Selective excitation of a symmetric interference plasmon mode in two close planar SiO2/Si interfaces observed by electron energy-loss spectroscopy

We have studied the effects of the SiO2/Si interface parallel to an electron beam on transmission electron energy-loss spectra of a SiO2 area for poly-Si/SiO2/Si samples. The dependence of the energy-loss spectra on the distance from the interface to the probe position and on the distance between two interfaces was investigated. Spectra obtained from the center of the thick (150 nm) SiO2 layer had no peak in the energy region of 4-10 eV. However, a peak at about 7 eV was observed in the spectra obtained at a position of 7.5 nm from the SiO2/Si interface for the same specimen. This peak was assigned to a SiO2/Si interface plasmon excitation. For the thin (15 nm) oxide poly-Si/SiO2/Si sample, on the other hand, the peak appeared at 8.7 eV. Furthermore, this peak shifts to higher energies as the oxide thickness decreases. This peak was assigned to an excitation of a symmetric interference plasmon mode in two close planar SiO2/Si interfaces.