Rodrigo Martins

Indium tin oxide (ITO) has been used as the prefered electrode material for the emerging area of transparent electronics, namely for thin-film transistors (TFTs) based on oxide semiconductors. This work pretends to investigate different materials to replace ITO in inverted-staggered TFTs based on gallium-indium-zinc oxide (GIZO), one of the most promissing oxide semiconductors for TFTs. The analyzed electrode materials are indium-zinc oxide (IZO), Ti, Mo and Ti/Au. Devices are analyzed with special focus on the contact resistance fundamentals, including the extraction of source/ drain series resistances and TFTs intrinsic parameters, such as intrinsic mobility (p\) and intrinsic threshold voltage (V Ti). The obtained contact resistance values are between 10 kΩ and 20 kΩ, and the best devices have field effect mobility ((μ FE) close to 25 cm 2/V s and on/off ratio close to 10 8. © 2008 WILEY-VCH Verlag GmbH & Co. KGaA.

This work refers to the electro-optical and structural characteristics of titanium oxide (TiOx) thin films produced by radio frequency (r.f.) magnetron sputtering that present promising performances for gate dielectric applications, alone or in mixed tandem structures, such as with Al yOz films, taking advantage of its high dielectric constant. Films produced with a O2/Ar ratio between 0.1 and 0.15 present an improved stochiometry and density where the resistivity overcomes 1011 Ω cm and the fixed charge density decreases below 10 12 cm-2. The deposition pressure influences greatly the growth rate that seems to be a determinant factor dictating the films properties. © 2004 Elsevier Ltd. All rights reserved.

Multicomponent amorphous oxides are starting to emerge as a class of appealing semiconductor materials for application in transparent electronics. In this work, a high performance bottom-gate n-type transparent thin-film transistors are reported, being the discussion primarily focused on the influence of the indium zinc oxide active layer thickness on the properties of the devices. For this purpose, transparent transistors with active layer thicknesses ranging from 15 nm to 60 nm were produced at room temperature using rf magnetron sputtering. Optical transmittance data in the visible range reveals average transmittance higher than 80%, including the glass substrate. The devices work in the enhancement mode and exhibit excellent saturation drain currents. On-off ratios above 107 are achieved, but this value tends to be lower for devices with thicker semiconductor films, as a result of the decrease in the resistance of the channel region with increasing thickness. Channel mobilities are also quite respectable, with some devices presenting values around 40 cm2/V s, even without any annealing or other post-deposition improvement processes. Concerning the evolution of threshold voltage with the thickness, this work shows that it increases from about 3 V in thicker films up to about 10 V in the thinnest ones. The interesting electrical properties obtained and the versatility arising from the fact that it is possible to modify them changing only the thickness of the semiconductor makes this new transparent transistors quite promising for future transparent ICs. © 2006 Elsevier B.V. All rights reserved.

Insensitivity to light irradiation is desirable for conventional applications of thin-film transistors, i.e., the active matrices of displays. However, if one produces a device presenting controlled sensitivity to light, many other applications can benefit or can even be created. In this work it is shown the influence of the photon energy on the optoelectronic properties presented by n-type bottom-gate thin-film transistors based on indium zinc oxide. In the dark, the devices present very good electrical properties, working in the enhancement mode, exhibiting on-off ratios higher than 107 and channel mobility above 30 cm2/V s. Remarkable results were achieved when the devices were exposed to light radiation, the most striking one is the possibility to switch between enhancement (in the dark) and depletion (illuminated) modes, with different threshold voltages and on/off ratios, function of the light power density and wavelength used. This type of behavior permits to design circuits where one can have the same transistor working either in the enhancement or depletion modes, function of the light beam and intensity impinging on it, highly important for short wavelength detector applications. © 2006 Elsevier B.V. All rights reserved.

