Rodrigo Martins

In this work we present results of structural composition and morphological characteristics of polymorphous silicon (pm-Si:H) films deposited by PECVD at 13.56 and 27.12 MHz. In addition, the role of the excitation frequency on the growth rate will be also analyzed. The results show that by using the 27.12 MHz excitation frequency the hydrogen dilution in the plasma needed to produce pm-Si:H can be reduced by more than 50% as well as the rf power density, leading to an increase on the growth rate to values higher than 3 Å/s. Spectroscopic ellipsometry and Raman spectroscopy show that the 27.12 MHz pm-Si:H films are more ordered than the pm-Si:H films produced at 13.56 MHz, while the infrared spectroscopy show that the SiH2 concentration in the films is strongly reduced. AFM measurements reveal that the films produced at 27.12 MHz films are more structured, presenting also higher roughness. © 2004 Elsevier B.V. All rights reserved.

This work refers to a study performed on polymorphous silicon (pm-Si:H) at excitation frequencies of 13.56 and 27.12 MHz in a large area PECVD reactor. The plasma was characterised by impedance probe measurements, aiming to identify the plasma conditions that lead to produce pm-Si:H films. The films produced were characterised by spectroscopic ellipsometry, infrared and Raman spectroscopy and hydrogen exodiffusion experiments, which are techniques that permit the structural characterisation of the pm-Si films and to study the possible differences between the films deposited at 13.56 and 27.12 MHz. Conductivity measurements were also performed to determine the transport properties of the films produced. The set of data obtained show that the 27.12 MHz pm-Si:H can be grown at higher rates with less hydrogen dilution and power density, being the resulting films denser, chemically more stable and with improved performances than the pm-Si:H films grown at 13.56 MHz.

In this work we present the current/voltage characteristics of Si:H/Pd Schottky structures using high quality, low defect density amorphous silicon (a-Si:H) deposited by a non-conventional, modified triode PECVD method. This new configuration allows the deposition of compact and high quality a-Si:H with a photosensitivity of 107, yielding films with low bulk defects. AFM measurements also revealed that these films have a very smooth surface allowing a low defect interface between the metal and the a-Si:H. As a result, we show that by using these a-Si:H films and by proper control of the i-layer thickness the reverse dark current of the diode can be highly reduced achieving signal to noise ratio of 106, surpassing the results usually achieved by p-i-n structures.

The linearity and sensitivity of linear Position Sensitive Detectors (PSD) are the two principal characteristics of sensors to be optimised in sensor fabrication. This work presents several efforts made to understand the internal and external parameters that influence the linearity and sensitivity of Metal Insulator Semiconductor (MIS) linear PSD with an active length of 6 cm. The use of long sensitive areas allows the PSD to achieve greater resolution without the need of a highly accurate light spot integration mechanism. The PSD is built in a multi-layered 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 Modified Triode Plasma Enhanced Chemical Vapour Deposition (MTPECVD), which allows the deposition of good electronic grade material with a low (≈ 1 × 1015 cm-3) defect density inferred by CPM. The sensor linearity and sensitivity shows dependence on the sensor width to length ratio, SiOx layer and on the value of the 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 in the near-UV region and has its maximum response at 540 nm. © 2005 Springer Science + Business Media, Inc.

This work presents a comparative study between MIS photodiodes produced using high quality amorphous silicon (a-Si:H), deposited by PECVD at 2Å/s, using 13.56 MHz frequency and polymorphous silicon (pm-Si) deposited at 3Å/s using a 27.12 MHz frequency. The results show that the pm-Si:H outperforms the a-Si:H MIS photodiodes by having a rectification ratio of 107, and photosensitivity at AM1.5 conditions of 107, under 1V reverse bias. Apart from that, the pm-Si:H photodiode presents a higher open circuit voltage and better fill factor than a-Si:H MIS photodiode. These results prove that quality devices can be produced at high growth rates by using pm-Si:H. In this work the photodiode performances were correlated to the films properties, aiming to determine the characteristics responsible for the performances exhibited by the pm-Si:H devices.

In this work, spectroscopic ellipsometry was used to characterise oxide films produced by anodic oxidation of amorphous silicon using an ethylene glycol (0.04 M KNO3) solution. The data obtained show that the growth of the oxide is not only a function of the voltage applied, but also of the current density and of the time process. An empiric model based on a power law is proposed for the growth of the oxide using, as parameters, the voltages and the time process. The oxide produced shows porosity of approximately 12%, which can be reduced down to 6% under well-controlled growth conditions. © 2002 Elsevier Science B.V. All rights reserved.

