Ultrathin flexible linear-piezoelectric ZnO thin film actuators: Tuning the piezoelectric responses by in-plane epitaxial strain

Linear piezoelectric materials and devices potentially serve as high-accuracy and fast-response sensors and actuators. However, linear piezoelectrics have been scarcely studied, while tuning the linear piezoelectric response of undoped ZnO remains a challenge. Here, linear piezoelectric zinc oxide (ZnO) thin film actuators are fabricated with a sandwich structure of AZO/ZnO/AZO epitaxially on CeO2/Hastelloy substrates, with AZO denoting the aluminum-doped ZnO electrodes and CeO2 denoting the cerium oxide buffer. Due to the lattice mismatch between ZnO and CeO2, compressive in-plane lattice strains are detected. Through varying the ZnO layer thickness, a strong correlation is found between the in-plane epitaxial strain and the piezoelectric constant which proves an effective method to improve the linear piezoelectric response of ZnO. Further, the better linear piezoelectric response of the thinner flexible ZnO layer resolves the controversy between the performance and the deformation-durability of flexible piezoelectric ZnO devices. The vertical piezoelectric constant of the ultrathin (∼32 nm) ZnO film is ∼ 8.14 pm/V, which is highly suitable for positioners working at the picometer scale. Future works harnessing the epitaxial strains to further improve the piezoelectric constant of ZnO materials potentially lead to the prosperity of linear piezoelectrics.