Optical properties of tungsten oxide thin films by non-reactive sputtering

Tungsten oxide thin films were grown on glass substrates by RF sputtering at room temperature using a tungsten trioxide target for several values of the argon pressure (P_Ar). The structural and morphological properties of these films were studied using X-ray diffraction and atomic force microscopy. The as-deposited films were amorphous irrespective of the argon pressure, and crystallized in a mixture of hexagonal and monoclinic phases after annealing at a temperature of 350 °C in air. Surface-roughness increased by an order of magnitude (from 1 nm to 20 nm) after thermal treatment. The argon pressure, however, had a strong influence on the optical properties of the films. Three different regions are clearly identified: deep blue films for P_Ar ≤ 2.67 Pa with low transmittance values, light blue films for 2.67 Pa < P_Ar < 6 Pa with intermediate transmittance values and transparent films for P_Ar ≥ 6 Pa with high transmittance values. We suggest that the observed changes in optical properties are due to an increasing number of oxygen vacancies as the growth argon pressure decreases.     

Effects of oxygen stoichiometry on electrochromic properties in amorphous tungsten oxide films

We have investigated the electrochromic properties of amorphous granular tungsten oxide (WO_3 + δ) thin films with over-stoichiometric oxygen content (δ), using LiClO₄ with propylene carbonate as an electrolyte. Different optical and electrochromic characteristics are observed with increasing δ. All the devices are electrochemically stable for more than 5000 color/bleach cycles without apparent degradation, and they have a faster response to coloration than to bleaching. WO_3 + δ films with an optimized δ value show an optical modulation of 86% at a wavelength of 630 nm and the highest coloration efficiency ever reported of ~ 213 cm²/C. The δ-dependent coloration mechanism is discussed using the site saturation model. It is proposed that WO_3 + δ films with the optimal δ value have favorable thickness and stoichiometry for the generation of Li⁺W⁺⁵ states.     

Effects of oxygen stoichiometry on resistive switching properties in amorphous tungsten oxide films

The bipolar resistance switching in WO_3 + δ films sandwiched by Al and Pt electrodes was investigated by changing additional oxygen content (δ). Reliable switching voltages and retention were observed for all samples. As δ increases the bi-stable current-voltage characteristics fluctuate leading to unstable switching power consumption. An analysis of the temperature dependence of the bi-stable resistance states revealed additional features that thermionic emission and metallic conduction co-contribute to the electrical transport of the resistance states. The authors propose that the observed resistance switching is due to the combined effects of potential modification near the interface and the formation of a metallic channel.     

Electrochromic materials and devices: Brief survey and new data on optical absorption in tungsten oxide and nickel oxide films

Current work on inorganic electrochromic materials and devices is reviewed briefly, and the emphasis on thin films of W oxide and Ni oxide is pointed out. New results are presented on the optical properties for sputter-deposited thin films of these two materials: for Li⁺ intercalated sub-stoichiometric W oxide, it is shown that the absorption can be reconciled with an extended “site saturation” model accounting for electronic transitions between W ions in 6+, 5+, and 4+ states, and for H⁺ deintercalated Ni oxide, it is found that the coloration efficiency is higher than in prior work presumably as a consequence of the nanocrystalline structure ensuing from the deposition conditions.     

Making tungsten targets using tungsten oxide powder

A straightforward method of making tungsten targets from tungsten oxide powder on Cu backing is presented. A ceramic crucible containing WO₃ powder placed on a copper foil is heated in hydrogen atmosphere at temperatures ranging from 650 up to 850 °C. The resulting thickness of tungsten deposited in our trials was in the range of 0.5-3.4 mg/cm².     

Radio frequency sputtered zinc oxide thin films with application to metal-semiconductor-metal photodetectors

Zinc oxide (ZnO) films were successfully deposited on silicon, silicon dioxide, and glass substrates by radio frequency magnetron sputtering at different deposition conditions. Field emission scanning electron microscopy, X-ray photoelectron spectroscopy, transmission and photoluminescence measurements were employed to analyze the effect of the deposition conditions and the postdeposition annealing treatment on the surface morphology, structure, chemical deposition and optical properties of ZnO thin films. It was found that the thickness of ZnO films decreased with increased ratio of oxygen/argon and increased temperature. The crystalline and stoichiometric quality of the film was improved by depositing at high temperature and low pressure. Crystals formed more tightly and uniformly with heat treatment under air ambient. The dark current of the ZnO metal-semiconductor-metal photodetector was reduced from 3.06 μA to 96.5 nA at 5 V after postdeposition annealing when compared with that of as-deposited ZnO. Its magnitude was found to be at least two orders lower than that of the as-deposited sample.     

