Rodrigo Martins

Diphasic silicon films (nc-Si/a-Si:H) have been prepared by a new regime of plasma enhanced chemical vapour deposition in the region adjacent of phase transition from amorphous to microcrystalline state. Comparing to the conventional amorphous silicon (a-Si:H), the nc-Si/a-Si:H has higher photoconductivity (σph), better stability, and a broader light spectral response range in the longer wavelength range. It can be found from Raman spectra that there is a notable improvement in the medium range order. The blue shift for the stretching mode and red shift for the wagging mode in the IR spectra also show the variation of the microstructure. By using this kind of film as intrinsic layer, a p-i-n junction solar cell was prepared with the initial efficiency of 8.51% and a stabilized efficiency of 8.01% (AM1.5, 100 mw/cm2) at room temperature. © 2006.

Thin films of molybdenum doped indium oxide (IMO) were rf sputtered onto glass substrates at room temperature. The films were studied as a function of oxygen volume percentage (OVP) ranging 1.4 - 10.0% in the sputtering chamber. The thickness of the films found varying between 180 and 260 nm. The X-ray diffraction pattern showed the films are polycrystalline with the peaks corresponding to (222) and (400) planes and one among them showing as a preferential orientation. It is observed that the preferred orientation changes from (222) plane to (400) as the OVP increases from 1.4 to 10.0%. The transmittance spectra were found to be in the range of 77 to 89%. The optical band gap calculated from the absorption coefficient of transmittance spectra was around 3.9 eV. The negative sign of Hall coefficient confirmed the films were n-type conducting. The bulk resistivity increased from 2.26×10 -3 to 4.08×-1 Ωcm for the increase in OVP from 1.4 to 4.1%, and thereafter increased dramatically so as the Hall coefficients were not detectable. From the AFM morphologies it is evaluated that the RMS roughness of the films ranges from 0.9 to 3.2 nm. © 2006 Materials Research Society.

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.

Electrochromic materials have attracted considerable attention during the last two decades as a consequence of their potential application in several different types of optical devices. Examples of these devices include intelligent windows and time labels. In this paper the authors describe results obtained with thin tungsten oxide films produced at room temperature by rf magnetron sputtering under an argon and oxygen atmosphere on transparent conductive oxide coated glass substrates. To protect the surface of the electrochromic film, prevent water absorption and obtain a good memory effect under open circuit voltages, a layer of Ta2O5 was deposited over the WO3 films. In this study, the effect of different electrolyte compositions on the open circuit memory of optical devices has been characterized. The best results were obtained for electrochromic devices with polymer gel p(TMC)3LiC1O4 and p(TMC)8LiClO 4 electrolytes. These prototype devices present an overall transmittance of ∼75% in their bleached state and after coloration 40.5 and 52.5% respectively. These devices also show memory effect and an optical density considered satisfactory for some electrochromic applications.

A series of amorphous silicon carbide films were prepared by plasma enhanced chemical vapor deposition technique on (100) silicon wafers by using methane, silane, and hydrogen as reactive resources. A very thin (around 15 Å) gold film was evaporated on the half area of the a-SiC:H films to investigate the metal induced crystallization effect. Then the a-SiC:H films were annealed at 1100°C for 1 hour in the nitrogen atmosphere. Fourier transform infrared spectroscopy (FTIR), X-Ray diffraction (XRD), and scanning electron microscopy (SEM) were employed to analyze the microstructure, composition and surface morphology of the films. The influences of the high temperature annealing on the microstructure of a-SiC:H film and the metal induced metallization were investigated.

Nanostructured silicon films were deposited on the amorphous to microcrystalline transition region by plasma enhanced chemical vapor deposition, using an rf frequency of 27.12 MHz. Micro-Raman spectroscopy data show that in the transition region the peaks typically associated with amorphous silicon are slightly shifted towards higher wavenumber and become narrow, which could be explained by the short range order improvement or by the incorporation of very small Si nanocrystallites. The hydrogen evolution spectra from samples deposited in this region show two peaks, one at low temperature (LT) and the other at high temperature (HT), around 698 K and 840 K, respectively. These peaks represent activation energies of 87 (LT) and 135 (HT) kJ/mol, respectively, as deduced from the so-called Kissinger's method. The solar cells fabricated using i-layers produced in this transition region show good performances, with current density = 14.96 mA/cm2, short circuit voltage = 0.95 V, and fill factor = 0.67, which leads to efficiencies of 9.52%. © 2006 Elsevier B.V. All rights reserved.

