Reduced thermal decomposition of OH-free LiNbO₃ substrates even in a dry gas atmosphere

A thermal diffusion process of Ti into a LiNbO₃ substrate for optical waveguides has generally been carried out under a wet gas atmosphere in order to prevent undesirable Li outdiffusion. In this work, such thermal decomposition was confirmed to be significantly suppressed for an OH-free LiNbO₃ substrate, even after a dry atmosphere annealing. No extra x-ray diffraction peak for LiNb₃O₈ was detected from the OH-free substrate after 10 h of annealing at 1000 °C in a dry O₂. Furthermore, the surface morphology of this sample, and as well an unannealed one, were smooth. In a conventional LiNbO₃ substrate containing many OH ions, subjected to a similar dry annealing, the presence of the LiNb₃O₈ phase and a surface coarsening were observed.