Synthesis, structural and mechanical characterization of sputtered tungsten oxide coatings

Tungsten oxide coatings were deposited without substrate bias by DC reactive magnetron sputtering of a tungsten target using oxygen as reactive gas. By tuning the partial pressure of oxygen (p_O2/p_Ar) between 0 and 4, the oxygen content of the films was changed from 0 to 75 at.%. The structure of the films (investigated by X-ray diffraction) depends on their oxygen content. For low oxygen contents, the -W and β-W₃O phases were observed (< 30 at.%), and with the increase of oxygen content (30 at.% < O < 67 at.%) the structure became amorphous. A transition region was obtained for oxygen content between 67 at.% and 75 at.%, and when O > 75 at.%, a nanocrystalline (WO₃) structure was reached.

The hardness and Young's modulus were evaluated by depth sensing indentation. The decrease in hardness followed the four different ranges of chemical compositions accordingly, from ≈ 23 GPa for pure W down to ≈ 7 GPa for WO₃ films. A similar behaviour was observed for the Young's modulus, which ranged from 450 GPa to 150 GPa. The cohesion/adhesion of the films were investigated using a scratch-test apparatus. These coatings displayed a low adhesion (critical load, L_c < 15 N) to the steel substrate because the depositions were carried out intentionally without an adhesion interfacial layer.     

Hydrogen sensing properties of Pd-doped SnO2 sputtered films with columnar nanostructures

Pd-doped SnO₂ sputtered films with columnar nanostructures were deposited using reactive magnetron sputtering at the substrate temperature of 300 °C and the discharge gas pressures of 1.5, 12, and 24 Pa. Structural characterization by means of X-ray diffraction and scanning electron microscopy shows that the films composed of columnar nanograins have a tetragonal SnO₂ structure. The films become porous as the discharge gas pressure increases. Gas sensing measurements demonstrate that the films show reversible response to H₂ gas. The sensitivity increases as the discharge gas pressure increases, and the operating temperature at which the sensitivity shows a maximum is lowered. The highest sensitivity defined by (R_a − R_g) / R_g, where R_a and R_g are the resistances before and after exposure to H₂, 84.3 is obtained for the Pd-doped film deposited at 24 Pa and 300 °C upon exposure to 1000 ppm H₂ gas at the operating temperature of 200 °C. The improved gas sensing properties were attributed to the porosity of columnar nanostructures and catalytic activities of Pd doping.     

Thermal stability of sputtered zirconium oxide films

In this work, the effect of post-growth annealing on the structural and optical properties of sputtered zirconium oxide films has been investigated. The temperature dependence of structure, density, and optical constants has been systematically studied by X-ray diffraction, X-ray reflectometry, atomic force microscopy (AFM) and optical spectroscopy. X-ray diffraction studies show no variation in the crystalline phase upon annealing except grain growth. X-ray reflectivity measurements determine a density increase of approximately 11% and a simultaneous thickness reduction of 10% upon annealing. The surface roughness of the films increases upon annealing as determined by XRR and confirmed by AFM measurements. Optical spectroscopy measurements confirm that the refractive index n of the films decreases with increasing annealing temperature. At the same time the optical band gap E_g of the films increases from 4.58 to 4.97 eV annealing at 900°C. The surprising decrease of refractive index upon annealing is attributed to both the intermixing of Si with ZrO₂ and the increasing surface roughness of the films.     

Field induced polarisation and relaxation of tungsten oxide thick films

We have investigated the time dependent conductance of a WO₃ film (∼ 700 K) after a voltage step. From the beginning to some 100 ms, the region next to the positive electrode shows a strong increase in resistance. The region next to the negative electrode shows a decrease in resistance. A possible explanation for this first effect is the depletion (respectively accumulation) of mobile donors. After 10 s, a decrease in resistance next to the positive electrode is observed. The region next to the negative electrode shows an increase in resistance. The resistance of the two regions nearly meets its value prior to the perturbation. A possible explanation for this second effect is desorption (respectively adsorption) of charged surface oxygen.     

