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 (similar to 32 nm) ZnO film is similar to 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.

Automated summary

Linear piezoelectric materials and devices have the potential to be high-accuracy and fast-response sensors and actuators. By varying the thickness of the ZnO layer, the linear piezoelectric response of ZnO can be improved, and thinner ZnO layers have better performance and deformation durability. Furthermore, utilizing epitaxial strains to enhance the piezoelectric constant of ZnO materials could lead to the prosperity of linear piezoelectrics.