  The critical impact of epitaxial stress on the stabilization of the
ferroelectric orthorhombic phase of hafnia is proved. Epitaxial bilayers of
Hf0.5Zr0.5O2 and La0.67Sr0.33MnO3 electrodes were grown on a set of single
crystalline oxide 001-oriented, cubic or pseudocubic setting, substrates with
lattice parameter in the 3.71 - 4.21 A range. The lattice strain of the
La0.67Sr0.33MnO3 electrode, determined by the lattice mismatch with the
substrate, is critical in the stabilization of the orthorhombic phase of
Hf0.5Zr0.5O2. On La0.67Sr0.33MnO3 electrodes tensile strained most of the
Hf0.5Zr0.5O2 film is orthorhombic, whereas the monoclinic phase is favored when
La0.67Sr0.33MnO3 is relaxed or compressively strained. Therefore, the
Hf0.5Zr0.5O2 films on TbScO3 and GdScO3 substrates present substantially
enhanced ferroelectric polarization in comparison to films on other substrates,
including the commonly used SrTiO3. The capability of having epitaxial doped
HfO2 films with controlled phase and polarization is of major interest for a
better understanding of the ferroelectric properties and paves the way for
fabrication of ferroelectric devices based on nanometric HfO2 films.
