Structures, Phase Equilibria, and Properties of HfO2

Hafnium dioxide (HfO2) has long been known as a refractory material due to its high melting temperature (~ 2800°C) and low thermal conductivity (1.5 W/m K) [1], [2]. Refractories are thermally insulating materials known to withstand high temperatures without being degraded and are used for high-temperature applications to reduce heat losses [3]. Due to the higher dielectric permittivity of HfO2 relative to SiO2, HfO2 was studied as a high-k gate dielectric [4], [5], [6]. Decades of research have been dedicated to understanding the dielectric, electrical, mechanical, and thermal properties in these applications. Since the discovery of ferroelectricity in HfO2-based thin films in 2011 [7], further research surrounding the dielectric, supercapacitive, piezoelectric, pyroelectric, and electrocaloric properties has emerged. Understanding the mechanisms underpinning these functionalities requires a basic understanding of the HfO2 crystal structures and HfO2-metal oxide phase equilibria. This chapter reviews these fundamentals, including a discussion on the role of dopants and phase stability of doped HfO2.