Sputtered Tungsten Oxide Films Research Articles

Bis(β-ketoiminate) dioxo tungsten(VI) complexes as precursors for growth of WOx by aerosol-assisted chemical vapor deposition

Bis(β-ketoiminate) dioxo tungsten(VI) complexes WO2L2 (L = acNac, acNacMe, acNacEt) have been synthesized and characterized. The thermal behaviors and possible decomposition mechanisms of these dioxo compounds were studied by TGA, MS and thermolysis. The performance of WO2(acNacMe)2 as an AACVD precursor was evaluated at growth temperatures of 250–550 °C using nitrogen as the carrier gas. The elemental compositions of the as-deposited and sputtered tungsten oxide films were determined by XPS. The morphology and stoichiometry of the as-deposited films were studied by SEM and XRD.

Improvement of sensing characteristics of radio-frequency sputtered tungsten oxide films through surface modification by laser irradiation

To enhance sensing characteristics of semiconductor gas sensors, the surface morphology of sensing layer modified by laser irradiation was performed. Tungsten oxide (WO3) thin-films grown on Si/SiO2 wafers by radio-frequency sputtering were annealed and then irradiated by an excimer laser under various laser power densities. The effect of the laser irradiation on film surface morphology was studied by X-ray diffraction and atomic force microscopy. Gas sensors based on the laser-irradiated WO3 films demonstrated better responses to low concentration NO2 compared with the sensors without laser irradiation.

Magnetron sputtered tungsten oxide films activated by dip-coated platinum for ppm-level hydrogen detection

Platinum-activated tungsten oxide (Pt–WO3) films are prepared for hydrogen (H2) sensing applications. In this study, WO3 films were fabricated by reactive magnetron sputtering and Pt clusters were deposited on them by dip-coating. The microstructure, chemical composition and phase structure of Pt–WO3 films were characterized by scanning electron microscopy, X-ray photoelectron spectroscopy and X-ray diffraction, respectively. It was observed that a reconditioning period is required after a rest period of the sensor for obtaining a stable signal. A thermal treatment at 450°C for 24h is proposed to solve this problem.

Progress in sputtered tungsten trioxide for photoelectrode applications

In the context of photoelectrochemical water-splitting, tungsten oxide films have been fabricated at low processing temperatures ( < 250 ∘ C ) by reactive sputtering from tungsten targets in an argon/oxygen ambient. The films have a dense, compact morphology and show columnar growth. Amorphous and highly polycrystalline films can be produced depending on the deposition conditions; polycrystalline phases appear only at higher temperatures and under certain sputter target conditions. Large crystallites proved beneficial to photoelectrochemical performance. A maximum photocurrent of 2.7 mA / cm 2 (at 1.6 V vs SCE) was observed in 0.33 M H 3 PO 4 under AM 1.5 Global illumination, exceeding published results for material fabricated at higher temperatures (in the 400 – 600 ∘ C range). Doping of sputtered tungsten oxide films with nitrogen results in a red-shifted absorption edge, but so far not in increased photocurrents. The maximum photocurrent of a nitrogen-doped sample was measured at 2.3 mA / cm 2 (at 1.6 V vs SCE). A multi-junction photoanode based on the best available sputtered WO 3 film and an amorphous silicon photovoltaic device is projected to operate at 2.2% solar-to-hydrogen efficiency.

Low-Temperature Reactively Sputtered Tungsten Oxide Films for Solar-Powered Water Splitting Applications

Tungsten oxide semiconductor films for photoelectrochemical water-splitting applications, reactively sputtered at low temperatures , have demonstrated photocurrents of under AM 1.5 illumination in 0.33M , exceeding published results for material pyrolytically fabricated at higher temperature, and approaching the theoretical maximum for tungsten trioxide based on optical absorption limits. The microstructural form of the reactively sputtered films, which is sensitive to the sputter process parameters, critically affects the photoconversion efficiency. It has also been determined that the sputtered films form true compact photoelectrodes in solution, with active charge generation and separation over the material's optical width.

Properties and electrochromic performances of reactively sputtered tungsten oxide films with water as reactive gas

A change of water vapour partial pressure from 0 to 4 Pa was carried out in order to prepare sputtered tungsten oxide compounds with various oxygen and hydrogen concentrations. Oxygen, tungsten and hydrogen concentrations were determined by Rutherford Backscattering Spectroscopy (RBS) and by Elastic Recoil Detection (ERD) analysis. Structure of tungsten oxide films was analyzed by X-ray diffraction. At low water vapour partial pressure, the films are crystallized and WO2 and W3O phases were observed. The electrochromic performances of such film/SnO2/glass substrate system were measured and discussed taking into account the influence of the water vapour partial pressure injected into the deposition process on the structure, chemical composition and optical properties of the films.

