Rodrigo Martins

The influence of U.V. light on the transport properties of a-Si : H films during its growth in a r.f. double chamber system was investigated by conductivity, optical absorption, I.R. absorption, spectral photoconductivity and X-ray diffraction measurements. It was concluded that the presence of U.V. light during the deposition process controls the way how hydrogen is incorporated in the structure as well as the impurity atoms. The microcrystalline films investigated present sharp peaks in the I.R. spectra. Both boron and phosphorus doped films show conductivities higher than 10 S cm-1 and estimated crystalline sizes of the order of 80 Å. © 1987.

A study of intrinsic zinc oxide thin film as ozone sensor based on the ultraviolet (UV) photoreduction and subsequent ozone re oxidation of zinc oxide as a fully reversible process was presented. It was found that the film described were produced by spray pyrolysis, dc and rf magnetron sputtering. The dc resistivity of the films changed more than eight orders of magnitude when exposed to an UV dose of 4 mW/cm2. Analysis shows that the porous and textured zinc oxide films produced by spray pyrolysis at low substrate exhibit an excellent ac impedance response.

This paper deals with a new process to improve the stability of a-Si:H pin solar cells deposited in a single batch process by proper passivation of the interfaces. The process consists in removing partially a deposited sacrificial oxide layer grown between the p/i or i/n interfaces by SF6 etching. This layer is an absorber of defects and impurities that are introduced in the interfaces, mainly from the chamber walls and the substrate surface. The results achieved in laboratory samples lead to devices in which the fill factor and short circuit current density were improved respectively towards 75% and 16.5 mA cm-2, with a final working efficiency of about 9.5%. © 2001 Elsevier Science Ltd. All rights reserved.

This work reports on the performances of undoped and doped amorphous/nanocrystalline silicon films grown by hot wire plasma assisted technique. The structure (including the presence of several nanoparticles with sizes ranging from 5 nm to 50 nm), composition (oxygen and hydrogen content) and transport properties of the films are highly dependent on the temperature of the filament and on the hydrogen dilution. The undoped films grown under low r.f. power (≈4mWcm-2) and filament temperatures around 1850 K present dark conductivities below 10-10 Scm-1, optical gaps of about 1.6 eV and photosensitivities above 105, (under AM 1.5 light intensities), with almost no traces of oxygen content. For the n- and the p-doped silicon films also fabricated under the same conditions the conductivities obtained are of about 10-2Scm-1 and 10-5Scm-1, respectively. © 1998 Elsevier Science Ltd. All rights reserved.

This paper discusses the effect of order and disorder on the electrical and optical performance of ionic oxide semiconductors based on zinc oxide. These materials are used as active thin films in electronic devices such as pn heterojunction solar cells and thin-film transistors. Considering the expected conduction mechanism in ordered and disordered semiconductors the role of the spherical symmetry of the s electron conduction bands will be analyzed and compared to covalent semiconductors. The obtained results show p-type c-Si/a-IZO/poly-ZGO solar cells exhibiting efficiencies above 14% in device areas of about 2.34 cm2. Amorphous oxide TFTs based on the Ga-Zn-Sn-0 system demonstrate superior performance than the polycrystalline TFTs based on ZnO, translated by ION/IOFF ratio exceeding 107, turn-on voltage below 1-2 V and saturation mobility above 25 cm2/Vs. Apart from that, preliminary data on p-type oxide TFT based on the Zn-Cu-O system will also be presented. © 2009 SPIE.

Today there is a strong interest in the scientific and industrial community concerning the use of biopolymers for electronic applications, driven mainly by low-cost and disposable applications. Adding to this interest, we must recognise the importance of the dream of wireless auto-sustained and low-energy-consumption electronics. 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. Most of the paper used up to now is optimised in terms of the required mechanical and physical properties to be used as the support of inks of different origins. In the future, specific electronic heterogeneous paper sheets should be fabricated aiming to get paper fibers with required bulk and surface functionalities, proper water/vapour barrier, size and diameter/thickness of the fibrils and full paper thickness. This will be the function of components/devices to be incorporated/integrated such as thin-film transistors, complementary metal oxide semiconductor devices, passive electronic components (resistances, inductors and capacitors), memory transistors, electrochromics and thin-film paper batteries. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

This paper deals with the engineering aspects related to the rf energy coupling in Plasma Enhanced Chemical Vapour Deposition (PECVD) processes, in a diode-type unit in which an extra grid is used. The main emphasis is given in the determination of the real power delivered to the gas and comparing it with the total power losses, besides determining the best way to control the powder formed during the process. © 1994.

