Evolution of phases and microstructure in optical waveguides of lithium niobate
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S. A. Dregia Department of Materials Science and Engineering, The Ohio State University, Columbus, Ohio 43210
W. E. Lee Department of Engineering Materials, University of Sheffield, Sheffield, England (Received 28 July 1993; accepted 11 April 1994)
The microstructural development of Ti: LiNbO3 optical waveguides, as a function of annealing time and temperature, was studied by x-ray diffraction, scanning and transmission electron microscopy, and Auger electron spectroscopy. The microstructure evolves in three major stages: oxidation, precipitation and abnormal grain growth, and interdiffusion. The deposited Ti film is oxidized at low temperatures through a series of intermediate TiOx phases until complete oxidation to rutile TiO2 occurs at ~500 °C. At intermediate temperatures, 500-800 °C, epitaxial precipitates of LiNb 3 O 8 are formed at the rutile/LiNbO 3 interface. At this stage abnormal grain growth occurs in the rutile film, causing multivariant epitaxy where all of the grains have a single orientation relationship to the substrate. Subsequent interdiffusion between TiO 2 and LiNb 3 O 8 produces a solid solution with the rutile structure which, at these temperatures, appears to coexist in equilibrium with the underlying lithium niobate substrate. This rutile solid solution serves as the source of Ti in the final stage of interdiffusion, which occurs only at higher temperatures ( a 1000 °C), and leads to consumption of the rutile layer by the substrate. Structural models are discussed for epitaxial grain growth and interdiffusion.
I. INTRODUCTION Lithium niobate (LN) is a widely developed material for hybrid optoelectronic devices. Waveguides can be produced by several techniques including proton exchange, ion implantation, and lithium outdiffusion. One of the most utilized techniques, Ti indiffusion,1'2 has been successfully used to produce low-loss optical waveguides by formation of a Ti: LN solid solution (ss) channel with a higher refractive index than the surrounding single crystal of congruent-melting LN. Devices made by this technique are commercially available for different applications, and much work has been done to understand the relationship between waveguide processing parameters and optical properties.3'4 Key to this understanding is careful characterization of microstructural changes as a function of processing parameters during waveguide fabrication. However, only recently have attempts been made to understand the microstructural changes that develop during waveguide fabrication en route to the Ti: LN (ss) and the role of fabrication parameters in these changes. There are few studies of structural changes during waveguide fabrication by indiffusion of Ti. Those that do exist do not give a complete description of the microstructural evolution. Read et al.5 studied the com2040
http://journals.cambridge.org
J. Mater. Res., Vol. 9, No. 8, Aug 1994
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positional changes during heat treatment of Ti films on LN and reported color changes which result from annealing at differ
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