The role of oxygen partial pressure and annealing temperature on the formation of W=O bonds in thin WO3 films

Thin films of tungsten oxide were deposited onto silicon substrates using reactive rf sputtering. The structure of the films is strongly dependent on the conditions of deposition and post-treatment. Important issues are the influences of oxygen pressure during deposition and of annealing temperature. We used x-ray photoelectron spectroscopy to investigate the in-depth composition of the films. The most surface sensitive O 1s core level spectra are made up of two structures, one generated by photoelectrons emitted from oxygen atoms in WO3 (O-W-O) and other at lower energy generated by the photoelectrons emitted from oxygen atoms located at the boundary of the grains (W=O). Using Raman spectroscopy, an increase of the W=O/O-W-O ratio was correlated to an increase in the oxygen partial pressure used during the deposition. A decrease of this ratio was observed while annealing temperature was increased, which was correlated to an increase in the size of the grains that form the films.