Rodrigo Martins

Highly conducting and transparent gallium doped zinc oxide thin films have been deposited at high growth rates by r.f. magnetron sputtering at room temperature on inexpensive soda lime glass substrates. The argon sputtering pressure was varied between 0.15 and 2.1 Pa. The lowest resistivity was 2.6 × 10-4 Ω cm (sheet resistance ≈6 Ω/sq. for a thickness ≈600 nm) and was obtained at an argon sputtering pressure of 0.15 Pa and a r.f. power of 175 W. The films present an overall transmittance in the visible spectra of approximately 90%. The increase on the resistivity for higher sputtering pressures is due to a decrease of both, mobility and carrier concentration, and is associated to a change on the surface morphology. The low resistivity, accomplished with a high growth rate (290 Å/min) and with a room temperature deposition enables these films deposition onto polymeric substrates for flexible optoelectronic devices. © 2002 Elsevier Science B.V. All rights reserved.

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.

We report the effect of a disperse carbon interlayer between the n-a-Si:H layer and an aluminium zinc oxide (AZO) back contact on the performance of amorphous silicon solar cells. Carbon was incorporated to the AZO film as revealed by x-ray photoelectron spectroscopy and energy-dispersive x-ray analysis. Solar cells fabricated on glass substrates using AZO in the back contact performed better when a disperse carbon interlayer was present in their structure. They exhibited an initial efficiency of 11%, open-circuit voltage Voc = 1.6 V, short-circuit current JSC = 11 mA cm -2 and a filling factor of 63%, that is, a 10% increase in the J SC and 20% increase in the efficiency compared to a standard solar cell. © 2013 National Institute for Materials Science.

Metal nanoparticles are of great interest for light trapping in photovoltaics. They are usually incorporated in the rear electrode of solar cells, providing strong light scattering at their surface plasmon resonances. In most cases, the nanoparticles are self-assembled by solid-state dewetting over a transparent conductive oxide (TCO) layer incorporated in the cell's rear electrode. Up to now, this process has been optimized mainly by tuning the thermal annealing parameters responsible for dewetting, or the thickness of the precursor metallic layer; but little attention has been paid to the influence of the underlying TCO layer properties on the morphology of the nanoparticles formed, which is the focus of the present article. This work investigates Ag nanoparticles structures produced on distinct surfaces by a simple, fast and highly reproducible method employing rapid thermal annealing. The results indicate that both the thermal conductivity and surface roughness of the TCO layer play a determinant role on the morphology of the nanostructures formed. This is of particular relevance, since we show in the study performed that the parasitic absorption of these Ag nanostructures is reduced, while the scattering is enhanced when the Ag nanostructures are formed on TCO layers with the highest conductivity and the lowest surface roughness (∼1 nm). These results unveil novel possibilities for the improvement of plasmonic nanostructures fabricated by thermal dewetting, via the careful adjustment of the physical properties of the underlying surface. © 2016 American Chemical Society.

Although there are a few research studies on solution-processed p-channel oxide thin-film transistors (TFTs), the strict fabrication conditions and the poor electrical properties have limited their applications in low-power complementary metal oxide semiconductor (CMOS) electronics. Here, the application of the polyol reduction method for processing p-type CuxO and NiOx channel layers and their implementation in TFT devices are reported. The optimized CuxO and NiOx TFTs were achieved at low annealing temperatures (∼300 °C) and exhibited decent electrical properties. Encouraged by the inspiring results obtained on SiO2/Si substrates, the TFT performance was further optimized by device engineering, employing high-k AlOx as the gate dielectric. The fully solution-processed NiOx/AlOx TFT could be operated at a low voltage of 3.5 V and exhibits a high hole mobility of around 25 cm2 V-1 s-1. Our work demonstrates the ability to grow high-quality p-type oxide films and devices via the polyol reduction method over large area substrates while at the same time it provides guidelines for further p-type oxide material and device improvements. © The Royal Society of Chemistry 2016.

