Rodrigo Martins

Amorphous Silicon solar cells have been produced by a two consecutive decomposition and deposition chamber system, using polished S. S. substrates. Through a systematic investigation of the electrical and optical properties of doped and undoped amorphous silicon layers (1) we observe that the deposition conditions (gas partial pressure, density of r. f. power, substrate temperature, electromagnetic static fields applied to the substrate, and gas flow rate) influence films properties. In the course of this investigation we have been studying the role of the sheet resistance, R//s, of the I. T. O. layer on the short circuit current, I//s//c, and on the open circuit voltage, V//o//c, of p. i. n. structures of 16cm**2 in area. The obtained results indicate that V//o//c is almost independent on R//s, while I//s//c variation approaches a square root dependence on R//s.

In this work we studied the influence of the power density of hydrogen plasma on electrical and optical properties (Hall mobility, free carrier concentration, sheet resistance, optical transmittance and a.c. impedance) of indium zinc oxide films, aiming to determine their chemical stability. This is an important factor for the optimization of amorphous/nanocrystalline p-i-n hydrogenated silicon (a/nc-Si:H) solar cells, since they should remain chemically highly stable during the p layer deposition. To perform this work the transparent conductive oxide was exposed to hydrogen plasma at substrate temperature of 473 K, 87 Pa of pressure and 20 seem of hydrogen flow. The results achieved show that IZO films were reduced for all plasma conditions used, which leads mainly to a decrease on films transmittance. For the lowest power density used in the first minute of plasma exposition the transmittance of the IZO films decreases about 29%.

In this paper we study the electro-optical behaviour and the structure of different TCOs, namely the ZGO, ITO and IZO films before and after being submitted to different hydrogen plasma power densities, for times up to 60 s, aiming their use in a/nc-Si:H solar cells. The results achieved show that ZGO films do not reduce for all plasma conditions used and so, the solar cells produced evidence high current density, about 17% larger that the one recorded in the other TCOs. Besides that, by combining the electrical and optical characteristics of the films through a figure of merit, the data reveal that for the ITO and IZO films even when exposed to very low hydrogen power plasma, the figure of merit is reduced up to 50%. © 2005 Elsevier B.V. All rights reserved.

In this work we present a study aiming to determine the role of Ga2O3-In2O3-ZnO (GIZO) channel layer composition on the electrical performance and stability exhibited by thin-film transistors (TFTs). The GIZO films were obtained by magnetron sputtering using ceramic targets of different compositions (Ga:In:Zn = 2:2:1, 2:2:2, 2:4:1 and 2:4:2 at.). Structural analysis corroborates the fully amorphous character of the GIZO deposited layers. For the target compositional range used we observe a Zn deficiency on the produced films, which affects the In/Ga atomic concentration ratios. Resistivity and mobility are found to show a general trend against the measured In/Ga ratio that reveals the role played by In and Ga cations on the transport mechanisms. Targets with increased In concentrations (2:4:1 and 2:4:2) allow to obtain the best TFT performances with field effect mobilities reaching values of 53.0 and 51.7cm2 V-1 s-1, respectively. In addition, the In-richer GIZO compositions result in considerably more stable TFTs, especially under positive gate bias stress conditions. Finally, it is verified that by using a target with a slightly lower In atomic composition (2:4:2 in comparison to 2:4:1), good stability and mobility can be achieved with potentially lower material costs.© 2011 Elsevier B.V. All rights reserved.

The aim of this paper is to present results concerning the role of the buffer layer on pin devices, deposited in a single chamber for plasma enhanced chemical vapor deposition, using high hydrogen dilution and pressures at 27.12 MHz. By doing so, we allow the incorporation of nanoparticles into the i-layer, during plasma process. The results show solar cells with 8.8% efficiency with a collection efficiency of 95% in the blue region of the spectra. Apart from that, the results from impedance spectroscopy, imaginary impedance vs. real impedance, show difference of a semicircle radius as function of sample temperatures, which could be explained by total device series resistance variation. © 2005 Elsevier B.V. All rights reserved.

