Rodrigo Martins

The aim of this work is to study the electrical characteristics of polycrystalline silicon (poly-Si) thin film transistors (TFTs) using spectroscopic impedance technique, where the poly-Si active layer was obtained by metal induced crystallization of amorphous silicon. From the study performed a theoretical model that fitted the impedance data is proposed, in order to obtain the separate contributions of each region that constitutes the TFT namely the channel, non accumulated region and contacts. © 2006 Elsevier B.V. All rights reserved.

In this work Spectroscopic Ellipsometry (SE) was used to study metal induced crystallization (MIC) on amorphous silicon films in order to analyze the influence of different annealing conditions on their structural properties. The variation of the metal thickness has shown to be determinant on the time needed to full crystallize silicon films. Films of 100 nm thickness crystallize after 2h at 500°C using 1 nm of Ni deposited on it. When reducing the average metal thickness down to 0.05 nm the same silicon film will need almost 10 hours to be totally crystallized. Using a new approach on the modelling procedure of the SE data we show to be possible to determine the Ni remaining inside the crystallized films. The method consists in using Ni as reference on the Bruggeman Effective Medium Approximation (BEMA) layer that will simulated the optical response of the crystallized silicon. Silicon samples and metal layers with different thicknesses were analyzed and this new method has shown to be sensible to changes on the initial metal/silicon ratio. The nickel distribution inside the silicon layers was independently measured by Rutherford Backscattering Spectroscopy (RBS) to check the data obtained from the proposed approach. © 2006 Materials Research Society.

In this work, metal induced crystallization using nickel was employed to obtain polycrystalline silicon by crystallization of amorphous films for thin film transistor applications. The devices were produced through only one lithographic process with a bottom gate configuration using a new gate dielectric consisting of a multi-layer of aluminum oxide/titanium oxide produced by atomic layer deposition. The best results were obtained for TFTs with the active layer of poly-Si crystallized for 20 h at 500 °C using a nickel layer of 0.5 nm where the effective mobility is 45.5 cm2 V-1 s-1. The threshold voltage, the on/off current ratio and the sub-threshold voltage are, respectively, 11.9 V, 5.55×104 and 2.49 V/dec. © 2005 Elsevier B.V. All rights reserved.

In this work metal induced crystallization (MIC) using nickel (Ni) was employed to obtain poly-Si by crystallization of amorphous films for application as active layer in TFTs. Ni layers with thicknesses of 0.5 nm, 1 nm and 2 nm were used to crystallize the silicon. The TFTs were produced with a bottom gate configuration using a multi-layer Al2O3/TiO2 insulator produced by atomic layer deposition (ALD) as gate dielectric. The best performances of the TFT produced were obtained when using very thin Ni layers for the crystallization. This is attributed to a lower metal contamination and to the enhancement of grain size, as a result of the lower nucleation density achieved, when using the thinnest Ni layer. Devices that exhibit effective mobility of 45.5 cm2V-1s-1 and an on/off ratio of 5.55×104 were produced using a 0.5 nm Ni layer to crystallize the active channel area.

In this work, the optical and structural properties of high k materials such as tantalum oxide and titanium oxide were studied by spectroscopic ellipsometry, where a Tauc-Lorentz dispersion model based in one (amorphous films) or two oscillators (microcrystalline films) was used. The samples were deposited at room temperature by radio frequency magnetron sputtering and then annealed at temperatures from 100 to 500 °C. Concerning the tantalum oxide films, the increase of the annealing temperature, up to 500 °C does not change the amorphous nature of the films, increasing, however, their density. The same does not happen with the titanium oxide films that are microcrystalline, even when deposited at room temperature. Data concerning the use of a four-layer model based on one and two Tauc-Lorentz dispersions is also discussed, emphasizing its use for the detection of an amorphous incubation layer, normally present on microcrystalline films grown by sputtering. © 2006 Elsevier B.V. All rights reserved.

