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Crystallization Process of Sr0. 7Bi2. 3Ta2O9 Thin Films with Different Crystal Orientation Prepared by Chemical Liquid Deposition Using Alkoxide Precursor
Ichiro KOIWA Takao KANEHARA Juro MITA Tetsuya OSAKA Sachiko ONO Akira SAKAKIBARA Tomonori SEKI
IEICE TRANSACTIONS on Electronics
Publication Date: 1998/04/25
Print ISSN: 0916-8516
Type of Manuscript: Special Section PAPER (Special Issue on Advanced Memory Devices Using High-Dielectric-Constant and Ferroelectric Thin Films)
ferroelectric memory, SrBi2Ta2O9 thin film, crystallization process, alkoxide precursor,
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The crystallization process of Sr0. 7Bi2. 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 fluorite 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 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 10-8 A/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 heat-treatment, both films consist of large grains with bismuth layer-structure and fine fluorite grains. The matrix of both films contains large grains with bismuth layer-structure that determines the leakage current density characteristics. Since the fluorite grain size after a 700 heat-treatment is the same as that after 650 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 heat-treatment. 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.