Tracer Diffusion in Proton-Exchanged Lithium Niobate

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Lithium niobate (LiNbO3) is one of the most technologically important materials for optical applications. Optical waveguides based on this material are a fundamental structure unit in building various photonic devices. The proton-exchange process has been found as an effective method of fabricating low-loss waveguides out of LiNbO3 crystals. During proton-exchange lithium is replaced by hydrogen at the surface during annealing of LiNbO3 in a hydrogen containing melt like benzoic acid at moderate temperatures (< 400 °C). Currently, the kinetic aspects of the formation of the hydrogen enriched zone are unknown and have to be understood on a fundamental way. This includes the knowledge of the tracer diffusivities of the individual Li and H ions and their mutual interplay for the determination of effective diffusivities governing the proton-exchange process. The aim of the present project is the determination of hydrogen and lithium tracer diffusivities in proton-exchanged LiNbO3 single crystals as a function of temperature below 500 °C and hydrogen concentration. For realization, two-step experiments will be done. First, a LiNbO3 single crystal will be proton-exchanged with the isotope 1H at a certain temperature. From this effective diffusivities governing proton-exchange will be extracted. Afterwards, stable 6Li and 2H tracers will be used to quantify diffusion in the 1H enriched zone. As tracer source 6LiNbO3 sputter layers and deuterated liquid benzoic acid will be used. Isotope depth profiles will be measured by Secondary Ion Mass Spectrometry (SIMS). The experiments will be carried out on congruent (48.5 mol% Li2O) and near-stoichiometric (about 50 mol% Li2O) LiNbO3 single crystals at different crystal orientations. The results will allow a comparison of diffusivities and activation energies for Li and H diffusion. In the framework of literature and based on these experimental data, a model will be presented to describe effective diffusivities governing the proton-exchange. The fundamental results gathered during the project should also contribute to optimized LiNbO3 device fabrication and performance.