Reduced thermal decomposition of OH-free LiNbO 3 substrates even in a dry gas atmosphere
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Reduced thermal decomposition of OH-free LiNbO3 substrates even in a dry gas atmosphere Hirotoshi Nagata, Toshihiro Sakamoto, Hideki Honda, and Junichiro Ichikawa Optoelectronics Division, Sumitomo Osaka Cement Co., Ltd., 585 Toyotomi-cho, Funabashi-shi, Chiba 274, Japan
Eungi Min Haga and Kaori Shima Central Research Laboratories, Sumitomo Osaka Cement Co., Ltd., 585 Toyotomi-cho, Funabashi-shi, Chiba 274, Japan
Nobuhiko Haga Faculty of Science, Himeji Institute of Technology, Kamigori-cho, Ako-gun, Hyogo 678-12, Japan (Received 17 October 1995; accepted 8 March 1996)
A thermal diffusion process of Ti into a LiNbO3 substrate for optical waveguides has generally been carried out under a wet gas atmosphere in order to prevent undesirable Li outdiffusion. In this work, such thermal decomposition was confirmed to be significantly suppressed for an OH-free LiNbO3 substrate, even after a dry atmosphere annealing. No extra x-ray diffraction peak for LiNb3 O8 was detected from the OH-free substrate after 10 h of annealing at 1000 ±C in a dry O2 . Furthermore, the surface morphology of this sample, and as well an unannealed one, were smooth. In a conventional LiNbO3 substrate containing many OH ions, subjected to a similar dry annealing, the presence of the LiNb3 O8 phase and a surface coarsening were observed.
I. INTRODUCTION
Optical waveguide devices based on LiNbO3 material have become widely used in areas such as optical communication systems and optical measurement applications.1,2 Like the proton exchange method at lower temperatures, thermal diffusion of Ti at high temperatures near the Curie temperature of LiNbO3 is also a common method for forming the optical waveguide on the LiNbO3 substrate.3–5 Recently, rare earth ions of Er, Nd, and Yb were codoped with Ti in order to integrate active waveguide devices, such as lasers.3,6–13 These high-temperature diffusion processes have been carried out under wet gas atmospheres, O2 (and N2 ), supplied via a water bubbler, in order to suppress a thermal outdiffusion of Li from the substrate surface.14–16 For the dry or vacuum annealed substrate, the presence of a LiNb3 O8 phase was reported, which was found to grow heteroepitaxially on the substrate due to decomposition.16–18 Such a chemically deteriorated layer is considered to be one of the origins for a higher optical propagation loss and dc drift.14,19,20 Compared to the above-mentioned conventional wet annealing process of the LiNbO3 , this paper describes the possibility of a dry annealing process for preparing Tiindiffused waveguides in the OH-free LiNbO3 substrates without significant surface decomposition. The OH ions were known to be inevitably introduced into the substrate during the growth and polarization processes of the LiNbO3 crystal.21 The OH-free LiNbO3 substrates are J. Mater. Res., Vol. 11, No. 8, Aug 1996
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originally proposed by Koide et al. to reduce the Li outdiffusion during the dry annealing.22 T
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