Understanding the Origins of Low-Frequency Instability in Thin-Film Lithium Niobate Nanophotonic Devices

Thin-film lithium niobate on insulator (TFLN) is 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 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 use a combination of materials characterization (XPS, Atom Probe Tomography, and electronic transport) and optical device characterization to explore the structure-device-processing parameter space in integrated lithium niobate devices and attempt to identify a mechanistic insight for low-frequency instability in our devices. All our devices are fabricated on 600 nm thick x-cut Lithium Niobate on insulator wafers, and we explore fabrication processes that impact both the bulk and interfaces in our devices. First, we show through XPS that the etch conditions and standard acid cleans used in our thin-film processing dramatically impact the surface by removing lithium from the surface, locally reducing Nb⁵⁺ to Nb⁴⁺, and creating a damaged amorphous Lithium Niobate layer. 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 with ~nm-scale resolution. Surprisingly, we find 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 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 or Hydrogen may indeed play a role in long-term stability of our devices.<br/><br/>Finally, we find that the metal-Lithium Niobate interface is important for the low-frequency response of our devices. Etching through the oxide cladding in our structures dramatically improves the electro-optic response but leaves behind an amorphous lithium niobate interface for the electrical contacts. Surprisingly, any attempts to remove redeposition or clean the surface semiconductor-metal interface reduces the low-frequency response and degrades the response flatness. Together, these results point to the important of surface and bulk conductivity within our electro-optic devices for stable low-frequency tuning, a requirement for deployable devices and systems in thin-film Lithium Niobate.