Rodrigo Martins

Engineering procedures governing the selection or development of printable nanostructured metal oxide nanoparticles for chromic, photovoltaic, photocatalytic, sensing, electrolyte-gated TFTs, and power storage applications are established in this chapter. The main focus is given on how to perform the material selection and formulation of printable dispersion in order to develop functional films for electrochemical applications. This chapter is divided into four main parts. Firstly, a brief introduction on electrochemically active nanocrystalline metal oxide films developed via printing techniques is given. This is followed by the description of the film morphology, structure, and required functionality. A theoretical approach to understand the impact of size and shape of nanoparticles on an ink formulation and electrochemical performance being the subject of the third section provides a greater control over the material selection. We attempt to describe these properties and show that for a given material, geometry and size of the nanoparticles have a major influence on the electrochemical reactivity and response time. This gives the ability to tune the performance of the film simply by varying the morphology of incorporated nanostructures. This section is completed by the recommendations on each major step of an ink formulation, together with imposed critical constraints concerning the fluid control. Finally, the performance of the ink-jetprinted dual-phase electrochromic films is discussed as a case study. By providing such a rather systematic survey, we aim to stress the importance of proper design strategy that would result in both improved physicochemical properties of nanoparticle-loaded inks and enhanced electrochemical performance of printed functional films. © Springer International Publishing Switzerland 2016.

Transparent electronics is emerging as one of the most promising technologies for the next generation of electronic products, away from the traditional silicon technology. It is essential for touch display panels, solar cells, LEDs and antistatic coatings. The book describes the concept of transparent electronics, passive and active oxide semiconductors, multicomponent dielectrics and their importance for a new era of novel electronic materials and products. This is followed by a short history of transistors, and how oxides have revolutionized this field. It concludes with a glance at low-cost, disposable and lightweight devices for the next generation of ergonomic and functional discrete devices. Chapters cover: Properties and applications of n-type oxide semiconductors P-type conductors and semiconductors, including copper oxide and tin monoxide Low-temperature processed dielectrics n and p-type thin film transistors (TFTs) - structure, physics and brief history Paper electronics - Paper transistors, paper memories and paper batteries Applications of oxide TFTs - transparent circuits, active matrices for displays and biosensors Written by a team of renowned world experts, Transparent Oxide Electronics: From Materials to Devices gives an overview of the world of transparent electronics, and showcases groundbreaking work on paper transistors. © 2012 John Wiley & Sons, Ltd.

We have developed in the past a transient technique called Flying Spot Technique (FST)[1], based on the lateral photoeffect. It allows to determine the ambipolar diffusion length and the effective lifetime of the photogenerated carriers, once the light spot velocity and geometry of the structure are known. We propose to apply this technique backwards in order to detect the path and velocity of an object that is moving toward a light source direction. The light back reflected is analyzed by a p.i.n structure measuring the transient transverse photovoltage which is dependent on the object movement (position and velocity). Details concerning material characterization and device geometry will be presented.

In this work we report experimental results on light induced metastability of a-Si: H p.i.n. devices with different microscopic/macroscopic structures and we discuss them in terms of improved stability through spatial control of charged defects grown during light exposure. By placing a thin (few A) intrinsic layer (i) between both p/i and i/n a-Si: H interfaces we are able to reduce the effective degradation rate through spatial modification of the electric field profile in the device. The electronic transport and the stability changes that accompany the change in microstructure (R) and hydrogen content (CH) of the i- and i′-layer, were monitored throughout the entire light induced degradation process and compared with the corresponding μT product (for both carriers) inferred through steady state photoconductivity and Flying Spot Technique (FST) measurements. Results show that the degradation rate is a function of CH and R of both layers and can be correlated with the density of microvoids and di-hydride bonding. Since the i′-layers have a higher CH bonded mainly as SiF2 radicals (R≈0.4), they act as an hindrance to the growth of the defect, in the active region, generating "gettering centers" whose localisation and density are tailored in such a way that they will control spatially the electric field profile during light exposure. Preliminary results show improvements in film's stability when the interfacial layer is included. So future progress toward more stable and efficient a-Si: H solar cells will depend on a careful engineering design of the devices. © 1994 Materials Research Society.