This work deals with the study of nanostructured/amorphous silicon solar cell deposited by plasma enhanced chemical vapor deposition at 27.12 MHz by impedance spectroscopy. The solar cell studied present fill factor of 0.67, open circuit voltage of 0.94 V and short-circuit current density of 14.48 mA/cm2, which leads to the efficiency of 9.12%. The impedance spectroscopy analysis was performed under dark and illumination conditions. The data obtained were used to define an electrical equivalent circuit model able to explain the role of the different solar cell components, including the interfaces, on the solar cell performance. © 2006 Elsevier B.V. All rights reserved.

The purpose of this work is to present in-depth electrical characterization on transparent TFTs, using zinc oxide produced at room temperature as the semiconductor material. Some of the studied aspects were the relation between the output conductance in the post-pinch-off regime and width-to-length ratios, the gate leakage current, the semiconductor/insulator interface traps density and its relation with threshold voltage. The main point of the analysis was focused on channel mobility. Values extracted using different methodologies, like effective, saturation and average mobility, are presented and discussed regarding their significance and validity. The evolution of the different types of mobility with the applied gate voltage was investigated and the obtained results are somehow in disagreement with the typical behavior found on classical silicon based MOSFETs, which is mainly attributed to the completely different structures of the semiconductor materials used in the two situations: while in MOSFETS we have monocrystalline silicon, our transparent TFTs use poly/nanocrystalline zinc oxide with grain sizes of about 10 nm.

High-quality SrBi2Nb2O9 (SBN) single crystals were grown from a melt using a high-temperature self-flux solution method and Bi2O3 added with B2O3 as a flux. A suitable thermal profile involving slow cooling rates allowed growing large and translucent SBN crystals exhibiting platelet morphology with typical size ∼5 × 5 mm2 and thickness approximately 400 μm. X-ray diffraction revealed a dominant (001)-orientation of the major face of the platelet crystals and edges oriented parallel to the [110] directions. The dielectric properties were evaluated along the ab-plane and in the c-axis direction. The ferro-paraelectric phase transition was observed at TC = 440°C with Curie-Weiss relationship above TC. The anisotropy of dielectric permittivity, i.e., the ratio between permittivity in the ab-plane and along c-axis was about 10 at TC-The obtained results are used to discuss the observed correlations between anisotropy, crystalline orientation, and electrical properties.

This work deals with the study of the role of the buffer layers thickness on the TCO/p-a-SiC:H/buffer1/buffer2/i(nc-Si/a-Si:H)/n-a- Si:H/Al solar cell I-V and impedance performances. The aim was to improve the p/i interface region, which has a large influence on the solar cell characteristics and stability. In order to match the difference between the p and i layers optical gaps, the buffer layers were deposited using, for each layer, different methane to silane mixtures, aiming to obtain a gradual match of the corresponding optical gaps. The intrinsic layer was deposited at high hydrogen dilution rates at 27.12 MHz in conditions that allowed the incorporation of nanoparticles/nanoclusters. Solar cells with fill factor of 0.63; open circuit voltage of 0.93 Volts; short circuit current density of 16.13 mA/cm2 and an efficiency of 9.4% were produced with buffer layers around 1.3 nm thick. When comparing these solar cells with conventional amorphous silicon solar cells we notice that the quantum efficiency from ultraviolet to green regions is improved up to 13%, in average. Concerning solar cell capacitance, the data show that the best solar cells exhibit the highest capacitance, meaning that the films are compact and dense, in-line with the other electrical characteristics obtained. ©2005 IEEE.

This work reports on the fabrication process and performances presented by metal insulator semiconductor (MIS) linear position sensitive detectors (PSD) with an active length of 6 cm. The use of long sensitive areas allows the PSD to achieve higher resolution without the need of a highly accurate light spot integration mechanism. The PSD is built in a multilayered structure consisting of Cr/a-Si:H (n+ doped)/a-Si:H (intrinsic)/SiOx (passivation layer)/Au, where the active a-Si:H layers were deposited by a modified triode plasma enhanced chemical vapour deposition (MTPECVD), which allows the deposition of highly electronic grade material with a low (≈ × 1015 cm-3) defect density inferred by CPM. The sensor linearity and sensitivity shows dependence on the sensor width to length ratio and on the value of load resistance. Sensitivities of more than 30 mV/cm were achieved with linearity near 99%. Besides that, this type of MIS structure allows an improved spectral response near the UV region and has the maximum response at 540 nm. © 2004 Published by Elsevier B.V.