In this work the electrical and structural properties of two high k materials as hafnium oxide (HfO2) and tantalum oxide (Ta2O5) produced at room temperature are exploited. Aiming low temperature processing two techniques were employed: r.f. sputtering and electron beam evaporation. The sputtered HfO2 films present a nanocrystalline structure when deposited at room temperature. The same does not happen for the evaporated films, which are essentially amorphous. The density and the electrical performance of both sputtered and evaporated films are improved after annealing them at 200 °C. On the other hand, the Ta2O5 samples deposited at room temperature are always amorphous, independently of the technique used. The density and electrical performance are not so sensitive to the annealing process. The set of data obtained show that these dielectrics processed at temperatures below 200 °C present promising properties aiming to produce devices at low temperature with improved interface properties and reduced leakage currents. © 2007 Elsevier B.V. All rights reserved.

In this work we have focused our attention on the role of the gas mixture (O2/Ar) used during HfO2 thin film processing by r.f. magnetron sputtering, to produce dielectrics with suitable characteristics to be used as gate dielectric. Increasing the O2/Ar ratio from 0 to 0.2, the films properties (optical gap, permittivity, resistivity and compactness) are improved. At these conditions, films with a band gap around 5 eV were produced, indicating a good stoichiometry. Also the flat band voltage has a reduction of almost three times indicating also a reduction of the same order on the fixed charge density at the semiconductor-insulator interface. The dielectric constant is around 16 which is very good, since the surface of the silicon where the HfO2 films were deposited contains a SiO 2 layer of about 3 nm that gives an effective dielectric constant above 20, close to the HfO2 stoichiometric value (∼25). Further increase on the O2/Ar ratio does not produce significant improvements. © 2004 Elsevier B.V. All rights reserved.

The aim of this work is to present a spectroscopic ellipsometry study focused on the annealing time effect on nickel metal induced crystallization of amorphous silicon thin films. For this purpose silicon layers with 80 and 125 nm were used on the top of which a 0.5 nm Ni thick layer was deposited. The ellipsometry simulation using a Bruggemann Effective Medium Approximation shows that films with 80 nm reach a crystalline fraction of 72% after 1 h annealing, appearing to be full crystallized after 2 h. No significant structural improvement is detected for longer annealing times. On the 125 nm samples the crystalline volume fraction after 1 h is only around 7%, requiring 5 h to get a similar crystalline fraction than the one achieved with the thinner film. This means that the time required for full crystallization will be strongly determined by the Si layer thickness. Using a new fitting approach the Ni content within the films was also determined by SE and related to the silicon film thickness. © 2006 Elsevier B.V. All rights reserved.

In this paper a new approach is presented for the simulation of spectroscopic ellipsometry (SE) data to estimate the level of nickel (Ni) contamination in silicon crystallized by metal induced crystallization (MIC). The method employs the addition of Ni as reference for a Bruggemann effective medium approximation (BEMA) to simulate the optical response of the crystallized silicon. This new approach is sensitive to changes in the initial average metal thickness used on the crystallization process to thickness values as low as 0.05 nm. This corresponds to a volume fraction of 0.24%, confirmed by Rutherford backscattering spectrometry (RBS) where it was observed that the Ni volume fraction detected by SE varies linearly with the metal amount inside the crystallized films determined by RBS. © 2008.

In this work hafnium oxide (HfO2) was deposited by r.f. magnetron sputtering at room temperature and then annealed at 200 °C in forming gas (N2+H2) and oxygen atmospheres, respectively for 2, 5 and 10 h. After 2 h annealing in forming gas an improvement in the interface properties occurs with the associated flat band voltage changing from -2.23 to -1.28 V. This means a reduction in the oxide charge density from 1.33×1012 to 7.62×1011 cm-2. After 5 h annealing only the dielectric constant improves due to densification of the film. Finally, after 10 h annealing we notice a degradation of the electrical film's properties, with the flat band voltage and fixed charge density being -2.96 V and 1.64×1012 cm-2, respectively. Besides that, the leakage current also increases due to crystallization. On the other hand, by depositing the films at 200 °C or annealing it in an oxidizing atmosphere no improvements are observed when comparing these data to the ones obtained by annealing the films in forming gas. Here the flat band voltage is more negative and the hysteresis on the C-V plot is larger than the one recorded on films annealed in forming gas, meaning a degradation of the interfacial properties. © 2006 Elsevier Ltd. All rights reserved.

