Boundary effects on the electrical conductivity of pure and doped cerium oxide thin films

Thin films of CeO2 (both nominally pure and 10 mol% gadolinium-doped) grown via pulsed-laser deposition were studied. The electrical conductivity of the samples was measured as a function of thickness, temperature and oxygen partial pressure (pO2) using impedance spectroscopy. As expected, undoped CeO2 exhibits electronic conductivity (with activation energy between 1.4 and 1.6 eV) whereas the highly doped samples are oxygen vacancy conductors (activation energy around 0.7 eV for epitaxial films). In order to investigate the influence of the nature of the substrate the thin films were grown on two different substrates, Al2O3 (0001) and SiO2 (0001), and compared. While the films grown on SiO2 exhibit a microstructure characterized by columnar grains, the films grown on Al2O3 are epitaxial. Notably, for films on both substrates the conductivity and activation energy vary with film thickness and exhibit remarkable differences when the films on different substrates are compared. In the case of the polycrystalline films (SiO2 substrate), the space charge layer effects of the grain boundaries dominate over the substrate–film interface effect. In the case of the epitaxial films (Al2O3 substrate), a small interface effect, probably due to a space charge layer or structural strain, is observed.