Rodrigo Martins

The working principle of silicon p-i-n structures with low conductivity (σd) doped layers as single element image sensors is based on the modulation, by the local illumination conditions of the photocurrent generated by a light beam scanning the active area of the device. A higher sensitivity is achieved using a wide band gap a-Si:C alloy in the doped layers, improving the light penetration into the intrinsic semiconductor and reducing the lateral currents in the structure, which are responsible by an image smearing effect observed in sensors with high σd doped layers. This work focuses on the transient response of such sensor and on the role of the carbon (C) content of the doped layers. A set of devices with different percentage of C incorporation in the doped layers is analyzed by measuring the scanner-induced photocurrent under different bias conditions, (ranging from -1.5V to 1V) in order to evaluate the response time.

A commercial sheet of paper based on natural cellulose fibers acting as permeable membrane with thin film metal cathode (Cu) and anode (Al) layers in each face was used to produce paper batteries that could be interconnected in series and rechargeable using water as electrolyte. Their electrical characteristics and the set of electrochemical reactions that support the experimental behavior observed are described in this paper. A series of integrated batteries able to supply a voltage of about 3 V and a current ranging from 0.7 μA to 25 μA in cells with sizes of 1.2 cm × 3.0 cm for a relative humidity in the range of 50-65% were produced in a single sheet of paper, and successfully applied to control the ON/OFF gate state of paper transistors. © 2010 Elsevier Ltd. All rights reserved.

The performances of amorphous and nano-crystalline porous silicon thin films as gas detector are pioneer reported in this work. The films were produced by the hot wire chemical vapour deposition (HW-CVD). These films present a porous like-structure, which is due to the uncompensated bonds and oxidise easily in the presence of air. This behaviour is a problem when the films are used for solar cells or thin film transistors. For as gas detectors, the oxidation is a benefit, since the CO, H2 or O2 molecules replace the OH adsorbed group. In the present study we observe the behaviour of amorphous and nano-crystalline porous silicon thin films under the presence of ethanol, at room temperature. The data obtained reveal a change in the current values recorded by more than three orders of magnitude, depending on the film preparation condition. This current behaviour is due to the adsorption of the OH chemical group by the Si uncompensated bonds as can be observed in the infrared spectra. Besides that, the current response and its recover time are done in few seconds.

Nanocrystalline p-doped silicon films were deposited at low substrate temperatures (around 200°C) in a hot wire reactor. In this paper we present the results on the role of the hydrogen dilution and filament temperature on the film's structure, composition, morphology and transport properties. The film's structure changes from honeycomb-like to a granular needle shape as the filament temperature changes from about 2000°C and hydrogen dilution 87%, to values above 2100°C and hydrogen dilution 90%, respectively. The nanocrystalline silicon-based films produced have optical gaps varying from 1.6 to 1.95 eV, with conductivities up to 0.2 S cm-1 and grain sizes (obtained by X-ray diffraction) in the range of 10-30 nm. © 1999 Elsevier Science 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.

This paper reports results on the role of high hydrogen dilution (above 80%) on the electro-optical and structural properties of boron doped silicon films produced by hot wire chemical vapor deposition (HW-CVD) technique, keeping constant the filament temperature. The structural, compositional, morphological, electrical and optical properties achieved show that the films present excellent homogeneity over the entire 10 x 10 cm deposited area. These results were obtained for films produced at gas pressures below 66.5 Pa, in spite of the high flow rate used. © 2001 Elsevier Science B.V. All rights reserved.

Amorphous undoped a-Si:H films have been produced by hot wire plasma assisted chemical vapour deposition (HWPA-CVD), which combines the hot wire chemical vapour deposition (HW-CVD) and plasma enhanced chemical vapour deposition techniques. In this work we analyse the dissociation mechanism of the gas during the film growth in both processes with a quadrupole mass spectrometer. Besides that, the energy delivered to the gas dissociation is determined and correlated with the films properties. Thus, based on the results of the dissociated species for each deposition condition and process, we explain why the growth rate is enhanced when the filament temperature rises in HW-CVD process and why it decreases as r.f. power is enhanced in HWPA-CVD process. © 2002 Elsevier Science B.V. All rights reserved.

