tin oxide (ITO) thin films were deposited at room temperature by radio-frequency plasma enhanced reactive thermal evaporation (rf-PERTE), a new technique recently developed in our laboratory using as evaporation source either In rods or a 90%In:10%Sn alloy, respectively. The two most important macroscopic properties-optical transparency and electrical resistivity-seem to be independent of the tin content in these deposition conditions. Results show that the films present a visible transmittance of the order of 82%, and an electrical resistivity of about 8 x 10(-4) Omega . cm. Surface morphology characterization made by atomic force microscopy (AFM) showed that homogeneity of the films deposited from a 90%In:10%Sn alloy is enhanced (a film with small and compact grains is produced) and consequently a smooth surface with reduced roughness and with similar grain size and shape is obtained. Films deposited from pure In rods evaporation source show the presence of aggregates randomly distributed above a film tissue formed of thinner grains.

Amaral, A., P. Brogueira, O. Conde, G. Lavareda, and C. Nunes de Carvalho. "Device quality InOx:Sn and InOx thin films deposited at room temperature with different rf-power densities." THIN SOLID FILMS 526 (2012): 221-224. Abstract

The influence of tin doping on the electrical, optical, structural and morphological properties of indium oxide films produced by radio-frequency plasma enhanced reactive thermal evaporation is studied, as transport properties are expected to improve with doping. Undoped and tin doped indium oxide thin films are deposited at room temperature using both pure In rods and (95-80) % In:(5-20) % Sn alloys as evaporation sources and 19.5 mW/cm(2) and 58.6 mW/cm(2) as rf-power densities. The two most important macroscopic properties - visible transparency and electrical resistivity - are relatively independent of tin content (0-20%). Visible transmittance of about 75% and electrical resistivity around 5 x 10(-4) Omega.cm can be observed in the films. The structural features are similar for all samples. Nevertheless, the surface morphology characterization shows that the homogeneity of the films varies according to the tin content. Moreover this variation is a balance between the rf-power and the tin content in the alloy: i) films with small and compact grains are produced at 58.6 mW/cm(2) from a 5% Sn alloy or at 19.5 mW/cm(2) from a 15% Sn alloy and consequently, smooth surfaces with reduced roughness and similar grain size and shape are obtained; ii) films showing the presence of aggregates randomly distributed above a tissue formed of thinner grains and higher roughness are produced at the other deposition conditions. (C) 2012 Elsevier B. V. All rights reserved.

C
de Calheiros Velozo, A., G. Lavareda, C. Nunes de Carvalho, and A. Amaral. "Thermal dehydrogenation of amorphous silicon deposited on c-Si: Effect of the substrate temperature during deposition." In PHYSICA STATUS SOLIDI C: CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 9, NO 10-11, edited by S. Pizzini, G. Kissinger, H. YamadaKaneta and J. Kang, 2198-2202. Vol. 9. Physica Status Solidi C-Current Topics in Solid State Physics 9. European Mat Res Soc (E-MRS), 2012. Abstract

Samples of doped and undoped a-Si: H were deposited at temperatures ranging from 100 degrees C to 350 degrees C and then submitted to different dehydrogenation temperatures (from 350 degrees C to 550 degrees C) and times (from 1 h to 4 h). a-Si: H films were characterised after deposition through the measurements of specific material parameters such as: the optical gap, the conductivity at 25 degrees C, the thermal activation energy of conductivity and its hydrogen content. Hydrogen content was measured after each thermal treatment. Substrate dopant contamination from phosphorus-doped a-Si thin films was evaluated by SIMS after complete dehydrogenation and a junction depth of 0.1 mu m was obtained. Dehydrogenation results show a strong dependence of the hydrogen content of the as-deposited film on the deposition temperature. Nevertheless, the dehydrogenation temperature seems to determine the final H content in a way almost independent from the initial content in the sample. H richer films dehydrogenate faster than films with lower hydrogen concentration. (C) 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

de Calheiros Velozo, A., G. Lavareda, C. Nunes de Carvalho, and A. Amaral. "Thermal dehydrogenation of amorphous silicon: A time-evolution study." THIN SOLID FILMS 543 (2013): 48-50. Abstract

A model is proposed to describe the decrease of H content in hydrogenated amorphous silicon (a-Si: H), during annealing at a fixed temperature. H content has been measured in several a-Si: H samples ( grown by plasma enhanced chemical vapor deposition) after being submitted to different annealing times at 400 degrees C. Obtained data has been fitted to the proposed model and initial diffusion coefficients of 3.2 x 10(-14) cm(2)/s for intrinsic films and 4.2 x 10(-14) cm(2)/s for n-type films were obtained. Reversely, H content evolution can be predicted during a thermal treatment if diffusion coefficients are previously known. (C) 2013 Elsevier B.V. All rights reserved.

