First-principles investigation of ferroelectricity and related properties of HfO2

Nonvolatile memories are in increasing demand as the world moves toward information digitization. The ferroelectric materials offer a promising alternative for this. Since the existing perovskite materials have various flaws, including incompatibility with complementary metal-oxide-semiconductor processes in memory applications, the discovery of new optimized FE thin films was necessary. In 2011, the disclosure of ferroelectricity in hafnia (HfO$_2$) reignited interest in ferroelectric memory devices because this material is well integrated with CMOS technology.
Although the reporting of ferroelectricity in HfO$_2$ has been a decade, researchers are still enthralled by this material's properties as well as its possible applications.
The ferroelectricity in HfO$_{2}$ has been attributed to the orthorhombic phase with spacegroup $Pca2_1$. This phase is believed to be the metastable phase of the system. Many experimental and theoretical research groups joined the effort to understand the root causes for the stability of this ferroelectric phase of HfO$_{2}$ by considering the role of the surface energy
effects, chemical dopants, local strain, oxygen vacancies. However, the understanding was not conclusive. In this part of this work, we will present our first-principles results, predicting a situation where the ferroelectric phase becomes the thermodynamic ground state in the presence of a ordered dopant forming layers.
Since the main focus was on understanding and optimizing the ferroelectricity in HfO$_{2}$, we observed that the electro-mechanical response of the system has garnered comparatively less attention. The recent discovery of the negative longitudinal piezoelectric effect in HfO$_2$ has challenged our thinking about piezoelectricity, which was molded by what we know about ferroelectric perovskites. In this work, we will discuss the atomistic underpinnings behind the negative longitudianl piezoelectric effect in HfO$_{2}$. We will also discuss the behavior of the longitudinal piezoelectric coefficient ($e_{33}$) under the application of epitaxial strain, where we find that $e_{33}$ changes sign even though the polarization does not switch.
Aside from a basic understanding of piezoelectric characteristics in HfO$_2$, the application aspect is also worth considering. The piezoelectric properties of the material can be tuned to meet the needs of the applications. In this work, we will describe our findings on how the piezoelectric characteristics of the material change as a function of isovalent dopants.