Abstract
Ferroelectric (FE) materials, which typically adopt the perovskite structure with non-centrosymmetry and exhibit spontaneous polarization, are promising for applications in memory, electromechanical and energy storage devices. However, these advanced applications suffer from the intrinsic limitations of perovskite FEs, including poor complementary metal oxide semiconductor (CMOS) compatibility and environmental issues associated with lead. Hafnium oxide (HfO2), with stable bulk centrosymmetric phases, possesses robust ferroelectricity in nanoscale thin films due to the formation of non-centrosymmetric phases. Owing to its high CMOS compatibility and high scalability, HfO2 has attracted significant attention. In the last decade, significant efforts have been made to explore the origin of the ferroelectricity and factors that influence the FE properties in HfO2 films, particularly regarding the role of microstructure, which is vital in clarifying these issues. Although several comprehensive reviews of HfO2 films have been published, there is currently no review focused on the relationship between microstructure and FE properties. This review focuses on the microstructure-property relationships in FE polycrystalline and epitaxial HfO2 films. The crystallographic structures and characterization methods for HfO2 polymorphs are first discussed. For polycrystalline HfO2 films, the microstructure-FE properties relationships, driving force and kinetic pathway of phase transformations under growth parameters or external stimuli are reviewed. For epitaxial films, the lattice matching relations between HfO2 films and substrates and the corresponding impact on the FE properties are discussed. The FE properties between polycrystalline and epitaxial HfO2 films are compared based on their different microstructural characteristics. Finally, a future perspective is given for further investigating FE HfO2 films.
Highlights
Ferroelectric (FE) materials have non-centrosymmetric structures and present spontaneous electrical polarity that can be reversed by an applied electric field, which makes them promising for electromechanical, memory and energy storage devices[1]
These FE devices suffer from various problems during the device manufacturing process and usage, including a requirement for large thicknesses (~100 nm)[10], integration difficulties with modern complementary metal oxide semiconductor (CMOS) technology[11], small bandgaps (3-4 eV)[12,13] and environmental issues due to toxic elements like Pb and Ba[14]
For polycrystalline HfO2 films, we review their growth methods, the impacts of growth parameters on the FE phase fraction and properties, and the phase transformations under external stimuli, e.g., temperature and electric loading
Summary
Ferroelectric (FE) materials have non-centrosymmetric structures and present spontaneous electrical polarity that can be reversed by an applied electric field, which makes them promising for electromechanical, memory and energy storage devices[1]. Compared with undoped bulk HfO2 crystals, atomic layer deposition (ALD) processed undoped HfO2 films with a thickness of 6 nm have robust ferroelectricity due to the formation of the FE O-phase under a nanoscale grain size effect[93]. HfO2 films with larger grain sizes are expected to possess higher endurance, while the M-phase fraction increases, resulting in a reduction in remanent polarization.
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