Processing of highly oriented lithium tantalate films by chemical solution deposition
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Shin-ichi Hirano Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8603, Japan (Received 1 May 2002; accepted 8 July 2002)
The synthesis of lithium tantalate films by a chemical solution deposition method was studied. A precursor solution was prepared by dissolving lithium ethoxide and tantalum pentaethoxide in ethanol. The addition of formic acid to this precursor solution was very effective in the preparation of homogeneous and transparent precursor films on substrates by spin coating. Lithium tantalate films crystallized on sapphire (001) substrates with a highly preferred orientation along the c axis with heat-treating at temperatures above 450 °C. The refractive index of the film prepared at 550 °C was 2.049, which is close to the value for single crystals of lithium tantalate (2.176). I. INTRODUCTION
II. EXPERIMENTAL
Lithium tantalate (LiTaO3) single crystals are widely used in electro-optic, piezoelectric, and pyroelectric devices, including optical waveguides, surface acoustic wave devices, and pyrosensor devices. Processing methods for high-quality LiTaO3 films are under intensive study to allow the miniaturization and integration of optical waveguide devices. Methods for preparing LiTaO3 films include pulsed laser deposition,1 chemical vapor deposition,2,3 solid-phase epitaxy,4,5 radio frequency (rf) magnetron sputtering,6 and the sol-gel method.7–15 Table I summarizes reports of highly oriented LiTaO3 films prepared on sapphire (001) substrates. The sol-gel method is superior in providing good-quality films at low temperatures. Synthesis of highly oriented LiTaO3 films on sapphire (001) substrates at 700 °C has been reported using a lithium tantalum double ethoxide precursor solution.12 However, the preparation of LiTaO3 films with highly preferred orientation and a smooth surface at lower temperatures around 500 °C has not yet been reported. We have previously reported the processing of lithium niobate from lithium and niobium double alkoxide with acetic acid.16–19 Carboxylic acids such as acetic acid and formic acid act not only as catalysts but also as ligands, allowing ready control of hydrolysis. It is important to regulate the rate of hydrolysis, since the hydrolysis rate of tantalum ethoxide is faster than that of niobium ethoxide, making it difficult to prepare a stable precursor solution.20 This paper describes the modification of an ethanol precursor solution by adding formic acid to prepare highly oriented LiTaO3 films on sapphire (001) substrates and evaluates the refractive indices of these films.
A. Preparation of LiTaO3 gel powders and films
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http://journals.cambridge.org
J. Mater. Res., Vol. 17, No. 10, Oct 2002 Downloaded: 30 Mar 2015
Gel powders were prepared by drying the precursor solution at room temperature. Figure 1 shows the preparation procedure for LiTaO3 films. Lithium ethoxide (99.9%), tantalum pentaethoxide (99.999%) (Kojundo Chemical Lab. Co., Ltd., Saitama, Japan), and acetic acid (99.7%) or formic acid (99%) were dissolved in a mo
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