Raman and XPS characterization of vanadium oxide thin films deposited by reactive RF sputtering

In this paper we report on Raman and XPS characterization of vanadium oxide thin films deposited by RF-sputtering. The samples were deposited by using a vanadium target in different oxygen fluxes, so that the stoichiometry (O/V ratio) of the oxide was varied. Several physical parameters of the films indicate a strong structural difference between the sample deposited at lower oxygen flux (1 scc m) and those obtained with higher flux (from 1.25 to 9 scc m). The increase of O/V ratio corresponds to a lower crystallinity of the thin films as indicated by the initial lowering and the final disappearance of the characteristic Raman mode of V₂O₅ (crystal) at about 140 cm⁻¹. For the highest flux samples new broad bands develop, typical of amorphous materials, both in polarized as well as in depolarized Raman spectra.     

Structural, morphological and electrochromic properties of Nb2O5 films deposited by reactive sputtering

Nb₂O₅ films were prepared successfully by DC reactive sputtering process. The relationships among structural, morphological and electrochromic properties were studied using XRD, AFM, AES and cyclic voltammograms. Results show that the films deposited on heated substrates are composed of columnar TT-Nb₂O₅ microcrystalline with many grain-to-grain boundaries. These structural characteristics provide films strong electrochemical stability, high Li⁺ insertion/extraction reversibility and good electrochromic properties. DC reactive sputtered Nb₂O₅ films are colorless in bleached state and brownish gray in colored state and may be a promising candidate for the application in electrochromic devices.     

XPS study of amorphous carbon nitride (a-C:N) thin films deposited by reactive RF sputtering

Amorphous carbon nitride (a-C:N) thin films were deposited by reactive radiofrequency (RF) sputtering. The a-C:N films were deposited, at room temperature, onto silicon substrates, from a graphite target of very high purity, in an atmosphere of pure nitrogen. The chemical properties of these films were studied by X-ray photoelectron spectroscopy (XPS). The XPS spectra of the a-C:N films reveal that nitrogen is well incorporated in the amorphous carbon network. The atomic percentage of nitrogen in the a-C:N films, calculated from the XPS spectrum, is about 32%. In addition to C–C and CC bonds, the analysis of the chemical shifts of C 1 s and N 1 s core level peaks show that nitrogen is bonded to carbon in CN double bonding and CN triple bonding configurations. The content of the CN triple bonds is found to be more important than the CN double bonds.     

The structural,electrical and optical properties of ZnO/Al2O3 multilayer deposited on PET substrates by RF sputtering

In this work ZnO/Al₂O₃ multilayer is deposited on PET(polyethylene terephthalate) substrate by RF sputtering in argon atmosphere and the effect of Al₂O₃ layer on optical, electrical and structural properties of ZnO is investigated. It is observed that the presence of Al₂O₃ layer between ZnO and PET reduce the transmission of light from ZnO/Al₂O₃/PET structure 11% but if a thin layer of SiO₂ ∼100 nm is applied between Al₂O₃ and PET it can be compensated. The XRD results show that the ZnO film had hexagonal wurtzite structure with (002) preferred orientation and the application of Al₂O₃ and SiO₂ layers do not have any effect on its structure. The effect of Al₂O₃ and SiO₂ layers on optical band gap of ZnO has been investigated, also the surface morphology of ZnO film is studied by scanning electron microscopy.     

Optical characterization of a-C:N thin films deposited by RF sputtering

In this work we present the main results of the optical properties study of amorphous carbon nitride (a-C:N) thin films prepared by reactive radio frequency (RF) sputtering. The a-C:N films were deposited, at room temperature, onto glass substrates, from a graphite target, in a pure nitrogen plasma. During the deposition, the pressure of nitrogen and the power density were maintained at 10⁻² mbar and 0.79 W cm⁻², respectively. Optical properties of these films were deduced from optical transmission spectra in the ultraviolet–visible–near infrared (UV–Vis–NIR) range. The refractive index follows well the Cauchy law with an extrapolated value of 1.68 in the far IR region. The optical band gap of the a-C:N films is about 1.2 eV. This value is relatively high in comparison with that of amorphous carbon films (0.8 eV) obtained in similar conditions. The incorporation of nitrogen in the amorphous carbon network leads to an increase of the optical band gap.     

