Low temperature dielectric properties of Ce0.8Gd0.2O1.9 films

Abstract

Although Gd-doped ceria is one of the most important and well-studied of oxygen ion conductors, the relationship between its mechanical and electrical properties is not completely understood. In particular the low temperature electrical behavior of Gd-doped ceria, and its response to mechanical strain, have not been characterized. We have used impedance spectroscopy (1Hz–1MHz) to investigate the dielectric properties of both Si substrate-supported and self-supported Ce0.8Gd0.2O1.9 thin (450±50nm) films in the temperature range of 35–440K. We find that the grain boundary electronic conductivity for both types of Ce0.8Gd0.2O1.9 films freezes out between 120 and 150K. Upon cooling to 40K, the effective dielectric constant of the substrate-supported films decreases uniformly, remaining within the range of 20.5±2.5. In contrast, all (17) self-supported films investigated exhibit small (~2%) but readily detectable instability of the dielectric constant between 80 and 140K. Furthermore, below 90K, the dielectric constant of the self-supported films depends on the applied voltage and displays hysteretic behavior. This strongly suggests that even below 100K, the self-supported films can undergo structural changes. Comparison of the lattice parameter at 300K and at 100K shows that the self-supported films contract upon cooling with a thermal expansion coefficient close to that of the bulk material, whereas the substrate-supported films exhibit a thermal expansion coefficient which is approximately twice as large. On the basis of our earlier findings concerning the inelastic behavior of Gd-doped ceria films, we propose that a probable explanation for the observed differences between the self-supported and the substrate-supported films is that in the self-supported films, oxygen vacancy-cerium complexes are able to undergo partial ordering. In substrate-supported films these changes are suppressed by the tensile strain imposed by the substrate upon cooling.