Rodrigo Martins

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.

N-doped ZnO films were deposited on glass substrates by RF magnetron sputtering with different deposition pressures. The samples were characterized by X-ray diffraction (XRD), atomic force morphology (AFM), X-ray photoelectron spectroscopy (XPS), Hall measurements and optical spectrophotometer. The XRD patterns confirmed that the films are polycrystalline and the influence of deposition pressure on the structural properties. AFM microstructures also authenticated the change in the size and shape of the grains as a function of deposition pressure; the root mean square (RMS) roughness has reached a maximum (10.65 nm) at 1.5 x 10 -2 mbar. XPS spectra revealed the change in the chemical composition. The amount of adsorbed oxygen and nitrogen at oxide sites has reached the maximum at 9.0 x 10 -3 mbar, where the film showed p-type conductivity. The optical transmittance spectra have indicated that the absorption edge is shifted towards the shorter wavelength at higher deposition pressure. Correspondingly, the optical band gap is increased from 2.17 to 2.80 eV. The average visible transmittance in the wavelength ranging 500-800 nm has been increased from 49% to 82%. Copyright © 2010 American Scientific Publishers All rights reserved.

N-doped ZnO films were radio frequency (RF) sputtered on glass substrates and studied as a function of oxygen partial pressure (OPP) ranging from 3.0×10-4 to 9.5×10-3 Pa. X-ray diffraction patters confirmed the polycrystalline nature of the deposited films. The crystalline structure is influenced by the variation of OPP. Atomic force microscopy analysis confirmed the agglomeration of the neighboring spherical grains with a sharp increase of root mean square (RMS) roughness when the OPP is increased above 1.4×10-3 Pa. X-ray photoelectron spectroscopy analysis revealed that the incorporation of N content into the film is decreased with the increase of OPP, noticeably N 1s XPS peaks are hardly identified at 9.5×10-3 Pa. The average visible transmittance (380-700 nm) is increased with the increase of OPP (from ∼17 to 70), and the optical absorption edge shifts towards the shorter wavelength. The films deposited with low OPP (≤ 3.0×10-4 Pa) show n-type conductivity and those deposited with high OPP (≥ 9.0×10-4 Pa) are highly resistive (>105 ·cm) © 2010 The Nonferrous Metals Society of China.

A dye sensitized TiO2 photodetector has been integrated with a DNA detection method based on non-cross-linking hybridization of DNA-functionalized gold nanoparticles, resulting in a disposable colorimetric biosensor. We present a new approach for the fabrication of dye sensitized TiO2 photodetectors by an inkjet printing technique-a non-contact digital, additive, no mask and no vacuum patterning method, ideal for cost efficient mass production. The developed biosensor was compared against a dye sensitized photodetector fabricated by the traditional "doctor blade" method. Detection of gold nanoparticle aggregation was possible for concentrations as low as 1.0 nM for the "doctor blade" system, and 1.5 nM for the inkjet printed photodetector. The demonstrated sensitivity limits of developed biosensors are comparable to those of spectrophotometric techniques (1.0 nM). Our results show that a difference higher than 17% by traditional photodetector and 6% by inkjet printed in the photoresponses for the complementary and non-complementary gold nanoprobe assays could be attained for a specific DNA sequence from Mycobacterium tuberculosis, the etiologic agent of human tuberculosis. The decrease of costs associated with molecular diagnostic provided by a platform such as the one presented here may prove of paramount importance in developing countries. © 2009 Elsevier B.V. All rights reserved.

{A nonvacuum and low temperature process for passivating transparent metal oxides based thin-film transistors is presented. This process uses the epoxy-based SU-8 resist which prevents device degradation against environmental conditions, vacuum or sputtering surface damage. The incorporation of SU-8 as a passivation layer is based on the ability of this polymer to provide features with high mechanical and chemical stability. With this approach, lithography is performed to pattern the resist over the active area of the device in order to form the passivation layer. The resulting transistors demonstrate very good electrical characteristics, such as μFE =61 cm2 /V s

