Electromechanical properties of sodium bismuth titanate thin films

Lead-free piezoelectric sodium bismuth titanate (BNT) films are promising candidates for actuators applications as an alternative to lead zirconate titanate films. We focused in this work on the growth and characteristics of BNT films deposited by sputtering on silicon substrates with LaNiO3 electrodes. The films are amorphous previous to post-annealing treatments. We studied the influence of the crystallization state and film thickness on their electromechanical properties. Namely, we evaluated the piezoelectric properties, reduced elastic modulus, and hardness. BNT films fully crystallized (f-c) in the desired perovskite structure, without second phase, after post-annealing at 650°C. Relative permittivity grew from 60 to 540 for an amorphous vs. a f-c film and the d33eff coefficient improved with film thickness to 60 pm/V for a 450 nm thick film. The BNT film's mechanical properties increased with film crystallization from the amorphous structure to the f-c: 134 to 149 GPa for reduced modulus and 7 to 9 GPa for hardness. However, the film thickness did not remarkably modify the mechanical properties, yet a slight decrease for the 450 nm film was explained by previous cracking on the surface.