Rodrigo Martins

The effect is considered of order and disorder on the electrical and optical performance of ionic oxide semiconductors used to produce optoelectronic devices such as p-n heterojunction solar cells and thin-film transistors (TFTs). The results obtained show that p-type c-Si/a-IZO/poly-ZGO solar cells exhibit efficiencies above 14% in device areas of about 2.34 cm2, whereas amorphous oxide TFTs based on the Ga-Zn-Sn-O system demonstrate superior performance to the polycrystalline ZnO TFTs, with ION/I OFF ratio exceeding 107, turn-on voltage below 1-2 V and saturation mobility above 25 cm2 V-1 s-1. In addition, preliminary data on a p-type oxide TFT based on the Zn-Cu-O system are presented. © 2009 Taylor & Francis.

The recent application of wide band gap oxide semiconductors to transparent thin film transistors (TTFTs) is making a fast and growing (r)evolution on the contemporary solid-state electronics. In this paper we present some of the recent results we have obtained using wide band gap oxide semiconductors, like indium zinc oxide, produced by rf sputtering at room temperature. The devices work in the enhancement mode and exhibit excellent saturation drain currents. On-off ratios above 106 are achieved. The optical transmittance data in the visible range reveals average transmittance higher than 80%, including the glass substrate. Channel mobilities are also quite respectable, with some devices presenting values around 25 cm2/Vs, even without any annealing or other post deposition improvement processes. The high performances presented by these TTFTs associated to a high electron mobility, at least two orders of magnitude higher than that of conventional amorphous silicon TFTs and a low threshold voltage, opens new doors for applications in flexible, wearable, disposable portable electronics as well as battery-powered applications.

Highly transparent and conductive ZnO:Ga thin films were produced by rf magnetron sputtering at room temperature on polyethylene naphthalate substrates. The films present a good electrical and optical stability, surface uniformity and a very good adhesion to the polymeric substrates. The lowest resistivity obtained was 5×10-4 Ωcm with a sheet resistance of 15 Ω/sqr and an average optical transmittance in the visible part of the spectra of 80%. It was also shown that by passivating the polymeric surface with a thin SiO2 layer, the electrical and structural properties of the films are improved nearly by a factor of 2.

Large-area thin-film position-sensitive detectors (TFPSDs) using the hydrogenated amorphous silicon (a-Si:H) technology are presented. The detection accuracy of these devices (lengths of about 80 mm) is better than ±0.5% of the value of the full scale of the sensor, the spatial resolution is better than ±20 μm, the non-linearities measured are below ±2% and the frequency response is in the range of a few kilohertz, compatible with the sampling frequency of most electromechanical assembling/control systems. The obtained results are quite promising regarding the application of these sensors to a wide variety of optical inspection systems. © 1998 Elsevier Science S.A. All rights reserved.

We report, for the first time, results on transparent ZnO:Al thin films deposited on polyester (Mylar type D, 100 μm thickness) substrates at room temperature by 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 (0 0 2) perpendicular to the substrate surface. The ZnO:Al thin films with 83% transmittance in the visible region and a resistivity as low as 3.6 × 10-2 Ωcm have been obtained, as deposited. The obtained results are comparable to those obtained on glass substrates, opening a new field of low cost, light weight, small volume, flexible and unbreakable large area optoelectronic devices. © 2002 Elsevier Science Ltd. All rights reserved.

The aim of this work is to present the main optoelectronic characteristics of large-area one-dimensional position-sensitive detectors (1D TFPSDs) based on amorphous silicon (a-Si) p-i-n diodes. From that, the device resolution, response time and detectivity (defined as being the reciprocal of the noise equivalent power pattern) are derived and discussed concerning the field of applications of the 1D TFPSDs. © 1996.

We have developed large area (up to 80mm×80mm) Thin Film Position Sensitive Detectors (TFPSD) based on hydrogenated amorphous silicon (a-Si:H). Although crystalline silicon PSDs have been realized and applied to optical systems, their detection area is small (less than 10mm×10mm), which implies the need of optical magnification systems for supporting their field of applications towards large area inspection systems, which does not happen by using a-Si:H devices. The key factors for the TFPSDs resolution are the thickness uniformity of the constituting layers, the geometry and the position of the contacts. In this paper we present data on single and dual-axis rectangular TFPSDs correlating, their performances with the different underlying lateral effects. For the single axis-detector, with two opposite extended contacts, the output photocurrent difference to sum ratio is a linear function of the position of a narrow incident light beam, even for low illumination levels (below 20 lux). For the dual-axis detector with extended contacts, at all four sides (except for small gaps at the vertices due to edge effects) an almost linear relation has been found between the incident light spot position along both axis and the corresponding output photocurrents. © 1993.