In this study, amorphous indium-tungsten oxide (IWO) semiconductor thin films were prepared by an eco-friendly spin-coating process using ethanol and water as solvents. The electrical properties of IWO thin-film transistors (TFTs), together with the structural and morphological characteristics of IWO thin films, were systematically investigated as functions of tungsten concentration and annealing temperature. The optimized IWO channel layer was then integrated on an aqueous aluminum oxide (AlOx) gate dielectric. It is observed that the solution-processed IWO/AlOx TFT presents high stability and improved characteristics, such as an on/off current ratio of 5 × 107, a field-effect mobility of 15.3 cm2 V-1 s-1, a small subthreshold slope of 68 mV dec-1, and a threshold voltage shift of 0.15 V under bias stress for 2 h. The IWO/AlOx TFT could be operated at a low voltage of 2 V, which was 15 times lower than that of conventional SiO2-based devices. The solution-processed IWO thin films synthesized in a green route would be promising candidates for large-area and high-performance low-cost devices. © The Royal Society of Chemistry 2016.

High-quality SrBi2Nb2O9 (SBN) single crystals were grown from a melt using a high-temperature self-flux solution method and Bi2O3 added with B2O3 as a flux. A suitable thermal profile involving slow cooling rates allowed growing large and translucent SBN crystals exhibiting platelet morphology with typical size ∼5 × 5 mm2 and thickness approximately 400 μm. X-ray diffraction revealed a dominant (001)-orientation of the major face of the platelet crystals and edges oriented parallel to the [110] directions. The dielectric properties were evaluated along the ab-plane and in the c-axis direction. The ferro-paraelectric phase transition was observed at TC = 440°C with Curie-Weiss relationship above TC. The anisotropy of dielectric permittivity, i.e., the ratio between permittivity in the ab-plane and along c-axis was about 10 at TC-The obtained results are used to discuss the observed correlations between anisotropy, crystalline orientation, and electrical properties.

The present work is focused on gelatin-based electrolytes doped with a range of concentration of zinc triflate (Zn(CF3SO3)2). The transparent-thin-film samples have been represented by the notation GelatinnZn(CF3SO3)2, where n represents the zinc triflate salt concentration in the electrolyte membranes from 0.00 wt% to 10.93 wt% The samples have been characterized by conductivity measurements, thermal analysis, cyclic voltammetry, X-ray diffraction (XRD), polarized optical microscopy (POM) and scanning electron microscopy (SEM). The gelatin-based electrolytes were also tested as ionic conductors in electrochromic devices with the glass/ITO/WO3/gelatin-based electrolyte/CeO2-TiO2/ITO/glass configuration. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Zinc oxide is getting an enormous attention due to its potential applications in a variety of fields such as optoelectronics, spintronics and sensors. The renewed interest in this wide band gap oxide semiconductor relies on its direct high energy gap (Eg ∼ 3.437 eV at low temperatures) and large exciton binding energy. However to reach the stage of device production the difficulty to produce in a reproducible way p-type doping must be overcome. In this study we discuss the structural and optical properties of ZnO films doped with nitrogen, a potential p-type dopant. The films were deposited by magnetron sputtering using different conditions and substrates. The composition and structural properties of the films were studied combining X-ray diffraction (XRD), Rutherford backscattering (RBS), and heavy ion elastic recoil detection analysis (HI-ERDA). The results show an improvement of the quality of the films deposited on sapphire with increasing radio-frequency (RF) power with a preferentially growth along the c-axis. The ERDA analysis reveals the presence of H in the films and a homogeneous composition over the entire thickness. The photoluminescence of annealed samples evidences an improvement on the optical quality as identified by the well structured near band edge recombination. © 2009 Elsevier Ltd. All rights reserved.

In the present work one host natural matrix - agar - has been doped with magnesium triflate (Mg(CF3SO3)2) with the goal of developing electrolytes for the fabrication of solid-state devices. The resulting samples have been represented by the notation Agar nMg(CF3SO3)2, where n represents the percentage of the magnesium triflate salt proportion in the electrolyte samples. The samples investigated, with n between 0.00% and 37.56%, have been obtained as transparent and thin films. The samples have been characterized by conductivity measurements, thermal analysis, cyclic voltammetry, X-ray diffraction (XRD), and scanning electron microscopy (SEM). The agar-based electrolytes were also tested as ionic conductor in an electrochromic device with the following configuration: glass/indium tin oxide (ITO)/WO 3/agar-based electrolyte/CeO2-TiO2/ITO/glass. © 2013 Copyright Taylor and Francis Group, LLC.

