Rodrigo Martins

In this work we present results of studies performed on Schottky and metal-insulator-semiconductor (MIS) position sensitive detectors (PSD) structures: substrate (glass)/ Cr (300 nm) / a-Si:H [n] (37 nm) / a-Si:H [i] (600 nm) / SiO2 (1.5 nm - for the MIS) / Au (7 nm). The effect of the interfacial oxide layer between Au and a-Si:H, for the MIS structures, was studied and compared with the Schottky, in order to determine how beneficial it could be for device performances and time degradation. For doing so, the Au thickness of 70Å was deposited by thermal evaporation on an oxide free (Schottky) and oxidized (≈20Å) (MIS) a-Si:H surfaces. These structures were characterized by SIMS, RBS, SEM and AFM in order to correlate the obtained diffusion profile of Au at the interface and the topography with the presence of the oxide at the interface. The results show that the Au inter-diffuses very easily in the oxide free a-Si:H surface, even at room temperature, degrading the devices performance. On the other hand, the MIS structures, with their interfacial oxide present no structural changes after annealing and the PSD produced are stable. We believe that this effect is associated with the barrier effect of the interfacial oxide that prevents the Au diffusion. The optimized 1D MIS sensors are stable and exhibit a linearity error as low as 0.8 % and sensitivities of 33 mV/cm for a 5 mW spot beam intensity at a wavelength of 532 nm, while the Schottky sensors showed a time degradation of their characteristics. © 2006 Materials Research Society.

In this work we present a study of influence of ion bombardment on the optical, electrical and compositional properties of the intrinsic amorphous silicon films deposited in a modified triode plasma-enhanced chemical vapour deposition (PECVD) reactor. The application of a DC voltage to a grid placed in front of the r.f. electrode allowed us to control the energy and polarity of the ions striking the substrate during film growth. The results show a variation by two orders of magnitude in the dark conductivity from 10-10 to 6.2 × 10-12 (Ω cm)-1, while the photosensitivity varied from 2 × 105 to 2 × 107. A process plasma that takes place in the γ-type regime was associated with the use of a negative bias, while a process plasma like the one of the α-type regime was associated with the use of a positive bias. The films deposited with a bias ≈ 38 V are highly intrinsic and the abrupt change in the conductivity properties observed at this bias is attributed to a change in the density of the states ascribed to the position of the Fermi level. That is, a precise control of the energy of the ions striking the substrate during the film growth leads to improved film optoelectronic properties, very important for device applications. © 2001 Elsevier Science Ltd. All rights reserved.

In this work, we study the influence of the hydrogenated amorphous silicon (a-Si:H) surface treatment on the J-V characteristics of a-Si:H/Pd Schottky barrier photodiodes. The a-Si:H surface were etched, thermally oxidised and wet oxidised by H2O2. The a-Si:H films were characterised by spectroscopic ellipsometry, were we found that all the oxidation techniques promote an increase of the surface oxide thickness that was confirmed by the increase of the barrier height. The highest barrier was achieved by the H2O2 oxidation where a value of 1.17 eV was found. As a result of the barrier height increase, the dark reverse current density decreases up to 10-10 A/cm2 and the signal to noise ratio increases up to 106. The open circuit voltage under AM1.5 illumination conditions also increases from 0.4 to 0.5 V. These results reveal the importance of the a-Si:H surface preparation prior to metallization to improve the Schottky photodiodes properties. © 2002 Elsevier Science B.V. All rights reserved.

In this work we present results of structural composition and morphological characteristics of polymorphous silicon (pm-Si:H) films deposited by PECVD at 13.56 and 27.12 MHz. In addition, the role of the excitation frequency on the growth rate will be also analyzed. The results show that by using the 27.12 MHz excitation frequency the hydrogen dilution in the plasma needed to produce pm-Si:H can be reduced by more than 50% as well as the rf power density, leading to an increase on the growth rate to values higher than 3 Å/s. Spectroscopic ellipsometry and Raman spectroscopy show that the 27.12 MHz pm-Si:H films are more ordered than the pm-Si:H films produced at 13.56 MHz, while the infrared spectroscopy show that the SiH2 concentration in the films is strongly reduced. AFM measurements reveal that the films produced at 27.12 MHz films are more structured, presenting also higher roughness. © 2004 Elsevier B.V. All rights reserved.