Carvalho, Carlos, Guilherme Lavareda, Ana Amaral, Carlos Nunes de Carvalho, and Nuno Paulino. "A CMOS micro power switched-capacitor DC-DC step-up converter for indoor light energy harvesting applications." ANALOG INTEGRATED CIRCUITS AND SIGNAL PROCESSING 78 (2014): 333-351. Abstract

This paper presents a micro power light energy harvesting system for indoor environments. Light energy is collected by amorphous silicon photovoltaic (a-Si:H PV) cells, processed by a switched capacitor (SC) voltage doubler circuit with maximum power point tracking (MPPT), and finally stored in a large capacitor. The MPPT fractional open circuit voltage (V-OC) technique is implemented by an asynchronous state machine (ASM) that creates and dynamically adjusts the clock frequency of the step-up SC circuit, matching the input impedance of the SC circuit to the maximum power point condition of the PV cells. The ASM has a separate local power supply to make it robust against load variations. In order to reduce the area occupied by the SC circuit, while maintaining an acceptable efficiency value, the SC circuit uses MOSFET capacitors with a charge sharing scheme for the bottom plate parasitic capacitors. The circuit occupies an area of 0.31 mm(2) in a 130 nm CMOS technology. The system was designed in order to work under realistic indoor light intensities. Experimental results show that the proposed system, using PV cells with an area of 14 cm(2), is capable of starting-up from a 0 V condition, with an irradiance of only 0.32 W/m(2). After starting-up, the system requires an irradiance of only 0.18 W/m(2) (18 mu W/cm(2)) to remain operating. The ASM circuit can operate correctly using a local power supply voltage of 453 mV, dissipating only 0.085 mu W. These values are, to the best of the authors' knowledge, the lowest reported in the literature. The maximum efficiency of the SC converter is 70.3 % for an input power of 48 mu W, which is comparable with reported values from circuits operating at similar power levels.

Costa, J., M. Fernandes, M. Vieira, G. Lavareda, CN Carvalho, and A. Karmali. "Field Effect and Light-Assisted a-Si:H Sensors for Detection of Ions in Solution." SENSOR LETTERS 8 (2010): 493-496. Abstract

In 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.

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L., Ricardo, Amaral A., Nunes Carvalho de C., and Lavareda G. "Dopant transfer from poly-Si thin films to c-Si: an alternative technique for device processing"." Materials Science in Semiconductor Processing 42 (2016): 210.
Lavareda, G., C. Nunes de Carvalho, A. M. Ferraria, A. M. Botelho Do Rego, and A. Amaral. "p-Type Cuo(X) Thin Films by rf-Plasma Enhanced Reactive Thermal Evaporation: Influence of rf-Power Density." JOURNAL OF NANOSCIENCE AND NANOTECHNOLOGY 12 (2012): 6754-6757. Abstract

Copper oxide is a well known p-type semiconductor material, usually obtained by thermal oxidation of copper thin-films within few minutes, at atmospheric pressure. In this paper, thin films of copper oxide that were deposited by radio-frequency plasma enhanced reactive thermal evaporation of copper at room temperature, without any post-deposition annealing treatment, are studied. The deposition of good quality p-type semiconductor oxide to be used in the fabrication of p-TFTs is the purpose of this work. The thickness of the films varies from 97 up to 160 nm. The influence of rf power density on chemical, electrical and optical properties of the films was studied. Samples present conductivity within the range of 6 x 10(-5) to 4 x 10(2) Omega(-1) cm(-1) (thermal activation energy in the interval 0.46 to 0.01 eV). The p-type conductivity of the films was confirmed by Seebeck effect in the more conductive samples. Surface composition obtained by XPS analysis was correlated with optical and electrical properties, showing that rf-power plays a main role in changes of material characteristics.

