Properties and microstructure of thin LiNbO 3 films prepared by a sol-gel process
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I. INTRODUCTION Thin-film LiNbO3 is of interest because of its usefulness as an optical waveguide in electrooptic devices such as waveguide modulators and switches in optical fiber communication systems. Virtually all thin-film formation techniques have been used to prepare LiNbO 3 films, e.g., liquid phase epitaxy,1 rf sputtering,2 chemical vapor deposition,3 epitaxial growth by melting,4 and molecular beam epitaxy.5 In this work the solgel method was used to deposit LiNbO3 films, with the initial goal of investigating whether clear films of appreciable thickness could indeed be attained and later focusing on achieving preferred orientation in such films. The sol-gel method is attractive because of its simplicity; films may be applied from clear liquid polymerized solutions by spinning or dipping techniques followed by low-temperature pyrolysis of the coated substrate in air. No sophisticated apparatus is required, and excellent purity can be obtained by distillation of the volatile organometallic starting materials prior to solution preparation. Films of optical quality have been produced in a number of other chemical systems, e.g., SiO2 and TiO 2 , where the preparation variables were judiciously controlled.6 We, therefore, began our study by investigating the effects of some of these variables in the LiNbO3 system.
II. EXPERIMENTAL A. Precursor solution preparation The starting alkoxides chosen were LiOCH 3 and Nb (C 2 H 5 O) 5 (Alfa Products, Morton Thiokol, Inc.) because they were readily available and easily dissolved in dry ethanol. The solution preparation technique used had been developed by Yoldas,7 who found that stable J. Mater. Res. 2 (5), Sep/Oct 1987
clear polymerized solutions containing Li could only be formed in an acidic environment, and best results were obtained when the Li and Nb alkoxides were dissolved in ethanol to which 0.64 mol acetic acid per mol alkoxides had been added. Hydrolysis was then accomplished by combining this solution with ethanol containing 1.0 mol H 2 O per mol alkoxides. This yielded a polymerized solution having an equivalent LiNbO3 concentration of 4% by weight. This solution was stable for ~ 4 days, after which clarity was lost and gellation occurred. A 5 % solution prepared similarly was stable for less than 1 day, but a 7% solution prepared using only 0.4 mol H 2 O per mol alkoxides was stable for several days.
B. Coating application and processing For most of this work, LiNbO 3 coatings were deposited on single-crystal silicon substrates. Polished wafers were obtained from Monsanto, St. Peters, Missouri, and dendritic web material had been grown at Westinghouse. Two crystal substrate orientations, [100] and [111], were used. Cleaning was done by standard semiconductor processing techniques, i.e., immersion in a heated solution of H 2 O:NH 4 OH:H 2 O 2 with a volume ratio of 5:1:1, rinsing with high-resistivity H 2 O, immersion in a solution of H2O:HC1:H2O2 (also 5:1:1), and rinsing again with H 2 O. Drying was accomplished by immersion in warm propanol vapor, and
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