Geometrical Anatomy for Oxygen Vacancies in Epitaxial Hf_0.5Zr_0.5O₂ Films Grown via Atomic Layer Deposition

The essential impact of epitaxial strain has been examined on stabilizing the ferroelectric orthorhombic phase of hafnia. The thermodynamic equilibrium of various polymorphs of HfO₂ under the influence of epitaxial strain has been widely studied in terms of the metastability of ferroelectric orthorhombic Pca2₁ phase. In addition, the primary defect in HfO₂, oxygen vacancy, has been extensively researched for its effects on functional properties such as ferroelectricity and resistive switching. However, the correlation between formation of oxygen vacancies and epitaxial strain in HfO₂ has not yet been thoroughly investigated. In this study, we investigate the selective impact of elastic strain on the formation of oxygen deficiencies, assessed through the optical response of epitaxial (001), (110), and (111)-oriented Hf_0.5Zr_0.5O₂ films grown by conventional atomic layer deposition for epitaxy. Optical spectroscopy was performed using ultraviolet–visible spectroscopic ellipsometry on Hf_0.5Zr_0.5O₂ films deposited on yttria-stabilized zirconia substrates. Notably, the (111)-oriented Hf_0.5Zr_0.5O₂ film exhibited a distinct shallow trap level. X-ray photoemission spectroscopy revealed a significant oxygen deficiency preference in the (111)-oriented thin film. Additionally, density functional theory calculations indicated that the formation energy of oxygen vacancies is lower under epitaxial strain conditions induced by a (111) substrate. This selective formation of oxygen vacancies in the (111)-oriented thin film suggests that the latent phenomena associated with oxygen defects in the functional Hf_0.5Zr_0.5O₂ film are attributed partly to the (111) directional strain.