MgH2 thin films deposited by one-step reactive plasma sputtering

The morphology, crystal structure, hydrogen content, and sorption properties of magnesium hydride thin films prepared by reactive plasma-assisted sputter deposition were investigated. Few micrometers-thick films were deposited on Si and SiO₂/Si substrates, at low pressure (0.4 Pa) and close to room temperature using (Ar + H₂) plasma with H₂ fraction in the range 15–70%. The microstructure and hydrogen content of the films are closely related to the surface temperature and hydrogen partial pressure during the deposition process. Operating in pulsed-plasma mode allows the hydrogenation rate of the MgH₂ thin film to top up to 98%, thereby producing a nearly fully hydrogenated film in a single-step process. The positive effect of the pulsed process is explained by the significant decrease in the whole energy flux incident on the surface and the favourable impact of the transient process for the rearrangement/relaxation of the materials. As for the hydrogen storage properties, desorption experiments and cycling of the films show the destabilizing effect of Mg₂Si formation at the interface between the film and the Si substrate resulting in a drastically increased desorption kinetics compared to less reactive SiO₂ substrate. However, the reaction is regrettably not reversible upon hydrogenation and the hydrogen storage capacity is consequently reduced upon cycling. Nevertheless, the deposition process carried out on inert substrates would offer true potential for reversible storage. Finally, our experimental results, which show the possibility to preferentially grow the metastable medium pressure γ-MgH₂ phase, open possibilities for the synthesis of more complex metastable phases such as magnesium-based ternary compounds.     

Solid electrolyte of tantalum oxide thin film deposited by reactive DC and RF magnetron sputtering for all-solid-state switchable mirror glass

All-solid-state switchable mirror glass was prepared by magnetron sputtering. The device exhibited the multi-layer structure of Mg₄Ni/Pd/Ta₂O₅ on WO₃/ITO/glass substrate. The Mg₄Ni, Pd, and Ta₂O₅ in the device acted as optical switches, proton injector and solid electrolyte, respectively. Reactive DC magnetron sputtering was employed as a new deposition method for Ta₂O₅ electrolyte thin film for the device. The transmittance of the device, at a wavelength of 670 nm using reactive DC-sputtered Ta₂O₅ thin film, reached 0.1% (a reflective state) to 48% (a transparent state). The transmittance change occurred in less than 40 s when 5 V was applied, and the switching speed was 60 times faster than that of the device using reactive RF-sputtered Ta₂O₅ thin film.     

Optical properties and thermal stability of TiAlN/AlON tandem absorber prepared by reactive DC/RF magnetron sputtering

Spectrally selective TiAlN/AlON tandem absorbers were deposited on copper and stainless steel substrates using a reactive DC/RF magnetron sputtering system. The compositions and thicknesses of the individual component layers were optimized to achieve high absorptance (α=0.931-0.942) and low emittance (ε=0.05-0.06) on copper substrate. The experimental spectroscopic ellipsometric data have been fitted with the theoretical models to derive the dispersion of the optical constants (n and k). In order to study the thermal stability of the tandem absorbers, they were subjected to heat treatment (in air and vacuum) for different durations and temperatures. The tandem absorber deposited on Cu substrates exhibited high solar selectivity (α/ε) of 0.946/0.07 even after heat treatment in air up to 600 °C for 2 h. At 625 °C, the solar selectivity decreased significantly on Cu substrates (e.g., α/ε=0.924/0.30). The tandem absorber on Cu substrates was also stable in air up to 100 h at 400 °C with a solar selectivity of 0.919/0.06. Studies on the accelerated aging tests indicated that the activation energy for the degradation of the tandem absorber is of the order of 100 kJ/mol.     

Structure–property relationships in electrochromic WO3 films deposited by reactive sputtering

WO_x electrochromic (EC) films deposited by DC magnetron sputtering technique were investigated by XRD and STM measurements. The reversible microstructure changes of the WO_x film between the bleached and colored EC states were revealed. The study indicates that the amorphous as-deposited WO_x film (a-WO_x) is of amorphous microstructure both in bleached and colored states; however, the crystalline WO_x (c-WO_x) is stoichiometric triclinic lattice WO₃ in bleached state (the lattice parameters: a=7.2944 Å, b=7.4855 Å, c=3.7958 Å, α=89.38°, β=90.42°, γ=90.80°), and changes into nonstoichiometric tetragonal lattice WO_2.9 in colored state (a=b=5.336 Å, c=3.788 Å, α=β=γ=90°). The surface morphologies of the colored WO_x films are very different from those of the bleached WO_x films.     

