Fabrication and Properties of Epitaxial Lithium Niobate Thin Films by Combustion Chemical Vapor Deposition (CCVD)
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Fabrication and Properties of Epitaxial Lithium Niobate Thin Films by Combustion Chemical Vapor Deposition (CCVD) Yong Dong Jiang, Jake McGee, Todd A. Polley, Robert E. Schwerzel, and Andrew T. Hunt, MicroCoating Technologies, 5315 Peachtree Industrial Blvd, Atlanta, GA 30341, USA ABSTRACT Lithium niobate has a wide variety of applications because of its excellent ferroelectric, piezoelectric and electrooptic properties. In this study, epitaxial lithium niobate thin films were deposited on c-sapphire (α-Al2O3) by the low-cost, open-atmosphere Combustion Chemical Vapor Deposition (CCVD) technique developed by MicroCoating Technologies, Inc. It was found that deposition temperature plays a critical role in determining the growth behavior and quality of the lithium niobate thin films. XRD measurements show that the lithium niobate films are epitaxial with two in-plane orientations (twin structure). A surface roughness (root mean square) of about 4 nm was obtained from the deposited film (about 200 nm thick), as measured by optical profilometry. INTRODUCTION Lithium niobate (LiNbO3) has been widely investigated because it has a great number of useful physical properties, such as excellent ferroelectricity and piezoelectricity, high electrooptic coefficient, high second harmonic generation coefficient, and large photorefractive effect. These characteristics make LiNbO3 an attractive material for electrooptic and acoustooptical applications such as waveguides, modulators, second harmonic generators, and transducers [1-6]. In addition, when LiNbO3 is doped with an optically active ion, it will exhibit lasing properties, which can be used in active waveguides and optical amplifiers [7,8]. Bulk LiNbO3 single crystals have already been widely utilized in acoustic and optical devices. However, growth of thin films has been attracting more and more attention. One of the key motivations for developing thin film technologies is that thin films can potentially provide a higher level of integration than can be achieved with discrete components [9]. The crystal structure of LiNbO3 is hexagonal with a space group of R3c at room temperature. It is a uniaxial negative crystal with no = 2.286 and ne = 2.200 at the wavelength of 632.8 nm. Since the electrooptic properties of LiNbO3 are anisotropic, the control of the films growth orientation is very important [10]. Epitaxial growth is desired for electrooptic applications, since light scattering due to a refractive index mismatch at grain boundaries leads to optical loss. Moreover, a different epitaxial orientation is required to obtain a large electrooptic effect in certain device configurations, depending on the light propagation direction and applied electric field [5,11]. Therefore, growing an epitaxial LiNbO3 film and controlling its growth orientation are very important issues. Sapphire (α-Al2O3) has the same hexagonal crystal structure as LiNbO3, so it is one of the substrate candidates for growing LiNbO3 thin films. Also sapphire has smaller refractive indices that those of LiNbO
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