Ultra-thin epitaxial orthorhombic ferroelectric Hf_0.97Y_0.03O₂ films on La_2/3Sr_1/3MnO₃/SrTiO₃ substrate with different orientations

Ferroelectric hafnium-oxide (HfO₂) films have a great potential for integrated non-volatile memories, neuromorphic computation and micro-electro-mechanical systems (MEMS), thanks to its compatibility with mainstream semiconductor fabrication process. However, the high-quality ferroelectric single-crystal HfO₂ films, deemed as the consequence of the non-centrosymmetric orthorhombic o (111) phase, are still challenging to be reliably prepared. It is because various phase structures like monoclinic phase, tetragonal phase or cubic phase are possible to be formed accompanying with orthorhombic o (111) phase. The crystalline phase of HfO₂ is related to the doping, the lattice strain, the film thickness and the orientation of substrate, however the comprehensive understandings of different factors are still lacking. We report in this work the growth and optimization of o (1 1 1) phase Y-doped HfO₂ (HYO) films deposited on La_2/3Sr_1/3MnO₃/SrTiO₃ and comprehensively clarify the influence of the substrate orientation, the film thickness, and the oxygen pressure on crystal behaviors of HYO films. By comparing HYO/STO (0 0 1) with HYO/STO (0 1 1) systems, this work reports an insight into the orientation-dependent epitaxial relation, interplanar spacing, lattice constant and ferroelectric properties of o-HfO₂ (1 1 1) films. Eventually, an ultra-thin ∼5.5 nm epitaxial single-crystal o-HYO (1 1 1) film with a large ferroelectric polarization of ∼35.9 μC cm⁻² was obtained using LSMO/STO (0 1 1) substrate. These results are of great significance for the preparation and realization of high-quality epitaxial single-crystal ferroelectric o-HYO (1 1 1) ultra-thin films.