Inelastic energy loss of light particles scattered by solid surfaces at low energy: influence of the 'gap'

The energy spectra of particles scattered by solid surfaces are used to det ermine the inelastic energy loss at low energy. Assuming the binary collisi on approximation, a modified TRIM code provides length distributions which are converted to time-of-flight (TOF) spectra by using the friction coeffic ient as an adjustable parameter. Owing to the nonlinear effects occurring i n this energy range, the theoretical value of the electronic stopping power is performed from electron-particle scattering cross-section using a scree ned potential and so, the phase shifts, obtained self-consistently in the f ramework of density functional theory (DFT). In the case of Hela:Si interac tion at 4 keV, the standard model leads to a largely overestimated value. T his fact has been attributed to the presence of the electron energy gap E-G and to the structure of the valence band. We verify this assumption in a n on-static model involving all electrons of the valence band with a threshol d condition upsilon(e)'(2) > upsilon(F)(2) + 2E(G), where upsilon(F) is the Fermi velocity and upsilon(e) the electron velocity after scattering (non- static extended collisional model). The theoretical results agree very well with the experimental ones for He colliding three targets: a:Si, a:Ge and polycrystalline Ni at 4 keV. The calculations performed for the velocity ra nge below 1 a.u. confirm the important role of the gap and the band structu re in the lowering of stopping power at low velocity. (C) 2000 Elsevier Sci ence B.V. All rights reserved.