Competition between bulk and interface plasmonic modes in valence electron energy-loss spectroscopy of ultrathin $\mathrm{Si}{\mathrm{O}}_{2}$ gate stacks

Competition between bulk and interface plasmonic modes in valence electron energy-loss spectroscopy of ultrathin $Si O_{2}$ gate stacks

M. Couillard, A. Yurtsever, and D. A. Muller

Phys. Rev. B 77, 085318 – Published 26 February 2008

Abstract

Low-energy excitations $(≲ 50 eV)$ induced by fast electrons in materials can exhibit a collective and delocalized nature. Here, we study such excitations in $Si ∕ Si O_{2} ∕ Si$ stacks by spatially resolved electron energy-loss spectroscopy with a sub- $2 Å$ electron beam. Experimental spectra acquired in the $Si O_{2}$ layer are found to display delocalized contributions originating from interface plasmons, interband transitions, and Čerenkov 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 $Si O_{2}$ layer is reduced. We demonstrate both experimentally and theoretically that when the electron probe is located at the center of a $2 nm$ $Si O_{2}$ layer, the optical response expected from a bulk $Si O_{2}$ 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 $Si O_{2}$ layer. This poses a major challenge for directly extracting local optical properties of ultrathin layers by electron energy-loss spectroscopy.