Trimethylboron (TMB) has been receiving attention as a valid alternative to diborane and methane mixtures for the deposition of p-type silicon films for applications in optoelectronic devices such as solar cells. In this paper we report on p-type hydrogenated nanocrystalline silicon carbide (nc-Si:C:H) films produced by standard 13.56 MHz plasma enhanced chemical vapour deposition technique, using TMB as gas source, under high hydrogen dilution (98%) and using high deposition pressures (3 Torr). The films obtained were characterized by spectroscopic ellipsometry (SE), Raman spectroscopy (RS), and electrical measurements to determine their optical, structural and electrical properties. We achieved conductivities as high as 8.3 (Ω cm) -1, one of the highest values of conductivity published to date using TMB with standard rf-PECVD. Spectroscopic ellipsometry modeling revealed that the films growth mechanism proceeds through a sub-surface layer mechanism that leads to the formation of nanocrystalline silicon. Copyright © 2010 American Scientific Publishers All rights reserved.

In this work we present the current/voltage characteristics of Si:H/Pd Schottky structures using high quality, low defect density amorphous silicon (a-Si:H) deposited by a non-conventional, modified triode PECVD method. This new configuration allows the deposition of compact and high quality a-Si:H with a photosensitivity of 107, yielding films with low bulk defects. AFM measurements also revealed that these films have a very smooth surface allowing a low defect interface between the metal and the a-Si:H. As a result, we show that by using these a-Si:H films and by proper control of the i-layer thickness the reverse dark current of the diode can be highly reduced achieving signal to noise ratio of 106, surpassing the results usually achieved by p-i-n structures.

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 tin oxide (ITO) has been used as the prefered electrode material for the emerging area of transparent electronics, namely for thin-film transistors (TFTs) based on oxide semiconductors. This work pretends to investigate different materials to replace ITO in inverted-staggered TFTs based on gallium-indium-zinc oxide (GIZO), one of the most promissing oxide semiconductors for TFTs. The analyzed electrode materials are indium-zinc oxide (IZO), Ti, Mo and Ti/Au. Devices are analyzed with special focus on the contact resistance fundamentals, including the extraction of source/ drain series resistances and TFTs intrinsic parameters, such as intrinsic mobility (p\) and intrinsic threshold voltage (V Ti). The obtained contact resistance values are between 10 kΩ and 20 kΩ, and the best devices have field effect mobility ((μ FE) close to 25 cm 2/V s and on/off ratio close to 10 8. © 2008 WILEY-VCH Verlag GmbH & Co. KGaA.

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.

This work deals with the study of the role of intra-gap density of states on the colour selection of the collection spectrum of glass/ITO/a-Six:C1-x:H/Al Schottky photodiodes. In order to optimise the voltage colour selection and to study the influence of intragap density of states in the final device performances, different undoped a-Six:C1-x:H films (1 μm thick) have been produced in a conventional Plasma Enhanced Chemical Vapour Deposition (PECVD) system using silane and a controlled mixtures of silane and methane as gas sources. The properties of the films were analysed by dark conductivity measurements, infrared spectroscopy, visible spectroscopy and constant photocurrent method (CPM), to determine the valence controllability and to correlate the silicon carbide layer composition with the performances of the devices. The performances obtained concerning the spectral response of the devices were correlated with the carbon content and the density of states of the a-Six:C1-x:H films.

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.

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.

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

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.

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.

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.

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.

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 2D amorphous silicon hydrogenated pin thin film position sensitive detectors. In addition, experimental results concerning the device linearity and spatial resolution are presented and checked against the predicted values of the analytical model proposed.