Tensile tests were performed on PET films coated with Al doped zinc oxide films by RF magnetron sputtering. During the tensile elongation, the electrical resistance of the oxide was evaluated in situ. The results indicate that the increase in the electrical resistance is related to the crack debsity and crack width, which also depends on the film thickness.

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.

Gallium-doped zinc oxide (GZO) thin films have been deposited onto polyethylene naphthalate (PEN) substrates by r.f. magnetron sputtering at room temperature. The influence of the film thickness (from 70 to 890 nm) on the electrical, structural and morphological properties are presented. The lowest resistivity obtained was 5 × 10-4 Ω cm with a Hall mobility of 13.7 cm2/Vs and a carrier concentration of 8.6 × 1020 cm-3. These values were obtained by passivating the surface of the polymer with a thin silicon dioxide, so preventing the moisture and oxygen permeation inside the film. © 2003 Elsevier B.V. All rights reserved.

This paper reports the performances of metal/insulator/semiconductor devices, simultaneously sensitive to hydrogen and to the visible region of the spectrum. The sensors used in this work are based on glass/Cr/a-SiH(n+)/a-Si:H(i)/SiOx/Pd structures, where the amorphous silicon was deposited by conventional r.f. techniques and the oxide grown thermally (in air) or chemically (in hydrogen peroxide). The proposed sensors present a response of ∼ 3 orders of magnitude change in the saturation current when in the presence of 400 ppm of hydrogen and an open circuit voltage that decreases in the presence of hydrogen, with a maximum spectral response at 500 nm. These sensors were also compared with equivalent crystalline silicon devices whose oxides were prepared exactly in the same conditions as the ones used for the a-Si:H devices. © 1998 Elsevier Science B.V. All rights reserved.

A novel compact linear thin film position sensitive detector with 128 elements, based on p-i-n a-Si:H devices was developed. The proper incorporation of this sensor into an optical inspection camera makes possible the acquisition of three dimension information of an object, using laser triangulation methods. The main advantages of this system, when compared with the conventional charge-coupled devices, are the low complexity of hardware and software used and that the information can be continuously processed (analogue detection).

Aluminium doped zinc oxide thin films (ZnO:Al) have been deposited on polyester (Mylar type D, 100 μm thickness) substrates at room temperature by r.f. magnetron sputtering. The structural, morphological, 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. The ZnO:Al thin films with 85% transmittance in the visible and infra-red region and a resistivity as low as 3.6×10-2 Ωcm have been obtained, as deposited. The obtained results are comparable to those ones obtained on glass substrates, opening a new field of low cost, light weight, small volume, flexible and unbreakable large area optoelectronic devices.

In this paper we report on some of the recent advances in transparent thin film oxide semiconductors, specifically zinc oxide produced by radio frequency magnetron sputtering at room temperature, with multifunctional properties. By controlling the deposition parameters it is possible to produce undoped material with electronic semiconductor properties, or by doping it to get either n-type or p-type semiconductor behavior. In this work we refer to our experience in producing n-type doped zinc oxide as transparent electrode to be used in optoelectronic applications such as solar cells and position sensitive detectors, while the undoped zinc oxide can be used as active layer of fully transparent thin film transistors. © 2009 Springer-Verlag.

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.

The increasing demand in automation processes in finishing techniques also calls for automatic measurement and inspection methods. These methods ought to be installed as close as possible to the production process and they ought to measure the values needed in a safe and fast way, without disturbing the process itself. Simultaneously they should be free of wear and insensitive against mechanical perturbations. This approach can be reached by proper combination of the laser triangulation technique with an array of linear position sensitive detectors, able to supply information about the surface finishing of an object. This is the aim of this paper that envisages to present experimental results of the performances exhibited by such an array constituting 128 elements. The analogue information supplied by this array is processed by an analogue/digital converter, directly coupled to the array and whose information is computer processed, concerning the recognition of patterns and the processing of information collected over the object to be inspected. © 1999 Elsevier Science S.A. 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.

The aim of this work is to present an analytical model able to interpret the role of the thin collecting resistive layer on the static performances exhibited by 1D amorphous silicon hydrogenated p-i-n thin film position sensitive detectors. The data obtained show that the devices present a linearity and a spatial resolution, of respectively, better than 99% and 20 μm for a spatial detection limit of about 80 mm, highly dependent on the characteristics exhibited by the collecting resistive layer that should have sheet resistivities in the range of 10 to 103 Ω/sq, as predicted by the model proposed. © 1996 American Institute of Physics.