Lavareda, G., P. Parreira, J. Valente, F. T. Nunes, A. Amaral, and C. Nunes de Carvalho. "Highly transparent undoped semiconducting ZnOx thin films deposited at room temperature by rf-PERTE - Influence of rf power." JOURNAL OF NON-CRYSTALLINE SOLIDS 356 (2010): 1392-1394. Abstract

Transparent, undoped semiconductive thin films of zinc oxide were deposited by radio frequency plasma enhanced reactive thermal evaporation of zinc rods in the presence of oxygen at room temperature, without any post-deposition annealing treatments. The study of the variation of rf power, in the range 5-200W, on the main properties of these films was made. A decrease of the electrical conductivity of these films with increasing rf power was observed (10(-3) to 10(-12) (Omega cm)(-1)), respectively, while the average visible transparency was kept practically constant (approximate to 80%). From this initial study, a set of bottom-gate type thin-film transistors were made using the most promising semiconductor materials. Preliminary I(V) measurements showed all transistors working, with best results obtained from zinc oxide material deposited at the lowest rf power (20 W), within the deposition conditions range which leads to semiconductive material. Such devices presented an I-on (20 V)/I-off (-5 V) ratio of 6 x 10(6) and a field-effect mobility of 0.37 cm(2)/(V s). (C) 2010 Elsevier B.V. All rights reserved.

Lavareda, G., A. de Calheiros Velozo, C. Nunes de Carvalho, and A. Amaral. "p/n junction depth control using amorphous silicon as a low temperature dopant source." THIN SOLID FILMS 543 (2013): 122-124. Abstract

Phosphorus-doped amorphous silicon thin films, deposited at low temperatures by Plasma Enhanced Chemical Vapour Deposition were used as a dopant source on p-type c-Si substrates. A careful step of dehydrogenation was done in order to maintain the a-Si thin-film integrity. Subsequently, a fine-controlled drive-in of dopant, from the amorphous layer to the crystalline wafer was done, to form the p/n junction, using different time periods and temperatures. Dopant profiling in c-Si wafers as well as dopant concentration in a-Si: H films prior to diffusion, both measured by Secondary Ion Mass Spectrometry, are presented. Junction depths obtained are in the range of 98 nm to 2.4 mu m and surface concentrations are in the range of 1.1 x 10(21) to 4.3 x 10(20) at/cm(3). A dual diffusion mechanism explains the ``kink-and-tail{''} shape found for dopant profile. (C) 2013 Elsevier B.V. All rights reserved.

M
M., Vieira, Fantoni A., Fernandes M., Louro P., Lavareda G., and Nunes Carvalho de C. "Pinpi'n and pinpii'n multilayer devices with voltage controlled readout"." Journal of Nanoscience and Nanotechnology 9 (2009): 4022.
M., Fernandes, Vygranenko Y., Vieira M., Lavareda G., Nunes Carvalho de C., and Amaral A. "Automated rf-PERTE system for room temperature deposition of TCO coatings." Energy Procedia 102 (2016): 96.
Martins, R., L. Guimarães, N. Carvalho, A. M. Andrade, S. L. L. Corgnier, and M. S. Sanematsu. "Analysis of a new production technique for amorphous silicon solar cells”. In Fifth European Communities Photovoltaic Solar Energy Conference. Athens, Greece, 1983.
Merino, E. G., G. Lavareda, P. Brogueira, A. Amaral, C. Nunes de Carvalho, and PL Almeida. "InOx thin films deposited by plasma assisted evaporation: Application in light shutters." VACUUM 107 (2014): 116-119. Abstract

{An integration of undoped InOx and commercial ITO thin films into laboratory assembled light shutter devices is made. Accordingly, undoped transparent conductive InOx thin films, about 100 nm thick, are deposited by radiofrequency plasma enhanced reactive thermal evaporation (rf-PERTE) of indium teardrops with no intentional heating of the glass substrates. The process of deposition occurs at very low deposition rates (0.1-0.3 nm/s) to establish an optimized reaction between the oxygen plasma and the metal vapor. These films show the following main characteristics: transparency of 87% (wavelength

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Nunes de Carvalho, C., P. Parreira, G. Lavareda, P. Brogueira, and A. Amaral. "P-type CuxS thin films: Integration in a thin film transistor structure." THIN SOLID FILMS 543 (2013): 3-6. Abstract