Staggered bottom gate transparent thin film transistors (TTFTs) have been produced by rf magnetron sputtering at room temperature, using amorphous indium-zinc-oxide (IZO) semiconductor, for the channel as well as for the drain and source regions. The obtained TTFTs operate in the enhancement mode with threshold voltages of 2.4 V, saturation mobility of 22.7 cm2/Vs, gate voltage swing of 0.44 V/dec and an ON/OFF current ratio of 7×10 7. The high performances presented by these TTFTs produced at room temperature, make these TFTs a promising candidate for flexible, wearable, disposable portable electronics as well as battery-powered applications.

This work deals with the study of the role of the film thickness and composition on the color selectivity of the collecting spectrum of glass/ZnO:Ga/p-a-Si1-xCx:H/ a-Si1-x C x:H /a-Si:H/n-a-Si:H/Al photoelectronic detectors produced in a single chamber plasma enhanced chemical vapor deposition (PECVD) system. The cross contaminations were minimized by a rotate-cover substrate holder system. The devices can detect the blue illumination at small reverse bias and detect red illumination at large reverse bias. The role of the process parameters, especially the thickness of the p-type and intrinsic a-Si1-x C x:H, and the intrinsic a-Si:H layers on the device performances were studied in detail aiming to achieve a better detectivity. © 2005 Materials Research Society.

This new production technique is based on the growth of a-Si films on a reactor where gas decomposition promoted by a capacitively coupled r. f. power system takes place in a chamber separated from that where amorphous films are deposited under the action of an electromagnetic static field. Using this method, we shall reduce films contamination caused by the residual gas desorbed from reactor walls. At the same time, there is a reduction plasma ion and electron damages on the deposited films. The main species impinging upon our substrates will be mainly composed of long life radicals with high mobilities and high diffusion rates, which will give origin to a random silicon network free of long poly-silane chains.

Today there is a strong interest in the scientific and industrial community concerning the use of biopolymers for electronic applications, mainly driven by low-cost and disposable applications. Adding to this interest, we must recognize the importance of the wireless auto sustained and low energy consumption electronics dream. This dream can be fulfilled by cellulose paper, the lightest and the cheapest known substrate material, as well as the Earth's major biopolymer and of tremendous global economic importance. The recent developments of oxide thin film transistors and in particular the production of paper transistors at room temperature had contributed, as a first step, for the development of disposable, low cost and flexible electronic devices. To fulfil the wireless demand, it is necessary to prove the concept of self powered devices. In the case of paper electronics, this implies demonstrating the idea of self regenerated thin film paper batteries and its integration with other electronic components. Here we demonstrate this possibility by actuating the gate of paper transistors by paper batteries. We found that when a sheet of cellulose paper is covered in both faces with thin layers of opposite electrochemical potential materials, a voltage appears between both electrodes - paper battery, which is also self-regenerated. The value of the potential depends upon the materials used for anode and cathode. An open circuit voltage of 0.5V and a short-circuit current density of 1μA/cm2 were obtained in the simplest structure produced (Cu/paper/Al). For actuating the gate of the paper transistor, seven paper batteries were integrated in the same substrate in series, supplying a voltage of 3.4V. This allows proper ON/OFF control of the paper transistor. Apart from that transparent conductive oxides can be also used as cathode/anode materials allowing so the production of thin film batteries with transparent electrodes compatible with flexible, invisible, self powered and wireless electronics. © 2011 SPIE.

This paper presents the characterization of fundamental analog and digital circuits with a-IGZO TFTs from measurements performed at normal ambient. The fundamental blocks considered in this work include digital logic gates, a low-power single stage high-gain amplifier with capcacitive bootstrapping and a level shifter/buffer. These circuits are important functional blocks in analog/Mixed signal IC design with oxide TFTs. Being fabricated at low temperature (< 200 °C), they can find potential applications in low-cost large-area flexible systems. © 2016 IEEE.

This paper addresses a modeling and simulation methodology for analog circuit design with amorphous-GIZO thin-film transistors (TFTs). To reach an effective circuit design flow, with commercially available tools, a TFT model has been first developed with an artificial neural network (ANN). Multilayer perceptron with backpropagation algorithm has been adopted to model the static behavior of the TFT devices, for different aspect ratios. The model was then implemented in Verilog-A, to allow a quick instantiation in circuit. Simulations using Cadence Spectre are performed to validate the model. On a second phase, simulation results of basic analog circuits, with this ANN model, are verified against the actual functional results, namely an adder, subtractor, and current mirror circuit. Results demonstrate not only the ANN model accuracy and compatibility with dc and transient analysis, but also show the a-GIZO TFT capability to perform analog operations. © 2012 IEEE.

