The Polar Phase and Ferroelectricity Properties of Hf0.5Zr0.5O2 Freestanding Thin Films

The polar phase and ferroelectricity properties of Hf0.5Zr0.5O2 freestanding thin films<br/>Xu-Heng Diao1, Siang-Hong Tan1, Shang-Jui Chiu2, Yen-Lin Huang1<br/>Department of Materials Science and Engineering, National Yang Ming Chiao Tung University Hsinchu, Taiwan.<br/>National Synchrotron Radiation Research Center, Hsinchu, Taiwan.<br/><br/>Hafnium-based oxide thin films, such as Hf0.5Zr0.5O2 (HZO) have great potential for nanoscale device applications with their low power, high-speed, non-volatility, and compatibility with modern CMOS technology. However, fabricating a stable phase of HZO thin films exhibiting robust ferroelectricity is not an easy task. The major mechanism to stabilize the ferroelectric phase of HZO is to utilize an epitaxial strain imposed from a lattice-mismatched substrate. This limits the process window for ferroelectric HZO thin films. In addition to strain effect, the chemical composition of the bottom electrodes and the oxygen vacancies level will also affect the polar stability of the HZO thin-film. How to stabilize the polar phase of HZO and explore the boundary of ferroelectricity of HZO thin films are the central challenges for HZO-related devices and applications.<br/>In this study, we demonstrate the robust ferroelectricity of HZO epitaxial thin films in three different types of phases, which are pure polar-O-phase(orthorhombic), pure nonpolar-M-phase (monoclinic) and mixed O and M-phases. We then obtain HZO films without substrates (freestanding) at a millimeter scale. Through XRR (X-ray Reflectivity) and XRD fringes, we found that the nonpolar-M-phase started to appear from the polar-O-phase after the thickness is thicker than 7 nm resulting in the suppression of ferroelectric polarization. Finally, we discuss the phase competition between the polar O-phase and non-polar M-phase with and without the substrate clamping effect. Our approach can provide fundamental insights into the limit of ferroelectric switching of HZO-based films and the role substrate plays in polar phase stabilization.