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.

This work describes the fabrication and characterization of an improved version of large area (5mm×80mm with an active length of 70mm) flexible position sensitive detectors deposited onto polymeric substrates (polyimide-Kapton® VN). The new configuration presented by the sensor is based on a heterostructure of a-Si:H/ZnO:Al. The sensors were characterized by spectral response, photocurrent dependence as a function of light intensity and position detection 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 0.12A/W at 500nm and a non-linearity of ±10% over 70mm length. The produced sensors present a non-linearity higher than those ones produced on glass substrates, due to the different thermal coefficients exhibited by the polymer and the amorphous silicon film. In order to prove this behaviour, it was measured the defect density obtained by the constant photocurrent method on amorphous silicon deposited on polymeric substrates bended with different radius of curvature. © 2004 Elsevier B.V. All rights reserved.

Highly textured transparent conducting ZnO:Al thin films have been prepared by r.f. magnetron sputtering. The films were deposited on polyester (Mylar type D, 100 μm thickness) and glass substrates at room temperature. Surface stylus profiling, X-ray diffraction, scanning electron microscopy, transmission electron microscope and Hall effect measurements as a function of temperature, using the van der Pauw technique have characterized the films. The samples are polycrystalline with a hexagonal wurtzite structure and a strong crystallographic c-axis orientation (002) perpendicular to the substrate surface (columnar structure). The ZnO:Al thin films with a resistivity as low as 3.6×10-2 Ωcm have been obtained, as deposited. © 2001 Materials Research Society.

Thin film transistors (TFTs) have been produced by rf magnetron sputtering at room temperature, using non conventional oxide materials like amorphous indium-zinc-oxide (IZO) semiconductor, for the channel as well as for the drain and source regions. The obtained TFTs operate in the enhancement mode with threshold voltages of 2.4 V, saturation mobility of 22.7 cm2/Vs, gate voltage swing of 0.44 V/dec and an ON/OFF current ratio of 7×10 7. The high performances presented by these TFTs associated to a high electron mobility, at least two orders of magnitude higher than that of conventional amorphous silicon TFTs and a low threshold voltage, opens new doors for applications in flexible, wearable, disposable portable electronics as well as battery-powered applications.

Transparent electronics is today one of the most advanced topics for a wide range of device applications. The key components are wide bandgap semiconductors, where oxides of different origins play an important role, not only as passive component but also as active component, similar to what is observed in conventional semiconductors like silicon. 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. In this paper the recent progress in n- and p-type oxide based thin-film transistors (TFT) is reviewed, with special emphasis on solution-processed and p-type, and the major milestones already achieved with this emerging and very promising technology are summarizeed. After a short introduction where the main advantages of these semiconductors are presented, as well as the industry expectations, the beautiful history of TFTs is revisited, including the main landmarks in the last 80 years, finishing by referring to some papers that have played an important role in shaping transparent electronics. Then, an overview is presented of state of the art n-type TFTs processed by physical vapour deposition methods, and finally one of the most exciting, promising, and low cost but powerful technologies is discussed: solution-processed oxide TFTs. Moreover, a more detailed focus analysis will be given concerning p-type oxide TFTs, mainly centred on two of the most promising semiconductor candidates: copper oxide and tin oxide. The most recent data related to the production of complementary metal oxide semiconductor (CMOS) devices based on n- and p-type oxide TFT is also be presented. The last topic of this review is devoted to some emerging applications, finalizing with the main conclusions. Related work that originated at CENIMAT|I3N during the last six years is included in more detail, which has led to the fabrication of high performance n- and p-type oxide transistors as well as the fabrication of CMOS devices with and on paper. Transparent electronics is one of the most advanced science topics for a broad range of device applications. In this article an overview is presented of state-of-the-art n- and p-type oxides for TFTs and their integration, processed by physical vapor deposition and by solution processing techniques. Some of the most relevant emerging applications are also presented, including CMOS devices based on oxide TFTs fabricated on glass and even on paper. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

We report high performance ZnO thin film transistor (ZnO-TFT) fabricated by rf magnetron sputtering at room temperature with a bottom gate configuration. The ZnO-TFT operates in the enhancement mode with a threshold voltage of 19 V, a field effect mobility of 28 cm2/Vs, a gate voltage swing of 1.39 V/decade and an on/off ratio of 3×105. The ZnO-TFT present an average optical transmission (including the glass substrate) of 80% in the visible part of the spectrum. The combination of transparency, high field-effect mobility and room temperature processing makes the ZnO-TFT a very promising low cost optoelectronic device for the next generation of invisible and flexible electronics.