Nitrogen Doping of Reactively Sputtered Tungsten Oxide Films

Nitrogen Doping of Reactively Sputtered Tungsten Oxide Films

Effects of Oxygen Partial Pressure and Annealing Temperature on the Formation of Sputtered Tungsten Oxide Films

Thin films of tungsten oxide were deposited on silicon substrates using reactive radio frequency sputtering. The structure of the films strongly depends on the conditions of deposition and post-treatment. Important issues are the influences of oxygen pressure during deposition and annealing temperature on the morphology. Atomic force microscopy and scanning electron microscopy revealed that films were formed by grains. The sample deposited with an partial pressure ratio of 1:1 showed the highest roughness and the smallest grains when annealed at 350°C. X-ray photoelectron spectroscopy analysis revealed that the films were close to their stoichiometric formulation irrespective of the oxygen partial pressure used during film deposition. The number of W=O bonds at the grain boundaries was found to be dependent on the oxygen partial pressure. Analysis by Raman spectroscopy suggested that the structure of the films was monoclinic. On the basis of these results, an annealing temperature of 350°C was selected as post-treatment for the fabrication of gas sensors. These sensors were highly sensitive, highly selective to ammonia vapors, and moderately responsive to humidity. © 2002 The Electrochemical Society. All rights reserved.

Effects of ion beam bombardment on electrochromic tungsten oxide films studied by X-ray photoelectron spectroscopy and Rutherford back-scattering

The effect of ion bombardment on thermally evaporated and magnetron sputtered tungsten oxide films were investigated using X-ray photoelectron spectroscopy (XPS). Results show that irrespective of the porosity and crystallinity of the film samples formed with different techniques and conditions, ion bombardment induced preferential sputtering of oxygen, resulting in a decrease of oxygen/tungsten (O/W) ratio with increasing sputtering time. Samples experienced electrochromic switching cycles also show the same effect, except that a higher O/W ratio is detected because the tungsten oxide film reacts with the LiClO 4-propylene carbonate electrolyte. Angle-resolved XPS experiments further confirm preferential sputtering of oxygen, suggesting that ion beam sputtering used in XPS for pre-cleaning and depth profile analysis of tungsten oxide must be used with caution. Rutherford back-scattering gives more reliable composition data of tungsten oxide, since it does not involve any sputtering process.

Effect of oxygen content on the electrochromic properties of sputtered tungsten oxide films with Li + insertion

Tungsten oxide films have been deposited at room temperature from metallic tungsten target onto ITO coated glass substrate with sheet resistance of 15 Ω⧹□ using reactive rf magnetron sputtering. The films formed in an Ar+2–25% O 2 gas mixture with total pressure of 10 m Torr and sputtering power 300 W. The films were subjected to electrochemical Li + insertion using in situ lithium triflate (trifluoromethanesulfonate) anhydrous solution. The electrochromic properties of the rf reactively sputtered tungsten oxide films have been investigated in relation to the oxygen concentration in the sputtering atmosphere. Transmittance measurements have been performed in situ during the electrochemical formation of the tungsten bronzes using Bruker IFS-66 Fourier spectrophotometer with integrating sphere. From the obtained results the best values of the electrochromic properties can be achieved at oxygen concentration of 15% with electrochromic parameters as, Δ T sol = 0.547, Δ (OD) s = 0.865, η s = 21.05 cm [2]⧹C, Q inj = 41.09 mC⧹cm [2]and Q lef. = 10.25 mC cm [2]respectively. The films deposited with oxygen concentration less than 5% were found in metallic state.

