Nanoporous cluster-assembled WOx films prepared by radio-frequency assisted laser ablation

The influence of substrate temperature and ambient gas pressure-composition on the characteristics of WO_x films synthesized by radio-frequency assisted pulsed laser deposition (RF-PLD) are studied with the aim to obtain nanostructured films with large surface area that appear promising for gas sensing applications. A tungsten target was ablated both in chemically reactive molecular oxygen at 5 Pa and in a mixed oxygen-helium atmosphere at 700 Pa. Corning glass was used as the substrate, at 473, 673 and 873 K. Other deposition parameters such as laser fluence (4.5 J/cm²), laser wavelength (355 nm), radio-frequency power (150 W), and target to substrate distance (4 cm) were kept fixed. The sensitivity on the deposition parameters of roughness, morphology, nanostructure and bond coordination of the deposited films were analysed by atomic force microscopy, scanning electron microscopy, transmission electron microscopy and micro-Raman spectroscopy. The role of the investigated process parameters to nanoparticle formation and to the development of an extended nanostructure is discussed.     

Physical properties of transparent conducting Cd-Te-In-O thin films: Outlining a thermodynamic system for transparent conducting oxides

Cd-Te-In-O thin films are grown by pulsed laser deposition using a composite target of CdTe powder embedded in an indium matrix. Oxygen pressures range from 2.00 to 6.67 Pa at a substrate temperature of 420 °C. The structure, optical transmission and sheet resistance of the films are measured. Substitutional compounds with In_2 − 2x(Cd,Te)_2xO₃ stoichiometry are found at high oxygen pressures. A ternary phase diagram of the CdO-In₂O₃-TeO₂ system shows the relationship between the structure and the stoichiometry of the films. To evaluate film performance, a figure of merit is proposed based on the relationship between the integral photonic flux and the sheet resistance. The best figure of merit values corresponds to a sample prepared at 3.8 Pa O₂ that consists of (In₂O₃)_0.3(CdTe₂O₅)_0.7 and exhibits an optical band gap of 3.0 eV. This sample is a suitable substrate for electrodeposition due to its good electrochemical stability.     

Influence of oxygen partial pressure on the structural and dielectric properties of Ba(Zr0.3Ti0.7)O3 thin films grown on (LaAlO3)0.3(Sr2AlTaO6)0.35 (001) using pulsed laser deposition

Epitaxial Ba(Zr_0.3Ti_0.7)O₃ thin films were grown on (LaAlO₃)_0.3(Sr₂AlTaO₆)_0.35 (001) single-crystal substrates by pulsed laser deposition at 700 °C in different oxygen partial pressures ranging from 6.7 Pa to 40.0 Pa. A strong correlation is observed between the structure and dielectric properties for the Ba(Zr_0.3Ti_0.7)O₃ thin films. The tetragonal distortion (ratio of in-plane and out-of-plane lattice parameter, a/c) of the films depends on the oxygen partial pressures. a/c varies from 0.989 at 6.7 Pa to 1.010 at 40.0 Pa, indicating the in-plain strain changes from compressive to tensile. The in-plain strain (either compressive or tensile) shifts the Curie temperature of the Ba(Zr_0.3Ti_0.7)O₃ thin films dramatically. Surface morphology and dielectric properties of Ba(Zr_0.3Ti_0.7)O₃ thin films have a strong dependence of the oxygen partial pressure. The film grown 26.7 Pa, which corresponds to a moderate in-plain tensile strain and a Curie temperature of ~ 30 °C, shows the largest relative permittivity, tunability and the best figure of merit in a broad frequency range (1 kHz-500 MHz), which may be a promising candidate for room-temperature microwave device applications.     

Structure of post-annealed ferroelectric PbZrxTi1-xO3 and SrBi2Ta2O9 thin films

In order to fabricate good quality ferroelectric thin films, PbZr_xTi_(1-x)O₃ (PZT) and SrBi₂Ta₂O₉ (SBT) films were fabricated on SiO₂/Si(100) substrates and on Pt/Ti/ SiO₂/Si(100) substrates by pulsed laser excimer deposition (PLD). X-ray diffraction, Rutherford backscattering analysis, and atomic force microscopy were used to characterize the structural properties of the samples, which were post-annealed at different temperatures. The results showed that the PZT and SBT films fabricated on Pt/Ti/SiO₂/Si(100) substrates and annealed at 700 °C exhibited optimum properties.     

