Microstructural evolution of tungsten oxide thin films

Tungsten oxide thin films are of great interest due to their promising applications in various optoelectronic thin film devices. We have investigated the microstructural evolution of tungsten oxide thin films grown by DC magnetron sputtering on silicon substrate. The structural characterization and surface morphology were carried out using X-ray diffraction and Scanning Electron Microscopy (SEM). The as deposited films were amorphous, where as, the films annealed above 400 °C were crystalline. In order to explain the microstructural changes due to annealing, we have proposed a “instability wheel” model for the evolution of the microstructure. This model explains the transformation of mater into various geometries within them selves, followed by external perturbation.     

Oxidation kinetics of thin titanium films grown on tungsten

Growth of thin Ti films on (100)W and the kinetics of their oxidation are studied using thermal-desorption spectroscopy and Auger electron spectroscopy. Titanium films grow nearly layer by layer on the (100)W face at room temperature. The activation energy for desorption of Ti atoms decreases from 5.2 eV for coverage θ=0.1 to 4.9 eV in a multilayer film. Oxidation of a thin (θ=6) titanium film starts with dissolution of oxygen atoms in its bulk to the limiting concentration for a given temperature, after which the film oxidizes to TiO, with the TiO₂ oxide starting to grow when exposure of the film to oxygen is prolonged. The thermal desorption of oxides follows zero-order kinetics and is characterized by desorption activation energies of 5.1 (TiO) and 5.9 eV (TiO₂).     

Crystallization kinetics of amorphous aluminum–tungsten thin films

82 W₁₈ to Al₆₂W₃₈ compounds ranged from 800 K to 920 K. The activation energy for the crystallization and the Avrami exponent were determined. The results indicated that the crystallization mechanism in films with higher tungsten content was a diffusion-controlled process, whereas in films with the composition similar to the stoichiometric compound (Al₄W), the interface-controlled crystallization probably occurred. Received: 30 March 1998/Accepted: 9 September 1998     

Electrochromic colour centres in amorphous tungsten trioxide thin films

Amorphous tungsten trioxide films, investigated by the Raman scattering method, are shown to be composed of a spatial network of tightly bound (WO₆)_n·mH₂O clusters with a large number of terminal oxygen W=O and W-O-W bonds between clusters. The injected electrons in an amorphous tungsten trioxide film are localized in the tungsten 5d orbitals in an axially distorted octahedron, as is shown by ESR analysis. The optical absorption of a coloured amorphous tungsten trioxide film, as has previously been proposed, can be satisfactorily described by an intervalence charge-transfer transition between localized W⁵⁺ and W⁶⁺ states.     

Transmission of high-frequency phonons through thin glass films

By means of phonon spectroscopy we measured the transmission of high-frequency phonons through thin glass films in the frequency range from 100 to 300 GHz. The films were prepared by thermal oxydation of silicon single crystals. Our data obtained from films with different thicknesses suggest that the observed phonon attenuation is due to scattering processes at the silicon-glass interface and not to a bulk effect in the glass film. The phonon mean free path at 300 GHz turns out to be larger than 4microm. We find evidence for the absence of inelastic scattering processes.     

Pulsed laser deposition of fluoride glass thin films

The development of integrated waveguide lasers for different applications such as marking, illumination or medical technology has become highly desirable. Diode pumped planar waveguide lasers emitting in the green visible spectral range, e.g. thin films from praseodymium doped fluorozirconate glass matrix (called ZBLAN, owing to the main components ZrF₄, BaF₂, LaF₃, AlF₃ and NaF) as the active material pumped by a blue laser diode, have aroused great interest. In this work we have investigated the deposition of Pr:ZBLAN thin films using pulsed laser radiation of λ = 193 and λ = 248 nm. The deposition has been carried out on MgF₂ single crystal substrates in a vacuum chamber by varying both processing gas pressure and energy fluence. The existence of an absorption line at 210 nm in Pr:ZBLAN leads to absorption and radiative relaxation of the absorbed laser energy of λ = 193 nm preventing the evaporation of target material. The deposited thin films consist of solidified and molten droplets and irregular particulates only. Furthermore, X-ray radiation has been applied to fluoride glass targets to enhance the absorption in the UV spectral region and to investigate the deposition of X-ray treated targets applying laser radiation of λ = 248 nm. It has been shown that induced F-centres near the target surface are not thermally stable and can be easily ablated. Therefore, λ = 248 nm is not suitable for evaporation of Pr:ZBLAN.     