The purpose of this work is to present in-depth electrical characterization on transparent TFTs, using zinc oxide produced at room temperature as the semiconductor material. Some of the studied aspects were the relation between the output conductance in the post-pinch-off regime and width-to-length ratios, the gate leakage current, the semiconductor/insulator interface traps density and its relation with threshold voltage. The main point of the analysis was focused on channel mobility. Values extracted using different methodologies, like effective, saturation and average mobility, are presented and discussed regarding their significance and validity. The evolution of the different types of mobility with the applied gate voltage was investigated and the obtained results are somehow in disagreement with the typical behavior found on classical silicon based MOSFETs, which is mainly attributed to the completely different structures of the semiconductor materials used in the two situations: while in MOSFETS we have monocrystalline silicon, our transparent TFTs use poly/nanocrystalline zinc oxide with grain sizes of about 10 nm.

This work analyzes the role of processing parameters on the electrical performance of GIZO (Ga2 O3: In2 O3:ZnO) films and thin-film transistors (TFTs). Parameters such as oxygen partial pressure, deposition pressure, target composition, thickness, and annealing temperature are studied. Generally, better devices are obtained when low oxygen partial pressure is used. This is related to the damage induced by oxygen ion bombardment and very high film's resistivity when higher oxygen partial pressures are used. Low deposition pressures and targets with richer indium compositions led to films with high carrier concentration, resulting in transistors with field-effect mobility as high as ∼80 cm2 Vs but poor channel conductivity modulation, becoming ineffective as switching devices. Nevertheless, it is demonstrated that reducing the GIZO thickness from 40 to 10 nm greatly enhances the switching behavior of those devices, due to the lower absolute number of free carriers and hence to their easier depletion. Annealing also proves to be crucial to control device performance, significantly modifying GIZO electrical resistivity and promoting local atomic rearrangement, being the optimal temperature determined by the as-produced films' properties. For the best-performing transistors, even with a low annealing temperature (150°C), remarkable properties such as μFE =73.9 cm2 Vs, onoff ratio≈7× 107, VT ≈0.2 V, and S=0.29 Vdec are achieved. © 2008 The Electrochemical Society.

During the last years, oxide semiconductors have shown that they will have a key role in the future of electronics. In fact, several research groups have already presented working devices with remarkable electrical and optical properties based on these materials, mainly thin-film transistors (TFTs). Most of these TFTs use indium-tin oxide (ITO) as the material for source/drain electrodes. This paper focuses on the investigation of different materials to replace ITO in inverted-staggered TFTs based on gallium-indium-zinc oxide (GIZO) semiconductor. The analyzed electrode materials were indium-zinc oxide, Ti, Al, Mo, and Ti/Au, with each of these materials used in two different kinds of devices: one was annealed after GIZO channel deposition but prior to source/drain deposition, and the other was annealed at the end of device production. The results show an improvement on the electrical properties when the annealing is performed at the end (for instance, with Ti/Au electrodes, mobility rises from 19 to 25 cm2/V · s, and turn-on voltage drops from 4 to 2 V). Using time-of-flight secondary ion mass spectrometry (TOF-SIMS), we could confirm that some diffusion exists in the source/drain electrodes/ semiconductor interface, which is in close agreement with the obtained electrical properties. In addition to TOF-SIMS results for relevant elements, electrical characterization is presented for each kind of device, including the extraction of source/drain series resistances and TFT intrinsic parameters, such as VTi (intrinsic threshold voltage). © 2008 IEEE.

High performance transparent thin-film transistors deposited on glass substrates and entirely processed at a low temperature not exceeding 150°C are presented and analyzed in this paper. Besides being based on an amorphous oxide semiconductor, the main innovation of this work relies on the use of sputtered multicomponent oxides as dielectric materials based on mixtures of Ta2O5 with SiO2 or Al2O3. These multicomponent dielectrics allow to obtain amorphous structures and low leakage currents while preserving a high dielectric constant. This results in transistors with remarkable electrical properties, such as field-effect mobility exceeding 35 cm2 V-1 s-1, close to 0 V turn-on voltage, on/off ratio higher than 106, and a subthreshold slope of 0.24 V decade-1, obtained with a Ta2O5: SiO2 dielectric. When subjected to severe current stress tests, optimized devices show little and reversible variation in their electrical characteristics. The devices presented here have properties comparable to the ones using plasma-enhanced chemical vapor deposited SiO2 at 400°C, reinforcing the success of this amorphous multicomponent dielectric approach for low temperature, high performance, and transparent electronic circuits. © 2009 The Electrochemical Society.

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.

