Rodrigo Martins

High mobility bottom gate thin film transistors (TFTs) with an amorphous gallium tin zinc oxide (a-GSZO) channel layer have been produced by rf magnetron cosputtering using a gallium zinc oxide (GZO) and tin (Sn) targets. The effect of the post annealing temperatures (200 °C, 250 °C and 300 °C) was evaluated and compared with two series of TFTs produced at room temperature and 150 °C during the channel deposition. From the results it was observed that the effect ofpos annealing is crucial for both series of TFTs either for stability as well as for improving the electrical characteristics. The a-GSZO TFTs operate in the enhancement mode (n-type), present a high saturation mobility of 24.6 cm2/Vs, a subthreshold gate swing voltage of 0.38 V/decade, a turn-on voltage of -0.5 V, a threshold voltage of 4.6 V and an ION/IOFF ratio of 8x107, satisfying all the requirements to be used in active-matrix backplane.

In this paper we present results of indium doped zinc oxide deposited at room temperature by rf magnetron sputtering, with electron mobility as high as 60 cm2/Vs. The films present a resistivity as low as 5×10 -4 Ωcm with an optical transmittance of 85%. The structure of these films look-like polymorphous (mixed of different amorphous and nanocrystalline phases from different origins) as detected from XRD patterns (no clear peak exists) with a high smooth surface, as detected from SEM micrographs, highly important to ensure long life time when used in display devices.

Hydrogenated amorphous silicon photochemical sensors based on Pd-MIS structures were produced by Plasma Enhanced Chemical Vapor Deposition with two different oxidized surfaces (thermal and chemical oxidation). The behaviour of dark and illuminated current-voltage characteristics in air and in the presence of a hydrogen atmosphere is explained by the changes induced by the gases in the work function of the metal, modifying the electrical properties of the interface. The photochemical sensors produced present more than 2 orders of magnitude variation on the reverse dark current when in presence of 400 ppm hydrogen to which it corresponds a decrease of 45% on the open circuit voltage.

The aim of this work is to present the main optoelectronic characteristics of large area 1D position sensitive detectors based on amorphous silicon p-i-n diodes. From that, the device resolution, response time and detectivity are derived and discussed concerning the field of applications of the 1D thin film position sensitive detectors.

A high-gain amplifier topology, with all single n-type enhancement transistors, is proposed in this paper. This type of circuits are essential in transparent TFT technologies, such as GIZO and ZnO that lack complementary type transistor. All circuits were simulated using BSIM3V3 model of a 0.35 μm CMOS technology, due to the absence of a complete electrical model for the TFTs. Results reveal that the proposed circuit promise more gain, lower power consumption and higher bandwidth than the existing solutions under identical bias conditions. © 2013 IEEE.

Transparent TFT technologies, with amorphous semiconductor oxides are lacking a complementary type transistor. This represents a real challenge, when the design of high-gain amplifiers are considered, without resorting to passive resistive elements. However, some solutions do exist to overcome the lack of a p-type transistor. This paper then presents a comparison analysis of two high-gain single-stage amplifier topologies using only n-type enhancement transistors. In these circuits, high gain is achieved using positive feedback for the load impedance. The comparison is carried out in terms of bandwidth, power consumption and complexity under identical bias conditions. Further, the same load impedance is used to develop a novel high-gain multiplier. All the circuits are simulated using a 0.35 μm CMOS technology, as it is easy to test the reliability of the methods, since CMOS transistors have trustworthy models. © 2013 IEEE.

Transparent electronics has gained special attention during the last few years and is today established as one of the most promising technologies for leading the next generation of flat panel display due to its excellent electronic performance. The key components are wide band gap semiconductors, where oxides of different origin play an important role, not only as passive component but also as active component, similar to what we observe in conventional semiconductors like silicon. In this paper we present the recent progress in n- and p-type oxide based thin film transistors (TFT) produced by rf magnetron sputtering and we will summarize the major milestones already achieved with this emerging and very promising technology. © 2015 IEEE.

In this work a new structure is proposed for position sensitive detectors consisting of glass/Cr/aSi:H(n+)/a-Si:H(i)/ZnO, where the ZnO forms an heterojunction with the a-Si:H(i). The results show that this structure works with success in the fabrication of linear position sensitive detectors. The devices present a good nonlinearity of ≈ 2% and a good sensitivity to the light intensity. The main advantages of this structure over the classical p-i-n are an easier to built topology and a higher yield due to a better immunity to the a-Si:H pinholes, since the ZnO does not diffuse so easily into a-Si:H as the metal does, which are the cause of frequent failure in the p-i-n devices due to short-circuits caused by the deposition of the metal over the a-Si:H. In this structure the illumination is made directly on the ZnO, so a transparent substrate is not needed and a larger range of substrates can be used.