This paper presents the role of the deposition pressure and the rf power density on the optimization of the electrical, optical and structural properties of large area (30×40 cm2) indium-tin oxide films produced by rf magnetron sputtering, with growth rates exceeding 30 nm/min. The films were produced at room temperature under reactive plasma, followed by a thermal annealing in air or formic gas. The best films' uniformity (≤±5%), compactness and surface smoothness (preferential growth orientation along (222)); and electro-optical properties (resistivity and mobility, respectively, of about 7×10-4 Ω cm and 19.6 cm2 V -1 s-1, with transmittance of about 92%) were achieved using a rf power density of 0.92 W cm-2 and a pressure of 8.5×10-2 Pa, followed by its annealing in air by about 2 h at 773 K. © 2005 Elsevier B.V. All rights reserved.

We report the effect of a disperse carbon interlayer between the n-a-Si:H layer and an aluminium zinc oxide (AZO) back contact on the performance of amorphous silicon solar cells. Carbon was incorporated to the AZO film as revealed by x-ray photoelectron spectroscopy and energy-dispersive x-ray analysis. Solar cells fabricated on glass substrates using AZO in the back contact performed better when a disperse carbon interlayer was present in their structure. They exhibited an initial efficiency of 11%, open-circuit voltage Voc = 1.6 V, short-circuit current JSC = 11 mA cm -2 and a filling factor of 63%, that is, a 10% increase in the J SC and 20% increase in the efficiency compared to a standard solar cell. © 2013 National Institute for Materials Science.

Indium molybdenum oxide (IMO) thin films were deposited by RF magnetron sputtering on glass substrates at room temperature. The deposition and argon partial pressures were maintained at 6.0 × 10-1 Pa and 3.0 × 10-1 Pa, respectively. The oxygen partial pressure (OPP) was varied in the range 1.0-6.0 × 10-3 Pa. The films were sputtered at 40 W for 30 min using the target consisted In2O3 (98 wt%): Mo (2 wt%). The films are polycrystalline with a slight preferential orientation along (2 2 2) plane. The crystallinity is increased with the increasing OPP. The negative sign of Hall coefficient confirmed the n-type conductivity. A maximum mobility ∼19 cm2 V-1 s-1 is obtained for the films deposited with OPP of 3.6 × 10-3 Pa. The average visible transmittance calculated in the wavelength ranging 500-800 nm is ranging between 2% and 77%. The optical band gap calculated from the absorption data is varied between 3.69 and 3.91 eV. A striking feature is that the work function of the films is wide ranging 4.61-4.93 eV. A possibility of using the produced IMO films as transparent conducting oxide in photovoltaic applications such as organic solar cells is discussed in this article. © 2008 Elsevier Ltd. All rights reserved.

Doped zinc oxide with aluminium are attractive alternative material as transparent conducting electrode because they are nontoxic and inexpensive compared with indium tin oxide (ITO) for diffrent applications: solar cells, tft. Transparent aluminumdoped zinc oxide (AZO) thin films were deposited on glass substrates by RF magnetron sputtering at room temperature and 100W from ceramic target ZnO-Al2O3 (98:2 weight percent). The structural, electrical and optical properties of these films were characterized as a function of deposition pressure. AZO films with low resistivity 2.02×10-3 Ωcm and high transmittance (over 80% in vizible range) were thus prepared with a deposition pressure of 3 mTorr.

The ability to process and dimensionally scale field-effect transistors with and on paper and to integrate them as a core component for low-power-consumption analog and digital circuits is demonstrated. Low-temperature-processed p- and n-channel integrated oxide thin-film transistors in the complementary metal oxide semiconductor (CMOS) inverter architecture are seamlessly layered on mechanically flexible, low-cost, recyclable paper substrates. The possibility of building these circuits using low-temperature processes opens the door to new applications ranging from smart labels and sensors on clothing and packaging to electronic displays printed on paper pages for use in newspapers, magazines, books, signs, and advertising billboards. Because the CMOS circuits reported constitute fundamental building blocks for analog and digital electronics, this development creates the potential to have flexible form factor computers seamlessly layered onto paper. The holistic approach of merging low-power circuitry with a recyclable substrate is an important step towards greener electronics. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Zinc oxide is a well-known wide band gap semiconductor material (3.4 eV at room temperature, in the crystalline form), which has many applications, such as for transparent conductors, varistors, surface acoustic waves, gas sensors, piezoelectric transducers and UV detectors. More recently, it is attracting considerable attention for its possible application to thin film transistors. In this paper, we present some of the recent results already obtained as well as the ones that are being developed in our laboratory. The main advantage presented by these new thin film transistors is the combination of high channel mobility and transparency produced at room temperature which makes these thin film transistors a very promising low cost device for the next generation of invisible and flexible electronics. Moreover, the processing technology used to fabricate this device is relatively simple and it is compatible with inexpensive plastic/flexible substrate technology. © 2005 Elsevier B.V. All rights reserved.

