Tracer Diffusion in Proton-Exchanged Lithium Niobate

铌酸锂（LiNbO3）是光学应用中最重要的技术材料之一。基于这种材料的光波导是构建各种光子器件的基本结构单元。质子交换法是一种制备低损耗铌酸锂光波导的有效方法。在质子交换过程中，LiNbO3在含氢熔体如苯甲酸中在中等温度（< 400 °C）下退火期间，锂在表面被氢取代。目前，富氢区形成的动力学方面是未知的，必须从根本上加以理解。这包括知识的示踪剂扩散系数的个别锂和H离子和它们的相互作用，用于确定有效的扩散系数管理的质子交换过程。本项目的目的是确定质子交换LiNbO3单晶中氢和锂示踪剂扩散率作为低于500 °C的温度和氢浓度的函数。为了实现，将进行两步实验。首先，LiNbO3单晶将在一定温度下与同位素1H进行质子交换。从这个有效的扩散率控制质子交换将被提取。之后，将使用稳定的6Li和2H示踪剂来量化1H富集区中的扩散。将使用6LiNbO3溅射层和氘代液体苯甲酸作为示踪源。同位素深度分布将通过二次离子质谱法（西姆斯）进行测量。实验将在全等（48.5mol%Li2O）和近化学计量（约50mol%Li2O）铌酸锂单晶在不同的晶体取向进行。结果将允许扩散系数和激活能的Li和H扩散的比较。在文献的框架内，并根据这些实验数据，将提出一个模型来描述有效的质子交换扩散系数。在项目期间收集的基本结果也将有助于优化LiNbO3器件的制造和性能。

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.