Macro and nanoscale properties of (001)-oriented Bi0.5Na0.5TiO3 lead-free piezoelectric thin films grown by sputtering on LaNiO3/Si substrates

In this paper, the effect of the post-annealing temperature on the structural and electrical properties at macro and nanoscale of Sodium Bismuth Titanate - Bi0.5Na0.5TiO3 (BNT) films is reported. Stoichiometric BNT thin films were deposited by ex-situ radio frequency (rf) sputtering at substrate temperature of 200 degrees C. The as-deposited films were amorphous and post-annealing treatment was necessary to crystallize the film in the perovskite phase. 400-nm-thick stoichiometric BNT films were annealed at various temperatures from 400 degrees C up to 700 degrees C. The BNT film annealed at 400 degrees C remains amorphous. Film starts to crystallize at 450 degrees C and then the crystallization increases with the post-annealing temperature to reach an optimal at 650 degrees C without any secondary phase. Dielectric, ferroelectric and piezoelectric properties were also improved with the increasing of the postannealing temperature. The piezoelectric coefficient d33eff, measured at macroscale, reaches a maximum value of 57 pm/V for 400 nm-thick film post-annealed at 650 degrees C. On nanoscale, measurements performed by piezoresponse force microscopy are in perfect concordance with the performances obtained at macroscale. The efficient polarization reversal for domains and their retention when locally manipulated are observed, combined to a strong piezo-activity.