The behaviour of a microcrystalline p-i-n junction locally illuminated with monochromatic radiation (incident power of 50 mW/cm2) is analysed by means of numerical experiences. The model used for the two-dimensional analysis of the transport properties of a μc-Si:H p-i-n photo-detector is based on the simultaneous solution of the continuity equations for holes and electrons together with the Poisson's equation. The solution is found on a rectangular domain, taking into account the dimension perpendicular to the junction plane and one on the parallel plane. The lateral effects occurring within the structure, due to the non-uniformity of the illumination, are outlined. The results we present show that the potential profile has a linear variation from the illuminated to the dark neutral region. The lateral components of the electric field and of the current density vectors reveal to be mainly localised inside the doped layers.

In this work Spectroscopic Ellipsometry (SE) was used to study metal induced crystallization (MIC) on amorphous silicon films in order to analyze the influence of different annealing conditions on their structural properties. The variation of the metal thickness has shown to be determinant on the time needed to full crystallize silicon films. Films of 100 nm thickness crystallize after 2h at 500°C using 1 nm of Ni deposited on it. When reducing the average metal thickness down to 0.05 nm the same silicon film will need almost 10 hours to be totally crystallized. Using a new approach on the modelling procedure of the SE data we show to be possible to determine the Ni remaining inside the crystallized films. The method consists in using Ni as reference on the Bruggeman Effective Medium Approximation (BEMA) layer that will simulated the optical response of the crystallized silicon. Silicon samples and metal layers with different thicknesses were analyzed and this new method has shown to be sensible to changes on the initial metal/silicon ratio. The nickel distribution inside the silicon layers was independently measured by Rutherford Backscattering Spectroscopy (RBS) to check the data obtained from the proposed approach. © 2006 Materials Research Society.

Gallium-doped zinc oxide films were prepared by rf 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 Ωcm, 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 about 85 %, in average. The low resistivity, accomplished with a high growth rate deposited at RT, enables the deposition of these films onto polymeric substrates for flexible applications.

Aluminium doped zinc oxide thin films (ZnO:Al) have been deposited on polyester (Mylar type D, 100 μm thickness) substrates at room temperature by r.f. magnetron sputtering. The structural, morphological, optical and electrical properties of the deposited films have been studied. The samples are polycrystalline with a hexagonal wurtzite structure and a strong crystallographic c-axis orientation (002) perpendicular to the substrate surface. The ZnO:Al thin films with 85% transmittance in the visible and infra-red region and a resistivity as low as 3.6×10-2 Ωcm have been obtained, as deposited. The obtained results are comparable to those ones obtained on glass substrates, opening a new field of low cost, light weight, small volume, flexible and unbreakable large area optoelectronic devices.

Aluminum Zinc Oxide has been extensively investigated as a cheap alternative to transparent conducting tin oxide films for electronic and optoelectronic applications. Thin films of Aluminum Zinc Oxide have been developed successfully through a combination of solution combustion synthesis and solution synthesis. Zn(NO3)3·6H2O as metal source was dissolved in 2-methoxyethanol as solvent through combustion synthesis with Urea as fuel while dopant source of AlCl3·6H2O was mixed separately in solvent to avoid aluminum oxide formation in the films. Precursor solutions were obtained mixing Zn & Al separate solutions in 9:1, 8:2, and 7:3 ratios respectively with oxide, fuel and dopant concentrations of 0.5, 0.25, 0.1, and 0.05 M. The film stacks have been prepared through spin-coating with heating at 400°C for 10 minutes after each deposition to remove residuals and evaporate solvents. Thermal annealing in oven at 600°C for 1 hour followed by rapid thermal annealing at 500°C & 600°C first in vacuum and then in N2-5%H2 environment respectively for 10 minutes each reduced the resistivity of film stacks. Film stack with 10 layers for an average thickness of 0.5μm gave the best Hall Effect resistivity of 3.2 × 10-2 -cm in the case of 0.5M solution with Zn:Al mixing ratio of 9:1 for RTA annealings at 600°C with an average total transparency of 80 % in the wavelength range of 400-1200 nm. The results show a clear trend that increasing the amount of ingredients resistivity could further be decreased. © 2015 IEEE.

