Compositional Stresses in Polycrystalline Titania Films

Certain transition metal oxides exhibit relatively large compositional changes in response to variations in the oxygen partial pressure. In thin film form, these composition changes can lead to stress because of the constraint imposed by the underlying substrate. To investigate these effects, polycrystalline titania (anatase phase) was grown by metal-organic chemical vapor deposition. The stress changes during oxidation and reduction of these films were then studied by monitoring substrate curvature at elevated temperatures in controlled atmospheres. Variations in the temperature and grain size of the films were investigated. These results were analyzed in terms of point defects. In particular, the total stress change observed during oxidation-reduction cycles increased with decreasing grain size. This suggests that grain boundaries are associated with significantly higher defect concentrations in these materials, and that compositional stress data can provide important information about the nature of these defects.