磷酸盐激光玻璃聚(CH3)2SiO/SiO2防潮膜改性研究

采用溶胶-凝胶方法制备的(CH3)2Si(OC2H5)2预聚体涂膜液中,掺入碱催化的SiO2悬胶体涂膜液,采用旋涂法在掺钕磷酸盐激光玻璃棒端面涂制了改性的防潮膜。当涂膜液中(CH3)2Si(OC2H5)2和SiO2物质的量之比为1∶1时,热处理后的膜层耐摩擦性能明显改善;表面粗糙度的均方根值为1.245 nm;膜层激光破坏阈值大于15 J/cm2(1 053 nm,1 ns);在80 ℃和95%RH的封闭环境中静置336 h,膜层的透过率、接触角基本不变。结果证明膜层具有稳定的疏水结构和很好的防潮性能,膜层强度增加,耐磨擦能力提高,膜层使用期延长。     

磷酸盐激光玻璃聚(CH3)2SiO/SiO2防潮膜改性研究

 采用溶胶-凝胶方法制备的(CH₃)₂Si(OC₂H₅)₂预聚体涂膜液中，掺入碱催化的SiO₂悬胶体涂膜液，采用旋涂法在掺钕磷酸盐激光玻璃棒端面涂制了改性的防潮膜。当涂膜液中(CH₃)₂Si(OC₂H₅)₂和SiO₂物质的量之比为1∶1时，热处理后的膜层耐摩擦性能明显改善；表面粗糙度的均方根值为1.245 nm；膜层激光破坏阈值大于15 J/cm²(1 053 nm，1 ns)；在80 ℃和95%RH的封闭环境中静置336 h，膜层的透过率、接触角基本不变。结果证明膜层具有稳定的疏水结构和很好的防潮性能，膜层强度增加，耐磨擦能力提高，膜层使用期延长。     

Domain microscopy in chalcogenide alloy glass thin films

We report the observation of very well resolved polygonal domain (not island) trough networks in thin glass films. The linear troughs are observed by electron microscopy to be as narrow as 30 A and the hexagonal domain diameters are as large as 1000 (± 100) A. The composition dependence of the average diameters of the polygonal structures correlates well with the glass-forming tendency and agrees very well with the predictions of a recent topological model.     

Dewetting of thin polymer films near the glass transition

Dewetting of ultrathin polymer films near the glass transition exhibits unexpected front morphologies [G. Reiter, Phys. Rev. Lett. 87, 186101 (2001)]. Here, we present the first theoretical attempt to understand these features, focusing on the shear-thinning behavior of these films. We analyze the profile of the dewetting film, and characterize the time evolution of the dry region radius, R(d)(t), and of the rim height, h(m)(t). After a transient time, depending on the initial thickness, h(m)(t) grows like square root[t] while R(d)(t) increases like exp(square root[t]). Different regimes of growth are expected, depending on the initial film thickness and experimental time range.     

Characterization of heat-treated tungsten thin films using picosecond duration thermoelastic transients

Picosecond duration thermoelastic transients generated with femtosecond lasers have seen considerable use as a means to characterize sub-surface properties of thin metal films and interfaces. Frequently only the acoustic information is analyzed, because valuable information is readily available by quantifying relative changes in the observed signal, such as film thickness as determined from acoustic time-of-flight measurements. The simulations presented here model material transients in metals with a two-temperature parabolic model of thermoelasticity, which shows good agreement with experimental measurements. Simulations are conducted such that direct comparisons with experimental measurements allow determination of the relevant physical properties of the sample. Using these comparisons, the variations of the thermal conductivity of tungsten thin films that occur as a result of heat treatment are reported and the accompanying variation in the electrical conductivity is calculated.     

Adsorption of thin films of titanium on tungsten (111) surface

The structural and electronic properties of titanium-covered (111) surface of tungsten are studied via first-principles density-functional calculations. The dominant role played by substrate–adlayer interactions in the formation of thin films of Ti on W(111) explains the observed isomorphous growth for Ti depositions up to a full physical overlayer, with adatoms most favorably arranged in substrate-lattice-continuation sites. The optimized atomic configurations indicate substantial interplane relaxations following the contraction–contraction–expansion pattern, characteristic also of the clean W(111) substrate. Modification of the surface electronic properties of the Ti/W(111) system is examined for increasing adsorbate coverage with emphasis placed on the formation and localization properties of adsorbate-induced resonances.     

Excimer laser ablation of thin titanium oxide films on glass

Thin titanium dioxide films are deposited on glass substrates by magnetron sputter deposition. They are irradiated in air, by means of a KrF excimer laser. The ablation rate is measured as a function of the laser fluence per pulse, F, and of the number of pulses, N. Above a fluence threshold, the films are partially ablated. The ablated thickness does not vary linearly with N. This is the signature of a negative feedback between the film thickness and the ablation rate. The origin of this negative feedback is shown to be due to either thermal or electronic effects, or both. At high F, the film detachs from the substrate.     