In this work we present results concerning the composition and structure of intrinsic thin film silicon carbide alloys obtained by hot wire and hot wire plasma assisted techniques using ethylene as carbon gas source. The data show that by increasing the percentage of ethylene in the gas mixture from 14% to 60% the optical band gap is enhanced from 1.8 eV to 2.3 eV, for films produced by hot wire technique at a filament temperature of 2123K (1850°C). This is attributed to the increase of carbon incorporation, which was confirmed by the infrared spectra data where an increase is observed in the SiC stretching vibration mode ascribed to the peak located at around 750cm-1. On the other hand, the films produced by combining hot wire and rf plasma show a more efficient carbon incorporation. The SEM photographs of samples produced with hot wire technique reveal an amorphous structure, confirmed by micro-Raman spectroscopy data, while the samples produced with plasma assisting the process show a granular structure with grain sizes in the range of 100-200nm. © 2002 Elsevier Science Ltd. All rights reserved.

In this work, we report the optical and compositional properties of hydrogenated amorphous silicon carbide (a-SiC:H) thin films produced by plasma-enhanced chemical vapor deposition (PE-CVD), hot wire CVD (HW-CVD) and hot wire plasma-assisted CVD (HWPA-CVD) processes. The optical band gap of a-SiC:H films was controlled from 1.85 to 3.5 eV by varying the percentage of ethylene in the silane gas mixture from 3 to 100%. Adding a rf plasma to the hot wire process the carbon gas source dissociation is implemented leading to an increase in bulk carbon incorporation. This evidence is proved by the enhancement of the peak ascribed to the SiC stretching vibration mode, the reduction of the peak related to the SiH wagging modes, the decrease in the refractive index and the increase of optical band gap. The influence of hydrogen gas dilution on the properties of the films obtained by the different methods is also reported. © 2001 Elsevier Science B.V. All rights reserved.

The role of the deposition pressure (p) and the type of filaments (tungsten, W or tantalum, Ta) used to produce large area (10cm×10cm) n-type Si:H films by hot wire chemical vapour (HW-CVD) deposition technique was investigated. The data show that the electro-optical properties of the films produced are dependent on the gas pressure used. In the pressure range of 1×10-3 Torr to 1.0 Torr, the room dark conductivity (σd) varies from 1×10-8 to 2 S/cm for films produced at the same hydrogen dilution and filament temperature (Tfil). On the other hand, the hydrogen concentration (CH) decreases from 10% to 2%, while the growth rate (R) shows an exponential increase, from 1 to 9 Å/s. The SIMS analysis, within the detection limits, does not reveal the existence of any significant W or Ta contamination in the films produced.

This work deals with the role of hydrogen dilution and filament temperature on the morphology, structure and electrical properties of nanocrystalline boron doped silicon carbide thin films produced by hot-wire technique. The structural and morphological data obtained by XRD, SEM and micro-Raman show that for filament temperatures and hydrogen dilutions above 2100 °C and 90%, respectively, the surface morphology of the films is granular with a needle shape, while for lower filament temperatures and hydrogen dilutions the surface morphology gets honeycomb like. The SIMS analysis reveals that films produced with filament temperatures of about 2200 °C and hydrogen dilution of 99% present a higher hydrogen and carbon incorporation than the films produced at lower temperatures and hydrogen dilutions. These results agree with the electrical and optical characteristics recorded that show that the films produced exhibit optical gaps in the range from 1.8 to 2 eV and transverse conductivities ranging from 10-1 S/cm to 10-3 S/cm, consistent with the degree of films crystallinity and carbon incorporation recorded.