In this work we show that it is possible to control the plasma regime in the region close to the substrate in r.f. silane discharges. The PECVD reactor works in a modified triode configuration, where the control over the plasma regime is performed by polarising a grid electrode, placed close to the r.f. electrode, with a DC power source. Besides that, the DC grid allows also to control the energy of the ion bombardment, because the plasma potential will be a function of the voltage (Vpol) applied to the DC grid. The silane plasma was characterised with a Langmuir probe and an impedance probe. We were able to identify three plasma regimes in the region close to the substrate: γ′ regime for Vpol<0 V; γ′-α regime for 0 V<Vpol<40 V; and α regime for Vpol40 V. The γ′ regime is associated with a high concentration of dust particles in plasma and high electron energy (≈8eV), while the α regime is associated with a free dust plasma and low electron energy (≈2eV). The intermediate regime, γ′-α, is characterised by the presence of smaller particles (≈2-5nm) that can be beneficial for the film's properties. © 2002 Elsevier Science Ltd. All rights reserved.

This work presents a study performed on several Au contacts deposited by evaporation on oxide free and oxidised (5-20Å of oxide) a-Si:H surfaces. The characterisation of the films was performed on as deposited, aged and annealed at 150°C structures. SIMS and RBS measurements show that the Au diffuses very easily on oxide free a-Si:H surfaces, even at room temperature, resulting in the formation of an oxide at the device surface that acquires a blue colour instead of the gold colour of the contacts. This was also visible in the SEM pictures of the cross section of the structures produced and on the changes of the surface morphology observed by AFM measurements. On the other hand, when the Au is deposited on oxidised a-Si:H surfaces, the results show that the oxide prevents the Au from diffusing and the nature of the contact is preserved. That is, better rectifying and stability performances are obtained in MIS like structures than in Schottky structures.

This work presents a study performed on the deposition of pm-Si:H by plasma enhanced chemical vapor deposition using excitation frequencies of 13.56 and 27.12 MHz, where the interest of increasing the excitation frequency relies on higher plasma dissociation and reduced energy of ion bombardment, thus allowing the deposition of superior grade material at higher growth rates. The plasma impedance, which allows the monitoring of particle formation in the plasma, was correlated to the film properties, characterized by spectroscopic ellipsometry and hydrogen exodiffusion experiments. The set of data obtained show that by using the 27.12-MHz excitation frequency the hydrogen dilution and the r.f. power density needed to produce pm-Si:H can be reduced. Growth rates above 3.1 Å/s were obtained, the films being more dense and chemically more stable than those obtained with the standard 13.56 MHz. © 2003 Elsevier B.V. All rights reserved.

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.

In this work we present results of studies performed on Schottky and metal-insulator-semiconductor (MIS) position sensitive detectors (PSD) structures: substrate (glass)/ Cr (300 nm) / a-Si:H [n] (37 nm) / a-Si:H [i] (600 nm) / SiO2 (1.5 nm - for the MIS) / Au (7 nm). The effect of the interfacial oxide layer between Au and a-Si:H, for the MIS structures, was studied and compared with the Schottky, in order to determine how beneficial it could be for device performances and time degradation. For doing so, the Au thickness of 70Å was deposited by thermal evaporation on an oxide free (Schottky) and oxidized (≈20Å) (MIS) a-Si:H surfaces. These structures were characterized by SIMS, RBS, SEM and AFM in order to correlate the obtained diffusion profile of Au at the interface and the topography with the presence of the oxide at the interface. The results show that the Au inter-diffuses very easily in the oxide free a-Si:H surface, even at room temperature, degrading the devices performance. On the other hand, the MIS structures, with their interfacial oxide present no structural changes after annealing and the PSD produced are stable. We believe that this effect is associated with the barrier effect of the interfacial oxide that prevents the Au diffusion. The optimized 1D MIS sensors are stable and exhibit a linearity error as low as 0.8 % and sensitivities of 33 mV/cm for a 5 mW spot beam intensity at a wavelength of 532 nm, while the Schottky sensors showed a time degradation of their characteristics. © 2006 Materials Research Society.

This work presents for the first time a study on the deposition of polymorphous silicon at an excitation frequency of 27.12 MHz in a large-area plasma enhanced chemical vapor deposition (PECVD) reactor. Moreover, the films produced at 13.56 MHz were also investigated to compare their performance with that of the films produced at 27.12 MHz. The SiH4/H2 plasma was characterized by impedance probe measurements, aiming to identify the plasma conditions that lead to produce polymorphous films, under quasi-isothermal conditions. The films were characterized by spectroscopic ellipsometry, infrared absorption, Raman spectroscopy, and hydrogen exodiffusion experiments. These techniques enable a detailed structural characterization of the polymorphous films and a study of the differences between the films deposited at 27.12 MHz and 13.56 MHz. Conductivity measurements were also performed to determine the transport properties of the films. The results show that by using a 27.12 MHz frequency, the growth rate increased by 70% and a more stable, relaxed and denser structure was obtained.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.