Near-Field Investigation of Hybrid Surface-Phonon-Plasmon Polariton in Doped Gallium Nitride for Enhanced Infrared Characterization

Wide band gap (WBG) semiconductors, particularly gallium nitride (GaN), are at the forefront of advancements in power electronic devices. GaN's unique material properties, including a wide, direct bandgap, high breakdown field, and considerable thermal conductivity, enhance performance for low- and high-voltage applications. These attributes make GaN a promising candidate for next-generation high-power and high-frequency devices. This work explores the underlying physical properties of GaN in the infrared (IR) spectral range, focusing on the interactions between light and matter mediated by polar lattice vibrations and free carrier plasmas in GaN's wurtzite crystal structure. The IR response of GaN is characterized by its polar optical phonons, which define a spectral region with high reflectivity, known as the Reststrahlen band, bounded by transverse optical (TO) and longitudinal optical (LO) phonon modes. GaN exhibits anisotropic IR dielectric permittivity within this band, modeled through the TOLO formalism. The free carriers in doped GaN also introduce a Drude response in the IR spectrum, allowing for the coexistence of surface phonon polaritons (SPhPs) and surface plasmon polaritons (SPPs) on the material’s surface. This study investigates hybridized surface phonon-plasmon polaritons (SPPPs) modes in doped GaN using near-field infrared reflection measurements. The results show that the hybridization of phonon and plasmon polaritons permits tunable control over polariton wavelengths, propagation characteristics, and resonant frequencies, offering opportunities for applications requiring adjustable IR properties. Experiments were conducted using a free electron laser (FEL) coupled with scattering-type scanning near-field optical microscopy (s-SNOM) to measure frequency-wavevector dispersion relations of the hybridized modes across GaN samples with varying doping densities. Results demonstrate the high sensitivity of polariton dispersion to changes in free carrier density due to the combined effects of lattice vibrations and free carrier contributions to the IR dielectric function. The experimental data show strong concordance with simulations, illustrating that the polariton-based approach could serve as a powerful tool for probing carrier density, strain, and defects in WBG materials. This work establishes a foundation for using hybrid polariton modes in developing advanced spectroscopic techniques for WBG semiconductor characterization.