Ionic diffusion as a matter of lattice-strain for electroceramic thin films

Ionic conducting metal oxide thin films for Si-wafer based electroceramic devices are of high relevance to allow for new applications, quicker response times, and higher efficiencies. Metal oxide thin films are deposited via various processes on substrates. Depending on the synthesis method their microstructures differ on an atomistic length scale in their anionic and cationic lattice displacement fields and packing densities. Local changes in ionic bond strength can affect the oxygen migration barriers and ionic diffusion is no longer to be assumed as equal to zero-strained bulk material. This article proposes lattice strain measurements to characterize the defect states of metal oxide thin films that depend on their processing history and to correlate this measure for atomistic disorder. It is suggested to discuss differences in ionic conductivity observed relative to lattice strain and atomistic disorder in addition to other microstructure characteristics such as the grain size. The interplay of grain size, degree of crystallinity, phase changes and ionic conductivity are discussed with respect to lattice-strain for state-of-the-art ionic conducting thin films, i.e. ceria or zirconia solid solutions. Previous findings on the fields of compressive or tensile strain in epitaxial heterolayers or free-standing membrane films are discussed and compared with processing-dependent lattice strain of fully crystalline ceramic thin films of more than 200 nm in thickness. Finally, guidelines for processing of highly strained films with high ionic conduction are given for functional oxide thin films in Si-based electroceramic devices.