(Invited) Phase Transition and Related Energy Applications of (Hf,Zr)O2 Films

Abstract

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 Hf0.4Zr0.6O2 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 Hf0.3Zr0.7O2 thin film could store electrostatic energy up to 46 J/cm3.[7] The temperature dependent change of polarization of Hf0.2Zr0.8O2 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 Hf0.5Zr0.5O2 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).