It must be noticed however, that in polycrystalline films the average grain size varies with the film thickness and this can result in misleading interpretations of the intercept value on the y axis (conductance) of the diagrams shown in Fig. 1. As a matter of fact only the intercept of the pure films grown on Al2O3 correspond to a depletion of the charge carrier (electrons) at the film/substrate interface .
Grain boundary (GB) effects are able to strongly influence the conduction properties in many oxides. Here CeO2 thin films of different doping content (pure, acceptor doped, donor doped) are investigated. After microstructure characterization (with XRD, FIB, TEM, etc.) the conductivity of the samples is investigated with electrochemical impedance spectroscopy (EIS) as a function of temperature and pO2. The films with the high GB density (and therefore with the strongest GB effects) show a drastic change in the conductivity for several orders of magnitude (Fig. 2) and other unusual phenomena, such as a partially electronic conductivity of heavily doped samples at high pO2 and low temperatures (Fig. 3) [2,4]. Such results can be quantitatively explained within the framework of the space charge model.