Shadowed Off-Axis Pulsed Laser Deposition of YBa2Cu3O7−x Thin Films

Using a shadowed pulsed laser deposition allows us to reduce the number of droplets on the YBa₂Cu₃O_7?x film surface obtained by pulsed laser deposition method. In this study, we present a new modification of pulsed laser deposition method: a shadowed off-axis pulsed laser deposition. The substrate surface lies in the plain defined by the laser beam. A rectangular shadow mask located between target and substrate reduce the quantity of large particles deposited from the diffusion flow. Such geometry allows us to carry out the laser plume scanning on the target surface for simultaneous one-step double-sided deposition films with a droplet-free surface. Both films had good superconducting properties.     

Physical properties of transparent conducting Cd–Te–In–O thin films: Outlining a thermodynamic system for transparent conducting oxides

Cd–Te–In–O thin films are grown by pulsed laser deposition using a composite target of CdTe powder embedded in an indium matrix. Oxygen pressures range from 2.00 to 6.67 Pa at a substrate temperature of 420 °C. The structure, optical transmission and sheet resistance of the films are measured. Substitutional compounds with In_2 − 2x(Cd,Te)_2xO₃ stoichiometry are found at high oxygen pressures. A ternary phase diagram of the CdO–In₂O₃–TeO₂ system shows the relationship between the structure and the stoichiometry of the films. To evaluate film performance, a figure of merit is proposed based on the relationship between the integral photonic flux and the sheet resistance. The best figure of merit values corresponds to a sample prepared at 3.8 Pa O₂ that consists of (In₂O₃)_0.3(CdTe₂O₅)_0.7 and exhibits an optical band gap of 3.0 eV. This sample is a suitable substrate for electrodeposition due to its good electrochemical stability.     

Pulsed laser deposition of (WO3)1 − x(Nb2O5)x thin films: Characterization and gasochromic studies

A series of (WO₃)_{1 − x}(Nb₂O₅)_x (x = 0, 0.05, 0.1 and 0.15) mixed oxide films were fabricated by pulsed laser deposition (PLD) at 27 Pa oxygen partial pressure on ITO glass substrates. The thickness of the (WO₃)_{1 − x}(Nb₂O₅)_x thin films is about 350 ± 30 nm and their surface has a uniform morphology. A layer of platinum (Pt) was then sputtered onto the surface of the film. The hydrogen gas sensing performance of Pt catalyst activated (WO₃)_{1 − x}(Nb₂O₅)_x thin films were investigated. The cycling of the coloration was obtained from UV–vis spectra. Gasochromic coloration of (WO₃)_{1 − x}(Nb₂O₅)_x thin films were investigated at room temperature in H₂–N₂ mixtures containing 1–100 mol% of H₂. The results show that the shortest response time of (WO₃)_{1 − x}(Nb₂O₅)_x/Pt hydrogen sensor is within 30 s and the highest transmittance change (ΔT) varies from 20% to 30%.     

Substitution du tungstene par le tantale dans les bronzes de tungstene NaxTayW1−yO3

In order to study the modification of the transport properties of Na_xWO₃ by substitution in the WO₃ sublattice, single crystals of tantalum-substituted tungsten bronzes of formula Na_xTa_yW_1?yO₃ have been grown by electrolytic reduction of fused Na₂WO₄WO₃Ta₂O₅ mixtures. The lattice constant of their cubic perovskite like structure increases with Ta content. Electric conductivity measurements on Na_xTa_yW_1?yO₃ single crystals characterize a metal-non metal transition when x-y, i.e. the number of d electrons, decreases. This transition seems to be of Anderson type.     

Shadowed Off-Axis Pulsed Laser Deposition of YBa2Cu3O7?x Thin Films

Using a shadowed pulsed laser deposition allows us to reduce the number of droplets on the YBa₂Cu₃O_7–x film surface obtained by pulsed laser deposition method. In this study, we present a new modification of pulsed laser deposition method: a shadowed off-axis pulsed laser deposition. The substrate surface lies in the plain defined by the laser beam. A rectangular shadow mask located between target and substrate reduce the quantity of large particles deposited from the diffusion flow. Such geometry allows us to carry out the laser plume scanning on the target surface for simultaneous one-step double-sided deposition films with a droplet-free surface. Both films had good superconducting properties.     

