Stress-Induced Crystallization of Thin Hf_1–<i>X</i>Zr_<i>X</i>O₂ Films: The Origin of Enhanced Energy Density with Minimized Energy Loss for Lead-Free Electrostatic Energy Storage Applications

Abstract

Increasing interest in the development of alternative energy storage technologies has led to efforts being taken to improve the energy density of dielectric capacitors with high power density. However, dielectric polymer materials still have low energy densities because of their low dielectric constant, whereas Pb-based materials are limited by environmental issues and regulations. Here, the energy storage behaviors of atomic layer-deposited Hf_{1- X}Zr _XO₂ ( X = 0-1) thin films (10 nm) and the phase transformation mechanism associated with an enhancement of their energy density are reported using unipolar pulse measurements. Based on electrical and material characterization, the energy density and energy efficiency are dependent on the Zr content, and stress-induced crystallization by the encapsulating Hf_{1- X}Zr _XO₂ films with TiN top electrodes prior to annealing can enhance the energy density (up to 47 J/cm³ at a small voltage value of 3.5 MV/cm) while minimizing energy loss even at low process temperatures (400 °C). This work will facilitate the realization of Hf_{1- X}Zr _XO₂-based capacitors for lead-free electrostatic energy storage applications.