The role of order and disorder on the electronic performances of n -type ionic oxides such as zinc oxide, gallium zinc oxide, and indium zinc oxide used as active (channel) or passive (drain/source) layers in thin film transistors (TFTs) processed at room temperature are discussed, taking as reference the known behavior observed in conventional covalent semiconductors such as silicon. The work performed shows that while in the oxide semiconductors the Fermi level can be pinned up within the conduction band, independent of the state of order, the same does not happen with silicon. Besides, in the oxide semiconductors the carrier mobility is not bandtail limited and so disorder does not affect so strongly the mobility as it happens in covalent semiconductors. The electrical properties of the oxide films (resistivity, carrier concentration, and mobility) are highly dependent on the oxygen vacancies (source of free carriers), which can be controlled by changing the oxygen partial pressure during the deposition process and/or by adding other metal ions to the matrix. In this case, we make the oxide matrix less sensitive to the presence of oxygen, widening the range of oxygen partial pressures that can be used and thus improving the process control of the film resistivity. The results obtained in fully transparent TFT using polycrystalline ZnO or amorphous indium zinc oxide (IZO) as channel layers and highly conductive poly/nanocrystalline ZGO films or amorphous IZO as drain/source layers show that both devices work in the enhancement mode, but the TFT with the highest electronic saturation mobility and on/off ratio 49.9 cm2 V s and 4.3× 108, respectively, are the ones in which the active and passive layers are amorphous. The ZnO TFT whose channel is based on polycrystalline ZnO, the mobility and on/off ratio are, respectively, 26 cm2 V s and 3× 106. This behavior is attributed to the fact that the electronic transport is governed by the s -like metal cation conduction bands, not significantly affected by any type of angular disorder promoted by the 2p O states related to the valence band, or small amounts of incorporated metal impurities that lead to a better control of vacancies and of the TFT off current. © 2007 American Institute of Physics.

This paper deals with the study of the role of gas temperature and of the ratio of r.f. power to gas flow on the particle’s formation in amorphous silicon films grown by plasma enhanced chemical vapour deposition technique, by monitoring the plasma impedance behaviour under different process conditions. The aim is to determine in which conditions the particles formed do not deteriorate the performances of the films grown or even can lead to an improvement of the properties of the films deposited. The results achieved show the existence of two main boundary regions (β- and θ-regions) separating the so-called α-regime (no powder formed) from the γ-regime (powder formed). Those regions are reached either by heating the gas, changing the gas pressure or using high power to gas flow ratios. In the β-region the probability of incorporating nanoparticles in the films is low and the films exhibit properties similar to those of the ones grown in the α-regime, with a low density of voids incorporated. In the θ-region small nanoparticles can be incorporated leading to films with density of states below 5×1015 cm-3, widened Urbach energies and photosensitivities about two orders of magnitude larger than that of conventional amorphous silicon grown in the α-regime. © 2000 Elsevier Science Ltd.

The objective of this work is to present an analytical model able to interpret the experimental dependence of the uniformity of films produced by the plasma-enhanced chemical vapour deposition technique on the deposition parameters (discharge pressure, gas flow temperature and rf power density). The model proposed is based on the Navier-Stokes equations applied to a gas flow considered to be quasi-incompressible and quasi-inviscous, whenever the Mach number is below 0·3. This condition leads to the establishment of the proper quasisteady-state gas flow equations, and the corresponding equations of energy and momentum balance ascribed to the mass profile of the species formed, under the presence of a low-rf-power plasma density, are able to predict the uniformity distribution of the film over the entire deposited substrate area.

This paper presents results of the role of the oxygen partial pressure used during the deposition process on the transport properties exhibited by doped microcrystalline silicon oxycarbide films produced by a Two Consecutive Decomposition and Deposition Chamber system, where a spatial separation between the plasma and the growth regions is achieved. This paper also presents the interpretative models of the optoelectronic behaviour observed in these films (highly conductive and transparent with suitable properties for optoelectronic applications) as well as the interpretation of the growth process that leads to film's microcrystallization.

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%.

The aim of this work is to provide the basis for the interpretation, under steady state, of the lateral photoeffect in p-i-n a-Si:H 1D Thin Film Position Sensitive Detectors (1D TFPSD) through an analytical model. 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 role of the oxygen partial pressure (pO2) used on the properties exhibited by doped μc silicon oxycarbide films produced by a Two Consecutive Decomposition and Deposition Chamber (TCDDC) system [1], where a spatial separation between the plasma and the growth regions is achieved. The films produced are highly conductive and transparent with suitable properties for optoelectronic applications.

The electron transport in n-type polycrystalline zinc oxide, nanocrystalline Zinc-Gallium-Oxygen and amorphous Indium-Zinc-Oxygen systems produced by rf magnetron sputtering at room temperature, under different oxygen partial pressure were investigated. It was found that the carrier transport is not band tail limited, being governed by metal cations irrespective to the film's structure. The highest net room temperature electron mobility was achieved on the amorphous films and noticed that for the single component oxides the mobility decreases as the carrier concentration increases, while the reverse behaviour was observed for the multicomponent oxides, independently of their structure. These behaviours are related to the role that negative charge defects in excess of 1010 cm- 2 generated on multicomponent oxides have on carriers scattering and so on their electronic performances. © 2007 Elsevier B.V. All rights reserved.

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.

