Tracer Diffusion in Proton-Exchanged Lithium Niobate
Final Report Abstract
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 and Li1-xHxNbO3 is formed at the crystal surface. Currently, mechanisms and kinetics of the proton-exchange process are unclear, primarily due to a lack in reliable tracer diffusion data. We studied lithium and hydrogen tracer diffusion in proton-exchanged, Z-cut congruent LiNbO3 single crystals in the temperature range between 130 – 230 °C. Proton-exchange was done in benzoic acid with 0, 1, 2, or 3.6 mol% lithium benzoate added, resulting in micrometer thick surface layers where Li is substituted by H with relative fractions between x = 0.45 and 0.85 as determined by Nuclear Reaction Analysis. For the diffusion experiments, ion-beam sputtered isotope enriched 6LiNbO3 was used as a Li tracer source and deuterated benzoic acid as a H tracer source. Isotope depth profile analysis was carried out by secondary ion mass spectrometry. From the experimental results, effective diffusivities governing the lithium/hydrogen exchange as well as individual hydrogen and lithium tracer diffusivities were extracted. All three types of diffusivities can be described by the Arrhenius law with an activation enthalpy of about 1.0 -1.2 eV and increase as a function of hydrogen concentration nearly independent of temperature. The effective diffusivities and the lithium tracer diffusivities are identical within a factor of two, while the hydrogen diffusivities are higher by three orders of magnitude. The results show that the diffusion of Li is the rate determining step governing the proton-exchange process. Exponential dependencies between diffusivities and hydrogen concentration are determined. Assuming a vacancy diffusion mechanism, the observed increase of Li tracer diffusivities and consequently of effective diffusivities as a function of hydrogen concentration is attributed to a continuous reduction of the migration enthalpy of diffusion by a maximum factor of about 0.2 eV. Hydrogen is assumed to diffuse via a trap-limed diffusion mechanism and the increase of hydrogen diffusivities as a function of hydrogen concentration is ascribed to a saturation of traps. Simulations based on the obtained results show that the step-like behavior of hydrogen penetration can be reproduced.
Publications
- Hydrogen diffusion in proton-exchanged lithium niobate single crystals, J. Appl. Phys. 129 (2021), 135105
L. Dörrer, P. Tuchel, E. Hüger, R. Heller, H. Schmidt
(See online at https://doi.org/10.1063/5.0047606) - Lithium tracer diffusion in proton-exchanged lithium niobate, Solid State Ionics 365 (2021), 115657
L. Dörrer, P. Tuchel, D. Uxa, H. Schmidt
(See online at https://doi.org/10.1016/j.ssi.2021.115657) - Proton exchange at LiNbO3 surfaces - diffusion investigations, Tagungsband, 4. Symposium Materialtechnik, Clausthal-Zellerfeld, Germany (2021), 874
L. Dörrer, H. Schmidt
- Tracer Diffusion in Proton-Exchanged Congruent LiNbO3 Crystals as a Function of Hydrogen Content, Phys. Chem. Chem. Phys. 24 (2022), 16139
L. Dörrer, R. Heller, H. Schmidt
(See online at https://doi.org/10.1039/D2CP01818G)
