Epitaxial ferroelectric Hf_0.5Zr_0.5O₂ thin film on a buffered YSZ substrate through interface reaction

In this study, we used pulsed laser deposition to successfully grow epitaxial Hf_0.5Zr_0.5O₂ (HZO) films on (001)-, (011)- and (111)-oriented yttria-stabilized zirconia (YSZ) substrates using TiN as the bottom electrode. It is found that the TiO₂ buffer layer formed by the interface reaction is the key to epitaxial growth. The epitaxial HZO films (∼15 nm in thickness) exhibit ferroelectric behaviour with a remnant polarization of 7-30 μC cm^-2 and a coercive field of 1.1-2.3 MV cm^-1. Using piezoresponse force microscopy, polar domains can be written/read and reversibly switched with a phase change of 180° in all the films. X-ray diffraction and high-resolution transmission electron microscopy reveal the presence of nano domains, and a clear epitaxial relation among different layers whose interfaces are relaxed by reconstruction. X-ray absorption spectroscopy provides deep insight into the microstructural origin of ferroelectricity in HZO. A large interface strain stabilized ferroelectric state is observed which is manifested as the non-centrosymmetric Pca2₁ phase.