  Ferroelectric HfO2 films are usually polycrystalline and contain a mixture of
polar and nonpolar phases. This challenges the understanding and control of
polar phase stabilization and ferroelectric properties. Several factors such as
dopants, oxygen vacancies, or stress, among others, have been investigated and
shown to have a crucial role on optimizing the ferroelectric response. Stress
generated during deposition or annealing of thin films is a main factor
determining the formed crystal phases and influences the lattice strain of the
polar orthorhombic phase. It is difficult to discriminate between stress and
strain effects on polycrystalline ferroelectric HfO2 films, and the direct
impact of orthorhombic lattice strain on ferroelectric polarization has yet to
be determined experimentally. Here, we analyze the crystalline phases and
lattice strain of several series of doped HfO2 epitaxial films. We conclude
that stress has a critical influence on metastable orthorhombic phase
stabilization and ferroelectric polarization. On the contrary, the lattice
deformation effects are much smaller than those caused by variations in the
orthorhombic phase content. The experimental results are confirmed by density
functional theory calculations on HfO2 and Hf0.5Zr0.5O2 ferroelectric phases.