The aim of this work is to present a process able to allow a fast method to clean plasma enhanced chemical vapour deposition (PECVD) systems used to produce amorphous silicon films and their alloys, and a proper device patterning when required. In this work we propose to study CF4/O2 or SF6 as etchant gases at room temperature to perform cleaning and films patterning. The aim is to select the process that leads to a faster cleaning process without formation of residual contaminants or to anisotropic patterning of very thin layers. The influence of some plasma parameters, such as pressure (p), power (P) and flow (f) for the etchant gases used will be also analysed.

In this work we report the study of spectral response on large area amorphous silicon solar cells (30×40 cm2), deposited through plasma enhanced chemical vapour deposition technique (PECVD) at excitation frequencies of 27.12MHz. To perform this work, the solar cells were split in units of area of 0.126 cm2, which allows determining the device homogeneity over all the entire solar cell. Emphasis of this work is put the role of thickness and optical band gap of p-doped layer on the collection efficiency, spectral response, current density-voltage curves under standard condition and spectroscopy impedance. The results show that high transparent p-doped layer can be deposited at 42mW/cm2, which allows increasing the collection efficiency in 45%, at the blue region. The spectroscopy impedance performed showed to be efficient in analyzing the device shunt resistance, interfaces role on the device performances and the behaviour of the device depletion region, for the range of frequencies analysed.

Silicon nitride films were produced on glass and crystalline silicon substrates using r.f. magnetron sputtering to select the best process conditions (substrate temperature, gas pressure and r.f. power) to grow dielectrics for device applications such as low temperature thin film transistors, where special care has to be taken concerning the film's compactness and bulk defects. The films produced were analysed by different techniques such as ultra violet - visible - near infrared spectroscopy Fourier transformed infrared spectroscopy and capacitance measurements, aiming to correlate the films properties with its composition and degree of compactness. The role of the deposition pressure is notorious since films deposited at high pressures are more compact, presenting low oxygen incorporation after deposition. The increase of the substrate temperature up to 373 °K has the same effect, not changing the film's amorphous structure. These data will be discussed aiming to produce films with the required compactness and stoichiometry to grow very thin insulating layers (<10 nm) to be used in MIS structures or devices like thin film transistors.

In this work, spectroscopic ellipsometry was used to characterise oxide films produced by anodic oxidation of amorphous silicon using an ethylene glycol (0.04 M KNO3) solution. The data obtained show that the growth of the oxide is not only a function of the voltage applied, but also of the current density and of the time process. An empiric model based on a power law is proposed for the growth of the oxide using, as parameters, the voltages and the time process. The oxide produced shows porosity of approximately 12%, which can be reduced down to 6% under well-controlled growth conditions. © 2002 Elsevier Science B.V. All rights reserved.

In this paper, we present the preliminary results concerning the deposition of highly transparent and conductive gallium-doped zinc oxide (GZO) deposited on transparent flexible substrate based on cellulose derivatives. Prior to the deposition of the GZO film, the surface of the polymer have been coated with a thin silicon dioxide (SiO2) layer deposited by thermal evaporation assisted by an electron gun. By doing this surface treatment, we succeeded in depositing highly conductive and transparent GZO with an electrical resistivity of 2.0 × 10-3 Ω cm and an average optical transmittance in the visible part of the spectrum (400-700 nm) of 70% by r.f. magnetron sputtering at room temperature. Besides the optoelectronic properties, the films are mechanically stable with a polycrystalline structure with a strong preferred (002) orientation, parallel to the substrate. © 2003 Elsevier B.V. All rights reserved.

This paper deals with the growth process of nanostructured silicon films produced by chemical vapour deposition technique, at or close to the γ-regime where powders are formed. There, besides the set of chemical reactions undertaken by the species decomposed on the growth surface, the importance of the physics of the plasma in managing the powders and on the final film performances will be shown. To identify the plasma region where Si nanoaggregates are formed, we propose the use of a new parameter that translates the energy coupling of the rf power to the species of the gas flow, per pressure range of the process. By doing so we could establish an excellent correlation between this ratio and the plasma parameters such as peak to peak rf voltage and plasma impedance, or with the films defect density and their transport properties. Apart from that, we also show that high compact Si nanoclusters could be grown under moderate ion bombardment. Finally, to allow the growth at high rates of controlled silicon nanostructures, a three cycling process based on hot wire chemical vapour deposition and plasma assisting the hot wire technique will be discussed. © 2002 Elsevier Science Ltd. All rights reserved.