Using the Flying Spot Technique (FST) we have studied minority carrier transport parallel and perpendicular to the surface of amorphous silicon films (a-Si:H). To reduce slow transients due to charge redistribution in low resistivity regions during the measurement we have applied a strong homogeneously absorbed bias light. The defect density was estimated from CPM measurements. The steady-state photocarrier grating technique (SSPG) is a 1-dimensional approach. However, the modulation depth of the carrier profile is also dependent on film surface properties, like surface recombination velocity. Both methods yield comparable diffusion lengths when applied to a-Si:H.

Wide band gap microcrystalline silicon films have aroused considerable interest since they combine some electro-optical advantages of amorphous and crystalline materials highly important to produce electro-optical devices such as TFTs and solar cells. In this paper we present results concerning the electro-optical characteristics of highly transparent and conductive n-type μc-Si based films. Here, emphasis is given to the production of n-type μc-films with optical gaps of 2.3 eV and dark conductivity's of 6.5 Scm-1.

In order to understand the kinetics of formation of interface/surface states and its correlation on the final device performance, a preliminary study was performed on MIS structures, before and after surface oxidation/passivation, using different oxidation techniques and oxides: thermal (in air), chemical (in H2O2) and oxygen plasma. The devices used in this work are based on a glass/Cr/a-Si:H(n+)/a-Si:H(i)/SiOx/Pd structures, where the amorphous silicon intrinsic layer (i a-Si:H) with a photosensitivity of 107 was deposited by a modified plasma enhanced chemical vapour deposition (PECVD) triode system. The electrical properties of a-Si:H MIS structures were investigated by measuring their diode current-voltage characteristics in the dark and under illumination as well as the spectral response, as a function of the various oxidation techniques. Infrared spectroscopy and spectroscopic ellipsometry were used as a complementary tool to characterise the oxidised surface.

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.

This work's aim is to report for the first time the cubic to hexagonal phase transition in tin-doped In2O3 films with a Sn/In atomic ratio of 0.03, fabricated at low temperature and normal pressure from alcoholic solution of InCl3 and SnCl4. The performed X-ray diffraction measurements show a difference between crystallographic symmetry of thin (100 nm) and thick (400 nm) films prepared in the same conditions: the structure of thick films can be related to high pressure In2O3 hexagonal system with a preferred orientation of c-axis parallel to the substrate surface, while thin films present a cubic symmetry with columnar (400) grain orientation. Phase transition nature is connected with non-axial tensile deformation of indium oxide grid due to insertion of chlorine ions in the position of two diagonally opposite oxygen vacancies in In2O3 network.

The current transport in metal-amorphous semiconductor barriers is examined by solving the proper Poisson's equation and transport equations within the semiconductor's space charge region taking into account the role of trap shallow states distribution function. The effect of metal is also included through appropriate boundary conditions of the above solutions. Generalized transport equations will be derived either when thermionic drift-diffusion emission process dominates or when the conduction mechanism is mainly due to drift-diffusion emission. Both situations will be analysed with or without neglecting carriers losses during their collision free path, from which a tractable expression for the current-voltage characteristic will be determined.

This paper presents a low-offset comparator based on n-type amorphous indium gallium zinc oxide thin-film transistors (TFTs). An a-Si:H TFT model was adapted to fit the electrical characterization data obtained for these devices. The proposed comparator comprises three pre-amplification stages, a positive-feedback analog latch and a fully dynamic digital latch. Simulation results show that the proposed circuit can work at several tens of kHz, with an accuracy of the order of 10 mV, considering a supply voltage of 10 V and a current consumption of 380 μA. Monte-Carlo simulations exhibit a 1-sigma random offset voltage smaller than 10 mV and 40 mV, respectively, with and without using autozeroing techniques. © 2015 IEEE.

The aim of this work is to provide the basis for the interpretation of the steady state lateral photoeffect observed in p-i-n a-Si:H 1D Thin Film Position Sensitive Detectors (1D TFPSD). The experimental data recorded in 1D TFPSD devices with different performances are compared with the predicted curves and the obtained correlation's discussed.

This paper presents results of the spatial and frequency detection limits of an integrated array of 32 one-dimensional amorphous silicon thin film position sensitive detectors with a nip structure, under continuous and pulsed laser operation conditions. The data obtained show that 0.45×0.06 cm arrays, occupying a total active area of about 1 cm2 have a spatial resolution better than 10 μm (modulation transfer function of about 0.2), with a cut-off frequency of about 6.8 KHz. Besides that, under pulsed laser conditions the device non-linearity has its minimum (about 1.6%), for a frequency of about 200Hz. Up to the limits of the cut-off frequency, the device non-linearity increases to values above 4%.