Fast-electron energy losses in thin films of vitreous Se-As alloys

Abstract

The dielectric constants of amorphous thin films of Se-As alloys with 36-, 40-, and 50-at.% As have been determined over the spectral range 3-100 eV by measurement of the fast-electron energy-loss function, $-\mathrm{Im}{\epsilon }^{-1\mathrm{}}\left(\omega \right)$. The optical parameters were determined by a Kramers-Kronig analysis of the loss profile corrected for energy resolution and higher-order plasmons. The energy-loss functions for these alloys all exhibited similar characteristics, which included a small peak from 2 to 7 eV due to valence-band structure, a large plasma-loss peak with a maximum at 18.5-19.3 eV, and several small peaks at high energies associated with core-state transitions. The decay time for the collective plasma oscillations was approximately 0.65 × ${10}^{-16\mathrm{}}$ sec. The number of effective valence electrons contributing to the optical resonances exceeded the average number of valence electrons per atom, primarily due to the oscillator strength coupling of the valence and $3d$ bands. The dielectric and optical constants were found to compare favorably with parameters determined by recent direct optical measurements. The dielectric constants implied that the valence band in these glasses is split into two sections with a gap of approximately 1 eV.