Achieving a high energy storage density in Ag(Nb,Ta)O ₃ antiferroelectric films via nanograin engineering

Abstract

Due to its lead-free composition and a unique double polarization hysteresis loop with a large maximum polarization (<i>P</i>_max) and a small remnant polarization (<i>P</i>_r), AgNbO₃-based antiferroelectrics (AFEs) have attracted extensive research interest for electric energy storage applications. However, a low dielectric breakdown field (<i>E</i>_b) limits an energy density and its further development. In this work, a highly efficient method was proposed to fabricate high-energy-density Ag(Nb,Ta)O₃ capacitor films on Si substrates, using a two-step process combining radio frequency (RF)-magnetron sputtering at 450 ℃ and post-deposition rapid thermal annealing (RTA). The RTA process at 700 ℃ led to sufficient crystallization of nanograins in the film, hindering their lateral growth by employing short annealing time of 5 min. The obtained Ag(Nb,Ta)O₃ films showed an average grain size (<i>D</i>) of ~14 nm (obtained by Debye–Scherrer formula) and a slender room temperature (RT) polarization–electric field (<i>P–E</i>) loop (<i>P</i>_r ≈ 3.8 μC·cm⁻² and <i>P</i>_max ≈ 38 μC·cm⁻² under an electric field of ~3.3 MV·cm⁻¹), the <i>P–E</i> loop corresponding to a high recoverable energy density (<i>W</i>_rec) of ~46.4 J·cm⁻³ and an energy efficiency (<i>η</i>) of ~80.3%. Additionally, by analyzing temperature-dependent dielectric property of the film, a significant downshift of the diffused phase transition temperature (<i>T</i>_M2–M3) was revealed, which indicated the existence of a stable relaxor-like AFE phase near the RT. The downshift of the <i>T</i>_M2–M3 could be attributed to a nanograin size and residual tensile strain of the film, and it led to excellent temperature stability (20–240 ℃) of the energy storage performance of the film. Our results indicate that the Ag(Nb,Ta)O₃ film is a promising candidate for electrical energy storage applications.