Hafnium oxide-aluminum oxide (HfAlO) dielectric films were cosputtered using HfO2 and Al2 O3 targets, and their properties are studied in comparison with pure HfO2 films. The X-ray diffraction studies confirmed that the HfO2 films are nanocrystalline with a monoclinic phase. The as-deposited HfAlO films with a chemical composition of (HfO2) 0.86 (Al2 O3) 0.14 are amorphous even after annealing at 500°C. Further, the cosputtered films show a slight reduction in leakage current. The leakage current density may be significantly reduced below 3× 10-10 A cm-2 at an electric field of 0.25 MV/cm when applying the proper radio-frequency bias to the substrate. © 2009 The Electrochemical Society.

Here we present for the first time a TiO2/Cu2O all-oxide heterojunction solar cell entirely produced by spray pyrolysis onto fluorine doped tin oxide (FTO) covered glass substrates, using silver as a back contact. A combinatorial approach was chosen to investigate the impact of the TiO2 window layer and the Cu2O light absorber thicknesses. We observe an open circuit voltage up to 350 mV and a short circuit current density which is strongly dependent of the Cu2O thickness, reaching a maximum of  0.4 mA/cm2. Optical investigation reveals that a thickness of 300 nm spray pyrolysis deposited Cu2O is sufficient to absorb most photons with an energy above the symmetry allowed optical transition of 2.5 eV, indicating that the low current densities are caused by strong recombination in the absorber that consists of small Cu2O grains. © 2014 Elsevier Ltd. All rights reserved.

Shape memory alloys (SMA) represents a class of metallic materials that has the capability of recovering a previously defined initial shape when subject to an adequate thermomechanical treatment. The present work aims to study the influence of thermal cycles on the transition temperatures of a Ni-Ti alloy. In this system, small variations around the equiatomic composition give rise to significant transformation temperature variations ranging from 173 to 373 K. SMA usually presents the shape memory effect after an annealing treatment at ca. 973 K. The optimisation of the thermomechanical treatment will allow to "tune" the material to different transformation temperature ranges from the same starting material, just by changing the processing conditions. Differential scanning calorimeter (DSC) and in situ high-temperature X-ray diffraction (XRD) have been used to identify the transformation temperatures and the phases that are present after different thermal cycles. The results concerning a series of thermal cycles with different heating and cooling rates (from 1.67×10-2 to 1.25×10-1 K/s) and different holding temperatures (from 473 to 1033 K) are presented. © 2004 Elsevier B.V. All rights reserved.

High visible to near infrared (NIR) transparent Mo (0-1 at%) doped In2O3 (IMO) thin films with high carrier mobility were deposited on Corning-1737 glass substrates at 400 °C by spray pyrolysis experimental technique. The films were annealed in vacuum (∼1×10-4 mbar) at 550 °C for 45 min. XRD analysis confirmed that indium oxide belongs to cubic bixbyite structure. The preferred growth orientation along (2 2 2) plane for low Mo doping level shifts to (4 0 0) for higher Mo doping levels. Crystallite sizes extracted from the XRD data corroborate the changes in full-width at half-maximum due to the variation in Mo doping. Scanning electron microscopy study illustrates the evolution in surface microstructures as a function of Mo doping. The negative sign of Hall coefficient confirmed n-type conductivity. Films with high mobility of ∼149 cm2/(V s), carrier concentration of ∼1.0×1020 cm-3, resistivity of ∼4.0×10-4 Ω cm and high figure of merit of ∼1.02×10-2 Ω-1 were observed for post-annealed films (0.5 at% Mo). The obtained high average transparency of ∼83% in the wavelength range 400-2500 nm confirms that transmittance is well extended into the NIR region. © 2009 Elsevier B.V. All rights reserved.

