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Project manager: Prof. Dr. Harald Schmidt
Funding period: 08/18 - 07/21
Lithium niobate (LiNbO3) is one of the technologically most important materials in the field of optical applications. Optical waveguides based on this material are a fundamental element in the manufacture of a wide range of photonic components. In this context, the proton exchange process has become an effective method for the production of waveguides with low losses. During the proton exchange process, lithium is replaced by hydrogen in a surface layer by annealing LiNbO3 in a hydrogen-rich melt (e.g. benzoic acid) at moderate temperatures (< 300 °C). Currently, the kinetic aspects of the formation of the hydrogen-enriched zone are unknown and need to be understood at a fundamental level. This includes the knowledge of tracer diffusion coefficients of Li and H ions and their interplay to determine effective diffusion coefficients describing the proton exchange process. The aim of the project is the determination of hydrogen and lithium tracer diffusion coefficients in proton exchanged LiNbO3 single crystals as a function of temperature and hydrogen concentration.
Two-stage experiments are carried out for this purpose. First, a LiNbO3 crystal is proton exchanged with the isotope 1Hat a defined temperature. From this, effective diffusion coefficients are determined. Then, stable 6Liand 2H tracersare used to quantify the diffusion in the 1H-enrichedzone. As tracer source 6LiNbO3 sputter layersand deuterated liquid benzoic acid are used. The isotope profiles are determined using secondary ion mass spectrometry. The investigations are carried out on congruent and (almost) stoichiometric LiNbO3 single crystals. The results allow a comparison of the diffusion coefficients and activation energies of Li and H diffusion. Based on literature studies and these experimental data, a model is presented that allows a description of effective diffusion coefficients describing the proton exchange process. The fundamental results obtained during the project should also contribute to an optimized fabrication and performance of LiNbO3 based devices.