In the past we have developed a transient technique, called the Flying Spot Technique (FST). FST allows, not only to infer the ambipolar diffusion length but also the effective lifetime of the photogenerated carriers once the light spot velocity and geometry of the structure were known. In this paper, we propose to apply this technique backwards in order to detect the path and velocity of an object that is moving in the direction of a light source. The light reflected back from the object is analyzed through a p.i.n structure being the transient transverse photovoltage dependent on the movement of the object (position and velocity). Assuming that the transport properties of the material and the geometry of the device are known and using a triangulation method we show that it is possible to map the movement of the object. Details concerning material characterization, simulation and device geometry are presented.

PIN devices based on hydrogenated amorphous silicon (a-Si:H) became fundamental elements of many different types sensors, based on either the transverse or the lateral photovoltaic effect. In the past we have developed a transient technique, called the Flying Spot Technique (FST), based on the lateral photoeffect. FST allows, not only to infer the ambipolar diffusion length but also the effective lifetime of the photogenerated carriers once the light spot velocity and geometry of the structure were known. In this paper we propose to apply this technique backwards in order to detect the path and velocity of an object that is moving in a light source direction. The light reflected back from the object is analyzed through p.i.n. structure being the transient transverse photovoltage dependent on the object movement (position and velocity). Assuming known the transport properties of the material and the geometry of the device and using a triangulation method we show that it is possible to map the object movement. Details concerning material characterization, simulation and device geometry are presented.

Hydrogenated nanocrystalline silicon (nc-Si:H) n-layers have been used to prepare heterojunction solar cells on flat p-type crystalline silicon (c-Si) wafers. The nc-Si:H n-layers were deposited by radio-frequency (RF) plasma enhanced chemical vapor deposition (PECVD), and characterized using Raman spectroscopy, optical transmittance and activation energy of dark-conductivity. The nc-Si:H n-layers obtained comprise fine grained nanocrystallites embedded in amorphous matrix, which have a wider bandgap and a smaller activation energy. Heterojunction solar cells incorporated with the nc-Si n-layer were fabricated using configuration of Ag (100 nm)/lT0 (80 nm)/n-nc-Si:H (15 nm)/buffer a-Si:H/p-c-Si (300 μm)/Al (200 nm), where a very thin intrinsic a-Si:H buffer layer was used to passivate the p-c-Si surface, followed by a hydrogen plasma treatment prior to the deposition of the thin nanocrystalline layer. The results show that heterojunction solar cells subjected to these surface treatments exhibit a remarkable increase in the efficiency, up to 14.1% on an area of 2.43 cm2. © 2006 Elsevier B.V. All rights reserved.

In this paper we present the first results of thin film transistors produced completely at room temperature using ZnO as the active channel and silicon oxynitride as the gate dielectric. The ZnO-based thin film transistors (ZnO-TFT) present an average optical transmission (including the glass substrate) of 84% in the visible part of the spectrum. The ZnO-TFT operates in the enhancement mode with a threshold voltage of 1.8 V. A field effect mobility of 70 cm2/Vs, a gate voltage swing of 0.68 V/decade and an on-off ratio of 5×105 were obtained. The combination of transparency, high field-effect mobility and room temperature processing makes the ZnO-TFT very promising for the next generation of invisible and flexible electronics. © 2004 Elsevier B.V. All rights reserved.

In this work the electrical and structural properties of two high k materials as hafnium oxide (HfO2) and tantalum oxide (Ta2O5) produced at room temperature are exploited. Aiming low temperature processing two techniques were employed: r.f. sputtering and electron beam evaporation. The sputtered HfO2 films present a nanocrystalline structure when deposited at room temperature. The same does not happen for the evaporated films, which are essentially amorphous. The density and the electrical performance of both sputtered and evaporated films are improved after annealing them at 200 °C. On the other hand, the Ta2O5 samples deposited at room temperature are always amorphous, independently of the technique used. The density and electrical performance are not so sensitive to the annealing process. The set of data obtained show that these dielectrics processed at temperatures below 200 °C present promising properties aiming to produce devices at low temperature with improved interface properties and reduced leakage currents. © 2007 Elsevier B.V. All rights reserved.

The effect of oxygen partial pressure on the properties of In2 O3 - Ga2 O3 thin films produced by sputtering at room temperature aimed at thin film transistor (TFT) application is reported in this work. When produced in the absence of oxygen, the films are polycrystalline, while in the presence of oxygen, the films are amorphous. The films' resistivity is tuned between 10-3 and 104 γ cm. Moreover, the films present a high transmittance (> 80%) and a smooth surface (rrms =1.2 nm). The high performance as-produced transistors present high saturation mobility (μsat ≈43 cm2 /V s) and a subthreshold gate-voltage swing of 0.51 V/dec, which is reduced to 0.27 V/dec after 150°C annealing. © 2009 The Electrochemical Society.

