Evolution of domain structure in epitaxial ferroelectric K0.5 N a0.5Nb O3 films grown by metal-organic vapor-phase epitaxy

Potassium sodium niobate lead-free piezoelectric thin films are of great potential for integrated functional devices. Metal-organic vapor-phase epitaxy enables high-quality wafer-scale fabrication of epitaxial piezoelectric thin films, whereas the film domain structure and evolution, which particularly originate from the strain and strongly impact the film properties, require further study. We report in this paper an in-depth study of the piezoelectric domain structure as well as the domain evolution induced by an external electric field in K0.5Na0.5NbO3 films. Epitaxially strained cpc oriented K0.5Na0.5NbO3 films were grown on SrRuO3/DyScO3 (110) substrates, and four different superdomain variants consisting of monoclinic 90∘ MC stripe domains were verified and studied in detail. The domain switching behavior was examined by piezoresponse force microscopy (PFM), which suggests that both the in-plane and out-of-plane polarization can be reversed by an external out-of-plane positive electric field. However, application of a negative bias leads to the accumulation of surface charges concurrently accompanied by small morphology changes. We tentatively attribute this observation to the injection of vacancies in the film surface under negative voltage. Therefore, the out-of-plane polarization component cannot be switched, and in-plane orientation of the stripe domains is only reoriented at the edges of the litho-window, which could be attributed to either an inhomogeneous electric field below the PFM tip or trailing fields caused by the movement of the PFM tip across the surface. However, surface charges dissipate within 1 h. These results are beneficial for understanding the domain evolution mechanism and intentional tuning of ferroelectric thin-film properties.