Processing of highly oriented lithium niobate films through chemical solution deposition
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Shin-ichi Hirano Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa, Nagoya, 464-8603, Japan (Received 13 July 2000; accepted 31 January 2001)
A precursor solution for processing lithium niobate (LiNbO3) films was prepared from lithium niobium ethoxides through modification with acetic acid in hydrolysis. Homogeneous gel films were spin-coated on various substrates. Highly oriented LiNbO3 films were crystallized on sapphire and Pt-coated substrates by heat treatment at 500 °C. Crystallization behavior of the LiNbO3 films depended on the orientation of platinum under-layer films. LiNbO3 films crystallized with preferred (001) orientation on Pt(111) layers, while LiNbO3 films crystallized with preferred (113) orientation on Pt(100) layers. LiNbO3 films prepared on Si(111), SiO2, and MgO(100) substrates showed polycrystalline characteristics with less orientation by heat treatment at 500 °C.
I. INTRODUCTION
LiNbO3 single crystals are widely used in electro-optic and nonlinear optic devices, such as optical waveguides, surface acoustic wave devices, holographic memory, and so on. Recently, the environmental issues have become critical, and environmentally friendly processing and materials are required in the new century. LiNbO3 is a promising candidate for lead-free piezoelectric and ferroelectric materials. The development of the processing method of highquality stoichiometric LiNbO3 films has been receiving great attention for miniaturizing and integrating optical waveguide devices. Several methods for preparing LiNbO3 films have been reported, such as radio frequency (rf) magnetron sputtering,1 pulsed laser deposition, 2–4 chemical vapor deposition, 5 liquid phase epitaxy,6–8 and sol-gel method.9–27 We have already reported the processing of lithium niobate films from an aqueous precursor solution, which was prepared from complete hydrolysis of the lithium and niobium double alkoxide.23 The stoichiometric, homogeneous and highly oriented LiNbO3 crystalline films were successfully processed on sapphire substrates at 350 °C from the lithium niobium hydroxide aqueous solution.23 However, since the aqueous solution has low wettability and viscosity, the preparation of homogeneous gel films on substrates by dip or spin coating has been difficult. To prepare the suitable solution for spin coating, modification of a lithium and niobium double alkoxide with acetic acid was studied. This paper describes the preparation of an ethanol precursor solution J. Mater. Res., Vol. 16, No. 4, Apr 2001
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with an addition of acetic acid and the crystallization behavior of LiNbO3 precursor films prepared on various substrates.
II. EXPERIMENTAL PROCEDURE A. Preparation of LiNbO3 films
Figure 1 shows the preparation procedure for LiNbO3 films. Equivalent molar ratio of lithium ethoxide (99.9%) and niobium ethoxide (99.9%) (Kojundo Chemical Lab. Co., Ltd., Saitama, Japan) were dissolved in absolute ethanol to form double alkoxide solution by stirring and refluxing
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