Characterization of Te/Zn/Te … multilayers deposited by RF-sputtering

Structures consisting of alternating Te and Zn layers were prepared by RF-sputtering. Grazing incidence X-ray diffraction (GIXD) measurements revealed the formation of ZnTe and the existence of free Zn in as-deposited samples. Similar measurements on samples annealed for 2 h at 300°C showed enhanced crystallinity of ZnTe as well as the formation of oxides when annealing time is extended to 8 h. UV-Visible-NIR transmission measurements showed that metallic-type absorption dominates in as-deposited samples. Annealed samples at 300°C showed well defined absorption edges with an energy gap of 2.50 eV for annealing times of 2 h and 2.24 eV when the annealing time is extended to 8 h.     

ZnO thin films deposited by RF magnetron sputtering: Effects of the annealing and atmosphere conditions on the photocatalytic hydrogen production

This work studied the effect of different annealing conditions of ZnO thin films grown by RF magnetron sputtering and their application as photocatalysts for hydrogen production without any sacrificial agent or co-catalyst. ZnO films were annealed in air, nitrogen, and argon atmospheres to study the effect of their physical properties in the photocatalytic activity. ZnO films showed high crystallinity and optical transparence of around 75–90% after annealing. Changes in composition and optical properties of the ZnO films were studied by x-ray photoelectron spectroscopy (XPS) and ellipsometry spectroscopy (SE), and results were correlated with the photocatalytic performance in hydrogen production. The highest photocatalytic hydrogen production was obtained with the ZnO thin film annealed in an air atmosphere with a result of 76 μmol.     

Structural, morphological and electrochemical properties of nanocrystalline V2O5 thin films deposited by means of radiofrequency magnetron sputtering

Due to easiness of preparation and high energy density, V₂O₅ nanocrystalline thin films are particularly attractive as cathode materials for all-solid-state rechargeable lithium microbatteries. However, their electrochemical performances are strictly related to the film microstructure, which, in turn, is related to the nature and parameters of the deposition technique. For this reason, the preparation of thin films with reproducible electrochemical properties is still an open problem.Here, we report on the deposition of V₂O₅ crystalline thin films by means of reactive radiofrequency (r.f.) magnetron sputtering, using vanadium metal as the target. Different deposition times and substrate temperatures were adopted. X-ray powder diffraction (XRD) and atomic force microscopy were used to investigate the structural and morphological features of the films. In particular, XRD analysis revealed that the deposition parameters affect the crystallographic orientation of the films. A h 0 0 orientation is observed in case of thin samples (about 100 nm) prepared at 300 °C, whereas a 1 1 0 preferential growth is obtained for thicker films. Films deposited at 500 °C display a 0 0 1 orientation irrespective on the deposition time.Reversible Li intercalation/deintercalation processes and high specific capacity are observed for the h 0 0-oriented V₂O₅ thinner films, with the ab plane arranged perpendicular to the substrate. In this case, the cycling behaviour is very promising, and a stable capacity higher than 300 mAh g⁻¹ was delivered in the potential range 3.8-1.5 V at 1C rate over at least 70 cycles.     

Structural, optical and electrical characterizations of μc-Si:H films deposited by PECVD

The effects of the silane concentration f on the structural, optical and electrical properties of undoped hydrogenated silicon films prepared in a plasma-enhanced chemical vapour deposition system have been studied. The electrical conductivity and Hall mobility appear to be controlled by microstructures induced by silane concentration and a clear electrical transport transition from crystalline to amorphous phase has been found when 3%<f<4%. A two-phase model has been used to discuss the electrical properties.     

Structural study of device quality silicon germanium thin films deposited by pulsed RF plasma CVD

Microstructures in silicon germanium thin films deposited by pulsed rf plasma CVD have been studied with the help of small-angle X-ray scattering (SAXS) and high-resolution transmission electron microscopy (HRTEM). With lowering of the pulse duty cycle the size of the particles incorporated in the films from the plasma decreases. However, the particles become more symmetric in shape and crystalline in nature. At 75% duty cycle the films have the highest photosensitivity. The increase in SAXS scattering at 75% has been explained by the formation of uniform-size nanocrystallites of SiGe. Urbach energy variation with the duty cycle also suggests the formation of nanocrystallites.     

