Transferable epitaxial freestanding Hf_0.5Zr_0.5O₂ thin films with robust dielectric and ferroelectric properties

Bo-Cia Chen¹, Yu-Chen Liu^1,3*, Chia-Chun Wei¹, Chang-Yang Kuo², Yi-Chun Chen^1,3, Jan-Chi Yang^1,3

¹Department of Physics, National Cheng Kung University, Tainan 70101, Taiwan
²Department of Electrophysics, National Yang Ming Chiao Tung University, Hsinchu 30010, Taiwan
³Center for Quantum Frontiers of Research & Technology (QFort), National Cheng Kung University, Tainan 70101, Taiwan

* Presenter:Yu-Chen Liu, email:bob60734@gmail.com

Over the past decade, ferroelectricity in Hf_0.5Zr_0.5O₂ (HZO) has been an attractive topic due to the compatibility to silicon-based ferroelectric applications, including ferroelectric random-access memory and ferroelectric field-effect transistors. HZO has high dielectric constant and maintains robust ferroelectricity even at ultra-thin thickness (<10 nm), making it a potential alternative material in complementary metal-oxide-semiconductor (CMOS) gate layers. Previous studies have explained that the ferroelectricity is attributed to the transformation from a tetragonal (T-phase) to orthorhombic (O-phase) structure during cooling. Conventional HZO thin films grown on silicon are polycrystalline, which reduces the polarization efficiency in CMOS devices. To address this problem, we present 7 nm thick epitaxial O-phase freestanding HZO (FS-HZO) thin films. These films are transferable without substrate constraints and cater to the needs of high-κ dielectric devices.
In this study, we conducted thorough analysis of FS-HZO films with varying thicknesses and observed minimal changes when compared to their as-grown counterparts. 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 of x-ray linear dichroism when the thickness increased. In addition, the dielectric constant of FS-HZO decreases by half compared to as-grown HZO due to increased leakage current from the cracks leaded by freestanding process. The development of single-crystalline O-phase FS-HZO films with strong dielectric and ferroelectric properties bodes well for efficient ferroelectric-based devices. This study presents a versatile method for integrating oxides with silicon-based semiconductors, offering a potential solution to compatibility concerns for next-generation nanoelectronics.

Keywords: transferable Hf_0.5Zr_0.5O₂, freestanding, ferroelectricity, high dielectric