High visible to near infrared (NIR) transparent Mo (0-1 at%) doped In2O3 (IMO) thin films with high carrier mobility were deposited on Corning-1737 glass substrates at 400 °C by spray pyrolysis experimental technique. The films were annealed in vacuum (∼1×10-4 mbar) at 550 °C for 45 min. XRD analysis confirmed that indium oxide belongs to cubic bixbyite structure. The preferred growth orientation along (2 2 2) plane for low Mo doping level shifts to (4 0 0) for higher Mo doping levels. Crystallite sizes extracted from the XRD data corroborate the changes in full-width at half-maximum due to the variation in Mo doping. Scanning electron microscopy study illustrates the evolution in surface microstructures as a function of Mo doping. The negative sign of Hall coefficient confirmed n-type conductivity. Films with high mobility of ∼149 cm2/(V s), carrier concentration of ∼1.0×1020 cm-3, resistivity of ∼4.0×10-4 Ω cm and high figure of merit of ∼1.02×10-2 Ω-1 were observed for post-annealed films (0.5 at% Mo). The obtained high average transparency of ∼83% in the wavelength range 400-2500 nm confirms that transmittance is well extended into the NIR region. © 2009 Elsevier B.V. All rights reserved.

We have investigated the electrical properties of metal-semiconductor field-effect transistors (MESFET) based on amorphous oxide semiconductor channels. All functional parts of the devices were sputter-deposited at room temperature. The influence on the electrical properties of a 150 °C annealing step of the gallium-indium-zinc-oxide channel is investigated. The MESFET technology offers a simple route for processing of the transistors with excellent electrical properties such as low subthreshold swing of 112 mV/decade, gate sweep voltages of 2.5 V, and channel mobilities up to 15 cm2 /V s. © 2010 American Institute of Physics.

This paper discusses the properties of sputtered multicomponent amorphous dielectrics based on mixtures of high-κ and high-bandgap materials and their integration in oxide TFTs, with processing temperatures not exceeding 1 50° C. Even if Ta2O5 films are already amorphous, multicomponent materials such as Ta2O5-SiO2 and Ta2O5-Al2O3 allow an increase in the bandgap and the smoothness of the films, reducing their leakage current and improving (in the case of Ta2O5-SiO2)the dielectric/semiconductor interface properties when these dielectrics are integrated in TFTs. For HfO2- based dielectrics, the advantages of multicomponent materials are even clearer: while HfO2 films present a polycrystalline structure and a rough surface, HfO2-SiO2 films exhibit an amorphous structure and a very smooth surface. The integration of the multicomponent dielectrics in GIZO TFTs allows remarkable performance, comparable with that of GIZO TFTs using SiO2 deposited at 400°C by PECVD. For instance, with Ta2O5-SiO2 as the dielectric layer, field-effect mobility of 35 cm2/(V-sec), close to 0 V turn-on voltage, an on/off ratio higher than 106, a subthreshold slope of 0.24 V/dec, and a small/recoverable threshold voltage shifts under constant current (ID = 10 μ,A) stress during 24 hours are achieved. Initial results with multilayers of SiO2/HfO 2-SiO2/SiO2 are also shown, allowing a lower leakage current with lower thickness and excellent device performance. © Copyright 2010 Society for Information Display.

The movement of a micro cantilever was detected via a self constructed portable data acquisition prototype system which integrates a linear array of 32 1D amorphous silicon position sensitive detectors (PSD). The system was mounted on a microscope using a metal structure platform and the movement of the 30 μm wide by 400 μm long cantilever was tracked by analyzing the signals acquired by the 32 sensor array electronic readout system and the relevant data algorithm. The obtained results show a linear behavior of the photocurrent relating X and Y movement, with a non-linearity of about 3%, a spatial resolution of less than 2μm along the lateral dimension of the sensor as well as of less than 3μm along the perpendicular dimension of the sensor, when detecting just the micro-cantilever, and a spatial resolution of less than 1μm when detecting the holding structure. © 2010 by the authors.

Nanostructured silicon (na-Si:H) thin films were fabricated using plasma enhanced chemical vapour deposition (PECVD) technique under high silane hydrogen dilution and a discharge frequency of 27 MHz, where the substrate was located in the dark region of the plasma, protected by a grounded metal grid. By not exposing the growth surface directly to the plasma we avoid the silicon growth surface to sustain a high ion bombardment leading to a less defective surface and highly compact films. The intrinsic films grown under these conditions were used to produce the channel region of thin film transistors (TFTs) with a bottom gate staggered configuration, integrating different dielectric layers. The devices produced exhibit a field effect mobility close to 1.84 cm 2 V -1S -1, threshold voltage around 2 V, on/off ratio above 10 7 and sub-threshold slope below 0.5 V/decade, depending on the dielectric used. Copyright © 2010 American Scientific Publishers All rights reserved.

Long-term electrical stability measurements, including current/bias stress and aging over 18 months of idle shelf life are presented for GIZO-based TFTs. The effects of oxygen partial pressure, annealing temperature and passivation are discussed. Optimized devices show highly stable properties, such as a recoverable ΔVT<0.5 V after 24h of 1D=10 μA stress, quite promising for integration in electronic circuits.

