Tracer diffusion in proton-exchanged lithium niobate

Project leader: Prof. Dr. Harald Schmidt Funding period: 08/18 - 07/21

Lithium niobate (LiNbO3) is one of the most technologically important materials in the field of optical applications. Optical waveguides based on this material form a fundamental element for the fabrication of a wide variety of photonic devices. In this context, the proton exchange process has become an effective method to fabricate 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 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 to determine hydrogen and lithium tracer diffusion coefficients in proton exchanged LiNbO3 single crystals as a function of temperature and hydrogen concentration.

Two-step experiments are performed 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 tracers}are used to quantify diffusion in the ^1H-enrichedzone. ^{6LiNbO3 sputtering layers}and deuterated liquid benzoic acid are used as tracer sources. Isotope profiles are determined by secondary ion mass spectrometry. The studies are performed on congruent and (nearly) stoichiometric LiNbO3 single crystals. The results allow a comparison of the diffusion coefficients and activation energies of Li and H diffusion. Based on literature work 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 will also contribute to an optimized fabrication and performance of LiNbO3 based devices.