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.

Large-area electronics for applications in environments with radioactive contamination or medical X-ray detectors require materials and devices resistant to continuous ionizing radiation exposure. Here the superior X-ray radiation hardness of oxide thin film transistors (TFTs) based on gallium-indium-zinc oxide is demonstrated, when compared to organic ones. In the experiments both TFTs are subjected to X-ray radiation and their performances are monitored as a function of total ionizing dose. Flexible oxide TFTs maintain a constant mobility of 10 cm2 V−1 s−1 even after exposure to doses of 410 krad(SiO2), whereas organic TFTs lose 55% of their transport performance. The exceptional resistance of oxide semiconductors ionization damage is attributed to their intrinsic properties such as independence of transport on long-range order and large heat of formation. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The most common techniques used to produce ZnO thin films are the spray pyrolysis and the magnetron sputtering techniques, low and high cost processes respectively. The aim of this work is to compare the properties of the films produced by these two techniques. The predominant difference observed was on the morphological properties. The films produced by spray pyrolysis have a rougher surface than the ones obtained by sputtering. Also the effect of the thermal annealing treatment is much more prononnced for the ZnO thin films produced by spray pyrolysis. After heat treatment films exhibit similar electrical properties and their application to optoelectronic devices is demonstrated.

Thin films of SnO2 deposited by spray pyrolysis as a function of temperature and the carrier gas flow have been produced, in order to evaluate the adequate deposition parameters for application in optoelectronic devices. The characterisation was centred mainly onto the structural, electrical and optical properties of the films. The obtained results showed that the films produced at 450°C and a gas flow of 101/min (as deposited) present an average transmittance (visible spectrum) of 90% and a bulk resistivity of 3.2×10-3 Ωcm.

In this work we present a study of the properties of ZnO thin films produced by spray pyrolysis and r.f. magnetron sputtering. Before the annealing treatment the properties of the films are very similar, which means that the films produced by both techniques could be used on optoelectronic devices. However spray pyrolysis is a more simple and cheap technique than sputtering, but with this last technique the thin films exhibit a higher uniformity. © 2001 Materials Research Society.

In order to study the properties of P-doped ZnO films deposited at low temperature substrates, P-doped ZnO films were RF-sputtered on sapphire substrates in the range from RT to 350°C. XRD spectra indicated the growth of the crystallites along the strongest <001> orientation. Further ZnO (002) peak became the weakest when the film was sputtered at 250°C. AFM pictures showed that the surface morphology varied with the deposition temperature. The sample RMS increased with the increase of substrate temperature. XPS spectra showed a clear broad P 2p peak at about 134 eV. Further the film composition varied with the substrate temperature. The average visible transmittance calculated in the wavelength ranging 400-600 nm was more than 60%. The optical band gap calculated from the absorption coefficient was about 3.2 eV. The Hall measurements confirm the n-type conductivity of the films. The carrier concentration in the films decreases with the increase of substrate temperature. The study is helpful for understanding the properties of P-doped ZnO films sputtered at lower substrate temperature and achieving p type ZnO films at lower temperature.

In this work, we present the properties of n-type silicon films obtained by hot wire plasma assisted technique produced at different rf power and gas flow rate. The films were produced at a filament temperature of 2000°C and the rf power was varied from 0 W to 200 W while gas flow rate was varied from 15 to 100 sccm keeping rf power at 50 W. In this flow rate range, the growth rate of the films varied from 5Å/s to 250Å/s and the corresponding electrical room dark conductivity varied from 10-2 to 10(Ωcm)-1. On the other hand, we observed that the electrical conductivity increased from 2 to 6(Ωcm)-1, and the Hall mobility from 0.1 to 2 cm2/V.s as rf power change from 0 W to 200 W. The infrared, EDS and XPS analyses revealed the existence of oxygen incorporation, which is not related to post-deposition oxidation. The X-ray diffraction and μRaman data show the presence of Si crystals in the films structure and the SEM micrographs reveal a granular surface morphology with grain sizes lower than 60 nm.