Characterization of ultrasonic spray pyrolyzed tungsten oxide thin films

The ultrasonic spray pyrolysis (USP) technique has been employed to deposit tungsten oxide (WO₃) thin films. The films were prepared by spraying 0.02 M ammonium metatungstate solution onto amorphous glass substrates kept at 250°C. These films were further annealed at 400°C for different time periods (1–5 h) in air. The films were characterized for structural, electrical and opto-electronic properties. X-ray diffraction technique was used to determine the crystallinity of the WO₃ films and identify the phases that form as a function of annealing time. The as-prepared WO₃ films were amorphous and crystallize when annealed at 400°C in air for 2 or more hours. From TEM, the grain size and lattice plane spacing are estimated. The films were further characterized by using time resolved microwave conductivity (TRMC) technique and decay time of the photogenerated charge carriers is calculated to be about 154 ns. The concentration and mobility of charge carriers are estimated from thermoelectric power (TEP) measurements.     

New hydrogen sensor based on sputtered Mg-Ni alloy thin film

Pd-capped magnesium-nickel alloy thin films were prepared by magnetron sputtering and their hydrogen sensing properties were investigated. By monitoring the resistance change or the transmittance change of the film, we can obtain the information on hydrogen concentration in air. The sensing range of this sensor is quite wide and it can measure the hydrogen concentration range from 10 ppm to 10% without heating. Using a certain protective coating, the durability of the film can be much improved. Also there is a unique application of Pd/Mg-Ni thin film as ‘hydrogen check sheet’, which can visualize the hydrogen flow.     

Heteroepitaxial growth of tungsten oxide films on sapphire for chemical gas sensors

The performance of chemiresistive gas sensors made from semiconducting metal oxide films is influenced by film stoichiometry, crystallographic structure, surface morphology and defect structure. To obtain well-defined microstructures, heteroepitaxial WO₃ films were grown on r-cut and c-cut single crystal sapphire substrates using rf magnetron Ar/O₂ reactive sputtering of a W target. On r-cut sapphire, an epitaxial tetragonal WO₃ phase is produced at a 450°C deposition temperature whereas 650°C growth stabilizes an epitaxial monoclinic WO₃ phase. On c-cut sapphire, a metastable hexagonal WO₃ phase is formed. RHEED and X-ray diffraction indicate that the films have a ‘polycrystalline epitaxial structure’ in which several grains are present, each having the same crystallographic orientation. STM analysis of the film surfaces reveals morphological features that appear to be derived from the substrate symmetries. The monoclinic phase has a step/terrace growth structure, has the smallest mosaic spread in XRD rocking curves and exhibits the highest degree of reproducibility suggesting that it is the best suited for sensor applications. Measurements of film conductivity versus temperature indicate that the charge transport mechanisms are also dependent on the crystallographic phase and microstructure of the WO₃ films.     

R.F. magnetron sputtered tungsten carbide thin films

Thin films of tungsten carbide have been deposited on stainless steel substrates held at 500°C by r.f. reactive magnetron sputtering in two different modes of introducing argon and acetylene gases called normal and high rate mode. A single phase fcc-WC is formed in the normal mode whereas a mixture of A-15-W₃C, hexagonal-WC and graphitic- and diamond-carbon is found in the high rate mode. A microhardness value as high as 3200 kgf/mm² (as compared to the bulk value of 1800 kgf/mm²) is obtained in the film deposited by normal mode.     

High rate direct current magnetron sputtered and texture-etched zinc oxide films for silicon thin film solar cells

Aluminum-doped zinc oxide (AZO) films were prepared by in-line direct current (dc) magnetron sputtering on glass substrates. Four types of ceramic targets with 0.5 wt.% or 1 wt.% of aluminum oxide and different preparation methods, namely normal sintered, soft sintered and hot pressed, were employed. The influence of different target manufacturing processes, aluminum concentration and sputtering conditions on AZO films were investigated. Depending on the type of targets and deposition conditions, highly transparent films with low resistivity values in the range of 3.6-11 × 10^− 4 Ω cm were obtained. The etching behaviour in hydrochloric acid and the resulting light scattering properties of the AZO films were strongly influenced by the choice of the target and the deposition conditions. The most favourable films have been successfully applied in thin film solar cells with 1.1-μm microcrystalline silicon absorber layer leading to an initial efficiency of 7.8%.     

