A new ferroelectric phase (space group: Pca2₁) in (Hf,Zr)O₂ thin film and doped/undoped HfO₂ has been intensively studied since 2011.[1,2] The films could show large remanent polarizations (They are generally ~20 μC/cm² and can be even larger than 40 μC/cm² for La-doped HfO₂) with extremely small film thickness (It is generally ~10 nm and can be even as thin as ~2.5 nm).[3,4] Moreover, they could endure more than 10⁹ times of switching with the electric field of 2.5 MV/cm.[2] The Curie temperature of (Hf,Zr)O₂ could be controlled by changing Zr:Hf ratio, and the (Hf,Zr)O₂ films showed first-order like phase transition.[5] Interestingly, the two-step polarization switching could be observed in rather wide temperature range near room temperature for Hf_0.4Zr_0.6O₂ film,[5] which could be observed in a very narrow temperature range for BaTiO₃in a previous study.[6]

 The phase transition involved with the new ferroelectric phase is believed to be highly promising for various applications such as electrostatic supercapacitor, energy harvester, and electrocaloric cooler.[7-9] The Hf_0.3Zr_0.7O₂ thin film could store electrostatic energy up to 46 J/cm³.[7] The temperature dependent change of polarization of Hf_0.2Zr_0.8O₂ thin film could harvest electrical energy of 11.6 J/cm³ cycle from heat using Olsen cycle.[8] This also can be used for solid state cooling based on electrocaloric effect (The adiabatic temperature change was 13.4 K) when the inverse Olsen cycle is used.[8] It was suggested that a monolithic device with aforementioned various functions can be achieved on Si substrate.[8] In 2016, the giant negative electrocaloric effect (The adiabatic temperature change was -10.8 K.) in Hf_0.5Zr_0.5O₂ film was also reported based on the abnormal pyroelectric properties.[9] It is expected that the combination of giant positive and negative electrocaloric effect in (Hf,Zr)O₂films can make the solid state cooling device more efficient.[10]

 In this presentation, the fundamentals for phase transition in (Hf,Zr)O₂films will be discussed based on the first-­order phase transition model involving the orthorhombic and tetragonal phases. The application of the phase transition behaviors for the various energy applications including energy storage/harvesting and solid state cooling will be presented in detail.

[1] T. Boescke et al. Appl. Phys. Lett. 99, 102903 (2011).

[2] M. H. Park et al. Adv. Mater. 27, 1811 (2015).

[3] J. Meuller et al. IEEE International Electron Devices Meeting (IEDM) (2013).

[4] A. Chernikova et al. ACS Appl. Mater. Interfaces 8, 7232 (2016).

[5] M. H. Park and H. J. Kim et al. Nanoscale 8, 13898 (2016).

[6] W. J. Merz, Phys. Rev. 91, 513–517 (1953).

[7] M. H. Park et al. Adv. Electron. Mater. 4, 1400610 (2014).

[8] M. H. Park et al. Nano Energy 12, 131 (2015).

[9] M. H. Park et al. Adv. Mater. 28, 7956 (2016).

[10] Li et al. Europhys. Lett. 102, 47004 (2013).