Dense and smooth epitaxial BaTiO 3 thin films by the dipping-pyrolysis process
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Dense and smooth epitaxial BaTiO3 thin films by the dipping-pyrolysis process S. Kim National Institute of Materials and Chemical Research, Tsukuba, Ibaraki 305, Japan and Department of Chemical Engineering, Yosu National University, Yosu 550-250, Korea
T. Fujimoto, T. Manabe, I. Yamaguchi, T. Kumagai, and S. Mizuta National Institute of Materials and Chemical Research, Tsukuba, Ibaraki 305, Japan (Received 16 February 1998; accepted 12 May 1998)
Dense and smooth BaTiO3 thin films were prepared on SrTiO3 (100) substrates by the dipping-pyrolysis process using a mixed precursor solution of barium and titanium naphthenates. Combination effects of prefiring [at 150–450 ±C in air or low oxygen partial pressure, p(O2 )] and final heat treatment [at 850 ±C in air or low p(O2 )] on preparation of BaTiO3 thin films were examined. An epitaxial BaTiO3 thin film with a dense and smooth surface consisting of nanosized grains about 70 nm was prepared by prefiring under low p(O2 ) at 250 ±C and final heat treatment under low p (O2 ) at 850 ±C.
I. INTRODUCTION
Ferroelectric thin films with perovskite structure such as BaTiO3 , SrTiO3 , PbTiO3 , Pb(Zr, Ti)O3 , etc. are of particular interest for device applications in the nonvolatile memories for integrated circuits, pyroelectric detectors, and thin film capacitors.1 Epitaxial ferroelectric thin films having a smooth and dense surface are required for the electro-optical applications such as optical waveguides because of their low propagation loss.1,2 Recently, a lot of attempts to prepare the epitaxial BaTiO3 thin films have been done by various techniques.3–11 The present authors have studied the preparation of epitaxial BaTiO3 thin films on SrTiO3 substrates by the dipping-pyrolysis (DP) process.12 The process has many advantages such as low processing cost, precise controllability of chemical composition of films, and easy application to substrates of larger size.12–14 In our previous work,12 epitaxial BaTiO3 thin films were successfully prepared on SrTiO3 substrates by the DP process, where the prefiring condition was 470 ±C for 10 min in air and the final heat-treatment condition was 800 or 850 ±C for 1 h in air. Prefiring (at 150–500 ±C) is the process for pyrolytic conversion of organic acid salts into carbonate or oxides, and final heat treatment (at 700–850 ±C) is the process for solid-state reaction between BaCO3 and TiO2 . However, the film contained a trace of polycrystalline phase. The free surface of the film was round-shaped islandlike grains with diameters about 0.3 mm, and voids were interspersed among grains. Although the BaTiO3 thin film showed epitaxy to substrate with low fluctuation of alignments, the morphology of the film was similar to typical polycrystalline films. Since the pores were recognized even in the precursor film just after the pyrolysis at 470 ±C, they were considered to form during pyrolysis of metal 592
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J. Mater. Res., Vol. 14, No. 2, Feb 1999
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