In this paper we present results of indium zinc oxide deposited at room temperature by rf magnetron sputtering, with an electron mobility as high as 60 cm2/Vs. The films present a resistivity as low as 5 × 10 - 4 Ω cm with an optical transmittance of 85%. The structure of these films seems to be polymorphous (mix of different amorphous and nanocrystalline phases from different origins) as detected from XRD patterns with a smooth surface and from SEM micrographs, is highly important to ensure a long lifetime when used in display devices. © 2005 Elsevier B.V. All rights reserved.

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/V s, gate voltage swing of 0.44 V/dec and an ON/OFF current ratio of 7 × 107. 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. © 2007 Elsevier Ltd. All rights reserved.

This paper deals with high transparent and high conductive oxides based on polycrystalline titanium (Ti) doped (0.5-3 at.%) indium oxide (IO) thin films produced on glass substrates at 400 °C by spray pyrolysis technique. X-ray diffraction analysis confirmed the cubic bixbyite structure of indium oxide. A high mobility of ∼ 97 cm2 V- 1 s- 1, a carrier concentration of ∼ 1.55 × 1020 cm- 3 and a resistivity of ∼ 4.11 × 10- 4 Ω-cm with ∼ 83% of transmittance in the wavelength ranging between 400 and 2500 nm were obtained for 2 at.% Ti doping films, rivalling so to the best known transparent conducting oxide based on indium tin oxide. Moreover, the transmittance in the broad wavelength ranging between 400 and 2500 nm is over 83%, leading so to an increasing carrier generation towards the near infrared region of the spectrum, as required for applications such as solar cells. We also notice that increasing the doping concentration widened the optical band gap and caused a small Burstein-Moss shift, due to mobility decrease, as expected. © 2012 Published by Elsevier B.V.

High mobility bottom gate thin film transistors (TFTs) with an amorphous gallium tin zinc oxide (a-GSZO) channel layer have been produced by rf magnetron cosputtering using a gallium zinc oxide (GZO) and tin (Sn) targets. The effect of postannealing temperatures (200, 250, and 300 °C) was evaluated and compared with two series of TFTs produced at room temperature (S1) and 150 °C (S2) during the channel deposition. From the results, it was observed that the effect of postannealing is crucial for both series of TFTs either for stability as well as for improving the electrical characteristics. The a-GSZO TFTs (WL=5050 μm) operate in the enhancement mode (n -type), present a high saturation mobility of 24.6 cm2 V s, a subthreshold gate swing voltage of 0.38 V /decade, a turn-on voltage of -0.5 V, a threshold voltage of 4.6 V, and an Ion Ioff ratio of 8× 107, satisfying all the requirements to be used as active-matrix backplane. © 2008 American Institute of Physics.

Molybdenum (0-1 at. %) doped indium oxide thin films with high near-infrared (NIR) transparency and high carrier mobility were deposited on Corning-1737 glass substrates at 400 °C by a spray pyrolysis experimental technique. X-ray diffraction (XRD) analysis confirmed the cubic bixbyite structure of indium oxide. The preferred growth orientation along the (222) plane for the low Mo doping level (0.5 at. %) shifts to (400) for higher Mo doping levels (<0.6 at. %). The crystallite size extracted from the XRD data corroborates the changes in full width at half maximum due to the variation in Mo doping. A scanning electron microscopy study illustrated the evolution in the surface microstructure as a function of Mo doping. The negative sign of the Hall coefficient confirmed the n -type conductivity. A high carrier mobility of ∼122.4 cm2 /V s, a carrier concentration of ∼9.5× 1019 cm-3, a resistivity of ∼5.3× 10-4cm, and a high figure of merit of ∼4.2× 10-2 -1 are observed for the films deposited with 0.5 at. % Mo. The obtained high average transparency of ∼83% in the wavelengths ranging from 400 to 2500 nm confirmed the extension of transmittance well into the NIR region. © 2009 American Institute of Physics.

Molybdenum-doped indium oxide (IMO) thin films were deposited at 450 °C for varying molybdenum concentrations in the range of 0.5-2 at% by the spray pyrolysis technique. These films confirmed the cubic bixbyite structure of polycrystalline In2O3. The preferred growth orientation along the (2 2 2) plane shifts to (4 0 0) on higher Mo doping levels. The films doped with 0.5 at% Mo showed high mobility of 76.9 cm2/(V s). The high visible transmittance extends well into the near-infrared region. A possibility of using the produced IMO films in nanocrystalline (nc) silicon solar cell applications is discussed in this article. The morphological studies showed a change in the microstructure, which is consistent with the change in crystallographic orientation. © 2008 Elsevier B.V. All rights reserved.

