EVOLUTION OF PHASES AND MICROSTRUCTURE IN OPTICAL WAVE-GUIDES OF LITHIUM-NIOBATE

The microstructural development of Ti: LiNbO3 optical waveguides, as a function of annealing time and temperature, was studied by x-ray diff raction, scanning and transmission electron microscopy, and Auger elec tron spectroscopy. The microstructure evolves in three major stages: o xidation, precipitation and abnormal grain growth, and interdiffusion. The deposited Ti film is oxidized at low temperatures through a serie s of intermediate TiO(x) phases until complete oxidation to rutile TiO 2 occurs at approximately 500-degrees-C. At intermediate temperatures, 500-800-degrees-C, epitaxial precipitates of LiNb3O8 are formed at th e rutile/LiNbO3 interface. At this stage abnormal grain growth occurs in the rutile film, causing multivariant epitaxy where all of the grai ns have a single orientation relationship to the substrate. Subsequent interdiffusion between TiO2 and LiNb3O8 produces a solid solution wit h the rutile structure which, at these temperatures, appears to coexis t in equilibrium with the underlying lithium niobate substrate. This r utile solid solution serves as the source of Ti in the final stage of interdiffusion, which occurs only at higher temperatures (greater-than -or-similar-to 1000-degrees-C), and leads to consumption of the rutile layer by the substrate. Structural models are discussed for epitaxial grain growth and interdiffusion.