Microstructure development of sol-gel derived epitaxial LiNbO 3 thin films
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Film growth and microstructural evolution were investigated for sol-gel derived LiNbO 3 thin films deposited on lattice-matched single-crystal substrates. Epitaxial LiNbO3 films of about 100 nm nominal thickness were prepared by spin coating a solution of the lithium niobium ethoxide on sapphire (0001) substrates and annealing at 400 °C or 700 °C in a humidified oxygen atmosphere. These films exhibited an epitaxial relationship with the substrate of the type LiNbO3 (0001) || a - A l 2 O 3 (0001) and LiNbO3 [100] || a - A l 2 O 3 [100] as determined by x-ray pole figure analysis. Transmission electron microscopy indicated the epitaxial films annealed at 400 °C consisted of slightly misoriented ~ 5 nm subgrains and of numerous ~10 nm enclosed pores. The microstructure and orientation development of these films was consistent with a heteroepitaxial nucleation and growth mechanism, in which epitaxial nuclei form at the substrate surface and grow upward into an amorphous and porous intermediate film. Epitaxial films annealed at 700 °C contained larger 150-200 nm subgrains and pinholes. Misorientations between adjacent subgrains appeared to be significantly smaller in films annealed at 700 °C than those in films annealed at 400 °C. Hydrolysis of the alkoxide precursor solution prior to spin coating promoted the development of polycrystalline films on single-crystal sapphire substrates. Infrared spectra and thermal analysis indicated that, independent of the degree of the solution hydrolysis, nucleation of LiNbO3 was immediately preceded by decomposition of an amorphous carbonate intermediate phase.
I. INTRODUCTION Lithium niobate has been actively investigated for electro-optic and surface acoustic wave (SAW) applications. The applications include active optical waveguides, optical modulators, optical switches, and SAW devices because of the material's large nonlinear optical coefficients, large birefringence, high electro-optic coefficient, high Curie temperature, strong piezoelectric effect, and excellent acousto-optic properties.1 For electro-optic applications, single-crystal LiNbO 3 is employed as the substrate for fabricating optical waveguides. However, fabrication of single-crystal lithium niobate devices is complicated by the following issues: (1) stoichiometric lithium niobate melts incongruently; therefore, single crystals are grown from a nonstoichiometric congruent melt with 48.45 mol % Li2O composition which can lead to crystals with variable composition; (2) the properties of LiNbO 3 , such as the Curie point and refractive indices, are very sensitive to fluctuations in its stoichiometry; and (3) fabricating optical waveguides by the usual titanium in-diffusion method requires high temperatures a)
Present Address: Fuji Xerox Co., Ltd., 3-3-5, Akasaka, Tokyo, Japan.
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J. Mater. Res., Vol. 10, No. 10, Oct 1995
http://journals.cambridge.org
Downloaded: 12 Mar 2015
(over 1000 °C). Therefore, there is a need for methods that can control the stoichiometry and provide low temperature processing of lithi
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