Molybdenum doped indium oxide (IMO) thin films were deposited on the glass substrates preheated to 450 °C by spray pyrolysis technique. The Mo doping was varied between 0 and 2.0 at.%. The films were characterized by their structural, electrical and optical properties. The films are confirmed to be cubic bixbyite In2O3 with a strongest orientation along (222) plane, which is shifted to (400) plane for the increase in Mo doping to 1.25 and 2 at.%. The film deposited with 0.5 at.% Mo doping shows high mobility of 76.9 cm2V- 1s- 1 , resistivity of 1.8 × 10- 3 Ω-cm and high carrier concentration of 4.6 × 1019 cm- 3 with 81.3% transmittance in the visible range between 500 and 800 nm. Further, the transparency extents well into the near-IR range. © 2008 Elsevier B.V. All rights reserved.

Molybdenum (0-1 at. %) doped indium oxide thin films with high near-infrared (NIR) transparency and high carrier mobility were deposited on Corning-1737 glass substrates at 400 °C by a spray pyrolysis experimental technique. X-ray diffraction (XRD) analysis confirmed the cubic bixbyite structure of indium oxide. The preferred growth orientation along the (222) plane for the low Mo doping level (0.5 at. %) shifts to (400) for higher Mo doping levels (<0.6 at. %). The crystallite size extracted from the XRD data corroborates the changes in full width at half maximum due to the variation in Mo doping. A scanning electron microscopy study illustrated the evolution in the surface microstructure as a function of Mo doping. The negative sign of the Hall coefficient confirmed the n -type conductivity. A high carrier mobility of ∼122.4 cm2 /V s, a carrier concentration of ∼9.5× 1019 cm-3, a resistivity of ∼5.3× 10-4cm, and a high figure of merit of ∼4.2× 10-2 -1 are observed for the films deposited with 0.5 at. % Mo. The obtained high average transparency of ∼83% in the wavelengths ranging from 400 to 2500 nm confirmed the extension of transmittance well into the NIR region. © 2009 American Institute of Physics.

Molybdenum-doped indium oxide (IMO) thin films were deposited at 450 °C for varying molybdenum concentrations in the range of 0.5-2 at% by the spray pyrolysis technique. These films confirmed the cubic bixbyite structure of polycrystalline In2O3. The preferred growth orientation along the (2 2 2) plane shifts to (4 0 0) on higher Mo doping levels. The films doped with 0.5 at% Mo showed high mobility of 76.9 cm2/(V s). The high visible transmittance extends well into the near-infrared region. A possibility of using the produced IMO films in nanocrystalline (nc) silicon solar cell applications is discussed in this article. The morphological studies showed a change in the microstructure, which is consistent with the change in crystallographic orientation. © 2008 Elsevier B.V. All rights reserved.

In this work, we report results concerning the performances of thin-film transistors (TFTs) where the channel layer is based on microcrystalline zinc tin oxide (ZTO) processed by spray pyrolysis technique. TFTs made with 30 nm thick ZTO channel layer deposited at a substrate temperature of 400 C and 300 Cexhibited, respectively, a saturation mobility of 2.9 cm V s and 1.45 cm V s ; voltage of 0.15 V, and 0.2 V; a sub-threshold swing of 400 mV/dec and 500 mV/dec; ON/OFF ratio at the onset of hard saturation current of 3.5 10 and 6 10 , for a drain to source voltage of 10 V (close to or below the gate to source voltage). This indicates that the substrate temperature is relevant in determining the devices' electronic performances. © 2013 IEEE.

The high visible-near infrared transparent and high carrier mobility (μ) Mo doped (0.5 at%) indium oxide (IMO) films were deposited by the spray pyrolysis technique. The deposited films were irradiated by 50MeV Li 3+ ions with different fluences of 1×1011, 1×1012 and 1×1013 ions cm-2. X-ray diffraction analysis confirmed the cubic bixbyite structure of indium oxide. A fascinating feature is that the ion irradiation process has introduced a fraction of the molybdenum oxide phase. The μ of as-deposited IMO films is decreased from ̃122.4 to 93.3 cm2 V-1 s-1, following the ion irradiation. The theoretically calculated μ and carrier density values were correlated with those measured experimentally. The transport mechanism has been analysed based on the ionized and neutral impurity scattering centres. The average transmittance (400-2500 nm) of the as-deposited IMO films is decreased from 83% to 60% following irradiation. © 2011 IOP Publishing Ltd.

