Role of Grain Size in Redox Induced Thin Film Stress in Ceria Systems

Transitioning from the micro to nano morphology scale can result in dramatic changes in electrical and optical properties of films. Space charge effects and creation of a significantly greater fraction of interfaces/grain boundaries are believed to give rise to such size scale effects. While electrical properties have generally dominated research in this area, mechanical properties, such as film stress and strain, also exhibit scaling effects. In-situ wafer curvature measurements are employed to measure these stresses on pure and 10% Praseodymium doped ceria (PCO) thin films during oxidation-reduction cycling and over a range of temperatures and oxygen partial pressures. These measured stress values are compared to the values reported for the bulk PCO and also to the thin film defect equillibria model developed by some of the co-authors and was found to fit well with the predicted values. In both the pure ceria and PCO thin films, the compositional stresses increased with decreasing grain sizes thus corroborating the idea that the space charge effects near surfaces and interfaces has a significant impact on the compositional stresses.