HfO2-based ferroelectrics have attracted a huge interest owing to their robust polarization at nanometer scale and their full compatibility with CMOS technology. Hafnia and related compounds exhibit quite unconventional properties as compared to the prototypical perovskite ferroelectrics, such as wake-up or fluid imprint. The piezoelectric behavior in hafnia-based thin films has also raised questions as different behaviors have been observed: there are reports of positive d33 coefficient for some samples and of negative ones for other samples. Moreover, apparent switching of domains against the applied electric field (“anomalous switching”) has been reported.
In this talk, we will present and discuss the observation of a unique phenomenon for HZO under electrical cycling: an electrically induced change of the piezoelectric coefficient d33 sign in W/HZO/W capacitors. The capacitors were characterized by piezoresponse force microscopy (PFM) and local PFM switching spectroscopy.
We show that, upon electric field cycling, the ferroelectric HZO capacitors undergo a complete uniform inversion of the piezoelectric d33 coefficient sign, from positive to negative. For half positive and negative regions, with proper training, the capacitor exhibits a net zero piezoelectric response while it keeps a robust and fully switchable polarization Pr of more than 20 µC/cm2. In addition, local spectroscopy measurements reveal a continuous decrease of the magnitude of d33 upon ac cycling, from positive down to zero, followed by the d33 sign inversion and by a gradual increase of the negative d33 magnitude. Every single location of the ferroelectric capacitor undergo such a change passing through zero local piezoelectricity upon suitable number of ac cycles.
Density functional theory calculations suggest a mechanism for the d33 sign inversion that will be discussed. They also predict a groundbreaking result: the possible occurrence of an intrinsic non-piezoelectric ferroelectric compound. Experimental results pointing to it will be discussed.
Our study clears the current mystery regarding the sign of piezoelectricity in HfO2-based ferroelectrics. Piezoelectricity in HZO is not an invariable parameter but is a dynamic entity that can be changed, in the very same material, by an electrical ac field. These findings open unprecedented perspective for engineering the electromechanical functionality of ferroelectric HfO2-based devices.

In metal-ferroelectric-insulator-metal stacks, the depolarizing field (Edep) emerging from incomplete screening of the polarization charges at the insulator side is of paramount importance. Ferroelectric tunnel junction (FTJ) exploits such asymmetric screening to obtain a different barrier of electron injection depending on the polarization direction, resulting in different tunneling currents. The change of the device resistance can then be used to store information in a remarkable energy efficient way. FTJs are therefore intensively explored for next generation low power memristive devices. The memristive functionality stems from the ability to gradually tune the current magnitude by tuning the fraction of switched ferroelectric domains. Thanks to its crystallization temperature fully compatible with the thermal limitations of CMOS circuits, bilayer Hf0.5Zr0.5O2(HZO)/Al2O3-based FTJs have gained a lot of interest. In this work, we discuss the role of Al2O3 processing conditions on the ferroelectric stability of BEOL-compatible W/Al2O3(3 nm)//HZO(10nm)/TiN FTJs. A sizeable ferroelectric polarization is only obtained when the top W/Al2O3 layers are annealed together with HZO in the same annealing process. X-ray photoemission and electron energy loss spectroscopy techniques were employed to investigate the physical chemistry of the buried interfaces and identify the possible origin of the ferroelectric stability. Investigations of Hf 3d and Al2p core level spectra in addition to Al L-2,3 near edge structure revealed signatures of increased oxygen vacancy (VO) amount at the Al2O3/HZO when both materials are annealed in the same process. A larger VO concentration at the insulator/ferroelectric interface implies increased density of trap states allowing in turn higher trapping/de-trapping of electric charges, essential in stabilizing the ferroelectric polarization. In the absence of sufficient trap density at the interface and 3 nm thick un-annealed Al2O3 insulating barrier, Edep in HZO is large and lead to the absence of a sizeable polarization. The work highlights the role of the processing condition and VO on the ferroelectric stability in bilayer structures and identifies the number of fixed charges at the ferroelectric/insulator interface as an engineering route to control Edep and its consequences on device metrics.

In the pursuit of advancing the dielectric materials, zirconium oxide and zirconium-rich ZrxHf1-xO2 exhibited antiferroelectric-like behavior. These films have attracted attention due to their potential for high switching stability, thus carrying significant implications for the advancement of cutting-edge technologies, including the development of high-performance non-volatile memory devices, high-density energy storage devices, and supercapacitors. In the context of these applications, the factors influencing the formation of the ferroelectric properties in these films are crucial. However, a limited understanding of the responsible mechanisms for the variation of ferroelectric properties has impeded their implementation in technology.

This study investigates the influence of hafnium content, biaxial strain, and thermodynamic conditions for the formation of parasitic interfacial layers, which in turn influences the anti-ferroelectric to ferroelectric phase transition energy barrier of zirconium-rich ZrxHf1-xO2 films. Incorporating hafnium into zirconium linearly reduces the energy barrier for the anti-ferroelectric to ferroelectric phase transition and reduces the thickness of the parasitic interface. This study not only highlights the impact of hafnium on phase transition energetics but also establishes a comprehensive correlation between in-plane tensile strain and phase transition voltage. In addition, charge trapping or de-trapping and defect redistribution were studied by tuning the electric field cycling amplitude. Bipolar electric field cycling at 4.1 MV/cm resulted in the lowering of the phase transition field in both positive and negative hysteresis branches, with the rate of change being higher in the positive hysteresis branch compared to that of the negative hysteresis branch. The lowering of the energy barrier with field cycling is linked to the breakdown of the interface, which in turn lowers the in-plane tensile strain. The investigation also showed hafnium incorporation reduced leakage current density, providing an insight into the direction of formation and regulation of oxygen vacancies and alternative conduction mechanisms in ZrxHf1-xO2 films.

Thus, this study provides a novel basis for the development of doped ZrO2/HfO2-based thin film stacks for commercial device applications. Here, a more complete understanding of the interface formation can lead to improved reliability.