In this work, we study the electro-optical behaviour of cellulose/liquid crystal-based composite systems, in particular the influence of the flexible substrates and its conductive layers in the electro-optical behaviour of these kind of cells. Four cells were made using, respectively, two different substrates (a flexible polymer (poly(ethylene terephthalate) (PET)) and a soda lime glass) and two different conductive layers (indium tin oxide (ITO) and aluminium zinc oxide (AZO)). The conductive layer (AZO) was deposited in both, flexible and rigid substrates, for the same conditions, and the same substrates coated with ITO are commercially available. The cells were prepared from solid films of hydroxypropylcellulose (HPC) (30 μm thick) cross linked with 1,4-diisocyanatobutane (BDI) (7% w/w) and the nematic liquid crystal E7 (Merck, UK). The four different substrates were electrically and morphologically characterised. We have analysed all samples by light transmission and determined the maximum transmission, contrast and Von. We show a comparison of the results obtained for both flexible and rigid cells and discuss them in terms of the proposed working mechanism for these systems. © 2002 Elsevier Science 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.

A glass was made using bottom ash produced by a Portuguese municipal solid waste (MSW) incinerator. The bottom ash was the single batch material used in the formation of the glass, which was obtained through a conventional melt-quenching method. The glass was then converted to glass-ceramic for further recycling to construction materials. After submitting the glass samples to several heat treatments, between 820 and 1050°C and during different times, it was verified that the optimum heat treatment schedule for the ceramization of the glass was at 1000°C for 10h, as confirmed by microstructural observation and by X-ray diffraction. The major crystalline phases precipitated in the glass-ceramic were wollastonite (CaSiO3) and diopside (Ca(Mg,Al)(Si,Al)2O6). Microstructural analysis of the glass-ceramic revealed that the crystalline phases were present as dendrites and fiber-like structures that were homogeneously distributed in the material. The glassceramic showed good mechanical properties with a hardness of 5.6 MPa and a bending strength of 101 MPa. This material had a density of 2.8 gcm-3 and a thermal expansion coefficient of 9.10-6°C-1. The glass and the glass-ceramic showed an excellent chemical stability against leaching in acidic solution and in alkaline solution. In summary, both the glass and the glass-ceramic have good chemical and mechanical properties and can, therefore, be applied as construction materials.

The driving force behind the work herein presented is the importance of budgeting in a competitive market. The problem at hands is the creation of a budgeting methodology for reverse engineering applications, involving laser scanning, that has the ability to generate budgets for different customer accuracy requirements and for parts of different morphologic characteristics, such as: shape, dimension and/or detail complexity. A breakup approach was used to implement the methodology: the reverse engineering process was broken in nine basic identified steps and elementary sources of cost were defined at the different reverse engineering stages as well. Particular budgeting methodologies for each step of the process were created. The obtained results so far point to the possibility of creating a complete budgeting system based on the proposed methodology. © 2008 Taylor & Francis Group.

The present development of non-wafer-based photovoltaics (PV) allows supporting thin film solar cells on a wide variety of low-cost recyclable and flexible substrates such as paper, thereby extending PV to a broad range of consumer-oriented disposable applications where autonomous energy harvesting is a bottleneck issue. However, their fibrous structure makes it challenging to fabricate good-performing inorganic PV devices on such substrates. The advances presented here demonstrate the viability of fabricating thin film silicon PV cells on paper coated with a hydrophilic mesoporous layer. Such layer can not only withstand the cells production temperature (150 °C), but also provide adequate paper sealing and surface finishing for the cell's layers deposition. The substances released from the paper substrate are continuously monitored during the cell deposition by mass spectrometry, which allows adapting the procedures to mitigate any contamination from the substrate. In this way, a proof-of-concept solar cell with 3.4% cell efficiency (41% fill factor, 0.82 V open-circuit voltage and 10.2 mA cm-2 short-circuit current density) is attained, opening the door to the use of paper as a reliable substrate to fabricate inorganic PV cells for a plethora of indoor applications with tremendous impact in multi-sectorial fields such as food, pharmacy and security. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

This work reports on the fabrication process and performances presented by metal insulator semiconductor (MIS) linear position sensitive detectors (PSD) with an active length of 6 cm. The use of long sensitive areas allows the PSD to achieve higher resolution without the need of a highly accurate light spot integration mechanism. The PSD is built in a multilayered structure consisting of Cr/a-Si:H (n+ doped)/a-Si:H (intrinsic)/SiOx (passivation layer)/Au, where the active a-Si:H layers were deposited by a modified triode plasma enhanced chemical vapour deposition (MTPECVD), which allows the deposition of highly electronic grade material with a low (≈ × 1015 cm-3) defect density inferred by CPM. The sensor linearity and sensitivity shows dependence on the sensor width to length ratio and on the value of load resistance. Sensitivities of more than 30 mV/cm were achieved with linearity near 99%. Besides that, this type of MIS structure allows an improved spectral response near the UV region and has the maximum response at 540 nm. © 2004 Published by Elsevier B.V.