Proton conductive amorphous thin films of tungsten oxide clusters with acidic ligands

Three kinds of peroxo-polytungstic acid (PPTA, C-PPTA and N-PPTA) were obtained by reacting hydrogen peroxide with metallic tungsten, tungsten carbide or tungsten nitride, respectively. Polytungstates, C-PPTA and N-PPTA, were found to contain oxalate and nitrate ligands. Their proton conductivities were compared using thin film specimens spin-coated from their water solution. Conductivity of each as-coated film was in the range from 10⁻³ to 10⁻⁴ S cm⁻¹ under the relative humidity of 40% (25 °C). A sharp decrease in conductivity (to less than 10⁻⁷ S cm⁻¹ at 25 °C) was observed for PPTA without acidic ligands after thermal treatment at 80 °C. However, the effect of thermal treatment on C-PPTA or N-PPTA was much milder. A 80 °C-treated C-PPTA film showed the conductivity of 1.0 × 10⁻⁵ S cm⁻¹ (25 °C) with a very weak dependency on ambient humidity.     

Investigation of electrochromic tungsten trioxide thin films prepared by the ILGAR method

An ion layer gas reaction dip coating process for the deposition of tungsten trioxide has been developed. Thin films of electrochromic tungsten trioxide with thicknesses of up to 150 nm were prepared. The films were found to be microcrystalline by X-ray diffraction analysis. The growth rate of the films was measured by profilometry. The chemical diffusion coefficient of lithium was investigated as a function of the concentration of lithium by the electrochemical galvanostatic intermittent titration technique. The chemical diffusion coefficient was found to increase slightly from 7×10⁻¹² to 3×10⁻¹ cm²/s, with x increasing from 0.2 to 0.8 in Li _x WO₃.     

BK7玻璃保护层掺钕磷酸盐玻璃波导特性

实验研究了基于KNO3稀释AgNO3混合熔盐离子交换法制备的镀有BK7玻璃保护层的N31型掺钕磷酸玻璃平面波导。采用棱镜耦合技术测试了其波导的有效折射率,应用IWBK方法拟合了其折射率分布。实验结果表明:在一定的扩散时间(5～7h)和交换温度(360～380℃)范围内,KNO3与AgNO3混合熔盐比对波导制备的影响起主导作用,引起的表面折射率的变化可达到0.025;BK7玻璃保护层对热离子交换掺钕磷酸盐玻璃表面起到了很好的保护作用,获得了良好的导模传输特性,其光传输损耗约为0.9dB/cm。     

Hydrogen adsorption on thin low temperature deposited unsintered gold films

It has been found that molecular hydrogen does adsorb at 78 K on unsintered thin gold films deposited at low temperature, while sintering of these films at 420 K precludes H₂ chemisorption. The adsorbate is characterized by a single TD peak with a maximum placed at 125 K. The rate of desorption fits neither the first nor the second order kinetic equations exactly.     

Nanostructured tungsten oxide thin films by the reactive pulsed laser deposition technique

Preparation of nanostructured tungsten oxide thin films using the reactive pulsed laser ablation technique is reported. The structural, morphological, optical and electrical properties of deposited films are systematically studied by changing the ambient oxygen pressure (pO₂). Structural dependence of tungsten oxide films on ambient oxygen pressure is discussed using grazing incidence X-ray diffraction (GIXRD) and micro-Raman spectra. The section analysis using atomic force microscopy exposed the smooth surface features of the deposited films. The blue shift in optical bandgap with an increase in ambient oxygen pressure is expounded in terms of electronic band structure of tungsten oxide. The influence of oxygen pressure on optical constants like extinction coefficient, band edge sharpness, refractive index and optical bandgap is also conveyed. The temperature variation of electrical resistance for films deposited at 0.12 mbar furnishes evidence for its semiconducting nature. PACS 68.55-a; 72.80.Ga; 81.15.Fg; 81.07.Bc; 78.68.+m; 78.20.Ci     

Dynamics of excimer laser ablation of thin tungsten films monitored by ultrafast photography

The time course of laser light induced transport of tungsten films from a glass support is followed by ultrafast photography using delayed dye laser pulses. The photographs provide unambiguous evidence that the material transport in the 40–200 mJ/cm² intensity domain takes place via removal of solid pieces from the film material. These results are consistent with heat flow calculations which predict the overall melting of the metal layer above 380 mJ/cm². The series of photographs presented give detailed insight into the melting process and have revealed an unexpected in-flight phase separation of solid fracture pieces and molten droplets throughout the 200–900 mJ/cm² domain. The faster propagating molten droplets form a condensed halo in front of the solid pieces, thereby providing an efficient shield between the processing laser light and the solid phase.