Invited
Hafnia films processed from solutions - towards lead-free piezoelectrics

Discovery of ferroelectric HfO₂ (hafnia) thin films sparked an immense amount of fundamental and applied research activities because the material is well-known in the semiconductor industry, it is CMOS-compatible and does not contain toxic elements. One of its main benefits is a robust ferroelectricity at nanometre thicknesses, which is beneficial for memory applications. On the other hand, ferroelectricity tends to diminish with increasing thickness, which limits their applications in piezoelectric devices. Piezoelectricity in hafnia remains poorly understood and even controversial [1,2].

Using chemical solution deposition (CSD) method for the film growth, the thickness limitation of hafnia can be overcome. Ferroelectricity has been proven in the films with μm-range thickness, with piezoelectric coefficient d_33,f in the range of 10 pm V^-1. In the presentation we will briefly review current state-of-the art in the understanding of piezoelectricity in hafnia films. Then we will focus on our activities on CSD-processed La-doped HfO₂ films. We will show how 1 μm-thick piezoelectric films (d_33,f = 7.7 pm V^-1) can be grown and how remanent polarization can be doubled (from 7 to 15 μC cm^-1) through control of the microstructure and orientation [3,4]. In the last part, we will show our most recent activities on the influence of different parameters, i.e., annealing atmosphere, choice of the electrodes and nano-structuring, on stability of ferroelectricity in CSD-processed hafnia.

[1] Dutta et al., Piezoelectricity in hafnia, Nat. Comm., 12, 7301 (2021).

[2] Buragohain et al., Quantification of the electromechanical measurements by piezoresponse force microscopy, Adv. Mater., 34, 2206237 (2022).

[3] Schenk et al., Toward thick piezoelectric HfO₂-based films, Phys. Status Solidi-R, 1900626 (2019).

[4] Schenk et al., Enhancement of ferroelectricity and orientation in solution-derived hafnia thin films through heterogeneous grain nucleation, Appl. Phys. Lett., 118, 162902 (2021).