Electro-chemo-mechanical interplay in hafnia-zirconia thin films and heterostructures

The ongoing climate crisis has brought to light the urgent necessity for transformative solutions in more efficient energy use and renewable energy generation. Ferroelectric materials are a constitutive component in many solutions to this struggle: thanks to their inherent electronic polarisation, they are at the forefront of novel computing concepts such as ferroelectric field effect transistors and ferroelectric-based 3D memories, which aim to reduce the energy consumption of integrated circuits, enable faster computations and increase the density of components. The piezoelectric properties of ferroelectrics also make them desirable for converting between mechanical and electrical energy, which is extensively used in energy harvesters, sensors, actuators and transducers for applications ranging from non-destructive testing to medical ultrasound.

HfO₂-based materials have become promising candidates for the next generation of ferroelectric devices following the recent discovery of their ferroelectric properties, particularly thanks to their proven compatibility with semiconductor processing techniques and extensive use as a gate dielectric. The ferroelectric properties of HfO₂, and the related solid solution Hf_0.5 Zr_0.5 O₂ (HZO), have been thoroughly studied, but less attention has been given to its piezoelectric, and particularly electrostrictive, properties. The mechanisms underlying HZO-based piezoelectricity have largely remained unclear. Still, electrochemical effects change the oxygen concentration and crystal phase of the film, which seem to have a pronounced effect on its ferroelectric and piezoelectric behaviour. It is therefore necessary to investigate the interplay between electronic, chemical and mechanical effects to push the understanding of ferroelectricity, piezoelectricity and electrostriction in HZO.

In this thesis, we investigate the effect of depositing HZO thin films onto electrochemically active layers: Gd-doped CeO₂ (CGO) and Y-stabilised ZrO₂ (YSZ). HZO grows cube-on-cube onto the CGO and YSZ layers, forming an epitaxially coherent system. We demonstrate the tuning of HZO’s piezoelectric and ferroelectric properties and analyse the electrochemical response of the devices. We find that asymmetric ionic conductivity within the heterostructure enhances the leakage current under positive bias despite the ferroelectricity being maintained. When combining HZO and CGO films to form a multilayer heterostructure with 11 interfaces, we find that the interface engineering results in increased ionic conductivity within the sample and we show successful tuning of the system’s piezoelectric properties and impedance behaviour.

Furthermore, we characterise the electromechanical coupling in HZO thin films on YSZ crystals by laser interferometry. We observe first and second-order responses, despite the films being in the non-polar monoclinic phase of HZO. The first-order linear response displays an activation effect whereby the piezoelectric effect is very high above a threshold voltage but absent below it. This first-order response is not present in amorphous HZO or undoped HfO₂ films but is still observed in columnar HZO films deposited on SrTiO₃ crystals. This activation effect is reminiscent of the field-induced phase transition between antiferroelectric and ferroelectric phases. We therefore hypothesise that the observed field-activated piezoelectric response is caused by an electric field or strain-induced phase transition from the monoclinic to a polar phase.

The second order response shows large electrostriction reaching M₁₁ = 1 × 10⁻¹⁴ m² V⁻² in amorphous, polycrystalline and epitaxial HZO films, but not in pure HfO₂ films. The electrostriction is thickness-dependent, with a decrease for thicker films. Extensive structural analysis of the samples suggests that thin HZO films have more local distortions and defects than other samples. We therefore propose that local distortions linked to defects induced by the Zr atoms in HZO play a pivotal role in the mechanism behind electrostriction.

Despite some lingering open questions, our results highlight the tight connection between electronic, chemical and mechanical effects in HZO. Our electrochemical tuning experiments show the key role played by oxygen vacancies in the ferroelectric switching process, as well as opening an avenue for using multilayer heterostructures as an unexplored method for engineering HZO’s properties via ferroionic effects. The electromechanical measurements suggest that HZO can show phase instability linked to large piezoelectricity under high electric fields. Furthermore, the demonstration of large electrostriction in HZO thin films cements it as a contender for microelectromechanical applications, particularly as a lead-free and silicon-compatible material.