This work presents the I-V results of a-Si:H/SiOx/Pd MIS (metal-insulator-semiconductor) structures. The a-Si:H was deposited by non-conventional modified triode PECVD. This new configuration allows the deposition of high quality a-Si:H with a photosensitivity of 106, indicating the presence of low density of defects. Spectroscopic ellipsometry measurements revealed that these films are highly dense and present a very smooth surface so allowing a low defect interface between the Pd and the a-Si:H. As a result, we could make MIS photodiodes with barrier heights of 1.17 eV, which give a high reduction of the reverse dark current, an increase of the signal to noise ratio of 106 and an open circuit voltage VOC = 0.5 V. © 2002 Elsevier Science B.V. All rights reserved.

Transparent and highly conducting gallium-doped zinc oxide films were successfully deposited by rf sputtering at room temperature. The lowest resistivity achieved was 2.6×10-4 Ω cm for a thickness of 1100 nm (sheet resistance ≈1.6 Ω/sq), with a Hall mobility of 18 cm2/Vs and a carrier concentration of 1.3×1021 cm-3. The films are polycrystalline with a hexagonal structure and a strongly preferred orientation along the c-axis. A linear dependence between the mobility and the crystallite size was obtained. The films present a transmittance in the visible spectra between 80 and 90% and a refractive index of approximately 2, which is very close to the value reported for bulk material. © 2003 Elsevier B.V. All rights reserved.

Molybdenum doped indium oxide (IMO) thin films were deposited on the glass substrates preheated to 450 °C by spray pyrolysis technique. The Mo doping was varied between 0 and 2.0 at.%. The films were characterized by their structural, electrical and optical properties. The films are confirmed to be cubic bixbyite In2O3 with a strongest orientation along (222) plane, which is shifted to (400) plane for the increase in Mo doping to 1.25 and 2 at.%. The film deposited with 0.5 at.% Mo doping shows high mobility of 76.9 cm2V- 1s- 1 , resistivity of 1.8 × 10- 3 Ω-cm and high carrier concentration of 4.6 × 1019 cm- 3 with 81.3% transmittance in the visible range between 500 and 800 nm. Further, the transparency extents well into the near-IR range. © 2008 Elsevier B.V. All rights reserved.

Although there are a few research studies on solution-processed p-channel oxide thin-film transistors (TFTs), the strict fabrication conditions and the poor electrical properties have limited their applications in low-power complementary metal oxide semiconductor (CMOS) electronics. Here, the application of the polyol reduction method for processing p-type CuxO and NiOx channel layers and their implementation in TFT devices are reported. The optimized CuxO and NiOx TFTs were achieved at low annealing temperatures (∼300 °C) and exhibited decent electrical properties. Encouraged by the inspiring results obtained on SiO2/Si substrates, the TFT performance was further optimized by device engineering, employing high-k AlOx as the gate dielectric. The fully solution-processed NiOx/AlOx TFT could be operated at a low voltage of 3.5 V and exhibits a high hole mobility of around 25 cm2 V-1 s-1. Our work demonstrates the ability to grow high-quality p-type oxide films and devices via the polyol reduction method over large area substrates while at the same time it provides guidelines for further p-type oxide material and device improvements. © The Royal Society of Chemistry 2016.

In this letter, we report for the first time the use of a sheet of cellulose-fiber-based paper as the dielectric layer used in oxide-based semiconductor thin-film field-effect transistors (FETs). In this new approach, we are using the cellulose-fiber-based paper in an "interstrate"structure since the device is built on both sides of the cellulose sheet. Such hybrid FETs present excellent operating characteristics such as high channel saturation mobility (> 30 cm2Vs), drain-source current on/off modulation ratio of approximately 104, near-zero threshold voltage, enhancement n-type operation, and subthreshold gate voltage swing of 0.8 V/decade. The cellulose-fiber-based paper FETs' characteristics have been measured in air ambient conditions and present good stability, after two months of being processed. The obtained results outpace those of amorphous Si thin-film transistors (TFTs) and rival with the same oxide-based TFTs produced on either glass or crystalline silicon substrates. The compatibility of these devices with large-scale/large-area deposition techniques and low-cost substrates as well as their very low operating bias delineates this as a promising approach to attain high-performance disposable electronics like paper displays, smart labels, smart packaging, RFID, and point-of-care systems for self-analysis in bioapplications, among others. © 2008 IEEE.