Long-term electrical stability measurements, including current/bias stress and aging over 18 months of idle shelf life are presented for GIZO-based TFTs. The effects of oxygen partial pressure, annealing temperature and passivation are discussed. Optimized devices show highly stable properties, such as a recoverable ΔV T<0.5 V after 24h of I D=10 μA stress, quite promising for integration in electronic circuits. © 2010 SID.

Trimethylboron (TMB) has been receiving attention as a valid alternative to diborane and methane mixtures for the deposition of p-type silicon films for applications in optoelectronic devices such as solar cells. In this paper we report on p-type hydrogenated nanocrystalline silicon carbide (nc-Si:C:H) films produced by standard 13.56 MHz plasma enhanced chemical vapour deposition technique, using TMB as gas source, under high hydrogen dilution (98%) and using high deposition pressures (3 Torr). The films obtained were characterized by spectroscopic ellipsometry (SE), Raman spectroscopy (RS), and electrical measurements to determine their optical, structural and electrical properties. We achieved conductivities as high as 8.3 (Ω cm) -1, one of the highest values of conductivity published to date using TMB with standard rf-PECVD. Spectroscopic ellipsometry modeling revealed that the films growth mechanism proceeds through a sub-surface layer mechanism that leads to the formation of nanocrystalline silicon. Copyright © 2010 American Scientific Publishers All rights reserved.

This paper reports on the use of cellulose paper simultaneously as electrolyte, separation of electrodes, and physical support of a rechargeable battery. The deposition on both faces of a paper sheet of metal or metal oxides thin layers with different electrochemical potentials, respectively as anode and cathode, such as Cu and Al, lead to an output voltage of 0.70 V and a current density that varies between 150 nA/cm2 and 0.5 mA/cm2, subject to the paper composition, thickness and the degree of OHx species adsorbed in the paper matrix. The electrical output of the paper battery is independent of the electrodes thickness but strongly depends on the atmospheric relative humidity (RH), with a current density enhancement by more than 3 orders of magnitude when RH changes from 60% to 85%. Besides flexibility, low cost, low material consumption, environmental friendly, the power output of paper batteries can be adapted to the desired voltagecurrent needed, by proper integration. A 3-V prototype was fabricated to control the ON/OFF state of a paper transistor. © 2006 IEEE.

Molybdenum doped indium oxide (IO) thin films were deposited on the Coring F1737 glass substrates at 400 °C by spray pyrolysis technique. TheModoping was varied between 0 and 4 at.%. The films were characterized by their structural, electrical and optical properties. The films are confirmed to be cubic bixbyite In 2O 3 with a strongest orientation along (222) for 0.5 at.% Mo, which is shifted to (400) plane when the Mo doping is increased to ≥1.2 at.%. The films deposited with 0.5 at.% Mo showed high mobility of ̃90 cm 2/Vs, resistivity of ̃6.8×10 -4ωcm and carrier concentration of ̃1.01× 1020 cm -3 with >̃73% transmittance in the visible range between 500 and 800 nm. The transmittance is well extended into near infrared region.

Copper oxide (Cu2 O) thin films were used to produce bottom gate p-type transparent thin-film transistors (TFTs). Cu2 O was deposited by reactive rf magnetron sputtering at room temperature and the films exhibit a polycrystalline structure with a strongest orientation along (111) plane. The TFTs exhibit improved electrical performance such as a field-effect mobility of 3.9 cm2 /V s and an on/off ratio of 2× 102. © 2010 American Institute of Physics.

P-type thin-film transistors (TFTs) using room temperature sputtered SnOx (x<2) as a transparent oxide semiconductor have been produced. The SnOx films show p-type conduction presenting a polycrystalline structure composed with a mixture of tetragonal Β-Sn and α -SnOx phases, after annealing at 200 °C. These films exhibit a hole carrier concentration in the range of ≈ 1016 - 1018 cm-3; electrical resistivity between 101 - 102 cm; Hall mobility around 4.8 cm2 /V s; optical band gap of 2.8 eV; and average transmittance ≈85% (400 to 2000 nm). The bottom gate p-type SnOx TFTs present a field-effect mobility above 1 cm2 /V s and an ON/OFF modulation ratio of 103. © 2010 American Institute of Physics.