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.

The core of this paper concerns the use of an amorphous transparent conductive oxide (a-TCO), whose performance is on par with the classical indium tin oxide (ITO) films as a transparent contact in organic light emitting diodes (OLEDs). The main advantage of indium zinc oxide (IZO) films relies on their amorphous structure and high mobility that turns them likely to be used with high conductivity and high transmittance even at the infrared region. The mobility of IZO films (47.8 cm2 · V-1 · s-1) surpasses the one exhibited by ITO films (26.4 cm2 · V-1 · s-1), which along with its smoother surface and better current distribution plays an important role on OLEDs performance. Besides their similar turn-on voltage, the devices using IZO anodes exhibit higher power efficiency than the ITO ones, which is 66, 18, and 62% for orange, green, and blue OLEDs, respectively. These results suggest that IZO can potentially be applied as an anode in full color displays based on OLEDs. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

The aim of this work is to present an analytical model which can to interpret the role of the collecting resistive layer on the static performances exhibited by 2D amorphous silicon hydrogenated p-i-n thin film position sensitive detectors. In addition, experimental results concerning the device linearity and spatial resolution are presented and checked against the predicted values of the analytical model proposed. © 1999 Elsevier Science S.A. All rights reserved.

This paper deals with the study of the role of gas temperature and of the ratio of r.f. power to gas flow on the particle's formation in amorphous silicon films grown by the plasma enhanced chemical vapour deposition technique, by monitoring the plasma impedance behaviour under different process conditions. The results achieved show the existence of two main boundary regions separating the so-called α-regime (no powder formed) from the γ-regime (powder formed). Those regions are reached either by heating the gas, changing the gas pressure or using high power to gas flow ratios, corresponding to the establishment of a balance between the plasma resistance and the plasma reactance. In the β-region the probability to incorporate nanoparticles in the films is low and the films exhibit photosensitivity's of about 105 with density of states determined by the constant photocurrent method below 6×1015 cm-3 with Urbach energies below 50 meV. In the θ-region small nanoparticles can be incorporated leading to films with density of states below 3×1015 cm-3, with Urbach energies above 50 meV and photosensitivity's above 106, about two orders of magnitude larger than that of conventional amorphous silicon grown in the α-regime.

This paper aims to discuss the effect of order and disorder on the electrical performances of covalent silicon semiconductors and ZnO based ionic oxide semiconductors used as active channel layers in thin film transistors. The effect of disorder on covalent semiconductors directly affects their electrical transport properties due to the asymmetric behaviour of sp states, while in ionic oxide semiconductors it is found that this effect is small due to the fact that angular disorder has no effect on the spherical symmetry of s states. To this we must add that the mobility of carriers in both systems is quite different, being also affected by electron-phonon interactions (weak in silicon and strong in ionic oxides leading to formation of polarons). Besides, the impurity doping effect and the presence of vacancies in disordered silicon and in ionic oxides behave differently, which will influence the thin film properties and so, the performances of the devices produced. © 2007 Springer-Verlag.

The aim of this paper was to present results of the role of the d.c. grid bias on the silane plasma impedance and its I-V characteristics to grow undoped amorphous silicon (a-Si:H) films by plasma enhanced chemical vapour deposition (PECVD) in conditions where some nanoparticles can be formed in the growth region of the deposition process, under proper ion bombardment. The results achieved show that the performances of the films produced are dependent on the self bias voltage that can present photosensitivities of approximately 107 (two orders of magnitude larger than the one exhibited by films grown under conventional conditions) with density of states determined by the constant photocurrent method below 4×1015 cm-3. Apart from that, the films grown are less affected by light soaking than the conventional films.

The aim of this work is to interpret the role of the lateral leakage current on the a-Si:H solar cell performances (J-V characteristics, responsivity and the apparent device degradation behaviour), under low illumination conditions.