Dependence of target-substrate distance on crystallographic and optical properties of WO3 films prepared by reactive radio frequency magnetron sputtering

WO₃ films were deposited on glass substrates using radio frequency magnetron sputtering in a mixed gas of 80%Ar-20%O₂. From the X-ray diffraction patterns, the WO₃ films deposited at source to substrate distance (D_T-S) of 40 mm were polycrystalline, crystallizing in the monoclinic crystal structure, with highly preferred c-axis orientation perpendicular to the film plane. On the other hand, amorphous films were observed in the WO₃ films deposited at 70 mm. The crystallite sizes of the WO₃ films deposited at D_T-S of 40 mm were larger for films deposited at lower working gas pressures (P_W). The optical absorption edge of the films shifted to shorter wavelength region with increase in P_W irrespective of the D_T-S. The oxygen deficient films are obtained when the films are deposited at D_T-S of 40 mm and P_W of 0.3 Pa and stoichiometric films are formed at higher P_W. The photocatalytic oxidation of methanol proceeds via CO and formaldehyde as the intermediate species on WO₃/TiO₂ bilayer films and CO₂ is identified as the final product.     

Preparation of inorganic solid state thin film for electrochromic application

All oxide solid state ITO (indium tin oxide)/Li_yWO_3−x/Li_1−zMn₂O₄/ITO stacked structure was deposited on a silica glass substrate by pulsed laser deposition for its electrochromic application. The Li doped amorphous tungsten trioxide Li_yWO_3−x thin film prepared at room temperature and in oxygen pressure of 7 Pa got the color of blue due to the mixture valence state of tungsten. We found that the amorphous Li_1−zMn₂O₄ thin film was suitable for the electrochromic application in spite of the low ion conductivity along in-plane direction. The ITO electrode thin film deposited at room temperature showed the relatively high transmittance and the usable conductivity. The transmittance at a wavelength of 750 nm for the ITO/Li_yWO_3−x/Li_1−zMn₂O₄/ITO stacked film changed from 50% to 80% by the applied voltage, while the transmittance at around 450 nm did not change. The blue-colored electrochromic property could be observed for the all oxide solid state film.     

Effects of ambient oxygen pressures and substrate temperatures in pulsed laser deposited Zn0.85Li0.15O thin films

Li doped zinc oxide Zn_1−xLi_xO (x = 0.15) thin films were grown by using the pulsed laser deposition method. The depositions were done onto Pt(111)/Ti/SiO₂/Si(100) substrate set at temperatures ranging from 300 °C to 700 °C, with varying the ambient O₂ pressure range of 3-20 mTorr. The effects of substrate temperatures and ambient O₂ pressures on the surface morphology and structural properties of the Zn_0.85Li_0.15O thin films were investigated by using the scanning probe microscopy and X-ray diffraction spectra, respectively. Also the chemical structures of the films were investigated by observing the X-ray photoelectron spectra of the core and shallower levels. We observed the deep blue PL emissions centered at about 390 nm (3.20 eV) from the Zn_0.85Li_0.15O thin films. It was investigated with respect to the ambient O₂ pressures during the deposition. It is considered that the deep blue PL emission in the Zn_0.85Li_0.15O thin film may be related to the incorporation of oxygen vacancies.     

Developments in hot-filament metal oxide deposition (HFMOD)