In this paper, we investigate the optoelectronic properties and the photodegradation of amorphous silicon films produced by the hot wire plasma assisted technique (HWPA-CVD). We observed that hydrogen dilution in the gas phase plays an important role in the time dependence of the photoconductivity, which is correlated with an enhancement of defect density. We also compare the degradation of these films with those produced by plasma enhanced and by hot wire chemical vapour deposition techniques (PECVD and HW-CVD) and we found lower time dependence for the photodegradation of the films produced by HWPA-CVD technique © 2003 Elsevier B.V. All rights reserved.

In this work we studied the influence of hydrogen dilution, rf power, and the filament and substrate temperatures on the electro-optical properties and composition of a-Si:H films produced by hot wire plasma assisted technique. The a-Si:H films were produced on Mylar substrates with growth rate of up to 37 Å/s, ημτ product of 1.6 × 10-7 cm2/V, photoconductivity to dark conductivity ratio of 1 × 104 (at AM1.5 radiation), and a dark conductivity of about 10-10 (Ω cm)-1 for substrate temperature of 130 °C, hydrogen dilution of 99%, filament temperature of 1700 °C, and rf power of 100 W. © 2002 Elsevier Science B.V. All rights reserved.

The aim of this paper is to present results concerning the morphology, structure, mechanical and electrical characteristics of the new proposed Cu-Sn metallurgical alloy, which may be used in electronic joins. By proper choice of process temperature and pressure, Cu coated surfaces are soldered using Sn as pre-form. The main results achieved indicate that the formation of Cu3Sn phase begins at a temperature of about 473 K and that the Sn thickness (dSn) needed is slightly above 7 μm. Due to join wettability, higher temperatures (between 523 and 573 K) and dSn above 35 μm are required to form joins within the specifications of the electronic industry.

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.

Flexible large area thin film position sensitive detectors based on amorphous silicon technology were prepared on polyimide substrates using the conventional plasma enhanced chemical vapor deposition technique. The sensors were characterized by spectral response, illuminated I-V characteristics position detectability measurements and atomic force microscopy. The obtained one-dimensional position sensors, 5-mm wide and 60-mm long, presented a maximum spectral response at 600 nm, an open circuit voltage of 0.6 V and a position detectability with a correlation of 0.9989 associated to a S.D. of 1×10-2, comparable to those produced on glass substrates.

In this work Laponite was combined with a modified abundant natural polymer, (caboxymethyl cellulose), acrylic sodium salt polymer and lithium perchlorate aiming to produce inexpensive and sustainable nanocomposite electrolytes for functional electrochemical devices. Optical, electrochemical, structural, morphological and rheological characterization was performed in order to evaluate their properties and potential advantages as electrolyte. It was verified that the addition of Laponite led to an ionic conductivity at room temperature (25 C) in the range of 6 to 9 × 10- 5 Scm - 1 this value being then determined by the composition of the nanocomposite. These electrolytes were applied to electrochromic devices using evaporated nickel oxide thin film as the electrochromic layer. The devices exhibited a significant transmittance modulation that exceeds 45 % at 600 nm. © 2013 Elsevier B.V.

Thin films of molybdenum-doped indium oxide (IMO) were prepared by a 3-source, cylindrical radio-frequency magnetron sputtering at room temperature. The films were post-annealed and were characterized by their structural (X-ray diffraction) and optical (UV-VIS-NIR spectrophotometer) properties. The films were studied as a function of oxygen volume percentage (O2 vol.%) ranging from 3.5 to 17.5. The structural studies revealed that the as-deposited amorphous films become crystalline on annealing. In most cases, the (222) reflection emerged as high intensive peak. The poor visible transmittance of the films as-deposited without oxygen was increased from ∼ 12% to over 80% on introducing oxygen (3.5 O2 vol.%). For the films annealed in open air, the average visible transmittance in the wavelength ranging 400-800 nm was varied between 77 and 84%. The films annealed at high temperatures (> 300 °C) decreased the transmittance to as low as < 1%. The optical band gap of the as-deposited films increased from the range 3.83-3.90 to 3.85-3.98 eV on annealing at different conditions. © 2007 Elsevier B.V. All rights reserved.