In this work we present a study concerning the influence of some of the most important external factors on the electrical properties of transparent thin-film transistors (TFTs), using zinc oxide produced at room temperature as the semiconductor material. Electrical characterization performed sixteen months after the production of the devices showed a decrease in the on/off ratio from 8×105 to 1×105, mainly due to the increase of the off-current. We also observed a small increase in the saturation mobility, a decrease in the threshold voltage and an increase in the gate voltage swing (by factors of about 1.2, 0.9 and 1.6, respectively). Exposure to ambient light does not have a noticeable effect on the electrical properties, which is an important point as regards the application of these devices in active matrix displays. Some variation of the electrical properties was only detectable under intense white light radiation. In order to evaluate the temperature effect on the TFTs, they were also characterized at 90 °C. At this temperature we noticed that the off-current increased more than two times, and the other electrical properties had a small variation, returning to their initial values after cooling, meaning that the process is totally reversible. © 2005 Elsevier Ltd. All rights reserved.

The room temperature ozone sensing properties of polycrystalline undoped zinc oxide (ZnO) thin films have been investigated. ZnO thin films have been produced by the d.c. and r.f. magnetron sputtering technique as well as with spray pyrolysis with a variety of parameters. The as-grown films were brought to a high conducting state through a reversible photoreduction process by UV light exposure and were subsequently exposed to ozone resulting in a strong resistivity increase caused by re-oxidation. The magnitude of the effect was largest for the sputtered films, which exhibited resistivity changes of more than 8 orders of magnitude, whereas films deposited by spray pyrolysis showed changes of less than 3 orders of magnitude. XRD and AFM analysis of the films revealed that all films were microcrystalline. The film texture, however, was strongly related to the growth technique and the parameters used. Best results were achieved with r.f.-sputtered films, which have been deposited at high total pressures. These films exhibited a sensor response of 1.2 × 108. © 2002 Elsevier Science B.V. All rights reserved.

In this letter we compare the room temperature ozone sensing properties of intrinsic zinc oxide (ZnO) thin films deposited by spray pyrolysis, dc and r.f. magnetron sputtering. Their sensor response exceeds 8 orders of magnitude when the film structure is constituted by nanocrystallites. These preliminary results clearly demonstrate that the films could be potentially used for ozone detection at room temperature.

The aim of presented work is to show the improvements obtained in the properties of TiO2 films for dye sensitized solar cells fabricated by inkjet printing using an innovative methodology. We describe the development and properties of TiO2-based inks used in a lab-scale printer, testing various commercial TiO2 pastes. The porosity of the deposited inkjet printed TiO2 films is much higher than using the conventional "doctor blade" deposition technique, as the ink solvent evaporates during the droplet fly from the nozzle to the substrate due to its picoliter volume and the applied heating of a printing stage (70°C). Thanks to higher surface area, the dye sensitized solar cells incorporating inkjet printed TiO2 film gave higher efficiencies (ηmax≈3.06%) than the more compact films obtained by the "doctor blade" method (ηmax≈2.56%). Furthermore, electrochemical analysis indicates that for whole tested thickness range, the inkjet printed layers have higher effective electron diffusion length indicating their better transport properties. © 2015 Elsevier Inc..

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.

We have projected and fabricated a microfluidic platform for DNA sensing that makes use of an optical colorimetric detection method based on gold nanoparticles. The platform was fabricated using replica moulding technology in PDMS patterned by high-aspect-ratio SU-8 moulds. Biochips of various geometries were tested and evaluated in order to find out the most efficient architecture, and the rational for design, microfabrication and detection performance is presented. The best biochip configuration has been successfully applied to the DNA detection of Mycobacterium tuberculosis using only 3. l on DNA solution (i.e. 90. ng of target DNA), therefore a 20-fold reduction of reagents volume is obtained when compared with the actual state of the art. © 2013 Elsevier B.V.

The authors report on the performance of polymer-based light-emitting diodes, LEDs, using amorphous zinc oxide-doped indium oxide, IZO, as anode. In particular, LEDs with poly[(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] as electroluminescent layer and aluminium cathodes, show higher efficiency with this IZO anode (0.015 cd/A) than with indium-tin oxide (ITO) (0.010 cd/A). Inspite of the higher resistance of this IZO electrode, compared with ITO, the fact that it can be processed at lower temperatures and allows similar or even higher efficiency values for polymer LEDs make this material a good candidate for display and other optoelectronic applications. © 2009 Elsevier B.V. All rights reserved.