CuxS thin films, 80 nm thick, are deposited by vacuum thermal evaporation of sulfur-rich powder mixture, Cu2S:S (50:50 wt.%) with no intentional heating of the substrate. The process of deposition occurs at very low deposition rates (0.1-0.3 nm/s) to avoid the formation of Cu or S-rich films. The evolution of CuxS films surface properties (morphology/roughness) under post deposition mild annealing in air at 270 degrees C and their integration in a thin film transistor (TFT) are the main objectives of this study. Accordingly, Scanning Electron Microscopy studies show CuxS films with different surface morphologies, depending on the post deposition annealing conditions. For the shortest annealing time, the CuxS films look to be constructed of grains with large dimension at the surface (approximately 100 nm) and consequently, irregular shape. For the longest annealing time, films with a fine-grained surface are found, with some randomly distributed large particles bound to this fine-grained surface. Atomic Force Microscopy results indicate an increase of the root-mean-square roughness of CuxS surface with annealing time, from 13.6 up to 37.4 nm, for 255 and 345 s, respectively. The preliminary integration of CuxS films in a TFT bottom-gate type structure allowed the study of the feasibility and compatibility of this material with the remaining stages of a TFT fabrication as well as the determination of the p-type characteristic of the CuxS material. (c) 2013 Elsevier B.V. All rights reserved.

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Parreira, P., G. Lavareda, A. Amaral, A. M. Botelho Do Rego, O. Conde, J. Valente, F. Nunes, and Nunes C. de Carvalho. "Transparent p-type CuxS thin films." JOURNAL OF ALLOYS AND COMPOUNDS 509 (2011): 5099-5104. Abstract

The effect of different mild post-annealing treatments in air, at 270 degrees C, for 4-6 min, on the optical, electrical, structural and chemical properties of copper sulphide (CuxS) thin films deposited at room temperature are investigated. CuxS films, 70nm thick, are deposited on glass substrates by vacuum thermal evaporation from a Cu2S:S (50:50 wt.%) sulphur rich powder mixture. The as-deposited highly conductive crystalline CuS (covellite) films show high carrier concentration (similar to 10(22) cm(-3)), low electrical resistivity (similar to 10(-4) Omega cm) and inconclusive p-type conduction. After the mild post-annealing, these films display increasing values of resistivity (similar to 10(-3) to similar to 10(-2) Omega cm) with annealing time and exhibit conclusive p-type conduction. An increase of copper content in CuxS phases towards the semiconductive Cu2S (chalcocite) compound with annealing time is reported, due to re-evaporation of sulphur from the films. However, the latter stoichiometry was not obtained, which indicates the presence of vacancies in the Cu lattice. In the most resistive films a Cu2O phase is also observed, diminishing the amount of available copper to combine with sulphur, and therefore the highest values of optical transmittance are reached (65%). The appearance on the surface of amorphous sulphates with annealing time increase is also detected as a consequence of sulphur oxidation and replacement of sulphur with oxygen. All annealed films are copper deficient in regards to the stoichiometric Cu2S and exhibit stable p-type conductivity. (C) 2011 Elsevier B.V. All rights reserved.

Parreira, P., G. Lavareda, J. Valente, F. T. Nunes, A. Amaral, and C. Nunes de Carvalho. "Optoelectronic properties of transparent p-type semiconductor CuxS thin films." PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE 207 (2010): 1652-1654. Abstract

Nowadays, among the available transparent semiconductors for device use, the great majority (if not all) have n-type conductivity. The fabrication of a transparent p-type semiconductor with good optoelectronic properties (comparable to those of n-type: InOx, ITO, ZnOx or FTO) would significantly broaden the application field of thin films. However, until now no material has yet presented all the required properties. Cu2S is a p-type narrow-band-gap material with an average optical transmittance of about 60% in the visible range for 50 nm thick films. However, due to its high conductivity at room temperature, 10 nm in thickness seems to be appropriate for device use. Cu2S thin films with 10 nm in thickness have an optical visible transmittance of about 85% rendering them as very good candidates for transparent p-type semiconductors. In this work CuxS thin films were deposited on alkali-free (AF) glass by thermal evaporation. The objective was not only the determination of its optoelectronic properties but also the feasibility of an active layer in a p-type thin film transistor. In our CuxS thin films, p-type high conductivity with a total visible transmittance of about 50% have been achieved. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Parreira, P., G. Lavareda, J. Valente, F. T. Nunes, A. Amaral, and C. Nunes de Carvalho. "Undoped InOx Films Deposited by Radio Frequency Plasma Enhanced Reactive Thermal Evaporation at Room Temperature: Importance of Substrate." JOURNAL OF NANOSCIENCE AND NANOTECHNOLOGY 10 (2010): 2701-2704. Abstract

