Substoichiometric hot-wire WOx films deposited in reducing environment

Substoichiometric tungsten oxide films were deposited on Si substrates by heating metallic filaments at temperatures of 920, 1020 and 1070 K at a total pressure of 133 Pa and H-2 partial pressure of 13.3 Pa at or near room temperature. Due to their substoichiometry and deposition method samples were named hot-wire WOx (hwWO(x)) films (x < 3). No optical gap was identified by optical (spectroscopic ellipsometry) measurements made on hwWO(x) films and resistivity was found to increase with temperature. This metallic character of hwWO(x) films was attributed to substoichiometry, which causes the creation of electronic states within the band gap and shifts the Fermi level in the conduction band. The growth of hwWO(x) films occurred from substoichiometric WOx vapors produced in two steps: a) by the H-2 reduction of the superficial (native) oxide of the heated tungsten filament to WOx and b) evaporation of the last. Vapors were composed of WOx particles with dimensions of 5-6 nm or clusters of such particles and condensing on the cold substrate form hwWO(x) films. The substoichiometry of hwWO(x) samples was related to the presence of many unsaturated bonds in them, which re-constructed easily upon thermal annealing above 420 K or even after exposure to the electron beam of the transmission electron microscope leading to the formation of crystallites with dimensions of 5 nm within samples. (c) 2011 Elsevier B.V. All rights reserved.