Publication
Flexoelectricity, the coupling between electric polarization and strain gradient, is a universal phenomenon in dielectric materials. When a material bends, electric charges are generated, and inversely, applying an electric voltage induces bending. This bidirectional electromechanical interaction becomes particularly significant at the nanoscale, making flexoelectricity a promising actuation and sensing technique for nanoelectromechanical systems (NEMS). Unlike piezoelectricity, flexoelectricity is not limited by the material symmetry or the Curie temperature, and its potential increases at the nanoscale. Despite these benefits, flexoelectricity is still in its infancy and faces some challenges. These include discrepancies between theoretical predictions and experimental measurements of flexoelectric coefficients, difficulties in isolating flexoelectric effects from piezoelectricity and other phenomena, and limited understanding of its behavior in amorphous materials and at nanoscale thickness. This thesis addresses these challenges through a comprehensive study focused on hafnium oxide (HfO2), a high dielectric constant material compatible with semiconductor processes. First, a new methodology capable of isolating the flexoelectric effect from piezoelectric, electrostatic, and electrostrictive contributions is introduced, achieving a detection threshold below 1 fC/m. This is six orders of magnitude lower than previously reported coefficients. Using this methodology, the flexoelectric coefficient of HfO2 is measured for the first time, obtaining 105⠯±⠯10⠯pC/m, which is also the first measurement of flexoelectricity in any amorphous material. Second, this thesis studies the influence of high-temperature annealing on the flexoelectric properties of HfO2. The measurements reveal a significant decrease in the flexoelectric coefficient after annealing, with samples annealed in a nitrogen atmosphere showing the lowest value of 26⠯±⠯4⠯pC/m and samples annealed in oxygen exhibiting an improved value of 54⠯±⠯6⠯pC/m. By using cross-sectional imaging, x-ray diffraction, resonance frequency analysis, and permittivity measurements, the changes are attributed to the formation of oxygen vacancies during annealing. These results suggest that oxygen vacancies could negatively impact the flexoelectric response, indicating that mitigating their effects could enhance performance. Third, this thesis presents the first measurement of flexoelectric currents in materials with nanoscale thickness, obtaining flexoelectric coefficients consistent with those obtained from inverse effect experiments. By changing the geometry of the measured devices, a 40% increase in the effective flexoelectric coefficient is achieved, emphasizing the role of device design in optimizing flexoelectric responses. An extensive literature review reveals a quadratic relationship between the flexoelectric coefficient and the relative permittivity, challenging the expected theo