The degradation of thin-film transistors (TFTs) caused by the self-heating effect constitutes a problem to be solved for the next generation of displays. Aluminum nitride (AlN) is a viable alternative for gate dielectric of TFTs due to its good thermal conductivity, matching coefficient of thermal expansion to indium-gallium-zinc-oxide, and excellent stability at high temperatures. Here, AlN thin films of different thicknesses were fabricated by a low temperature reactive radio-frequency magnetron sputtering process, using a low cost, metallic Al target. Their electrical properties have been thoroughly assessed. Furthermore, the 200 nm and 500 nm thick AlN layers have been integrated as gate-dielectric in transparent TFTs with indium-gallium-zinc-oxide as channel semiconductor. Our study emphasizes the potential of AlN thin films for transparent electronics, whilst the functionality of the fabricated field-effect transistors is explored and discussed. © 2016 Elsevier B.V. All rights reserved.

The effect of post-deposition annealing temperature on the pH sensitivity of room temperature RF sputtered +{\hbox{Ta}}-{2}{\hbox{O}}5 was investigated. Structural and morphological features of these films were analyzed before and after annealing at various temperatures. The deposited films are amorphous up to 600 +^{\circ}{\hbox{C}}+ and crystallize at 700 +^{\circ}{\hbox{C}}+ in an orthorhombic phase. Electrolyte-insulator- semiconductor (EIS) field effect based sensors with an amorphous +{\hbox{Ta}}-{2}{\hbox{O}}5 sensing layer showed pH sensitivity above 50 mV/pH. For sensors annealed above 200 +^{\circ}{\hbox{C}}+ pH sensitivity decreased with increasing temperature. Stabilized sensor response and maximum pH sensitivity was achieved after low temperature annealing at 200 +^{\circ}{\hbox{C}}+ , which is compatible with the use of polymeric substrates and application as sensitive layer in oxides TFT-based sensors. © 2005-2012 IEEE.

We present a new approach for real-time monitoring of PCR amplification of a specific sequence from the human c-MYC proto-oncogene using a Ta 2O 5 electrolyte-insulator-semiconductor (EIS) sensor. The response of the fabricated EIS sensor to cycle DNA amplification was evaluated and compared to standard SYBR-green fluorescence incorporation, showing it was possible to detect DNA concentration variations with 30mV/μM sensitivity. The sensor's response was then optimized to follow in real-time the PCR amplification of c-MYC sequence from a genomic DNA sample attaining an amplification profile comparable to that of a standard real-time PCR. Owing to the small size, ease of fabrication and low-cost, the developed Ta 2O 5 sensor may be incorporated onto a microfluidic device and then used for real-time PCR. Our approach may circumvent the practical and economical obstacles posed by current platforms that require an external fluorescence detector difficult to miniaturize and incorporate into a lab-on-chip system. © 2011 Elsevier B.V.

Solution based deposition has been recently considered as a viable option for low-cost flexible electronics. In this context research efforts have been increasingly centred on the development of suitable solution-processed materials for oxide based transistors. Nevertheless, the majority of synthetic routes reported require the use of toxic organic solvents. In this work we report on a new environmental friendly solution combustion synthesis route, using ethanol as solvent, for the preparation of indium/gallium free amorphous zinc-tin oxide (ZTO) thin film transistors (TFTs) including AlOx gate dielectric. The decomposition of ZTO and AlOx precursor solutions, electrical characterization and stability of solution processed ZTO/AlOx TFTs under gate-bias stress, in both air and vacuum atmosphere, were investigated. The devices demonstrated low hysteresis (ΔV=0.23 V), close to zero turn on voltage, low threshold voltage (VT=0.36 V) and a saturation mobility of 0.8 cm2 V-1 s-1 at low operation voltages. Ethanol based ZTO/AlOx TFTs are a promising alternative for applications in disposable, low cost and environmental friendly electronics. © 2015 IOP Publishing Ltd.

