Experimental and Computational Investigations of Surface Polaritons at Metal-Oxide-Semiconductor Interfaces.

Planar surface plasmon polaritons (SPPs) at metal-oxide-semiconductor (MOS) interfaces are central to plasmon-assisted optoelectronic and tunneling devices, where ultrathin dielectrics govern field confinement and energy transfer. However, a direct link between electron energy-loss spectroscopy (EELS) measurements and electromagnetic power dissipation at buried interfaces remains unclear. Here, we combine STEM-EELS with full-wave electromagnetic simulations to investigate SPP excitation in planar A u / S i O 2 / S i and A u / A l 2 O 3 / S i stacks with 10 nm dielectric spacers. Using a finite-difference time-domain approach, the fast electron is modeled as a broadband electromagnetic source, enabling decomposition of extinction into absorbed and scattered power and direct comparison with EEL spectra. We show that extinction is dominated by non-radiative absorption, confirming that the observed resonances originate from SPPs confined to the buried Au-oxide interface. Increasing dielectric permittivity induces a redshift and modifies modal confinement, while losses remain Au-dominated. Kramers-Kronig analysis and simulations further confirm dielectric dispersion effects and exponential evanescent decay across the oxide layer.