Hot-filament metal oxide deposition (HFMOD) is a variant of conventional hot-filament chemical vapor deposition (HFCVD) recently developed in our laboratory and successfully used to obtain high-quality, uniform films of MO_x, WO_x and VO_x. The method employs the controlled oxidation of a filament of a transition metal heated to 1000 °C or more in a rarefied oxygen atmosphere (typically, of about 1 Pa). Metal oxide vapor formed on the surface of the filament is transported a few centimetres to deposit on a suitable substrate. Key system parameters include the choice of filament material and diameter, the applied current and the partial pressures of oxygen in the chamber. Relatively high film deposition rates, such as 31 nm min^− 1 for MoO_x, are obtained. The film stoichiometry depends on the exact deposition conditions. MoO_x films, for example, present a mixture of MoO₂ and MoO₃ phases, as revealed by XPS. As determined by Li⁺ intercalation using an electrochemical cell, these films also show a colouration efficiency of 19.5 cm² C^− 1 at a wavelength of 700 nm. MO_x and WO_x films are promising in applications involving electrochromism and characteristics of their colouring/bleaching cycles are presented. The chemical composition and structure of VO_x films examined using IRRAS (infrared reflection-absorption spectroscopy), RBS (Rutherford backscattering spectrometry) and XPS (X-ray photoelectron spectrometry) are also presented.     

Plasma properties of CrOx films synthesized by a cathodic arc evaporation process

The plasma in a cathodic arc evaporation process used for the deposition of Cr_1−xO_x films was studied by an optical emission spectroscopy (OES). With the introduction of Ar and oxygen into the chamber at deposition pressures from 0.7 Pa to 2.7 Pa, high density of evaporated chromium catalyzes the decomposition of oxygen reactive gas, and induces the formation of Cr_1−xO_x films. Optical emission spectra including atomic and ionized Cr, excited and ionized oxygen revealed that excitation, ionization and charge transfer reactions of the Cr-O plasma occurred during the Cr_1−xO_x deposition process. A simplified empirical model which incorporates the relevant atomic processes in the gas phase with the chemical composition and deposition rate of the deposited Cr_1−xO_x coating was developed. Rhombohedral Cr₂O₃ and tetragonal CrO₂ were observed in the Cr_1−xO_x coatings deposited at higher pressure than 1.3 Pa. The Cr_1−xO_x coating depicted a dense and compact microstructure with well-attached interface.     

ZnO nanostructured film deposition using the separated pulsed laser deposition (SPLD) assisted by electric and magnetic drift motion

We have developed the separated pulsed laser deposition (SPLD) technique to prepare high quality ZnO based films exhibiting uniform and droplet-free properties. This SPLD consists of an ablation chamber and a deposition chamber which can be independently evacuated under different ambient gases.The gas species and the pressures in both chambers can be arbitrarily chosen for the specific deposition such as nanostructured films and nanoparticles. The ablation chamber is a stainless steel globe and the deposition chamber is a quartz tube connected to a metallic conic wall with an orifice. We used a KrF excimer laser with λ = 248 nm and 25 ns pulse duration. The different gas conditions in two chambers allow us to realize optimal control of the plasma plume, the gas phase reaction and the film growth by applying the bias voltage between the conic wall and the substrate under the magnetic field. We can expect that at appropriate pressures the electric and magnetic field motion (E × B azimuthal drift velocity) gives significant influences on film growth.We have deposited ZnO thin films at various pressures of ablation chamber (Pab) and deposition chamber (Pd). The deposition conditions used here were laser fluence of 3 J/cm², laser shot number of 30,000, Pab of 0.67-2.67 Pa (O₂ or Ar), Pd of 0.399-2.67 Pa (O₂), and substrate temperature of 400 °C. Particle-free and uniform ZnO films were obtained at Pab of 0.67 Pa (Ar) and Pd of 1.33 Pa (O₂). The ZnO film showed high preferential orientation of (002) plane, optical band gap of 2.7 eV, grain size of 42 nm and surface roughness of 1.2 nm.     

Dependence of ferromagnetic properties on growth oxygen partial pressure in boron-doped ZnO thin films

Boron-doped ZnO films were prepared by pulsed laser deposition technique. Magnetic, electrical, and optical properties of Zn_1?x B _x O films have been studied. It is found that the magnetic properties of the Zn_1?x B _x O films are sensitive to growth oxygen partial pressure. The films deposited under a high oxygen partial pressure of about 10?Pa appear to be ferromagnetic insulators at room temperature (RT). However, when the oxygen partial pressure decreases to 1.2?Pa, the films are non-ferromagnetic conductors at RT. Zn vacancies, which can be controlled by the oxygen partial pressure, are shown to be essential for realizing ferromagnetism (FM); on the other hand, the n-type nature of ZnO has no contribution to the FM observed in the B-doped ZnO films.     