In this work we show that it is possible to control the plasma species present near the substrate surface, from what is usually associated with an α regime (a plasma free of particles) to a γ' regime (a plasma where particles are present) and simultaneously control the energy of the ions striking the substrate during a-Si:H deposition from a silane glow discharge in a modified triode (MT) type PECVD reactor, where a DC mesh electrode biased with Vpol is located in front of the r.f electrode. The presence of large particles in the plasma leads to the deposition of the films with the poorest optoelectronic properties. When the particle size in the plasma decrease the film properties improve, but, when particles are no longer present in the plasma region close to the substrate, like in a α like regime, the properties of the films deteriorate again. The results show that the best transport properties are achieved for the films deposited in the α-γ' transition regime corresponding to 0V<Vpol<51V. Under this condition the films present a dark conductivity, σ d ≈ 10-11 (Ωcm)-1, photosensitivity, S ≈ 107, activation energy, ΔE ≈ 0.9 eV, hydrogen content, CH ≈ 10%, factor of microstructure, R ≈ 0.085 and an optical gap, Eop ≈ 1.77 eV.

Despite of a growing interest in this material, until now the studies on polymorphous silicon (pm-Si:H) have been performed on small laboratory reactors working at 13.56 MHz. Envisaging an industrial application of pm-Si:H, the technology was transferred to a large area plasma enhanced chemical vapour deposition reactor (25 × 40 cm2) working at excitation frequencies of 13.56 and 27.12 MHz. The plasma was characterized by impedance probe measurements and the films were characterized by spectroscopic ellipsometry, infrared spectroscopy and hydrogen evolution experiments, which are techniques that allow a rapid and reliable identification of pm-Si:H structure. Conductivity measurements were also performed to determine their transport properties. The results show that scaling up using the 13.56 MHz was successfully done and pm-Si:H films were deposited at a growth rate of ≈ 12 nm/min. Moreover, by using the 27.12 MHz excitation frequency the growth rate was even further increased to above 18 nm/min, as desired for industrial production. © 2002 Elsevier Science B.V. All rights reserved.

In this work we present a study performed in a plasma-enhanced chemical vapour deposition reactor, where different rf electrode configurations were used with the aim of achieving conditions that lead to growth of highly uniform amorphous silicon films, with the required electronic quality, at high growth rates. This study consists in determining the plasma characteristics under different electrode configurations, in an argon plasma, using as diagnostic tools a Langmuir probe and impedance probe. These results were correlated with the hydrogenated amorphous silicon films produced, thereby allowing us to establish the best electrode configuration to grow electronic-grade-quality amorphous silicon films.

In this work we investigate the properties of a polymorphous silicon (pm-Si:H) metal-insulator-semiconductor (MIS) structure used in 3D position sensitive detectors (PSD). For the first time a 3D sensor made-up by pm-Si:H/SiO2/Au layers is presented. MIS structures present several advantages over p-i-n structures, such as easier fabrication, fast response time and higher resolution. The 1D MIS PSD that constitute the array were extensively studied aiming its application in 3D pattern recognition. The results obtained show that MIS PSD can achieve non-linearities below 2% and sensitivities of 3.2 μA/cm over 6 mm length sensors. The miniaturization of the sensors length to arrays of 6 and 16 mm, respectively showed average non-linearities of about 1.9% for the 16 mm sensor which proved to be the best solution for this MIS structure. © 2006 Elsevier B.V. All rights reserved.

In this work we present results of a study performed on MIS diodes with the following structure: substrate (glass) / Cr (2000Å) / a-Si:H n + (400Å) / a-Si:H i (5500Å) / oxide (0-40Å) / Au (100Å) to determine the influence of the oxide passivation layer grown by different techniques on the electrical performance of MIS devices. The results achieved show that the diodes with oxides grown using hydrogen peroxide present higher rectification factor (2×106) and signal to noise (S/N) ratio (1×107 at -1V) than the diodes with oxides obtained by the evaporation of SiO2, or by the chemical deposition of SiO 2 by plasma of HMDSO (hexamethyldisiloxane), but in the case of deposited oxides, the breakdown voltage is higher, 30V instead of 3-10 V for grown oxides. The ideal oxide thickness, determined by spectroscopic ellipsometry, is dependent on the method used to grow the oxide layer and is in the range between 6 and 20 Å. The reason for this variation is related to the degree of compactation of the oxide produced, which is not relevant for applications of the diodes in the range of ± 1V, but is relevant when high breakdown voltages are required.

