Composition control of lead-free piezoelectric BNT thin ceramic films deposited by ex situ sputtering

(Bi_0.5Na_0.5)TiO₃ thin film growth by ex situ sputtering has been investigated and reported in this paper. An original approach, based on the growth process, was used in order to precisely control the film composition, which has never been reported in BNT growth. The bismuth content in the films and so the composition of amorphous sputtered films was controlled by a slight heating of the substrate during the growth (150–240°C). Then, films were crystallized, obviously without any change in composition, by a post-annealing treatment. More precisely, without substrate heating and using a stoichiometric target, the film presents an excess of Bi but when it is deposited at 200°C the film becomes stoichiometric. It was shown that the sticking coefficient of Bi is particularly sensitive even at low substrate temperatures, whereas Na and Ti sticking coefficients are not impacted. Followed by a post-annealing in air at 650°C, the composition of the amorphous BNT films deposited at 200°C remains stoichiometric and the film exhibits a high (100) preferred orientation in a pure perovskite phase and a dense microstructure. The evaluation of the electrical properties as a function of the Bi content in the film, adjusted by the deposition temperature, shows a strong impact on the ferroelectric properties where the best performances were obtained with the stoichiometric BNT film deposited at 200°C.