Effect of doping and substrate temperature on the structural and optical properties of reactive pulsed laser ablated tin oxide doped tantalum oxide thin films

5% SnO₂ doped tantalum oxide (Ta₂O₅) films are deposited on quartz substrates at different substrate temperatures of 300 K, 773 K, 873 K and 973 K using pulsed laser deposition in an oxygen ambient of 0.002 mbar. Undoped Ta₂O₅ films are also deposited on quartz substrates kept at substrate temperature 973 K under the same oxygen ambient using PLD. The films are characterized using GIXRD, AFM, FTIR, micro-Raman and UV-visible spectroscopy. Undoped films show an amorphous nature even at a substrate temperature of 973 K, whereas, SnO₂ doped films show crystalline nature even for deposition at 300 K. As far as our knowledge goes, this is the first report of crystalline Ta₂O₅ films deposited at room temperature. The average size of the crystallites calculated using the Debye-Scherrer formula, shows that the size of the crystallite decreases with increase in substrate temperature. FTIR and micro-Raman spectroscopic analysis reveals the presence of Ta-O-Ta, O-Ta and O-Ta-O vibrational bands in the films. Raman analysis indicates that the addition of SnO₂ suppresses the bond formation and changes the magnitude of bonds in Ta₂O₅. AFM patterns reveal the formation of Ta₂O₅ nanorods of diameter about 100 nm for the doped film deposited at 973 K. Optical transmittance of the films is found to be sensitive to substrate temperature as well as to the presence of SnO₂. A blue shift in the band-gap of the doped films is observed. The decrease of band-gap with decrease of particle size observed for SnO₂ doped films can be due to a band-bending effect. The transmittance of the films is found to depend on SnO₂ doping and substrate temperature.     

Oxygen pressure and measurement temperature dependence of defects related bands in zinc oxide films

The ZnO films were fabricated by pulsed laser deposition at various oxygen pressure on single crystal silicon substrate. The structural and optical properties were investigated at various measurement temperature. The results showed that all the films have good c-axis preferred orientation. The different defects in films were fabricated which can be caused by various oxygen pressure. The films deposited at 1 Pa oxygen pressure have the most intense and narrow UV emission, and did not exhibit the deep band emission at the various measurement temperature. With the decrease of measurement temperature, the V_O-, O_i- and O_Zn-related band energy decreases, which is opposed to the V_Zn-related defects, meanwhile, the intensity of O_i-related emission peak has a sharp increase.     

Characterization of high quality tantalum pentoxide film synthesized by oxygen plasma enhanced pulsed laser deposition

Tantalum pentoxide films were deposited on BK7 glass substrates using oxygen plasma enhanced pulsed laser deposition (OPE-PLD). X-ray diffraction, atomic force microscopy, ultraviolet–visible–near infrared scanning spectrophotometry, and spectroscopic ellipsometry were used to characterize the crystallinity, microscopic morphology and optical properties of films. Results show that the film roughness increased with the increase of oxygen pressure, and decreased with the application of OPE. Meanwhile the use of oxygen plasma in a 2 Pa O₂ pressure resulted in the transmittance of the thin film of 91.8% at its peak position (the transmittance of bare substrate). Moreover, the root-mean-square roughness as low as 0.736 nm, and refractive index of 2.18 at 633 nm wavelength, close to the refractive index of bulk Ta₂O₅ (~ 2.20 at 633 nm wavelength), were obtained.     

Reduction of microwave dielectric losses in KTa1 − xNbxO3 thin films by MgO-doping

KTa_1 − xNb_xO₃ (KTN) thin films were grown by pulsed laser deposition on sapphire and MgO substrates. Their structural and high frequency dielectric characteristics evidenced the strong influence of the substrate and suggested possible KTN/MgO interdiffusion that could be responsible for the lower dielectric losses obtained on this substrate. Both undoped and 6% MgO-doped KTN thin films were then grown on sapphire. Dielectric measurements performed at 12.5 GHz by a resonant cavity perturbation method evidenced reduction of losses by MgO-doping. Loss tangent (tan δ) was reduced by a factor of 3 in comparison with undoped films grown on sapphire.