Deposition Temperature Dependence of Optical Gap and Coloration Efficiency Spectrum in Electrochromic Tungsten Oxide Films

Optical gaps and electrochromic efficiencies of sputtered tungsten oxide films are studied by focusing attention on the cluster size of the film. The cluster consists of O‐W‐O network with terminal W=O bonds on its boundary. The quantity of W=O bonds increased with the surface area of the clusters. Raman scattering bands of the O‐W‐O and W=O are observed between 500 and 1100 cm−1. These characteristic Raman scattering bands of the film are well reproduced by the combination of four Gaussian shaped Raman bands of the O‐W‐O and W=O modes. The ratio of integrated Raman scattering intensities (W=O/O‐W‐O) of these modes are employed as the measure of the cluster size. The cluster is found to increase in size with the elevation of the film deposition temperature. Optical gap was observed to increase inversely proportional to the cluster size of the film. This cluster size dependence appears to have the same origin assigned in nanocrysallites. The electrochemical reduction of the film produces a broad asymmetric polaron absorption band in the optical region from near IR to visible. The coloration efficiency spectrum of this absorption band becomes higher with the cluster size, and its maximum position shifts to a lower energy side. This behavior is explained in terms of a polaron‐polaron interaction and a polaron confinement in the cluster.

Determining thin film properties by fitting optical transmittance

The optical transmission spectra of rf sputtered tungsten oxide films on glass substrates were modeled to determine absorption edge behavior, film thickness, and index of refraction. Removal of substrate reflection and absorption phenomena from the experimental spectra allowed direct examination of thin film optical characteristics. The interference fringe pattern allows determination of the film thickness and the dependence of the real index of refraction on wavelength. Knowledge of the interference fringe behavior in the vicinity of the absorption edge was found essential to unambiguous determination of the optical band gap. In particular, the apparently random deviations commonly observed in the extrapolation of as-acquired data are eliminated by explicitly considering interference fringe phenomena. The multivariable optimization fitting scheme employed allows air-film-substrate reflection losses to be compensated without making reflectance measurements.

Preparation and properties of the dc reactively sputtered tungsten oxide films

Preparation and properties of the dc reactively sputtered tungsten oxide films

Infrared and Raman spectroscopies of rf sputtered tungsten oxide films

Infrared and Raman spectroscopies have been applied to study rf sputtered tungsten oxide films as prepared and after the chemical modifications induced by successive coloration/bleaching cycles. As seen from UV-visible spectra and X-ray diffraction patterns, these films remain essentially amorphous, but it is shown that infrared spectroscopy used in absorbance and reflectance modes and with s and p polarized light is particularly well adapted to provide new information about their aging and variation of composition during the cycles. Aging effects are characterized by water adsorption and hydroxyl group formation. The modification of the amorphous initial film after annealing has also been investigated. The infrared spectra of the film after coloration or bleaching in a sulfuric acid solution show the important role of water content equilibration during the first coloration and then the reversible water extraction/injection during the following bleaching/coloration cycles. The structure of the colored film is also characterized by WO terminal groups which disappear in the bleached state. Some coloration/bleaching cycles have also been performed in organic electrolytes for comparison.

Preparation and properties of the dc reactively sputtered tungsten oxide films

Preparation and properties of the dc reactively sputtered tungsten oxide films have been investigated in relation to the oxygen concentration in sputtering atmosphere. The films with 1500–20 700 Å thickness were deposited on the glass substrates maintained at 200 °C by dc reactive sputtering from a metallic tungsten target under a constant operating pressure of 6.5×10−2 Torr in Ar-1%–30% O2 gas mixture. The films formed in an Ar-3%–20% O2 gas mixture are crystalline WO3, and have an electrical resistivity of 107–1011 Ω cm which is dependent on the oxygen concentration of the sputtering atmosphere. These films have a spectral transmittance above 80% in the visible and near-infrared regions, and optical band gap of the films ranges from 3.15 to 2.98 eV, depending on the oxygen concentration. Densities of the film deposited in Ar-3% O2 and in Ar-20% O2 gas mixtures are 5.85 g/cm3 and 6.65 g/cm3, respectively. Electrochemichromic properties of the transparent-crystalline WO3 films were studied using asymmetric cells, and were found to be dependent on the crystal orientation of the films. The films with the orientation of WO3 (020) and WO3 (021) formed in an Ar-3%–6% O2 gas mixture have a very good electrochromic property, and the cells composed of the crystalline WO3 films have a higher coloration rate than the cells composed of the vacuum-evaporated amorphous film. The films with the orientation of WO3 (001) and WO3 (021) formed in an Ar-8%–20% O2 gas mixture were found to have a poor electrochromic property.

