Effect of Annealing on Flexoelectricity in Hafnium Oxide (HfO2)

Flexoelectricity is universal in all dielectrics, effective at high temperatures, and a promising transduction technique for nanoelectromechanical systems (NEMS). However, as flexoelectricity is still in its early stages, many aspects require further investigation. Understanding how flexoelectricity depends on material parameters like crystallographic phase and how temperature might affect it, is important for selecting and optimizing the right material for technological applications. This work studies the influence of high-temperature annealing (and the consequent crystallization) in the flexoelectricity of hafnium oxide (HfO2), a material with significant technological relevance. We measure the flexoelectric coefficient for amorphous (not annealed) and annealed (slightly crystalline) phases of HfO2, with samples annealed in nitrogen or oxygen atmospheres. Our results indicate that the amorphous phase of HfO2 exhibits the highest flexoelectric coefficient (105 $\pm$ 10 pC/m), while annealed samples show a significant decrease, with the lowest value in nitrogen-annealed samples (26 $\pm$ 4 pC/m). Samples annealed in an oxygen atmosphere improve flexoelectric properties (54 $\pm$ 6 pC/m) compared to those annealed in nitrogen. Using cross-sectional imaging, X-ray diffraction, resonance frequency characterization, and relative permittivity measurements, we find that annealing promotes crystallization into the tetragonal phase and increases internal stress within the HfO2 layer, while most other parameters remain constant. We attribute the differences in flexoelectricity from the annealed samples to the quantity of oxygen vacancies in hafnium oxide. These oxygen vacancies in hafnium oxide seem to negatively affect the flexoelectric coefficient. This finding can be applied to optimize materials to enhance their flexoelectric properties.