Cooperative Effects of Interface Symmetry, Redox Conditions and Low-Thickness to Improve Polarization in Ferroelectric Hf_0.5Zr_0.5O₂ Films

The ferroelectric phase of hafnia is metastable, and its stabilization is achieved by appropriate doping and generally only in ultrathin films where the contribution of surface energy is relevant. Other factors, such as interfaces and point defects such as oxygen vacancies, can affect the formation energy of competing polymorphs. Understanding the role of these factors is important to achieve further control over the stabilized phases and, thereby, improve ferroelectric polarization. To gain insight into the role of defects and stress at interfaces, we have compared a series of Hf_0.5Zr_0.5O₂ epitaxial films of various thicknesses. The films were grown on (001) and (110) oriented SrTiO₃ substrates to impose different symmetries at the interface and were deposited in a pure O₂ or a mixed O₂/Ar atmosphere to vary the oxidation conditions. We find that both factors are critical, with polarization maximized in films on (110)-oriented substrates and prepared under reducing conditions. Irrespective of the used substrate and atmosphere, polarization rapidly decays for thicknesses above 10 nm, indicating the relevance of the surface energy. Strain is thickness dependent, varying differently depending on the substrate orientation, but not on the deposition conditions investigated. Strain-thickness and polarization-thickness dependencies are not correlated, signaling that strain does not have a direct influence on the ferroelectricity of the films. Thickness, oxidation conditions, and epitaxial stress can contribute synergistically, and films with an optimal selection of these parameters have the ferroelectric polarization expected for pure orthorhombic phase films.