Microstructure and microstructural evolution in BaTiO 3 films fabricated using the precursor method
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J.H. Scott Chemical Science and Technology Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899
K. Chang and I. Takeuchi Department of Materials Science and Engineering and Center for Superconductivity Research, Department of Physics, University of Maryland, College Park, Maryland 20742 (Received 5 November 2001; accepted 24 June 2002)
Pulsed laser deposition of TiO2 and BaF2 layers at room temperature and subsequent annealing in flowing oxygen were used to form homogeneous epitaxial BaTiO3 films on LaAlO3. This oxide film synthesis method, known as the precursor technique, is frequently used for making combinatorial libraries. In this paper, we investigated the microstructures of the films at different stages of annealing using cross-sectional transmission electron microscopy, high-resolution imaging, and electron energy loss spectroscopy. It was shown that epitaxial BaTiO3 thin films with large grains could be formed on a LaAlO3 substrate. Their formation process consists of the following stages: At 200 °C, the BaF2 layer is partially oxidized. At 400 °C, the amorphous TiO2 layer crystallizes, further transformation of BaF2 into BaO takes place, and interdiffusion begins. At 700 °C, the formation of a polycrystalline structure with different Ba–Ti oxides occurs, epitaxial BaTiO3 grains nucleate on the film/substrate interface, and significant interdiffusion takes place. Finally, at 900 °C, the interdiffusion is completed, and the epitaxial BaTiO3 grains coalesce and grow. The presence of nonepitaxial polycrystalline regions in fully annealed films can be explained as the following: (i) stoichiometric transient regions not yet consumed by recrystallization of BaTiO3; (ii) nonstoichiometric regions resulting from inhomogeneous deposition of BaF2.
I. INTRODUCTION
Combinatorial thin-film methods have recently been applied to exploring new and improved properties and compositions of different electronic materials.1,2 In particular, the method was successfully used to improve and optimize properties of ferroelectric and dielectric oxide materials.3–6 For example, using a discrete combinatorial library, it was found that W doping of (Ba,Sr)TiO3 (BST) results in lower leakage current and lower microwave loss compared to the undoped BST.3,4 Also Ba0.12–0.25Sr0.35–0.47Ca0.32–0.53TiO3 was identified from a continuous composition spread of (Ba,Sr,Ca)TiO3 as the compositional region with the lowest microwave loss.6 The central part of the methodology is the synthesis of materials libraries where on a single substrate a large number of different compositions, or a continuous spread J. Mater. Res., Vol. 17, No. 10, Oct 2002
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of compositions, are deposited. There are several different methods of creating a combinatorial library using thin-film deposition techniques. Precisely positioned shadow masks or automated shutters are used to control the amount of thin-film materials delivered to selected regions on a substrate. Different physi
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