Dielectric properties of BaTiO₃ thin films prepared by laser ablation

To study the minimum particle size for a ferroelectric material at which ferroeiectricity is retained, thin films composed of stoichiometric BaTiO ₃ nanoparticles were prepared by laser ablation of a Ba-Ti-O ceramic target using a differential mobility analyzer. Highly densified films of various uniform grain sizes were obtained after post-annealing at 600, 700, 800, and 900 °C in an oxygen atmosphere. The dielectric constants of the post-annealed films varied from 100 to 300, depending on the grain size from 65 to 136 nm. These values are comparable to those of BaTiO₃ nano-granular films fabricated by metallorganic decomposition or the sol-crystal method. From the capacitance vs bias-voltage relationship and its hysteresis observed for the post-annealed films, it became clear that a phase transformation from paraelectric to ferroelectric occurs at a grain size of approximately 76 nm. It was also clarified from piezoresponse images of the post-annealed films obtained using a scanning probe microscope that ferroelectricity emerges in BaTiO₃ nano-size grains at a grain size of approximately 55 nm.