In this paper, we present results of the role of laser beam energy and shot density on the electro-optical and structural properties of undoped and doped recrystallized amorphous silicon thin films, generated by pulsed Nd-YAG laser (λ = 532 nm). The data reveal that the structure and electrical characteristics of the recrystallized thin films are mainly dependent on the energy and shot density of the laser beam, while the morphology of the obtained films are mainly governed by the number of shots used. The data also show that the electrical conductivity of undoped and doped recrystallized films can be varied up to 6 orders of magnitude, by the proper choice of the recrystallization conditions. Doped samples with conductivities in the amorphous states in the range of 10-5 Ω-1 cm-1 present, after recrystallization, conductivities of about 300 Ω-1 cm-1. The SEM micro-chemical analysis also shows that the obtained crystalline grains are constituted by pure silicon. © 1998 Elsevier Science S.A.

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.

The role of the deposition pressure and hydrogen dilution in the production of p-type Si:H films by hot wire chemical vapor deposition, was investigated. The system used permits to obtain uniform and homogeneous films properties over a 10cm×10cm substrate area. As heated filament we used Ta, since Ta filaments have longer life period without deteriorating than the W filaments ones. In this work, we show that the electrical properties of the films produced are dependent on the process gas pressure. In the pressure range of 13.3 Pa (0.1 Torr) to 66.5 Pa (0.5Torr), the film's coplanar electrical conductivity at room temperature varies by more than two orders of magnitude, for films produced at same hydrogen dilution and filament temperature, reaching values of about 0.1 (Ωcm)-1, at deposition pressures of about 40-53Pa (0.3-0.4Torr). On the other hand, the increase in hydrogen dilution (from 87% to 96%) promotes the surface roughness due to an enlargement of grain sizes in the direction of the {220} diffraction planes as observed by SEM micrographs without changing the crystalline fraction (48-50%) obtained by micro-Raman analysis.

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.

The combination of high transparency and good thermoelectric properties of SnO2 can open new field of applications for the thin film thermoelectric materials. Here we report on SnO2 thin films with transmittance above 90%, resistivity bellow 10-3Ωm and a Power Factor around 10-4 W/m.K2, for a Seebeck of -255μV/K, at room temperature. The effect of film thickness and post-deposition annealing on the thermoelectric properties were analysed. The performances of a single layer thermoelectric device are also presented. © 2015 .

Doped a/μc-Si:H films were produced in different starting deposition conditions by the hot wire chemical vapor deposition technique. In this paper, we show that by changing the initial onset deposition conditions of the process and maintaining the overall pressure, hydrogen dilution and filament temperature, it is possible to control the compactness of the films. As the films nucleation is the key parameter to produce compact films, we show that starting the process with hydrogen and progressively introducing the process gas enhances the compactness and improve the electrical properties of the films produced. © 2002 Elsevier Science B.V. All rights reserved.

This paper reports on the use of cellulose paper simultaneously as electrolyte, separation of electrodes, and physical support of a rechargeable battery. The deposition on both faces of a paper sheet of metal or metal oxides thin layers with different electrochemical potentials, respectively as anode and cathode, such as Cu and Al, lead to an output voltage of 0.70 V and a current density that varies between 150 nA/cm2 and 0.5 mA/cm2, subject to the paper composition, thickness and the degree of OHx species adsorbed in the paper matrix. The electrical output of the paper battery is independent of the electrodes thickness but strongly depends on the atmospheric relative humidity (RH), with a current density enhancement by more than 3 orders of magnitude when RH changes from 60% to 85%. Besides flexibility, low cost, low material consumption, environmental friendly, the power output of paper batteries can be adapted to the desired voltagecurrent needed, by proper integration. A 3-V prototype was fabricated to control the ON/OFF state of a paper transistor. © 2006 IEEE.

