Room Temperature Polarization Phenomena in Nanocrystalline and Epitaxial Thin Films of Gd-Doped Ceria Studied by Kelvin Probe Force Microscopy

Abstract

Thin films of the composition Ce0.9Gd0.1O2-δ and with a thickness of 200 nm were prepared on an Al2O3 (0001) substrate at 720°C or room temperature, to yield epitaxial or nanocrystalline materials. The effect of the microstructure on room temperature polarization were assessed by a combination of polarization with up to ±5 V using an AFM tip and a large silver back contact followed by surface potential mapping with Kelvin probe force microscopy. The nanocrystalline sample showed a low degree of polarization, which was reversible, and a fast relaxation process. The epitaxial thin film, in contrast, was already polarized at smaller biases and showed irreversible changes of the surface potential after polarization with +5 V. For lower biases, two distinct relaxation reactions were identified: a fast redistribution of charge over a large area in the first seconds after end of polarization and a slower process in the vicinity of the polarization contact, which took several minutes to relax. The slower polarization behavior is due to a bulk chemical diffusion process with a coefficient Dδ on the order of 10−17 m2·s−1, whereas the faster polarization can be assigned to a charge carrier redistribution within a space charge layer.