Electrical dark conductivity (σd) and surface composition of undoped and doped a-Si:H thin films have been investigated, using coplanar I−V as a function of temperature and Auger electron spectroscopy (AES). The films were prepared by rf glow discharge deposition on standard soda-lime glass and on alkali-free glass substrates. Comparing these two sets of substrates for undoped films, we find that σd of the films deposited on soda-lime glass substrates at room temperature is higher by more than two orders of magnitude, their activation energy is lower by about a factor of 3, and their photosensitivity (σph/σd) is lower by two orders of magnitude than that of the films deposited on alkali-free glass substrates. We suggest that Na ions, leached from the glass into the a-Si:H overlayer play a significant role in determining the film conductivity by creating electrically active donorlike states. This conclusion is supported by similar measurements on p- and n-type a-Si:H films on the same substrates and by AES results. Films of a-Si:H, grown on thin a-Si:C:H interlayers on soda-lime glass, showed very low Na concentrations and low dark conductivities as found by AES and electrical measurements, respectively. The role of the a-Si:C:H interlayers as diffusion barriers is discussed. © 1989, American Vacuum Society. All rights reserved.

In this paper we present an improved version of large area (5 mm × 80 mm) flexible position sensitive detectors deposited on polyimide (Kapton® VN) substrates with 75 μm thickness, produced by plasma enhanced chemical vapor deposition (PECVD). The structures presented by the sensors are Kapton/ZnO:Al/(pin)a-Si:H/Al and the heterostructure Kapton/Cr/(in)a-Si:H/ZnO:Al. These sensors were characterized by spectral response, photocurrent dependence as a function of light intensity and position detectability measurements. The set of data obtained on one-dimensional position sensitive detectors based on the heterostructure show excellent performances with a maximum spectral response of 0.12 A/W at 500 nm and a non-linearity of ±10%. © 2002 Elsevier Science B.V. All rights reserved.

This work deals with the study of the role of intra-gap density of states on the colour selection of the collection spectrum of glass/ITO/a-Six:C1-x:H/Al Schottky photodiodes. In order to optimise the voltage colour selection and to study the influence of intragap density of states in the final device performances, different undoped a-Six:C1-x:H films (1 μm thick) have been produced in a conventional Plasma Enhanced Chemical Vapour Deposition (PECVD) system using silane and a controlled mixtures of silane and methane as gas sources. The properties of the films were analysed by dark conductivity measurements, infrared spectroscopy, visible spectroscopy and constant photocurrent method (CPM), to determine the valence controllability and to correlate the silicon carbide layer composition with the performances of the devices. The performances obtained concerning the spectral response of the devices were correlated with the carbon content and the density of states of the a-Six:C1-x:H films.

Position sensitive detectors (PSD) using hydrogenated amorphous silicon as the active layer have been widely proposed either with the p-i-n or the Schottky structure. In this case, the devices are tailored to respond to light in the range 620-650 nm. Little is known about the use of silicon carbide active layers in such devices, which is important when the detected light is in the blue region of the light spectrum. In this paper we present for the first time the electro-optical properties of the a-Six:C1-x:H/Pd and p-ic-n PSD, using a-Six:C1-x:H layers deposited by plasma enhanced chemical vapour deposition (PECVD). These sensors are able to distinguish the wavelength of the impinging visible radiation (from red to blue light). In addition, the sensors respond to light intensities as lower as 1 × 10-6 W cm-2 with a resolution better than 0.04 mm and a linearity between ±0.12% and ±0.8%. © 2002 Elsevier Science B.V. All rights reserved.

This work deals with the study of the role of intra-gap density of states and films composition on the colour selection of the collection spectrum of glass/ITO/a-Six:C1-x:H/Al Schottky photodiodes produced in a conventional plasma-enhanced chemical vapour deposition (PECVD) system using as gas sources silane and a controlled mixtures of silane and methane. To do so, properties of the films were investigated, especially the one concerning the determination of the valence controllability of the films produced and the density of bulk states. Besides that, a PINIP device was also produced, using the a-Six:C1-x:layer that lead to the best Schottky diode performances. © 2001 Elsevier Science B.V. All rights reserved.