Currently, microactuators are being developed using shape memory alloys (SMAs), which allow simple design geometries and provide large work outputs in restricted space. Several techniques have been used to produce NiTi shape memory alloy thin films, but from the practical point of view, only the sputter deposition method has succeeded so far. Vacuum evaporation of NiTi binary alloy entails the potential problem of the evaporation rates of each component not being the same due to differences in vapour pressure. Aiming to study the possible applications of SMAs to microfabrication, NiTi thin films were produced at CENIMAT by sputter and vacuum evaporation using raw materials from different sources. The films were analysed by differential scanning calorimetry (DSC) and X-ray diffraction (XRD) at room temperature, as well as in situ high temperature, in order to characterise the temperature ranges at which the different structural transformations occur. © 2002 Elsevier Science B.V. All rights reserved.

In this work, we present a study of the effect of temperature, type and concentration of the dopant on the structural characteristics of ZnO thin films produced by spray pyrolysis; the crystallite size has been determined from profile peak shape analysis. These results are compared to the electrical characterisation performed on these materials. The effect of the dopant on the properties of ZnO thin films depends on its characteristics, mainly its ionic radius. Al, Ga and In have been studied as dopants, the best one being In, since it leads to the lowest resistivity.

Hall effect measurements are one of the most powerful techniques for obtaining information about the conduction mechanism in polycrystalline semiconductor materials, which is the basis for understanding semiconductor gas sensors. In order to investigate the correlation between the microscopic characteristics and the macroscopic performances exhibited by undoped tin oxide gas sensors deposited by spray pyrolysis, Hall effect measurements were performed at different temperatures, from room temperature up to 500 K, and in the presence of two different atmospheres, air and methane. From these measurements, it was possible to infer the potential barrier and its dependence with the used atmosphere. The obtained results were analysed in terms of the oxygen mechanism at grain boundaries on the basis of the grain boundary-trapping model. In the presence of methane gas, the electrical resistivity decreases due to the lowering of the inter-grain boundary barrier height.

The surfaces of cordierite and glass particles were modified by coating them with an alumina precursor using a precipitation process in the presence of urea. Scanning electron microscopy (SEM), high-resolution transmission electron microscopy, X-ray diffraction, electrophoresis, and rheological measurements were used to characterize the coated powders. SEM and transmission electron microscopy morphologies of the coated powders revealed that amorphous and homogeneous coatings have been formed around the particles. The morphology of the coated powders showed a coiled wormlike surface. The coating Al2O3 layer dominated the surface properties of the coated glass and cordierite powders. The influence of the coating layer on the processing ability of cordierite-based glass-ceramics substrates by tape casting was studied in aqueous media. It could be concluded that the coating of the powders facilitates the processing and yields green and sintered tapes with denser, more homogeneous microstructures compared with the uncoated powders.

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.

This work deals with the study of the role of intra-gap density of states and films composition on the colour selection of the collection spectrum of glass/ITO/a-Six:C1-x:H/Al Schottky photodiodes produced in a conventional plasma-enhanced chemical vapour deposition (PECVD) system using as gas sources silane and a controlled mixtures of silane and methane. To do so, properties of the films were investigated, especially the one concerning the determination of the valence controllability of the films produced and the density of bulk states. Besides that, a PINIP device was also produced, using the a-Six:C1-x:layer that lead to the best Schottky diode performances. © 2001 Elsevier Science B.V. All rights reserved.

The process conditions of growing thin silicon films by plasma enhanced chemical vapour deposition (PECVD) were presented. The plasma impedance was found to monitor the powders in the PECVD systems and good quality silicon films were grown close to the plasma regime where the powders were formed. The silicon films exhibited properties which were interpreted based on a two-phase model where silicon nanostructures were embedded in a disordered network.

In this work we present a study performed in a plasma-enhanced chemical vapour deposition reactor, where different rf electrode configurations were used with the aim of achieving conditions that lead to growth of highly uniform amorphous silicon films, with the required electronic quality, at high growth rates. This study consists in determining the plasma characteristics under different electrode configurations, in an argon plasma, using as diagnostic tools a Langmuir probe and impedance probe. These results were correlated with the hydrogenated amorphous silicon films produced, thereby allowing us to establish the best electrode configuration to grow electronic-grade-quality amorphous silicon films.