This paper deals with a new design method for the interfaces of a-Si:H pin solar cells that improves the stability and performances of devices deposited in a single batch chamber process. The method consists in removing a deposited sacrificial layer placed between the p/i and/or i/n interfaces by etching. This layer is an absorber of defects and impurities that are introduced in the interfaces, mainly from the chamber walls cross-contamination and the substrate surface. The results achieved increase the device fill factor and short circuit current density, respectively towards 75% and 16.3 mA cm-2, with a final efficiency of about 10%, before light soaking experiments. © 2000 Elsevier Science B.V. All rights reserved.

One of the main problems in producing large area amorphous silicon devices concerns films uniformity. In this paper we present data concerning the role of reactor geometry and design and on the film performances as well as the problems related to mechanical mismatches in scaling up the reactor size. © 1991 Elsevier Science Publishers B.V. All rights reserved.

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.

The electrosynthesis of polypyrrole (PPy) has been achieved on aluminium in aqueous medium of tartaric acid by means of cyclic voltammetry, potentiostatic and galvanostatic techniques. Scanning electron microscopy (SEM) and X-ray microanalysis by energy spectroscopy dispersion (EDS) applying on surfaces show that the PPy coating is developed from the metal surface through the cracks of the initial Al2O3 layer. A mechanism involving the participation of the supporting electrolyte and the pyrrole (Py) in distinct active sites was proposed based on the linear sweep voltammetry. It is observed for all applied electrochemical techniques that the pyrrole concentration has to be higher than 0.1 M to allow the polypyrrole electrodeposition in acid medium. Scanning electron microscopy, secondary electrons (SE) and backscattering electrons (BE), shows that the PPy coating obtained in galvanostatic and potentiostatic modes starts with small islands at weak applied potentials or current densities. Moreover, EDS reveals a good homogeneity and compactness of the film achieved in galvanostatic method. The corrosion results in 3% NaCl medium show that the PPy coating decreases the corrosion behaviour of the aluminium. The bilayer Al2O3/PPy shows a capacitor with future applications. © 2006 Elsevier Ltd. All rights reserved.

Curcuma longa L., also known as turmeric, is widely used as a food colorant and has been reported to have antioxidant, anti-inflammatory, anti-mutagenic and anti-cancer properties. The aim of this study was to evaluate the effects of the spray drying on curcuminoid and curcumin contents, antioxidant activity, process yield, the morphology and solubility of the microparticulated solid dispersion containing curcuma extract using a Box Behnken design. The microparticles were spherical in shape, and an increase in outlet temperature from 40 to 80 °C resulted in a significant increase in the yield of microparticles from 16 to 53%. The total curcuminoid content (17.15 to 19.57. mg/g), curcumin content (3.24 to 4.25. mg/g) and antioxidant activity (530.1 to 860.3 μg/mL) were also affected by the spray drying process. The solubility of curcuminoid from C. longa remarkably improved 100-fold in the microparticles, confirming the potential of the ternary solid dispersion technique to improve the dyeing and nutraceutical properties of these compounds. Furthermore, the microparticles were obtained using the spray drying process, can be easily scaled up. © 2011 Elsevier Ltd.

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.

Reported herein is a nonvolatile n-type floating gate memory paper field-effect transistor, emphasizing the role of the paper structure and properties on the device performance recorded such as in the high capacitance per unit area at low frequencies (>2.5 μFcm-2) and so on the set of high charge retention times achieved (>16000 hours). The device was built via the hybrid integration of natural cellulose fibers, which act simultaneously as substrate and gate dielectric, using amorphous indium zinc and gallium indium zinc oxides respectively for the gate electrode and channel layer. This was complemented by the use of continuous patterned metal layers as source/drain electrodes. © 2010 Copyright SPIE - The International Society for Optical Engineering.

The ability to process and dimensionally scale field-effect transistors with and on paper and to integrate them as a core component for low-power-consumption analog and digital circuits is demonstrated. Low-temperature-processed p- and n-channel integrated oxide thin-film transistors in the complementary metal oxide semiconductor (CMOS) inverter architecture are seamlessly layered on mechanically flexible, low-cost, recyclable paper substrates. The possibility of building these circuits using low-temperature processes opens the door to new applications ranging from smart labels and sensors on clothing and packaging to electronic displays printed on paper pages for use in newspapers, magazines, books, signs, and advertising billboards. Because the CMOS circuits reported constitute fundamental building blocks for analog and digital electronics, this development creates the potential to have flexible form factor computers seamlessly layered onto paper. The holistic approach of merging low-power circuitry with a recyclable substrate is an important step towards greener electronics. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

In this work, we present a model to interpret the steady-state and the dynamic detection limits of 1-D Thin Film Position Sensitive Detectors (1-D TFPSD) based on p-i-n a-Si:H devices. From this, an equivalent electric circuit is derived and the predicted values are compared with the experimental results obtained in 1-D TFPSD devices, with different sizes. The model is also able to determine the device characteristics that influence the spatial limits and the response time of the device. © 1996 IEEE.