Probing the Material Impact of Nanofabrication on Integrated Thin-Film Lithium Niobate Photonic Device Performance

Thin-film lithium niobate on insulator (TFLN) has emerged as a promising platform for integrated classical and quantum photonics due to its intrinsically large electro-optic effect and wafer-scale availability. The direct connection between driving electric fields and refractive index in this platform has enabled new schemes for creating pulsed lasers on chip, high-bandwidth and energy-efficient modulators, and devices for integrated nonlinear photonics. However, it is generally recognized throughout the community that the reliability of thin-film lithium niobate modulators is subject to unstable electro-optic response at low frequencies, and undesired photorefractive index changes due to defect absorption at high optical powers. Post-processing methods such as annealing have been developed to reduce these deleterious effects, but their microscopic origins remain unclear. It is also not known if these effects relate to fundamental materials issues, might be exacerbated for materials in thin-film form with possible strain inhomogeneities, and indeed might arise from thin-film preparation, which involves ion implantation, and chemical-mechanical polishing.<br/><br/>Here, we employ a combination of X-ray photoelectron spectroscopy (XPS) and atom probe tomography (APT) to analyze a commercial x-cut congruently-grown lithium niobate on insulator wafer, and thereby illuminate the surface and bulk composition as a function of nanofabrication process conditions. First, we show through XPS that the etch conditions and standard acid cleans used in our thin-film processing dramatically impact the surface, either by removing lithium from the surface or by locally reducing Nb⁵⁺ to Nb⁴⁺. Specific chemical cleans remove the damaged layer, while annealing at moderate temperatures in an oxygen environment restores the surface Li:Nb ratio to its unprocessed status. We gained additional insight by using APT to map out the material composition in 3D. Surprisingly, we found that the lithium composition within the unprocessed material varies substantially over length scales on the order of 10 nanometers, suggesting that local defect structures such as the small polaron (reduced Nb antisite defect: Nb^…._Li) may exist in appreciable concentrations that could give rise to photorefraction. Annealing our samples temporarily increases quality factors and reduces photorefraction, indicating that diffusion of mobile species such as Lithium may indeed play a role in long-term stability of our devices. These results suggest that simple fabrication modifications can help reduce the leaky dielectric nature of our integrated films, while posing questions about the fundamental limits of this platform for integrated photonics.