WO3 films prepared by thermal oxidation of magnetron-sputtered tungsten: Synthesis and properties

X-ray powder diffraction shows that a monoclinic WO2.90 film is formed during the thermal oxidation of 200-nm-thick magnetron-sputtered metallic tungsten on quartz substrates at T = 793 K. Temperature elevation to T = 840 K yields orthorhombic WO3 with preferred (001) orientation. Adsorption spectroscopy shows that these films have high transparency (∼90%) in the wavelength range 450–900 nm, and interference is observed in the transparency range. Two types of transitions are discovered: indirect transitions with the energies Egi = 2.77 and 2.41 eV and direct transitions with the energies Egd = 5.49 and 4.82 eV for the oxide films formed at 793 and 840 K, respectively. The tendency toward the increase in the transition energy with increasing annealing temperature proves that the crystallinity and order of the film improve.