G. Viera

Mechanical properties of nanometric structures of Si/SiC, C/SiC and C/SiN produced by PECVD

Abstract Nanometric multilayered structures were deposited by PECVD. Deposition on the cathode resulted in amorphous multilayers (a-C/a-SiC/…/a-SiC and a-C/a-SiN/…/a-SiN), whereas nanostructured multilayers (ns-Si/ns-SiC/…/ns-Si) were obtained at the anode. Previous studies on multilayer structures of ns-Si/ns-SiC deposited at room temperature on c-Si wafers by modulated PECVD revealed the mechanical and wear characteristics of these structures, which showed improved adherence to the substrate and blocking of the cracks induced by nanoindentation. Another characteristic of these structures was the absence of oxygen in the SiC layers after exposure to the atmosphere. In the present study, the mechanical characterization of the nanometric multilayer ns-Si/ns-SiC structures, after annealing under an inert atmosphere, has shown an increase in hardness due to: (a) material densification, with an increase in density after dehydrogenation; and (b) crystallization of the layers. Although the films were deposited at low temperature, the need to anneal them to improve their mechanical properties requires the use of temperature-resistant substrates. To avoid the need for post-thermal treatments, we have chosen the deposition of nanometric multilayer structures of a-C/a-SiC and a-C/a-SiN (5–10 nm/layer) using PECVD at room temperature and depositing them on the cathode (higher ion bombardment). The microstructure and morphology of the hybrid amorphous layers were examined by TEM. Hardness and Youngs modulus were measured by the nanoindentation technique. Wear properties were evaluated using a pin-on-disc system. The structures containing approximately 20 layers had better mechanical properties than the corresponding thick monolayers of their components. Their mechanical characteristics, along with their ability to block crack propagation and wear resistance, are useful for applications such as protective coatings for optical (fibers and lenses), electronic and magnetic devices.

Surface analysis of nanostructured ceramic coatings containing silicon carbide nanoparticles produced by plasma modulation chemical vapour deposition

Abstract Ceramic nanometric multilayer structures of nanostructured particles of SiCx:H layers and amorphous Si films were obtained by chemical vapour deposition using modulated rf plasma. This technology has been extensively used for producing ceramic Si-based nanoparticles (SiCxNy) with unique characteristics including spherical morphology, composition and controlled ultrafine particle size in the range 2–100 nm. Hybrid multilayer nanostructures of ceramic coatings containing Si and SiC were produced to study their structural, mechanical and surface properties. Low densities of crystalline nanoparticles were embedded in a-Si matrix during the growth of these structures and they were intercalated between amorphous Si layers. The phase structure, microstructure and morphology of the hybrid multilayered films were examined by transmission electron microscopy and selected area electron diffraction, which revealed the presence and distribution of the nanoparticles in the multilayered structure of the films. The hardness and Youngs modulus were measured by the nanoindentation technique, and the wear properties were evaluated using an improved pin-on-disc system. These results showed that the mechanical properties of the films (hardness, friction, propagation of cracks and wear resistance) were notably enhanced by the presence of the nanoparticles. Potential applications of these coatings based on ceramic multilayers include the production of tough and hard coatings, protective and wear-resistant coatings for mechanical tools, gears and mechanical parts, optical surfaces and fibres, corrosion and high temperature-resistant coatings, as well as inorganic membranes, buffer layers for heterogeneous coatings, and coatings with anisotropic properties.

Hard coatings for mechanical applications

Abstract Thin film hard coatings are produced by either intensive ion bombardment of surfaces during the deposition of hard materials or ion bombardment (primarily nitrogen) of metallic surfaces, under vacuum conditions. These techniques can be applied to mechanical tools, pieces subjected to wear and surfaces needing protective coatings such as optical recording devices and fibres. We describe an experiment combining physical and chemical vapour deposition to produce hard coatings for mechanical applications. The system comprises a hybrid plasma process combining reactive magnetron sputtering and ion beam bombardment from a capacitively coupled rf ion source in a vacuum chamber. To illustrate the results, a carbon target was used to deposit amorphous carbon (a-C) and amorphous carbon nitride (a-CN) thin films using a variable Ar and N 2 gas mixture. These were then compared with films produced by rf-plasma CVD. Single and multilayer structures of these materials were produced and characterized. The deposition techniques are compared in terms of film properties and their own parameters.

SiCN nanometric powder produced in square-wave modulated RF glow discharges

Abstract Square-wave modulation of the electrical power supplied to low pressure and temperature radio frequency (RF) plasmas has been revealed as a suitable method to produce nanometric powder of high purity and nanometric size (10–100 nm). Here, we present a study involving the production and characterisation of silicon carbon nitride (SiCN) nanometric powder, obtained from square-wave modulated RF plasmas of SiH4+CH4+NH3 and SiH4 + CH4 + N2 gas mixtures. Electron diffraction patterns and transmission electron micrographs showed that the SiCN powder particles are amorphous and have a size distribution in the range 25–45 nm. X-ray photoelectron spectroscopy, elemental analysis and Fourier-transform infrared spectroscopy showed important differences in the structure, hydrogenation, chemical composition and bond distribution (SiC, SiN) of the samples owing to the use of different gas mixture. This powder could be used as a raw material to produce advanced ceramics and composites with higher thermal and chemical stability and improved mechanical properties.