In this work we present a study of influence of ion bombardment on the optical, electrical and compositional properties of the intrinsic amorphous silicon films deposited in a modified triode plasma-enhanced chemical vapour deposition (PECVD) reactor. The application of a DC voltage to a grid placed in front of the r.f. electrode allowed us to control the energy and polarity of the ions striking the substrate during film growth. The results show a variation by two orders of magnitude in the dark conductivity from 10-10 to 6.2 × 10-12 (Ω cm)-1, while the photosensitivity varied from 2 × 105 to 2 × 107. A process plasma that takes place in the γ-type regime was associated with the use of a negative bias, while a process plasma like the one of the α-type regime was associated with the use of a positive bias. The films deposited with a bias ≈ 38 V are highly intrinsic and the abrupt change in the conductivity properties observed at this bias is attributed to a change in the density of the states ascribed to the position of the Fermi level. That is, a precise control of the energy of the ions striking the substrate during the film growth leads to improved film optoelectronic properties, very important for device applications. © 2001 Elsevier Science Ltd. All rights reserved.

To evaluate the evolution of a dark film formation on silver surface objects, several coupons were catalogued and place inside a museum, located in an urban area. The changes on these samples were measured by spectroscopic ellipsometry, in periods of months. This technique allows the reduction of the coupons exposure time, in several months, due to its high sensitivity to surface modifications, with acceptable results for the evaluation of its degradation. The thicknesses of the degradation layers and the optical properties of silver oxide, chloride and sulphide reference samples were determined using a mixture of Tauc-Lorentz and Drude models. The composition of the silver corrosion layer was determined by fitting the layer using a Bruggeman Effective Medium Approximation (BEMA) of the three products plus voids. It was found that the thickness of the layer depends in the placement of the coupons, namely, inside or outside displayers. The average film thickness after 6 months was of 180 Å and 280 Å, inside and outside the displayers, respectively. The main compounds found in the layers were the silver chlorides and sulphides, which composition changed with the thickness of the layer, and the exposition time. © 2008 Wiley-VCH Verlag GmbH & Co. KGaA.

In this work we present results of a study performed on metal-insulator-semiconductor (MIS) diodes with the following structure: substrate (glass)/Cr (2000 Å)/a-Si:H n+(400 Å)/a-Si:H i (5500 Å)/oxide (0-40 Å)/Au (100 Å) to determine the influence of the oxide passivation layer grown by different techniques on the electrical performance of MIS devices. The results achieved show that the diodes with oxides grown using hydrogen peroxide present higher rectification factor (2×106) and signal to noise (S/N) ratio (1×10 7 at -1 V) than the diodes with oxides obtained by the evaporation of SiO2, or by the chemical deposition of SiO2 by plasma of hexamethyldisiloxane. However, in the case of deposited oxides, the breakdown voltage is higher, 30 V instead of 3-10 V for grown oxides. The ideal oxide thickness, determined by spectroscopic ellipsometry, is dependent on the method used to grow the oxide layer and is in the range between 6 and 20 Å. The reason for this variation is related to the degree of compactation of the oxide produced, which is not relevant for applications of the diodes in the range of ±1 V, but is relevant when high breakdown voltages are required. © 2003 Elsevier B.V. All rights reserved.

In this work a new structure is proposed for position sensitive detectors consisting of glass/Cr/aSi:H(n+)/a-Si:H(i)/ZnO, where the ZnO forms an heterojunction with the a-Si:H(i). The results show that this structure works with success in the fabrication of linear position sensitive detectors. The devices present a good nonlinearity of ≈ 2% and a good sensitivity to the light intensity. The main advantages of this structure over the classical p-i-n are an easier to built topology and a higher yield due to a better immunity to the a-Si:H pinholes, since the ZnO does not diffuse so easily into a-Si:H as the metal does, which are the cause of frequent failure in the p-i-n devices due to short-circuits caused by the deposition of the metal over the a-Si:H. In this structure the illumination is made directly on the ZnO, so a transparent substrate is not needed and a larger range of substrates can be used.