Effect of Annealing on Flexoelectricity in Hafnium Oxide (HfO2)

TL;DR

This study investigates how high-temperature annealing affects the flexoelectric properties of hafnium oxide (HfO2), revealing that amorphous HfO2 has higher flexoelectricity than crystalline phases, with oxygen vacancies playing a key role.

Contribution

It provides new insights into the relationship between annealing-induced crystallization, oxygen vacancies, and flexoelectricity in HfO2, guiding material optimization.

Findings

Amorphous HfO2 exhibits the highest flexoelectric coefficient (105 pC/m).

Annealing reduces flexoelectricity, especially in nitrogen atmospheres.

Oxygen vacancies negatively impact the flexoelectric coefficient.

Abstract

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.