Following the first report on ferroelectricity in Si-doped HfO2 thin film in 2011 , fluorite-type ferroelectrics have attracted great interest in the field of ferroelectricity . Even with an extremely small film thickness of ~10 nm, they could have remanent polarization as high as 15 – 30 μC/cm2, which is already comparable to that of much thicker perovskite-type ferroelectric films . Moreover, they are environmental-friendly with an absence of Pb and have deposition techniques (atomic layer deposition), mature enough for mass production . Thus, they are considered highly promising for both memory and energy devices . Especially, the field-induced phase transition in fluorite-type ferroelectrics enables energy storage, energy harvesting and solid-state-cooling comparable to or even better than the previous candidates .
The crystallographic origin of the unexpected ferroelectricity is now believed to be a formation of Pca21 orthorhombic phase . However, this phase cannot be found in phase diagrams of bulk HfO2 and ZrO2, and the monoclinic phase is always a stable phase under process conditions. To understand the formation of the unexpected orthorhombic phase, various thermodynamic causes such as surface energy, interface/grain boundary energy, stress, and doping were suggested [2,4]. From a comprehensive comparison of our experimental results to the theoretical models, we suggested that the kinetic mechanism of phase transition should also be considered [5,6]. In this presentation, therefore, the thermodynamic/kinetic origin of the formation of orthorhombic phase in fluorite-type ferroelectrics and their properties will be comprehensively examined.
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 M. H. Park et al., Adv. Mater. 28, 7956 (2015).
 M. H. Park et al., Nano Energy 12, 131 (2015).
 R. Materlik et al., J. Appl. Phys. 117, 134109 (2016).
 M. H. Park et al., Nanoscale, 9, 9973 (2017).
 M. H. Park et al., submitted to Nanoscale.