Preparation and characteristics of seeded epitaxial (Sr,Ba)Nb 2 O 6 optical waveguide thin films using sol-gel method
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Highly c-axis-oriented (Sr,Ba)Nb2O6 (SBN) films were grown on a seeded MgO(100) substrate via sol-gel method. The substrate was preseeded with epitaxial islands of SBN made by breaking up a continuous film into single-crystal islands by pores. Since the number of epitaxial nuclei was increased at the interface between the film and the substrate, the film on a seeded substrate had better highly orientation than that on unseeded substrate. The film having low Sr content exhibited better epitaxial growth because of the distorted unit-cell network and the change of lattice parameters of SBN thin film. For obtaining excellent optical properties, SBN:75 film was prepared on MgO substrate with SBN:25 composition seed layer. Because of low birefringence of refractive indices in the film having high Sr content, the optical scattering loss by the anisotropy of refractive indices was suppressed.
I. INTRODUCTION
Strontium barium niobate (SrxBa1−x Nb2O6, referred to as SBN:100x, where 0.25 ⱕx ⱕ 0.75) is currently being investigated as a potential ferroelectric material for many microdevice applications, such as pyroelectric infrared detectors, electro-optic modulators, holographic storage, and beam steering, because of its large pyroelectric coefficient, excellent piezoelectric and electro-optic properties, and photorefractive sensitivity.1– 4 Compared with other well-known ferroelectric materials, SBN has an extremely high electro-optic coefficient, for example, the figure of merit is more than 50 times higher than that of LiNbO3, offering the possibility of much smaller devices.3 SBN is a solid solution between BaNb2O6 and SrNb2O6 phases with a tetragonal tungsten bronze (TTB) structure, and its physical properties vary with its composition.5 These properties and their applications have been investigated mainly for single crystals and polycrystalline ceramics. However, the demand for thin film processing has increased due to the development of electronic and optical integrated devices. SBN thin films, especially highly c-axis-oriented SBN thin films, are desired for optical applications, such as electro optic properties, photorefractive, and nonlinear optical applications because these applications can take full advantage of SBN films’ ferroelectricity.6–8 There are several kinds of fabrication techniques for thin films such as radio frequency (rf) sputtering, metalorganic chemical vapor deposition (MOCVD), the sol-gel method, and pulsed 430
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J. Mater. Res., Vol. 16, No. 2, Feb 2001 Downloaded: 27 Mar 2015
laser depositions. Among these, the sol-gel process has been developed for ferroelectric thin film processes due to excellent homogeneity, ease of chemical composition control, high purity, low processing temperature, and film uniformity over a large area. Therefore, many papers recently reported on the synthesis of SBN thin films by using the sol-gel method.4,8–10 However, it was hard to obtain fully densified and oriented SBN thin films, and the quality of sol-gel-derived films was not be
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