The Growth of Optical Quality LiNbo 3 Thin Films On Sapphire and LiTao 3 Substrates Using Solid-Source Mocvd
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"**Centerfor Materials Research,
Stanford University, CA 94305-4045
ABSTRACT High quality lithium niobate (LiNbO 3 ) epitaxial thin films have been grown on c-plane sapphire and LiTaO3 substrates by solid-source MOCVD using the tetramethyl-heptanedionate sources, Li(thd) and Nb(thd) 4 . Phase content was controllable, and stoichiometric films were reproducibly deposited over a broad temperature range from a Li(thd)-rich source. Using sapphire substrates, with which the occurrence of multiple in-plane orientations is typically a problem, LiNbO 3 films with only one in-plane orientation could be prepared within a narrow range of growth conditions. XRD rocking curve FWHM values as low as 0.04', and optical waveguiding losses near 2dB/cm (1200A thick film, TM0 mode, 632.8nm) were obtained. However, film thicknesses on sapphire were limited to 2000A because of cracking caused by the large thermal expansion mismatch, and in such thin films, optical confinement is poor. In contrast, LiTaO 3 has almost same lattice constants and thermal expansion coefficients as LiNbO 3 , making it a potentially superior substrate material. Lithium niobate films up to 6000A were successfully deposited on LiTaO 3 substrates without cracking. Film quality was greatly improved, with FWHM values as low as 0.01', and rms surface roughness less than 10 A. Preliminary optical waveguiding losses less than 6 dB/cm for the TEO mode have been achieved.
INTRODUCTION LiNbO3 is one of the most attractive materials for integrated and guided wave optics because it has excellent ferroelectric and piezoelectric properties and large electro-optic and nonlinear optical coefficients[l I]. Currently, integrated optical devices are being fabricated from Tiindiffused or proton-exchanged surface layers on single crystal LiNbO 3 wafers[2]. Since these waveguides are made by diffusion processes, they do not have sharp interfaces or good compositional uniformity, which leads to spatial variations in refractive index. A technology to deposit high quality LiNbO 3 thin films on an appropriate substrate would offer major advantages, including precise dimensional control, the ability to vary the refractive index difference between the film and substrate, and increased power density through tighter beam confinement made possible by the sharper index profile at the film / substrate interface. Another attractive feature would be the possibile integration with III-V solid state laser sources. A number of thin film fabrication techniques have been used for the growth of LiNbO 3 thin films including liquid phase epitaxy (LPE)[3,4], epitaxial growth from melts[5], sol-gel processing[6], sputtering[7,8], laser ablation[9], molecular beam epitaxy (MBE)[10], and metalorganic chemical vapor deposition (MOCVD)[ 11,12]. Sapphire has been used most often 177 Mat. Res. Soc. Symp. Proc. Vol. 392 0 1995 Materials Research Society
as the substrate material because of its low optical index and chemical inertness. Unfortunately, there is a large lattice mismatch (7.7%), and a large ther
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