Physical and electrochemichromic properties of rf sputtered tungsten oxide films

The physical and electrochemical coloration characteristics of tungsten oxide films rf sputtered from a compressed powder WO3 target have been investigated. Oxide films with 3600–9800 Å thickness were deposited on substrates maintained at 200 °C at a total pressure of 0.5–8×10−2 Torr in Ar gas or an Ar-0.5-50% O2 gas mixture. Physical properties of the oxide films depend on the oxygen concentration and total pressure of the sputtering atmosphere. The films prepared at 4×10−2 Torr in a mixture of Ar-0.5-20% O2 gas are transparent and amorphous, and their electrical resistivity ranges from 6.5×108 to 2.4×1011 Ω cm. The films prepared at pressures between 4 and 6×10−2 Torr in an Ar-50% O2 gas mixture are transparent, and have crystallites with a composition of WO3. The films prepared at 1×10−2 Torr in a mixture of Ar-0.5 and 5.0% O2 gas are blue colored and transparent, respectively, and these films are crystallites with a composition of WO2.83. Electrochemichromic properties of the rf sputtered tungsten oxide films depend on the film structure, or on the sputtering conditions under which the films are prepared. The amorphous and crystalline WO3 films formed at a high total pressure of 3–8×10−2 Torr have good electrochemichromic properties and are colored deep blue. However, the crystalline WO2.83 films formed at a low total pressure of 0.5–1.5×10−2 Torr have poor electrochemichromic properties and are hardly colored. The amorphous oxide films with resistivity of 108–109 Ω cm and low density of ∼6.0 g/cm3, formed at a deposition rate higher than ∼100 Å/min, have very good coloration characteristics. The optical properties and density of the oxide films are also described.

Physical and electrochemichromic properties of rf sputtered tungsten oxide films

The physical and electrochemichromic properties of rf sputtered tungsten oxide films from a compressed powder WO3 target have been investigated. This report mainly concerns the dependence of the properties on the total pressure of the sputtering atmosphere. The oxide films, 5000–5300 Å thick, were deposited on substrates maintained at 200 °C under total pressures of 5×10−3–8×10−2 Torr in an atmosphere of Ar gas or Ar–5% O2 mixture. The physical and electrochromic properties of the oxide films prepared are dependent on the total pressure during deposition. The electrical resistivity ranges from 4.75×107 to 3.33×1011 Ω cm depending on the pressure. From x-ray analysis, the films prepared under pressures of 5×10−3–1.5×10−2 Torr were found to be crystalline and assigned a composition of WO2.83. The films prepared under pressures of 1.8×10−2–8×10−2 Torr were found to be amorphous. Electrochemical coloration of these oxide films was observed using an asymmetric cell, and the coloration characteristics of the oxide films were found to be dependent on the electrical resistivity and structure of the films. The amorphous films with high resistivity have good electrochromic properties, but the crystalline films with low resistivity are hardly colored. The optical band gap, refractive index, and density of the oxide films are described.

Electrical and optical properties of reactively sputtered tungsten oxide films: K Miyake et al, J Appl Phys, 53 (3), 1982, 1511–1515

Electrical and optical properties of reactively sputtered tungsten oxide films: K Miyake et al, J Appl Phys, 53 (3), 1982, 1511–1515

Preparation and properties of reactively sputtered tungsten oxide films: H Kaneko et al, J Appl Phys, 53 (4), 1982, 3070–3075

Preparation and properties of reactively sputtered tungsten oxide films: H Kaneko et al, J Appl Phys, 53 (4), 1982, 3070–3075

Preparation and properties of reactively sputtered tungsten oxide films

Properties of reactively sputtered tungsten oxide films for electrochromic display devices have been investigated. The films (2000–23000 Å thick) were formed by rf sputtering from a metallic tungsten target under a constant operating pressure ranging from 5×10−3 to 6×10−2 Torr using a mixture of Ar-4–50% O2. Electrical resistivity of the films formed increases remarkably with increasing oxygen concentration and total operating pressure of the sputtering atmosphere, ranging from 10−3 to 1011 Ω⋅cm. The films 5900–9200 Å thick prepared in a sputtering atmosphere of Ar-10% O2 under a constant operating pressure higher than 2×10−2 Torr are transparent and have a spectral transmittance of about 85% in the visible and infrared region. Optical band gap and refractive index of these films are 3.01–3.08 eV, and 2.2–2.4, respectively. Electrochemichromic properties of the films were also studied using asymmetric cells, and it was found that a good electrochromic performance was achieved usually by the cells composed of the transparent films with a resistivity of 106–1010 Ω⋅cm.