Competition between bulk and interface plasmonic modes in valence electron energy-loss spectroscopy of ultrathin SiO2 gate stacks

Low-energy excitations (less than or similar to 50 eV) induced by fast electrons in materials can exhibit a collective and delocalized nature. Here, we study such excitations in Si/SiO2/Si stacks by spatially resolved electron energy-loss spectroscopy with a sub-2 A electron beam. Experimental spectra acquired in the SiO2 layer are found to display delocalized contributions originating from interface plasmons, interband transitions, and Cerenkov radiation. A comparison with simulations based on a local semiclassical dielectric model, which includes relativistic effects, highlights the changes in interface plasmon coupling as the thickness of the central SiO2 layer is reduced. We demonstrate both experimentally and theoretically that when the electron probe is located at the center of a 2 nm SiO2 layer, the optical response expected from a bulk SiO2 layer is suppressed and delocalized contributions dominate. As the layer thickness is reduced, the spectra become more like that of bulk Si even if the incident electrons travel only in the SiO2 layer. This poses a major challenge for directly extracting local optical properties of ultrathin layers by electron energy-loss spectroscopy.