The structure and electrical conductivity of vacuum-annealed WO3 thin films

• Published in: Thin solid films Vol. 467; no. 1-2; pp. 239 - 246
• Main Authors: GILLET, M, AGUIR, K, LEMIRE, C, GILLET, E, SCHIERBAUM, K
• Format: Journal Article
• Language: English
• Published: Lausanne Elsevier Science 22.11.2004
• Subjects: Cold working, work hardening; annealing, post-deformation annealing, quenching, tempering recovery, and crystallization; Cold working, work hardening; annealing, quenching, tempering, recovery, and recrystallization; textures; Condensed matter: electronic structure, electrical, magnetic, and optical properties; Conductivity of specific materials; Cross-disciplinary physics: materials science; rheology; Electronic transport in condensed matter; Exact sciences and technology; Materials science; Other crystalline inorganic semiconductors; Physics; Treatment of materials and its effects on microstructure and properties; Temperature dependence; Vacuum annealing; Cubic lattices; Inorganic compounds; Electrical conductivity; Vacancies; Transition element compounds; Binary compounds; Tungsten oxides; Crystal growth from vapors; Evaporation; RHEED; Experimental study; Monoclinic lattices; Surface structure; Thin films; Magnetrons; EEL spectroscopy; Acceptor center; 73.01A Tungsten oxide; Surface conductivity; Sensors; Radiofrequency sputtering
• ISSN: 0040-6090; 1879-2731
• DOI: 10.1016/j.tsf.2004.04.018

We have investigated the surface structure of monoclinic WO3 thin films on *a-Al2O3(0001) substrates during thermal treatments in ultra high vacuum and oxygen exposure. For an annealing temperature of 450 DGC, reflection high-energy electron diffraction shows the formation of a second WO3 phase with a pseudo cubic structure and the partial reduction to metallic tungsten clusters on the WO3 surface. Electron energy loss spectrometry confirms these changes of the surface structure. The reverse process, i.e. the oxidation of W, occurs if the samples are heated in air at 450 DGC. Under oxygen exposure films in the monoclinic WO3 phase follow a semiconductor like behaviour with an activation energy of -0.34 eV. Reduced WO3 films exhibit a metal-like conductivity with an activation energy of approximately zero. The temperature-dependent conductivities as well as the conductivity effects that we found upon CO exposures to the WO3 thin films depend much on the thermal pre-treatments and the associated surface structures. CO exposure leads to an increase of the conductivity; this acceptor-type interaction of CO with samples that have been annealed at 450 DGC is explained by a dissociation into carbon and oxygen, the latter interacting with oxygen vacancies.