This paper deals with high transparent and high conductive oxides based on polycrystalline titanium (Ti) doped (0.5-3 at.%) indium oxide (IO) thin films produced on glass substrates at 400 °C by spray pyrolysis technique. X-ray diffraction analysis confirmed the cubic bixbyite structure of indium oxide. A high mobility of ∼ 97 cm2 V- 1 s- 1, a carrier concentration of ∼ 1.55 × 1020 cm- 3 and a resistivity of ∼ 4.11 × 10- 4 Ω-cm with ∼ 83% of transmittance in the wavelength ranging between 400 and 2500 nm were obtained for 2 at.% Ti doping films, rivalling so to the best known transparent conducting oxide based on indium tin oxide. Moreover, the transmittance in the broad wavelength ranging between 400 and 2500 nm is over 83%, leading so to an increasing carrier generation towards the near infrared region of the spectrum, as required for applications such as solar cells. We also notice that increasing the doping concentration widened the optical band gap and caused a small Burstein-Moss shift, due to mobility decrease, as expected. © 2012 Published by Elsevier B.V.

Molybdenum doped indium oxide (IO) thin films were deposited on the Coring F1737 glass substrates at 400 °C by spray pyrolysis technique. TheModoping was varied between 0 and 4 at.%. The films were characterized by their structural, electrical and optical properties. The films are confirmed to be cubic bixbyite In 2O 3 with a strongest orientation along (222) for 0.5 at.% Mo, which is shifted to (400) plane when the Mo doping is increased to ≥1.2 at.%. The films deposited with 0.5 at.% Mo showed high mobility of ̃90 cm 2/Vs, resistivity of ̃6.8×10 -4ωcm and carrier concentration of ̃1.01× 1020 cm -3 with >̃73% transmittance in the visible range between 500 and 800 nm. The transmittance is well extended into near infrared region.

Molybdenum doped (0-1 at. %) indium oxide thin films with high near infrared (NIR) transparency and carrier mobility were deposited on Corning-1737 glass substrates at 400 °C by spray pyrolysis experimental technique. Films with mobility as high as ∼149 cm2 /V s were obtained when annealed in vacuum at 550 °C, which also possess carrier concentration of ∼1× 1020 cm-3 and resistivity as low as ∼4.0× 10-4 cm. Further, both the average visible transmittance (500-800 nm) and the average NIR transmittance are >83%. This clearly shows that the transmittance is extended well into the NIR region. © 2009 American Institute of Physics.

The charge transfer dynamics at interfaces are fundamental to know the mechanism of photovoltaic processes. The internal potential in solar cell devices depends on the basic processes of photovoltaic effect such as charge carrier generation, separation, transport, recombination, etc. Here we report the direct observation of the surface potential depth profile over the cross-section of the ZnO nanorods/Cu2O based solar cell for two different layer thicknesses at different wavelengths of light using Kelvin probe force microscopy. The topography and phase images across the cross-section of the solar cell are also observed, where the interfaces are well-defined on the nanoscale. The potential profiling results demonstrate that under white light illumination, the photoinduced electrons in Cu2O inject into ZnO due to the interfacial electric field, which results in the large difference in surface potential between two active layers. However, under a single wavelength illumination, the charge carrier generation, separation, and transport processes between two active layers are limited, which affect the surface potential images and corresponding potential depth profile. Because of changes in the active layer thicknesses, small variations have been observed in the charge carrier transport mechanism inside the device. These results provide the clear idea about the charge carrier distribution inside the solar cell in different conditions and show the perfect illumination condition for large carrier transport in a high performance solar cell. © 2016 American Chemical Society.

{A nonvacuum and low temperature process for passivating transparent metal oxides based thin-film transistors is presented. This process uses the epoxy-based SU-8 resist which prevents device degradation against environmental conditions, vacuum or sputtering surface damage. The incorporation of SU-8 as a passivation layer is based on the ability of this polymer to provide features with high mechanical and chemical stability. With this approach, lithography is performed to pattern the resist over the active area of the device in order to form the passivation layer. The resulting transistors demonstrate very good electrical characteristics, such as μFE =61 cm2 /V s