Achieving a high energy storage density in Ag(Nb,Ta)O₃ antiferroelectric films via nanograin engineering

Due to its lead-free composition and a unique double polarization hysteresis loop with a large maximum polarization (P_max) and a small remnant polarization (P_r), AgNbO₃-based antiferroelectrics (AFEs) have attracted extensive research interest for electric energy storage applications. However, a low dielectric breakdown field (E_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 (D) of ~14 nm (obtained by Debye–Scherrer formula) and a slender room temperature (RT) polarization–electric field (P–E) loop (P_r ≈ 3.8 μC·cm⁻² and P_max ≈ 38 μC·cm⁻² under an electric field of ~3.3 MV·cm⁻¹), the P–E loop corresponding to a high recoverable energy density (W_rec) of ~46.4 J·cm⁻³ and an energy efficiency (η) of ~80.3%. Additionally, by analyzing temperature-dependent dielectric property of the film, a significant downshift of the diffused phase transition temperature (T_M2–M3) was revealed, which indicated the existence of a stable relaxor-like AFE phase near the RT. The downshift of the T_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.