Probing The Impact of Interfaces on Thin-Film Lithium Niobate Electro-Optic Device Performance

Thin-film lithium niobate on insulator (TFLN) has emerged as a strong candidate platform for integrated classical and quantum photonics due to its large linear electro-optic (EO) effect and wafer-scale availability. Driven by breakthroughs in nanofabrication, the EO interaction strength has greatly improved over the bulk, unlocking a new class of devices such as high-bandwidth and energy-efficient modulators, ultrafast pulse generators, and microwave-to-optical transducers. Despite rapid advances in device functionality, however, material understanding has fallen behind. In particular, it is generally recognized that the reliability of TFLN modulators is subject to unstable EO response at dc and low frequencies. This instability is especially disruptive to the advancement of large-scale photonic circuits and cryogenic applications such as quantum photonics, which require precise index reconfigurability and low-power operation. Post-processing techniques such as thermal annealing have been developed to reduce these deleterious effects, but the microscopic origins of instability remain unclear.<br/><br/>Here, we combine measurements of the material structure, electronic properties, and EO device performance to identify a mechanistic insight for low-frequency instability in TFLN. All devices are fabricated on 600 nm thick x-cut lithium niobate on insulator wafers, and we explore modifications to the fabrication process that affect both bulk and interface properties. First, we correlate electronic transport measurements with improvements in the EO response magnitude to show that annealing reduces charge leakage pathways in TFLN. However, SIMS measurements of the elemental composition show that simultaneously additional dielectric relaxation pathways are introduced; annealing in the presence of a cladding oxide creates an interface by which lithium can diffuse out of the TFLN.<br/><br/>Next, we show through XPS that the etch chemistry and acid cleans used in our thin-film processing dramatically affect the surface chemistry of TFLN. Dry etching with an Ar plasma reduces Nb and creates a damaged amorphous surface layer, whereas employing a C₃F₈-based chemistry creates a prominent F peak stemming from the formation of LiF_x salts or fluoropolymers. The surface Li:Nb ratio can be restored both by specific chemical cleans that remove the damaged layer or thermal annealing. Surprisingly, forming metal contacts to LN surfaces that have been cleaned or annealed actually reduces the EO response magnitude and degrades the stability. Together, these measurements indicate that the metal-TFLN interface also plays a key role in determining EO device performance.