Energy storage performance of ferroelectric ZrO2 film capacitors: effect of HfO2:Al2O3 dielectric insert layer
The present work reports for the first time, the employment of ferroelectric ZrO2 films as energy storage capacitors utilized in pulsed power systems. Furthermore, the effect of insertion of a low permittivity dielectric HfO2:Al2O3 (HAO) layer, with a thickness ranging from 2 to 8 nm, on the tunability of ferroelectric and energy storage characteristics of ZrO2 films is assessed. The increase in thickness of the HAO layer gave rise to distorted ferroelectric loops with decreased polarization, coercive field, and hysteresis loss of the films. These results are correlated with the depolarization field induced by the insertion of the dielectric HAO layer. An optimum combination of high energy density of 54.3 J cm−3 and good storage efficiency of 51.3% are obtained for the ZrO2 film capacitors with 2 nm-thick HAO insert layer. These values correspond to an increase of ∼55% and ∼92%, from the respective values of pure ZrO2 film capacitors. In addition, the HAO/ZrO2 films showed a good fatigue endurance of energy storage performance over 109 electric field cycles. The energy storage density obtained from HAO(2 nm)/ZrO2 film capacitor is found to be higher than that reported for several Pb-based as well as Pb-free ferroelectric ceramic films with complex compositions. The present study demonstrates that simple binary oxides such as ZrO2 with ferroelectric behavior could be potential candidates for developing high performance energy storage capacitors.