Non-diffusive behavior of aluminum and yttrium dopants in ZrO2/Al2O3 and ZrO2/Y2O3 bilayer thin films†

Abstract

This study investigates the diffusive and non-diffusive behaviors of Al and Y dopants in ZrO₂/Al₂O₃ and ZrO₂/Y₂O₃ stacked thin films grown via atomic layer deposition (ALD), focusing on their interaction with the film's crystallization and grain growth. Contrary to the conventional diffusion theory, this work reveals that the diffusion in these nano-scale thin films is strongly influenced by the formation of through-grain structures rather than concentration gradients. Various thin film stacks analyzed by grazing incidence X-ray diffraction confirm this phenomenon. In the bilayer configurations, the Al and Y dopant layers do not diffuse into the adjacent ZrO₂ lattice since they do not necessarily interfere with the continuous grain growth of the ZrO₂ layer. However, when the dopant layers are embedded within the ZrO₂, which disrupt ZrO₂ grain growth at the insertion site, they must diffuse away to form the through-grains and thus lower the grain boundary energy. These findings indicate that the primary driving force for the Al and Y dopant diffusion in ZrO₂ thin films is lowering the grain boundary energy, not the concentration gradient. In contrast, thicker (>0.3 nm) Al–O layers maintain structural integrity and inhibit through-grain formation, resulting in no Al diffusion. These results offer insights for implementing dopant layers in various thin film applications.