High-Performance Plasmonic Hafnium Nitride Nanocavity and Nanodisk Arrays for Enhanced Refractometric Sensing

We report on the deposition and thorough characterization of plasmonic hafnium nitride (HfN) thin films, along with the fabrication of HfN nanocavity and nanodisk arrays for refractometric sensing in the visible-near-infrared (Vis-NIR) range. By optimizing reactive RF magnetron sputtering parameters, we achieved high-quality HfN thin films with tunable properties, confirmed through extensive structural, compositional, and optical analyses: grazing-incidence X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, Hall-effect measurements, and variable angle spectroscopic ellipsometry. Our optimized HfN films display a gold-like color with metallic behavior down to ∼360 nm, high free-carrier concentration, and minimal energy losses. Using electron-beam lithography, we patterned HfN nanocavity and nanodisk arrays on fused silica substrates. The nanocavity arrays exhibit a grating-coupled surface plasmon polariton (SPP) tunable by adjusting the lattice periodicity, yielding a bulk refractive index sensitivity of up to 636 nm·RIU^-1 and a figure of merit (FOM) of 17.3. In nanodisk arrays, coupling between LSPR and diffractive orders leads to surface lattice resonances (SLRs), giving rise to narrower spectral line widths and a quality factor exceeding 60. Both array types show significant spectral red-shifts in response to incremental changes in surrounding media refractive indices, demonstrating strong promise for high-performance refractometric sensing. These findings highlight that HfN─a CMOS-compatible, mechanically stable, and cost-effective alternative to noble metals─enables tunable plasmonic devices for biosensing and photonic applications. By bridging a key gap in the exploration of refractory transition metal nitrides, this work emphasizes the potential of HfN in next-generation plasmonic platforms.