Correlation between surface chemistry, density, and band gap in nanocrystalline WO3 thin films.

Nanocrystalline WO(3) thin films were produced by sputter-deposition by varying the ratio of argon to oxygen in the reactive gas mixture during deposition. The surface chemistry, physical characteristics, and optical properties of nanocrystalline WO(3) films were evaluated using X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray reflectivity (XRR), and spectrophotometric measurements. The effect of ultramicrostructure was significant on the optical properties of WO(3) films. The XPS analyses indicate the formation of stoichiometric WO(3) with tungsten existing in fully oxidized valence state (W(6+)). However, WO(3) films grown at high oxygen concentration (>60%) in the sputtering gas mixture were over stoichiometric with excess oxygen. XRR simulations based on isotropic WO(3) film-SiO(2) interface-Si substrate modeling indicate that the density of WO(3) films is sensitive to the oxygen content in the sputtering gas. The spectral transmission of the films increased with increasing oxygen. The band gap of these films increases from 2.78 to 3.25 eV with increasing oxygen. A direct correlation between the film density and band gap in nanocrystalline WO(3) films is established on the basis of the observed results.

X-ray photoelectron spectroscopy depth profiling of La2O3/Si thin films deposited by reactive magnetron sputtering.

The La(2)O(3)/Si thin films have been deposited by reactive DC magnetron sputtering. Amorphous state of La(2)O(3) layer has been shown by RHEED observation. Top surface chemistry of the a-La(2)O(3) has been evaluated with layer-by-layer depth profiling by ion bombardment and XPS measurements. It was found by core level spectroscopy that the top surface of the a-La(2)O(3) film consists of hydrocarbon admixture, lanthanum carbonate, and hydroxides that formed as a result of contact with air atmosphere. Thickness of this top surface modified layer is below 1 nm for a contact time of ~1.5 h with air at normal conditions.

Effect of structure and size on the electrical properties of nanocrystalline WO3 films.

Nanocrystalline WO3 films were grown by reactive magnetron sputter-deposition by varying the substrate temperature in the range of 303(RT)-673 K. The structure and electrical transport properties of WO3 films were evaluated using X-ray diffraction and dc electrical conductivity measurements. The effect of ultramicrostructure and grain-size was significant on the electrical properties of WO3 films. DC conductivity variation of the WO3 films measured in the temperature range of 120-300 K reveals their semiconducting nature. The temperature dependent electrical conductivity curves exhibit two distinct regions indicative of two different types of electrical transport mechanisms. Analysis of the conductivity indicates that the small polaron and variable-range-hopping mechanisms are operative in 180-300 K and 120-180 K temperature regions, respectively. The density of localized states at the Fermi level, N(EF), has been calculated and it was found to be ∼1×10(19) eV(-1) cm(-3) for all the films.