Inelastic electron scattering from clean and arsenic-overcoated GaAs(100)

High-resolution electron-energy-loss spectra have been recorded for the (4×{}6), c(2×{}8), and c(4×{}4) surface reconstructions of GaAs(100). Spectra were also measured following the deposition of As₂ and As₄ molecular species at 100 K onto the c(2×{}8) reconstructed surface for coverages ranging from 0.25 to 6 monolayers. For the clean-surface reconstructions, low-frequency quasielastic scattering increases systematically and reproducibly in the order (4×{}6)(2×{}8)(4×{}4). Arsenic dimer deposition induces additional quasielastic scattering, which is approximately linear in the arsenic coverage, whereas tetramer deposition displayed minimal linewidth broadening for coverages below {~}3 monolayers. Vibrational modes associated with isolated As₂ or As₄ molecules were not detected. A mode observed near 26 meV after deposition of {~}6 monolayers of As₄ has been tentatively assigned as a film-substrate interface phonon.