The effects of zinc concentration on the performance of solution processed amorphous indium gallium zinc oxide (a-IGZO) thin film transistors (TFTs) have been investigated using high-k aluminum titanium oxide as gate dielectric. The x-ray diffraction results confirmed that all the IGZO channel layers are amorphous. The performance of a-IGZO TFTs were investigated in the linear regime operation. Highest linear field-effect mobility of 5.8 cm2 /V s with an Ion / Ioff ratio of 6× 107 and subthreshold swing of 0.28 V/dec were obtained for the a-IGZO (311) TFTs. The obtained performance of the a-IGZO TFTs is very promising for low-voltage display applications. © 2010 American Institute of Physics.

Zinc oxide (ZnO) has attracted recent interest for a range of applications, including use as a transparent conductive oxide (TCO) and in gas sensor devices. This paper compares ZnO films grown using two methods designed for the production of thin films, namely sol-gel and aerosol assisted chemical vapour deposition (AACVD) for potential use in sensor and TCO applications. Materials produced by the sol-gel route were observed to be amorphous when annealed at 350 °C, but were crystalline when annealed at higher temperatures and had a relatively open grain structure when compared to the AACVD films. Electrical characterization showed that materials were highly resistive, but that their properties varied considerably when the measurements were performed in vacuum or in air. This behaviour was rapidly reversible and reproducible for room temperature measurement. In contrast materials grown by aerosol-assisted CVD were non-porous, polycrystalline and conductive. Measured electrical properties did not vary with changing measurement atmosphere. These differences are discussed in terms of the structural characterisation of the films and some comments are made regarding the suitability of both approaches for the growth of ZnO thin film sensor materials. © 2009 Elsevier B.V. All rights reserved.

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.

Indium doped, and undoped, zinc oxide films were deposited using aerosol assisted chemical vapour deposition (AACVD) at atmospheric pressure on glass substrates. Electrical measurements (I-V) showed a reduction in resistivity following the addition of indium, and XRD analysis revealed an associated switch to c-axis preferred crystal orientation. The ability of the films to oxidise organic material on their surface was analysed using stearic acid as the model contaminant under ultra-violet (UV, 365 nm) irradiation. The In-doped films displayed a greater rate of organic decomposition, which we attribute to the formation of a platelet surface structure having a larger surface area than the undoped films, on which the UV generated electrons and holes may react to form active photocatalytic species. In addition we suggest that the switch to c-axis crystal orientation may reduce the electron-hole pair recombination rate at the grain boundaries, due to an improvement in crystallinity and related reduction in carrier scattering losses, leading to an increase in photocatalytic organic decomposition rate. © 2011 Elsevier B.V. All rights reserved.

A complementary metal oxide semiconductor (CMOS) device is described. The device is based on n-(In-Ga-Zn-O) and p-type (SnO x) active oxide semiconductors and uses a transparent conductive oxide (In-Zn-O) as gate electrode that sits on a flexible, recyclable paper substrate that is simultaneously the substrate and the dielectric. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

The influence of doping on the morphology, physical and electrical properties of zinc oxide produced by the sol-gel method was examined. Undoped zinc oxide was observed to form relatively porous films. Addition of an Al dopant influenced the sheet resistance, but did not result in a change in morphology, examined by atomic force microscopy when compared to undoped films. In the case of electrical measurements, undoped ZnO films were extremely resistive. A minimum dopant concentration of 2 at.%. Al was required to produce materials which were more conductive, as observed by sheet resistance measurements, which were shown to vary with annealing temperature. The versatile nature of sol-gel processing was demonstrated by selective ink-jet deposition of sol-gel droplets which were annealed to form oxide materials. © 2011 Elsevier B.V. All rights reserved.

The high visible-near infrared transparent and high carrier mobility (μ) Mo doped (0.5 at%) indium oxide (IMO) films were deposited by the spray pyrolysis technique. The deposited films were irradiated by 50MeV Li 3+ ions with different fluences of 1×1011, 1×1012 and 1×1013 ions cm-2. X-ray diffraction analysis confirmed the cubic bixbyite structure of indium oxide. A fascinating feature is that the ion irradiation process has introduced a fraction of the molybdenum oxide phase. The μ of as-deposited IMO films is decreased from ̃122.4 to 93.3 cm2 V-1 s-1, following the ion irradiation. The theoretically calculated μ and carrier density values were correlated with those measured experimentally. The transport mechanism has been analysed based on the ionized and neutral impurity scattering centres. The average transmittance (400-2500 nm) of the as-deposited IMO films is decreased from 83% to 60% following irradiation. © 2011 IOP Publishing Ltd.