Enhancement of oxidation rate of a-Si nanoparticles during dehydrogenation

Oxidation of amorphous silicon (a-Si) nanoparticles grown by plasma-enhanced chemical vapor deposition were investigated. Their hydrogen content has a great influence on the oxidation rate at low temperature. When the mass gain is recorded during a heating ramp in dry air, an oxidation process at low temperature is identified with an onset around 250 °C. This temperature onset is similar to that of hydrogen desorption. It is shown that the oxygen uptake during this process almost equals the number of hydrogen atoms present in the nanoparticles. To explain this correlation, we propose that oxidation at low temperature is triggered by the process of hydrogen desorption.

Nanoparticles of Si–C–N from low temperature RF plasmas: selective size, composition and structure

Abstract Silane-based rf plasma CVD is used to produce nanostructured materials. Depending on the discharge conditions, either ultrafine powder of alloys (Si–C–N), with potential applications for advanced ceramics, or nanostructured thin films grown in the presence of nanoparticles, can be obtained. The present study deals with the synthesis of nanoparticles based on the Si–C–N system in plasmas of SiH4, CH4, NH3 and Ar. Ultrafine quasi-monodisperse powder particles of controlled mean size were produced in conditions of fast particle development. Before the onset of particle coagulation, non-agglomerated amorphous clusters smaller than 10 nm were obtained.

Deep profiles of lithium in electrolytic structures of ITO/WO3 for electrochromic applications

Lithium doped WO3 electrochromic films have been deposited by thermal reactive evaporation of LiClO4 and WO3 ultra-pure powder mixture for different concentrations of the former from a Joule effect source, using an oxygen partial pressure of 5 × 10−2 Pa. The substrate was a transparent conducting (20–25 Ω/) ITO film previously deposited on standard glass in the same run as the WO3 deposition, and it was kept at 20Q °C of temperature during the WO3 evaporation. Secondary ion mass spectrometry (SIMS) and X-ray photoelectron spectroscopy (XPS) analysis revealed intercalation of Li+ and Cl− ions on the WO3 layer. Chrono-amperometric measurements with simultaneous optical transmittance analysis were performed in order to evaluate the electrochromic colouring efficiency and also the role played by the Li+ cations and the Cl− anions in the electrochromic structures.

Optical emission spectroscopy of rf glow discharges of methane–silane mixtures

Abstract Optical emission spectroscopy (OES) measurements of radio frequency (rf) SiH 4 –CH 4 plasmas were carried out during the growth of a-Si 1−x C x :H thin films in order to correlate the plasma emission characteristics with the composition of the deposited thin films. The relative flow fraction of SiH 4 in the gas mixture was varied from 0.02 to 0.4 at three different values of rf power (30 W, 50 W and 70 W). Real time OES measurements were performed using a diode array-based optical multi-channel analyzer working in the visible range (300–800 nm). The main features of OES spectra corresponded to SiH * (414 nm) and CH * (430 nm) emissions whose intensities were strongly dependent on the gas feed mixture composition and on the rf power supplied to the discharge. The gas mixture composition dependence of the SiH * and CH * emission intensities where similar to those of [Si] and [C] contents in the films as measured by X-ray photoelectron spectroscopy. In particular, a linear relationship was found between the ratio of the SiH * to CH * emission intensities and the [Si]:[C] composition ratio in the films.

Nanopowder of silicon nitride produced in radio frequency modulated glow discharges from SiH4 and NH3

Abstract Square-wave modulated radio frequency (RF) plasmas have been shown to be a suitable source of nanometric size powder with high purity and controllable nanostructure. This paper reports the production of SiN nanopowder at room temperature and low pressure by RF glow discharge decomposition of SiH 4 and NH 3 gases. The RF power was modulated at 0.5 Hz with a duty cycle of 20% in order to control the size of the expelled particles. A study of the particle formation process using in-situ monitoring of the RF dissipated power is presented. Transmission electron microscopy showed a narrow size distribution of spherical particles from 25 to 45 nm and their electron diffraction provided evidence of a short range order in Si or SiN units depending on the precursor gas mixture. Analysis by Fourier transform infrared spectroscopy revealed the presence of SiN, NH and SiH bonds and the elemental analysis of the powder determined its chemical composition.

Effects of thermal and laser annealing on silicon carbide nanopowder produced in radio frequency glow discharge

Abstract Hydrogenated SiC nanoparticles of high purity have been produced in radio frequency (RF) glow discharges from silane and methane gas mixtures. These nanoparticles show high surface reactivity and can undergo spontaneous oxidation when exposed to the atmosphere. In order to analyze the chemical stability of SiC particles, we present a study of the structural, chemical and compositional modifications induced by thermal annealing at different temperatures (400, 600 and 800 °C). The post-treated powder was analyzed by RAMAN, FTIR and XPS spectroscopy. The effects of the thermal annealing of SiC powder include: change of colour from yellow to dark brown; hydrogen desorption; and an increase in CC and SiC bonds in detriment to SiSi bonds. In addition, annealing with a KrF excimer laser induced the presence of Si and SiC nanocrystallites.