Thickness-Dependent Ferroelectricity in Freestanding Hf_0.5Zr_0.5O₂ Membranes

In the recent years, ferroelectricity in Hf_0.5Zr_0.5O₂ (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.