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Min Hyuk Park1 Cheol Seong Hwang1

1, Seoul National University, Seoul, , Korea (the Republic of)

Following the first report on ferroelectricity in Si-doped HfO2 thin film in 2011 [1], fluorite-type ferroelectrics have attracted great interest in the field of ferroelectricity [2]. 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 [2]. Moreover, they are environmental-friendly with an absence of Pb and have deposition techniques (atomic layer deposition), mature enough for mass production [2]. Thus, they are considered highly promising for both memory and energy devices [2]. 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 [3].
The crystallographic origin of the unexpected ferroelectricity is now believed to be a formation of Pca21 orthorhombic phase [2]. 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.
[1] T. S. Boescke et al., Appl. Phys. Lett. 99, 102903 (2011).
[2] M. H. Park et al., Adv. Mater. 28, 7956 (2015).
[3] M. H. Park et al., Nano Energy 12, 131 (2015).
[4] R. Materlik et al., J. Appl. Phys. 117, 134109 (2016).
[5] M. H. Park et al., Nanoscale, 9, 9973 (2017).
[6] M. H. Park et al., submitted to Nanoscale.

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