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 H₂ partial pressure of 13.3 Pa at or near room temperature. Due to their substoichiometry and deposition method samples were named hot-wire WO_x (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 WO_x vapors produced in two steps: a) by the H₂ reduction of the superficial (native) oxide of the heated tungsten filament to WO_x and b) evaporation of the last. Vapors were composed of WO_x 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.