Rodrigo Martins

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.

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.

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.

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.

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.

High conductivity in the off-state and low field-effect mobility compared to bulk properties is widely observed in the p-type thin-film transistors of Cu2O, especially when processed at moderate temperature. This work presents results from in situ conductance measurements at thicknesses from sub-nm to around 250 nm with parallel X-ray photoelectron spectroscopy. An enhanced conductivity at low thickness is explained by the occurrence of Cu(II), which is segregated in the grain boundary and locally causes a conductivity similar to CuO, although the surface of the thick film has Cu2O stoichiometry. Since grains grow with an increasing film thickness, the effect of an apparent oxygen excess is most pronounced in vicinity to the substrate interface. Electrical properties of Cu2O grains are at least partially short-circuited by this effect. The study focuses on properties inherent to copper oxide, although interface effects cannot be ruled out. This non-destructive, bottom-up analysis reveals phenomena which are commonly not observable after device fabrication, but clearly dominate electrical properties of polycrystalline thin films. © 2016 Author(s).

Hydrogenated silicon carbon nitride (SiCN:H) thin film alloys were produced by hot wire (HWCVD), plasma assisted hot wire (PA-HWCVD) and plasma enhanced chemical vapor (PECVD) deposition techniques using a Ni buffer layer as catalyst for inducing crystallization. The silicon carbon nitride films were grown using C2H4, SiH4 and NH3 gas mixtures and a deposition temperature of 300 °C. Prior to the deposition of the SiCN:H film a hydrogen etching of 10 min was performed in order to etch the catalyst material and to facilitate the crystallization. We report the influence of each deposition process on compositional, structural and morphological properties of the films. Scanning Electron Microscope-SEM and Atomic Force Measurement-AFM images show their morphology; the chemical composition was obtained by Rutherford Backscattering Spectrometry-RBS, Elastic Recoil Detection-ERD and the structure by Infrared-IR analysis. The thickness of the catalyst material determines the growth process and whether or not islands form. The production of micro-structured SiCN:H films is also dependent on the gas pressure, gas mixture and deposition process used. © 2006 Elsevier B.V. All rights reserved.

We have produced amorphous intrinsic silicon thin films by hot-wire plasma assisted chemical vapor deposition, a process that combines the traditional rf plasma and the recent hot-wire techniques. In this work we have studied the influence of hydrogen gas dilution and rf power on the surface morphology, composition, structure and electro-optical properties of these films. The results show that by using this deposition technique it is possible to obtain at moderate rf power and filament temperature, compact i-type silicon films with ημτ of the order of 10 -5cm 2V -1, without hydrogen dilution. © 2002 American Institute of Physics.

In this paper we present a study of boron-doped nc-Si:H films prepared by PECVD at high deposition pressure (≥4 mbar), high plasma power and low substrate temperature (≤200 °C) using trimethylboron (TMB) as a dopant gas. The influence of deposition parameters on electrical, structural and optical properties is investigated. We determine the deposition conditions that lead to the formation of p-type nanocrystalline silicon thin films with very high crystallinity, high value of dark conductivity (>7 (Ω cm)-1) and high optical band gap (≥1.7 eV). Modeling of ellipsometry spectra reveals that the film growth mechanism should proceed through a sub-surface layer mechanism that leads to silicon crystallization. The obtained films are very good candidates for application in amorphous and nanocrystalline silicon solar cells as a p-type window layer. © 2009 Elsevier Ltd. All rights reserved.

We have characterized the structure and electrical properties of p-type nanocrystalline silicon films prepared by radio-frequency plasma-enhanced chemical vapor deposition and explored optimization methods of such layers for potential applications in thin-film solar cells. Particular attention was paid to the characterization of very thin (∼20nm) films. The cross-sectional morphology of the layers was studied by fitting the ellipsometry spectra using a multilayer model. The results suggest that the crystallization process in a high-pressure growth regime is mostly realized through a subsurface mechanism in the absence of the incubation layer at the substrate-film interface. Hydrogen plasma treatment of a 22-nm-thick film improved its electrical properties (conductivity increased more than ten times) owing to hydrogen insertion and Si structure rearrangements throughout the entire thickness of the film. © 2012 National Institute for Materials Science.

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.

Transparent and highly conducting gallium zinc oxide (GZO) films were successfully deposited by RF sputtering at room temperature. A lowest resistivity of∼2.8 × 10-4 ωcm was achieved for a film thickness of 1100nm (sheet resistance ∼2.5ω/□), with a Hall mobility of 18cm2/Vs and a carrier concentration of 1.3 × 1021 cm-3. The films are polycrystalline with a hexagonal structure having a strong crystallographic c-axis orientation. A linear dependence between the mobility and the crystallite size was obtained. The films are highly transparent (between 80% and 90% including the glass substrate) in the visible spectra with a refractive index of about 2, very similar to the value reported for the bulk material. These films were applied to single glass/TCO/pin hydrogenated amorphous silicon solar cells as front layer contact, leading to solar cells with efficiencies of about 9.52%. With the optimized deposition conditions, GZO films were also deposited on polymer (PEN) substrates and the obtained results are discussed. © 2008 Elsevier B.V. All rights reserved.

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.

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.

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.

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

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.

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.

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.

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.

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.

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.

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.

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.