Structural studies on the microcrystallization of Si:H network developed by hot-wire CVD

Effect of H₂-dilution to the SiH₄ plasma, R(H₂), on the microcrystallization of Si:H network at a low substrate temperature (180 °C) has been studied by using hot-wire CVD. Structural characterization of the films has been performed by micro-Raman, ellipsometry, infrared absorption and X-ray diffraction studies. A dramatic structural transformation from amorphous to microcrystalline phase has been identified at an H₂-dilution beyond 92.0%, induced by high atomic H density in the plasma. A virtual saturation in overall crystallinity has been attained for H₂-dilution in the range 92.75⩽R(H₂) (%)⩽93.75, contributing crystalline volume fraction changing between 60% and 64%, the average crystalline grain size varying between 150 and 200 Å and bonded hydrogen content maintaining between 3.3 and 2.6 at%. A crystalline volume fraction of 86.6% was obtained along with a low bonded H-content of 1.76 at% at R(H₂)=98.0%. However, at such extremely high H₂-dilution, overall crystallization is hindered due to enormous polyhydrogenation and formation of lesser dense network full of voids. Hence, microcrystallization in Si-network can be easily obtained in HWCVD, at a relatively low hydrogen dilution and low substrate temperature, without compromising much with the deposition rate arising out of those two stringent factors affecting in the conventional technique; and thereby, enhancing the technological acceptability of the deposition process presently dealt with.     

Experimental analysis of the absorption and desorption rates of HCOOK/H2O and LiBr/H2O

This paper evaluates the absorption and desorption rates of a potassium/water binary mixture and compares them to those of lithium bromide/water. The experimental procedure involved small-scale absorption chiller test rig. Extensive instrumentation was used so that identical operating conditions could be obtained for all tests to ensure a good comparison. Analysis of results has shown that the absorption rates for potassium formate/water is approximately 5% lower than lithium bromide/water. The desorption rates of potassium formate/water was found to be higher than that of lithium bromide/water. Heats of absorption and condensation were also recorded. © 1998 John Wiley & Sons, Ltd.     

A thin-film solar cell of high-quality -FeSi2/Si heterojunction prepared by sputtering

High-quality (1 1 0)/(1 0 1)-oriented epitaxial β-FeSi₂ films were fabricated on Si (1 1 1) substrate by the sputtering method. The critical feature was the formation of a high-quality thin β-FeSi₂ template buffer layer on Si (1 1 1) substrate at low temperature. It was demonstrated that the template is very important for the epitaxial growth of thick β-FeSi₂ films and for the blocking of Fe diffusion into the Si at the β-FeSi₂/Si interface. Hall effect measurements for β-FeSi₂ films showed n-type conductivity, with residual electron concentration around 2.0 × 10¹⁷ cm⁻³ and mobility of 50–400 cm²/V s. A prototype thin-film solar cell was fabricated by depositing n-β-FeSi₂ on p-Si (1 1 1). Under 100 mW/cm² sunlight, an energy conversion efficiency of 3.7%, with an open-circuit voltage of 0.45 V, a short-circuit current density of 14.8 mA/cm² and a fill factor of 0.55, was obtained.     

Effects of H2/O2 and H2/O3 gases on PtMo/C cathode PEMFCs performance operating at different temperatures

This study reported the activity of catalysts synthesized from platinum and molybdenum alloys in different atomic ratios and used as cathode electrocatalysts in the PEMFC. The structural properties of PtMo/C and Pt/C catalysts were analyzed by XRD analysis. The composition and distribution of these alloys in Vulcan XC-72R Carbon were determined by SEM and EDX techniques. CV studies assessed electrochemical properties such as ORR and ECSA activity. The performance of PEMFC cathodes that supplied pure hydrogen and oxygen was examined using polarization curves at different temperatures. Another way to improve the cathodic reaction is to use ozone as a potent oxidizing agent. It was measured that the OCV of the H₂/O₃ PEM fuel cell was 1.60 V, much greater than the open circuit voltage of the traditional H₂/O₂ PEM fuel cell. The PtMo/C catalyst achieved its highest power density of 137 mWcm⁻² at 70 °C, 128 mWcm⁻² at 60 °C, 101 mWcm⁻² at 50 °C, and 85 mWcm⁻² at 40 °C when exposed to H₂/O₂. As the temperature of the cell was raised, it was seen that the catalyst's catalytic activity increased.The maximum power density was detected to be inversely related to the rise in temperature when ozone was used. At low current densities, however, ozone was observed to greatly boost activation polarization.     

Thermally activated conversion of methane and carbon dioxide gas mixtures: Complex C/H/O modelling

Reforming of natural gas and carbon dioxide into H₂ and CO is an efficient method to convert heat into chemical energy, thus offering interesting perspectives for capturing solar energy. The CHEMKIN and SENKIN codes have been applied to a model of the system C/H/O represented by 47 species and 215 reactions. Equilibrium states have been calculated as a function of temperature and pressure. It has been found that at pressures higher than 10 bar conversion efficiency is reduced considerably. Non-catalytic conversion kinetics have been derived for 5 bar pressure and temperatures from 800 to 1200°C, clearly demonstrating the necessity of a catalytic converter.