Transmission electron microscopy observation and optical property of sol-gel derived LiNbO 3 films
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Transmission electron microscopy observation and optical property of sol-gel derived LiNbO3 films K. Terabe National Institute for Research in Inorganic Materials, Namiki 1-1, Tsukuba-shi, Ibaraki 305, Japan
A. Gruverman National Institute for Advanced Interdisciplinary Research, Higashi 1-1-4, Tsukuba-shi, Ibaraki 305, Japan
Y. Matsui, N. Iyi, and K. Kitamura National Institute for Research in Inorganic Materials, Namiki 1-1, Tsukuba-shi, Ibaraki 305, Japan (Received 21 December 1995; accepted 15 March 1996)
Crystallization behavior, defects, and interface structures of sol-gel derived LiNbO3 films on three kinds of substrates were examined. The nucleation was found to occur epitaxially at the interface between the film and the substrate. The continuous film is formed by coalescence of the island-like crystallites. When sapphire substrate is used, which has large lattice mismatch with the LiNbO3 , the resulting film contains a large amount of micropores, twin structures, and misfit dislocations. On the other hand, while LiTaO3 and 5% MgO-doped LiNbO3 substrates with smaller mismatch are used as substrates, the films show no evidence of the formation of dislocations and twins. The film on 5% MgO-doped LiNbO3 substrate shows better optical waveguiding property.
I. INTRODUCTION
Since lithium niobate (LiNbO3 ) has excellent electro-optical and nonlinear optical properties, it is one of the most promising materials for the application in active integrated devices, such as optical modulators and second harmonic generators. Preparations of LiNbO3 films by a sol-gel method, using metal alkoxide solution as starting material, have been reported recently.1–5 This method has many advantages, such as controllability of composition, high productivity, low temperature processing, etc. Hirano and Kato1,2 prepared the homogeneous and dense LiNbO3 films with a high degree of orientation on the sapphire substrate by controlling the synthetic conditions. Later, we reported that the films of higher crystallinity, as revealed by x-ray diffraction analyses, could be prepared using LiTaO3 and 5%-MgO doped LiNbO3 substrates, because they have smaller discrepancies in the lattice constant and thermal expansion with LiNbO3 than those of sapphire.3 Optical and electrical properties of LiNbO3 films are strongly influenced by the surface morphology and structural defects. A surface roughness, pores, grain boundaries, and misfit dislocations usually worsen the properties of the LiNbO3 films. Thus, the structural characterization of the prepared film is very important. The surface morphologies of sol-gel derived LiNbO3 films on sapphire, LiTaO3 , and 5% MgO-doped LiNbO3 have been examined by using SEM (scanning electron microscopy) and AFM (atomic force microscopy).3 The prepared LiNbO3 films after heat treatment at 500 ±C 3152
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J. Mater. Res., Vol. 11, No. 12, Dec 1996
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showed a smooth surface and no large pores. The value of the average r
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