Structural investigation by the Rietveld method of sputtered tungsten carbide thin films

Thin films of tungsten carbides deposited by reactive radio-frequency sputtering were investigated by X-ray diffraction using the Rietveld method. Two films were selected for the structural refinement. One was biased, the other unbiased. The unbiased film was found to consist of a cubic phase WC_1 − x (C_0.9) in the space group Fm3m with a lattice parameter of 4.263 (5) Å. A negative substrate bias of − 40 V leads to a multiphasic film: a cubic phase WC_1 − x with a lattice parameter of 4.301 (6) Å and a hexagonal phase W₂C (P-3m1) with lattice parameters of a = b = 2.787 (1) and c = 4.549 (2) Å. The domain size was found to be of ~ 5 nm. The coexistence of nanocrystalline phases WC_1 − x and W₂C is in accordance with the decrease of the carbon content (WC_0.7) in the biased film.     

Electrochromic properties of N-doped tungsten oxide thin films prepared by reactive DC-pulsed sputtering

In depositing nitrogen doped tungsten oxide thin films by using reactive DC-pulsed magnetron sputtering process, nitrous oxide gas (N₂O) was employed instead of nitrogen (N₂) as the nitrogen dopant source. The nitrogen doping effect on the structural and electrochromic properties of WO₃ thin films was investigated. X-ray diffraction (XRD) results show that the films are amorphous. Morphological images reveal that the films are characterized by a hybrid structure comprising nanoparticles embedded in amorphous matrix and open channels between the agglomerated nanoparticles, which promotes rapid charge transport through the film. Increasing the nitrogen doping concentration is found to decrease the nanoparticle size and the band gap energy. The electrochromic properties were studied using cyclic voltammetric and spectroeletrochemical techniques. The film with N content of ~ 5 at.% exhibits higher optical modulation and coloration efficiency as well as faster ion transport kinetics. The results reveal that electrochromic and lithium ion transport properties are moderately enhanced relative to the un-doped tungsten oxide thin films by appropriate content of dopant, due to the effects of nitrogen doping.     

Electrochromic performance of hybrid tungsten oxide films with multiwalled-CNT additions

In this study, tungsten oxide films were prepared by sol–gel technique. Various amounts of multiwalled carbon nanotubes (MWCNTs) were added during sol–gel process to obtain hybrid WO₃/MWCNT films. The original and hybrid films were characterized by thermogravimetric analysis, X-ray diffraction analysis, and scanning electron microscopy analysis, whereas the electrochromic performance was evaluated by measuring changes in the optical transmittance caused by potentiostatic charge–discharge intercalation. The influence on the structure and properties of tungsten oxide film due to MWCNT addition was also investigated. The results showed that all of the films were amorphous and exhibited porous microstructure. The electrochromic performance of pristine WO₃ film was improved by adding MWCNTs that served as a template for the growth of WO₃ and resulted in more porous microstructure. The hybrid tungsten oxide films with 0.1 wt.% MWCNT addition exhibited the best electrochromic performance.     

H2S sensing property of porous SnO2 sputtered films coated with various doping films

Pure SnO₂ films and Ag-, Cu-, Pt-, and Pd-doped SnO₂ films were investigated for H₂S sensing properties. SnO₂ films were deposited by DC magnetron sputtering at various substrate temperatures and discharge gas pressures. As the discharge gas pressure increased and the substrate temperature decreased, the film became porous. Doping with Cu or Ag film improved the sensitivity, and the highest sensitivity was obtained in the porous SnO₂ film coated with an Ag film 16 nm thick. According to the X-ray diffraction (XRD) pattern, Ag deposited on SnO₂ film transformed to Ag₂S upon exposure to H₂S. When the Ag-doped film sensor was operated at a low temperature, the sensitivity was extremely high, but the recovery was insufficient. By increasing the operation temperature, the recovery was improved but the sensitivity decreased.     

Electrochromic properties of nanocrystalline tungsten oxide thin films

Tungsten trioxide films formed of aggregates in the nanometric range have been deposited by thermal evaporation and condensation method. From the atomic force microscopy study, the aggregate dimensions were found to be in the range between 350 nm and 450 nm. The optical, structural and electrochromic performance of these films under lithium intercalation is studied in detail and their properties are compared to those of continuous films of tungsten trioxide. The films show a slightly elevated diffuse reflectance in the lower wavelength range and a very high transmission in the solar and visible wavelengths. Under lithium intercalation, the films exhibit a very good degree of electrochromic optical modulation in both these spectral ranges and hence, are very suitable for electrochromic device application. The high degree of change between the clear state and the colored state render such nanocrystalline films more efficient in their electrochromic performance than the conventional continuous films.