Thickness-Dependent Ferroelectricity in Freestanding Hf<sub>0.5</sub>Zr<sub>0.5</sub>O<sub>2</sub> Membranes
Yuchen Liu, Bo‐Cia Chen, Chia-Chun Wei, Sheng‐Zhu Ho, Yi‐De Liou, Puneet Kaur, Rahul, Yi‐Chun Chen, Jan‐Chi Yang
Abstract
High Resolution Image Download MS PowerPoint Slide In the recent years, ferroelectricity in Hf 0.5 Zr 0.5 O 2 (HZO) has been intensively studied due to its compatibility with silicon-based ferroelectric applications, high dielectric constant, and maintenance of robust ferroelectricity even at ultrathin thickness (<10 nm). This makes it a potential alternative material in ferroelectric field-effect transistors, complementary metal-oxide-semiconductor (CMOS) gate layers, and other applications. Previous studies have explained that ferroelectricity is attributed to the transformation from a tetragonal (t-phase) to orthorhombic (o-phase) structure during cooling. Conventional HZO thin films that are directly grown on silicon are polycrystalline, which reduces the polarization efficiency in CMOS devices. To address this problem, there is a demand for epitaxial freestanding HZO (FS-HZO) thin films that can be transferred without substrate constraints. In this study, we conducted comprehensive analysis of FS-HZO membranes with varying thicknesses to investigate the relationship between ferroelectric properties and thin film thickness. With monoclinic phase (m-phase) substitution, we notice a reduced o-phase fraction and gradual suppression of ferroelectricity when the thickness is over 20 nm. Owing to the large structure anisotropy, we further observed an increase in X-ray linear dichroism with increasing thickness. In addition, the dielectric constant of FS-HZO decreases by half compared to as-grown HZO due to increased leakage current from the cracks led by the freestanding process. This study presents an important method to combine oxides with silicon-based semiconductors and detailed information on various thicknesses of FS-HZO in aspects of structure and ferroelectricity, providing a potential solution to compatibility concerns in the context of next-generation nanoelectronics.