Effect of substrates on the crystallinity and morphology of sol-gel-derived epitaxial LiNbO 3 films
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Epitaxial thin films of LiNbO3 were prepared by the sol-gel method on (OOOl)-sapphire, (0001)-LiTaO3 and (0001)-5% MgO-doped LiNbO 3 substrates. The precursor films crystallized with the highly preferred orientation on all substrates. When sapphire substrates, which have large discrepancies in the lattice constant and thermal expansion percentage with the film were used, the resulting films showed a low crystallinity after heat treatment at 500 °C and grain growth at 650 °C. On the other hand, when using LiTaO3 and 5% MgO-doped LiNbO 3 substrates, with smaller discrepancies, the formed films, after heat treatment at 500 °C, showed better crystallinity with the smooth surface.
I. INTRODUCTION Heteroepitaxial LiNbO 3 thin films have attracted considerable interest because of their usefulness in integrated electro-optical devices. Homoepitaxial and heteroepitaxial films have been prepared by the sol-gel method with many advantages such as a lowtemperature process, controllability of the composition, high productivity, etc.1"3 Good quality LiNbO 3 films are required for practical applications. Recently, Nashimoto et al. reported that epitaxial LiNbO 3 films with smooth and dense texture were prepared on sapphire substrates using nonhydrolyzed alkoxides, but grain growth occurred with increasing temperature of the heat treatments.3-4 Furthermore, dislocations, twins, and moire fringe patterns were observed in epitaxial LiNbO 3 films on sapphire substrates.5'6 It appears to be still very important to improve the quality of LiNbO3 films. For epitaxial growth processing, it is known that the differences in the lattice constant or thermal expansion coefficient, between a film and a substrate, considerably affect the quality of the resulting films. In the case of growth by liquid phase epitaxy, substrates of LiTaO3 and MgO-doped LiNbO 3 , with smaller lattice mismatch than sapphire, have been used to make better LiNbO3 films.78 Quite recently, epitaxial LiNbO3 films on LiTaO3 also were prepared by spray metal-organic chemical vapor deposition (MOCVD).9 The films prepared on these substrates can be used as optical waveguides, because the refractive indices of the substrates are smaller than that of the films. So far, however, only sapphire has been used as the substrate for the preparation of heteroepitaxial LiNbO 3 films by the sol-gel method.2"5'10 Single crystals of Si and MgO, with different crystal systems from that of the LiNbO 3 films, were tried as substrates, but only polycrystalline films without preferred orientation were obtained.10'11 J. Mater. Res., Vol. 10, No. 7, Jul 1995 http://journals.cambridge.org
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In the present study, three kinds of substrates with crystal structures similar to that of the films, but showing different lattice constants and thermal expansion percentages, were used. The influence of the substrates on the crystallinity and morphology of the resulting films was investigated. Thus, the conditions for preparing better quality films by the sol-gel method were clarified
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