In this work we present data concerning the structure, composition and electro-optical performances of nanocrystalline silicon carbide doped films produced at the different filament temperatures and hydrogen dilution ratios. The XRD spectra reveal the presence of the typical Si peaks ascribed to (111) (220) and (311) diffraction planes, where no traces of the carbon peaks were found. The average grain sizes ranges from 10 nm to 30 nm, depending on the temperature of filament and hydrogen dilution used. We observed an enhancement of the peak ascribed to the (220) plane when high H dilution rates are used, meaning that the film starts being textured. The infrared data reveal the typical silicon carbide modes and a hydrogen content that varies from 3% to 1%, with the increase of the filament temperature. Besides that, the IR spectra show the typical SiO2 and SiO modes, associated to the oxide species that are mainly incorporated in the surface of the films and can be removed by proper wet etching. The planar conductivity is enhanced as the temperature of the filament is increased, being the highest conductivity achieved in the range of 0.2 (Ωcm)-1 and almost non activated. © 1998 Elsevier Science Ltd. 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 work, we show the possibility to use undoped porous silicon (PS) thin films produced by hot wire chemical vapour deposition technique (HW-CVD) as ethanol detector. Silicon thins films produced by HW-CVD technique, under certain deposition conditions, have a porous structure [Vacuum 52 (1999) 147]. Therefore, in the presence of an alcohol, the OH group is adsorbed by the uncompensated bonds behaving as donor-like carriers leading to an increase in the current flowing through the material. This current enhancement is bias dependent in glass/ITO/i-a-Si:H/Al sensor and increases as the ethanol vapour pressure increases from 10-1mbar to atmospheric pressure. The response time of the current of the sensor and its recovery time are in the range of 10-50s at room temperature. Ethanol quantities above 50ppm can be detected. © Published by Elsevier B.V.

In this work, we show results concerning electro-optical properties, composition and morphology of nanocrystalline hydrogenated undoped silicon (nc-Si:H) films produced by hot wire plasma assisted chemical vapour deposition process (HWPA-CVD) and exhibiting a compact granular structure, as revealed by SEM micrographs. This was also inferred by infrared spectra, which does not present the SiO vibration band located at 1050-1200 cm-1, even when samples have long atmospheric exposition. The photoconductivity measured at room temperature also does not change when samples have a long time exposition to the air or to the light irradiation. The influence of hydrogen dilution on the properties of the films was also investigated.

This work reports on the performances of undoped and n doped amorphous/nano-crystalline silicon films grown by hot wire plasma assisted technique. The film's structure (including the presence of several nanoparticles with sizes ranging from 5 nm to 50 nm), the composition (oxygen and hydrogen content) and the transport properties are highly dependent on the filament temperature and on the hydrogen dilution. The undoped films grown under low r.f. power (≈4 mWcm-2) and with filament temperatures around 1850 °K have dark conductivities below 10-10 Scm-1, optical gaps of about 1.5 eV and photo-sensitivities above 105, (under AM1.5), with almost no traces of oxygen content. N-doped silicon films were also fabricated under the same conditions which attained conductivities of about 10-2 Scm-1.

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.

Nanocrystalline hydrogenated silicon (nc-Si:H) thin films are generally accepted to be a two phase material-Si crystalline and Si:H amorphous. This work reports the use of impedance spectroscopy to determine the amorphous and crystalline electrical conductivity of a/nc-Si:H films obtained by hot wire chemical vapour deposition. Different relaxation time or time constants are detected, if the film is composed by inhomogeneous material, by measuring ac impedance in a wide range of frequencies. Relating the conduction mechanism of the film to a series of two RC circuits constituted by a resistance and a capacitor in parallel, we may determine distinct ac conductivities and correlate that to the crystalline, amorphous and interface components. The amorphous films analysed exhibit one ac conductivity component while for nanocrystalline films two ac conductivity components are observed. The average value of ac conductivities is in agreement with that of dc conductivity. © 2006.