Effect of grain size of sputtered cerium-oxide films on their electrical and kinetic behavior at high temperatures

Abstract

Following an annealing process of several hours duration at a temperature of at least 1100 °C, reactively sputtered cerium-oxide films with film thicknesses ranging from 0.5 to 3 μm show a dependence of electric conductivity on oxygen partial pressure similar to that of polycrystalline bulk material within the temperature range studied (700 to 1000 °C). But films with comparatively small grain structures have specific electrical conductivities that are as much as an order of magnitude higher than those of large-grained structures, let alone bulk materials. This outcome justifies the supposition that the carrier transport in CeO2−x thin films occurs in a grain barrier layer within which electrons are enriched. This negative carrier enrichment layer may be due to a positive surface charge. An investigation of the interaction between oxygen vacancies of CeO2−x thin films and the oxygen of the environment showed that for layer thickness of from 1 to 3 μm and temperatures of 700–1000 °C, the reaction of the oxygen molecules at the surface is always the kinetics-governing step. Above 950 °C the transport reaction through the laminar boundary zone on the surface determines the reaction kinetics. Under these conditions the volume diffusion of oxygen vacancies in the thin film proceeds more rapidly than the surface reaction or the gas-phase transport of oxygen molecules.