High Energy Efficiency and Thermal Stability of BaTiO3 BiScO3 Thin Films Based on Texture and Defects Engineering

Abstract

ABO₃ perovskite ferroelectric thin films have gained wide attention in recent years for their high density capacitive energy storage applications. Thin-film dielectrics with high energy density and storage efficiency, as well as good thermal stability in the temperature range of 0–200 °C, must be developed for deployment in high-temperature power electronics systems. In addition, the development of new Pb-free high energy-density dielectric materials is desired due to concerns over the adverse environmental effects of Pb.

In this regard, thin films of weakly coupled relaxors based on solid solutions of BaTiO₃-BiMeO₃ have shown good promise, since they exhibit remarkably large polarization with small hysteresis. Nevertheless, the performance of Pb-free thin films has lagged behind those of their Pb-based counterparts with regard to the thermal stability of their energy conversion efficiency. However, the broader adoption of BaTiO₃-BiMeO₃ thin films in advanced electronics applications requires high thermal stability, energy density and energy storage efficiency. Towards this end, most of the recent studies on BaTiO₃-BiMeO₃ systems have focused on optimizing material composition, while considerations such as defect chemistry, crystallographic texture and film-electrode interaction have received less attention. Here we examine the effects of diffused elements in the Pt-bottom layer as a function of deposition temperature on film-electrode interaction and A-site vacancy, B-site ionic substitution and crystallographic texture on the energy storage performance of BaTiO₃-BiScO₃ (BSBT) thin films. In this study, the (1-x)BaTiO₃-xBiScO₃ thin films with x = 0.1(10BSBT) and 0.15(15BSBT) were synthesized using pulsed laser deposition (PLD) at different temperatures in the range of 510–550 ºC. Using (TEM)-based spectroscopy and conventional imaging, we unfold the subtle, but still critical dissimilarities in the bottom electrode’s chemistry and microstructure at the interface of the BSBT films deposited at different temperatures. We find moderate diffusion of elements in the Pt-bottom layer to be helpful for the semi-epitaxial growth of the BSBT films along 110 orientations on Pt with a high quality interface. However, an amorphous interfacial layer with a bumpy interface is observed with a very limited diffusion of elements at a low deposition temperature (i.e., 510 ºC). We show that defects such as A-site vacancies in the BSBT films can be created using the deposition parameters (i.e., temperature and pressure). For BSBT (x = 0.1 deposited at 550 ºC) films, a combination of moderate A-site vacancy concentration and (110) crystallographic texture lead to a high energy storage density with an excellent efficiency. However, this strategy worked in the temperature rage up to ~170 °C, after which the performance of the thin films diminished. A superior combination of these characteristics (high energy-storage density, excellent efficiency, and outstanding thermal stability) can be obtained through the control of different defect concentrations, namely A-site cation vacancies (V_A) and B-site ionic substitutions (MeTi). For BSBT (x = 0.15 deposited at 510 ºC) thin films, we show that an optimum combination of V_A and Sc_Ti leads to a high energy-storage density of 40.5 J cm⁻³ and an efficiency higher than 85%, which could be maintained from room temperature to 200 °C. We propose a mechanistic understanding of enhanced energy storage performance based on synergistic effect of random fields introduced by A-site vacancies and strong hole trapping by Sc_Ti acceptor centres.

Our results provide practical recommendations for minimizing the harmful effects of the bumpy interface and dielectric amorphous interfacial layer, and we elucidate the texture growth mechanism on Pt-electrodes in nanoscale devices and reveal changes in the platinum electrode’s operational features due to diffusion at varying growth conditions. Moreover, perovskite ferroelectric thin films capable of maintaining high performance at high temperatures may facilitate the advancement of power electronics applications in harsh environments.

Date of Award	15 Jun 2021
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Abhijit PRAMANICK (Supervisor)