Conductive and transparent undoped thin films of indium oxide (InOx), 120 nm average thick, were deposited by radio frequency plasma enhanced reactive thermal evaporation (rf-PERTE) of indium in the presence of oxygen at room temperature. Several substrates were used in order to study their influence on the main properties of these films: alkali free (AF) glass, fused silica, crystalline silicon and polyethylene terephthalate (PET). Surface morphology of the InOx films as a function of the substrates was observed by SEM and showed that the undoped InOx films obtained are nano-structured. For the c-Si substrate, InOx films with increased grain size are obtained, induced by the crystalline substrate. Films deposited on fused silica and AF glass substrates show a nano-grainy surface with similar surface morphologies. The InOx films deposited on AF glass show the highest values of both: electrical conductivity of about 1100 (Omega cm)(-1) and visible transmittance of 85%. The substrate has a greater influence on the surface morphology of the films when a polymer (PET) is used. InOx films deposited on PET show a decrease in the electrical conductivity (90 (Omega cm)(-1)) and a slight decrease in the average visible transmittance (78%).

Parreira, P., E. Torres, C. Nunes, C. Nunes de Carvalho, G. Lavareda, A. Amaral, and M. J. Brites. "Dye-sensitized 1D anatase TiO2 nanorods for tunable efficient photodetection in the visible range." SENSORS AND ACTUATORS B-CHEMICAL 161 (2012): 901-907. Abstract

TiO2 films with enhanced photosensitivity were deposited on alkali free glass substrates without intentional substrate heating by pulsed DC magnetron reactive sputtering with an average thickness of about 2 mu m. Three dyes, commercial N719 and two new organic dyes were impregnated in order to control the optical spectral selectivity of such films. The type of dye used proved to dramatically influence the device's response to radiation pulses. The practical breakthrough is the use of different dyes according to the region of the electromagnetic spectrum one wants to detect. Devices with photocurrent 6 orders of magnitude higher than the dark current (from similar to 2 x 10(-12) to 2 x 10(-6) A for a 100 V bias) were fabricated with a spectral response within the visible range of the electromagnetic spectrum. In addition, this approach is likely to allow for the fabrication of hybrid photodetectors on cheap heat sensible flexible polymeric substrates. (C) 2011 Elsevier B.V. All rights reserved.

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Ribeiro, Celso, Pedro Brogueira, Guilherme Lavareda, Carlos Nunes de Carvalho, Ana Amaral, Luis Santos, Jorge Morgado, Ulrich Scherf, and Vasco D. B. Bonifacio. "Ultrasensitive microchip sensor based on boron-containing polyfluorene nanofilms." BIOSENSORS & BIOELECTRONICS 26 (2010): 1662-1665. Abstract

A fluorene-based pi-conjugated copolymer with on-chain dibenzoborole units was used in the development of a nanocoated gold interdigitated microelectrode array device which successfully detects fluoride in a broad range of concentrations (10(-11)-10(-4) M) in aqueous solution, upon impedance spectroscopy measurements. A calibration curve obtained over this range of concentrations and a new analytical method based on impedance spectroscopy measurements in aqueous solution is proposed. The sensor nanofilm was produced by spin-coating and diagnosed via spectroscopic ellipsometry, AFM, and electrically conductivity techniques. Changes in the conductivity due to the boron-fluoride complex formation seem to be the major mechanism behind the dependence of impedimetric results on the fluoride concentration. (C) 2010 Elsevier B.V. All rights reserved.

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Valente, J., G. Lavareda, O. Conde, P. Parreira, A. Amaral, and C. Nunes de Carvalho. "Role of rf power on the properties of undopedSnOx films deposited by rf-PERTE at low substrate temperature”." Surface and Coatings Technology 202 (2008): 3893.