Thickness- and temperature-dependent structural and electromechanical properties of (100)-oriented Sc-doped (Na0.85K0.15)0.5Bi0.5TiO3 ferroelectric films

Abstract

(100)-oriented Sc-doped (Na_0.85K_0.15)_0.5Bi_0.5TiO₃ (NKBT-Sc) films with different thicknesses of about 100–620 nm were grown on Pt(111)/Ti/SiO₂/Si substrates by a sol–gel method. A LaNiO₃ (LNO) layer of about 20 nm thickness was introduced at the film–substrate interface or alternatively with NKBT-Sc layers to form a sandwich multilayer structured film. The film thickness- and temperature-dependent structural and electromechanical properties of the films were investigated. A rhombohedral–tetragonal phase transition phase occurred with the variation of NKBT-Sc film thickness, and during this process a two-phase coexistence existed in an appropriate film thickness region, inducing an increased remnant polarization (P_r) value at an intermediate thickness (∼460 nm) of about 23.7 μC cm⁻². The effective piezoelectric coefficient initially increased from about 32 pm V⁻¹ for the thinnest film (∼100 nm) to a peak value of about 73 pm V⁻¹ (∼460 nm), but then decreased as the film thickness further increased. With the insertion of the LNO layers alternatively inside the NKBT-Sc film, the sandwich multilayer structure further enhanced the degree of the (001)-orientation and was beneficial for the crystallization process, leading to better electromechanical properties than the NKBT-Sc/LNO/substrate structure composite film. A high piezoelectric coefficient of about 82 pm V⁻¹, dielectric constant ε_r of about 523 and remnant polarization (P_r) of about 26.2 μC cm⁻², which gradually increased with the decrease of the testing temperature from 20 °C to −120 °C, together with a with a low dielectric loss (tan δ) of about 0.055 were simultaneously obtained in the sandwich multilayer structured film with NKBT-Sc film thickness of about 460 nm.