Epitaxially grown LiNbO 3 thin films by polymeric precursor method
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Varela Instituto de Quı´mica, Universidade Estadual Paulista, P.O. Box 355, 14884-970 Araraquara, SP, Brazil
M. Guilloux Viry and A. Perrin Laboratoire Chimie du Solide et Inorganique Mole´culaire, UMR CNRS 6511, Universite´ de Rennes 1, Av. du Ge´ne´ral Leclerc, 35 042 Rennes cedex, France (Received 9 December 1999; accepted 9 August 2000)
LiNbO3 thin films were grown on (0001) sapphire substrates by a chemical route, using the polymeric precursor method. The overall process consists of preparing a coating solution from the Pechini process, based on metallic citrate polymerization. The precursor films, deposited by dip coating, are then heat treated to eliminate the organic material and to synthesize the phase. In this work, we studied the influence of the heat treatment on the structural and optical properties of single-layered films. Two routes were also investigated to increase the film thickness: increasing the viscosity of the coating solution and/or increasing the number of successively deposited layers. The x-ray diffraction -2 scans revealed the c-axis orientation of the single- and multilayered films and showed that efficient crystallization can be obtained at temperatures as low as 400 °C. The phi-scan diffraction evidenced the epitaxial growth with two in-plane variants. A microstructural study revealed that the films were crack free, homogeneous, and relatively dense. Finally, the investigation of the optical properties (optical transmittance and refractive index) confirmed the good quality of the films. These results indicate that the polymeric precursor method is a promising process to develop lithium niobate waveguides.
I. INTRODUCTION
In addition to its excellent ferroelectric and piezoelectric properties, lithium niobate (LiNbO3 or LN) has large electro-optic and nonlinear optical coefficients.1 These characteristics make LN an attractive material for optoelectronic and acousto-optical applications such as waveguides, modulators, second harmonic generators, and transducers.2 For miniaturizing and integrating these devices, different processing methods have been developed to prepare LN thin films, including sol-gel process,3–6 metalorganic decomposition (MOD),7 chemical vapor deposition (CVD),8 metalorganic chemical vapor deposition (MOCVD),9 liquid phase epitaxy (LPE),10 sputtering,11,12 and pulsed laser deposition.13–16 Compared to bulk devices, thin films are of much interest because they potentially allow monolithic integration with microelec-
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J. Mater. Res., Vol. 15, No. 11, Nov 2000 Downloaded: 22 Mar 2015
tronic or optoelectronic components. Moreover, microoptics such as channels and lenses can also be fabricated in thin films.17,18 For the above mentioned devices, the film must present well-defined features. In particular, the LN thin film should be epitaxially grown or at least highly textured since the properties of this anisotropic material depend on the crystallog
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