Exploring Ferroelectricity of Buffer-Free Epitaxial Hf_1-xZr_xO₂ Films

Ferroelectricity of hafnia-zirconia Hf_1-xZr_xO₂ (HZO) thin films is attracting a remarkable scientific interest due to its fundamental aspects and potential technological applications [1]. The solid solution of HZO films has been extensively investigated in polycrystalline form. On the other hand, fewer research groups have explored the stabilization of the ferroelectric phases by epitaxial growth. The most studied cases are HZO films grown on perovskite- and fluorite-type substrates buffered with bottom electrodes to enable the out-of-plane electrical characterization [2]. Consequently, the crystal structure and the orientation of the conducting layer dictate the specific phase, orientation and properties of the growing HZO films.

We report here our recent investigation of the structural and ferroelectric properties of epitaxial HZO films grown directly on single-crystalline fluorite substrates of yttria-stabilized zirconia (YSZ) without any bottom electrode. We have used two different deposition techniques, pulsed laser deposition (PLD) and polymer assisted deposition (PAD). This approach allows for the study of the effects of substrate symmetry and orientation, induced strain and growth thermodynamics on the stabilization of the various polymorphs of HZO. It is worth noting that PLD is a growth process far from thermodynamic equilibrium, thus favoring the stabilization of metastable ferroelectric phases of HZO, while PAD is a chemical solution deposition method based on the slow thermal decomposition of a polymer precursor and subsequent crystallization of the film in a near-equilibrium process.

Buffer-free epitaxial HZO films thus prepared exhibit a good crystal quality and remarkable structural selectivity depending on the substrate orientation. PLD-grown films show an orthorhombic structure and ferroelectric behavior on YSZ(111), the optimized thickness being approximately 7 nm. However, the growth on YSZ(001) stabilizes the monoclinic, paraelectric structure on [3]. ZrO_{<span style="font-size:10.8333px">2</span>} thin films have grown by PAD and show the same phase selectivity with the substrate orientation, regardless of the radically different growth dynamics of PLD and PAD. In spite of the absence of a metallic bottom electrode to perform the electrical characterization in a parallel-plate configuration, the thermally-activated oxygen conductivity of YSZ has been exploited to use the substrate as a buried floating electrode. Out-of-plane ferroelectric switching has been observed in 7 nm-thick orthorhombic Hf_0.5Zr_0.5O₂ films on YSZ(111) at a temperature of 185 °C and a frequency of 0.01 Hz. Monoclinic HZO films grown on YSZ(001) characterized by the same procedure show the expected paraelectric behavior [3].

References:
[1] U. Schroeder, M. H. Park, T. Mikolajick, C. S. Hwang, “The fundamentals and applications of ferroelectric HfO2” Nature Reviews Materials 7, 653–669 (2022)
[2] I. Fina, F. Sánchez: “Epitaxial Ferroelectric HfO₂ Films: Growth, Properties, and Devices”, ACS Appl. Electron. Mater. 3, 1530 (2021)
[3] E. Barriuso, R. Jiménez, E. Langenberg, P. Koutsogiannis, A. Larrea, M. Varela, C. Magén, P. A. Algarabel, M. Algueró, J. A. Pardo: “Epitaxy-Driven Ferroelectric/Non-Ferroelectric Polymorph Selection in an All-Fluorite System”, Adv. Electron. Mater. 10, 2300522 (2024)