Crystallization process of Sr_0.7Bi_2.3Ta₂O₉ thin films with different crystal orientation prepared by chemical liquid deposition using alkoxide precursor

The crystallization process of Sr_0.7Bi_2.3Ta₂O₉ (SBT) ferroelectric thin films with different crystal orientations formed by chemical liquid deposition using an alkoxide precursor was investigated. One film showed strong c-axis orientation (a-type film), while another shows scarcely any c-axis orientation (b-type film). We report that the crystallization process was the same even when crystal orientation differed. Thin films first change from amorphous to fluorite fine grains; the fiuorite grains then change to bismuth layer-structure grains. The different orientation of the SBT films is not caused by different crystallization process. Both SBT films with different crystal orientations consist of fine fluorite grains after 650°C heat-treatment. Their leakage current density characteristics differ, however. The leakage current density of the a-type film was independent of the electric field, and showed a low value of 10^-8A/cm². The leakage current density of the b-type film, however, was dependent on the electric field, and increased continuously with the increasing electric field. After 700°C heat-treatment, both films consist of large grains with bismuth layer-structure and fine fiuorite grains. The matrix of both films contains large grains with bismuth layer-structure that determines the leakage current density characteristics. Since the fiuorite grain size after a 700°C heat-treatment is the same as that after 650°C heat-treatment, nucleation is predominant at the structural phase boundary from amorphous to fluorite. The bismuth layer-structure grains are large and single-crystal grains after both a 700 and 800°C heattreatment. Increased grain size predominates at the structural phase boundary from fluorite to bismuth layer-structure grains. Clearly, ferroelectric SBT films with bismuth layer-structure are crystallized in two steps, each having a different predominant crystal growth mechanism.