  Doped HfO2 thin films exhibit robust ferroelectric properties even for
nanometric thicknesses, are compatible with current Si technology and thus have
great potential for the revival of integrated ferroelectrics. Phase control and
reliability are core issues for their applications. Here we show that, in
(111)-oriented 5%La:HfO2 (HLO) epitaxial thin films deposited on
(La0.3Sr0.7)(Al0.65Ta0.35)O3 substrates, the flexoelectric effect, arising from
the strain gradient along the films normal, induces a rhombohedral distortion
in the otherwise Pca21 orthorhombic structure. Density functional calculations
reveal that the distorted structure is indeed more stable than the pure Pca21
structure, when applying an electric field mimicking the flexoelectric field.
This rhombohedral distortion greatly improves the fatigue endurance of HLO thin
films by further stabilizing the metastable ferroelectric phase against the
transition to the thermodynamically stable non-polar monoclinic phase during
repetitive cycling. Our results demonstrate that the flexoelectric effect,
though negligibly weak in bulk, is crucial to optimize the structure and
properties of doped HfO2 thin films with nanometric thicknesses for integrated
ferroelectric applications.
