Pronounced surface peaks were observed in the backscattering spectra measured with a toroidal electrostatic analyzer for medium energy He + incidence on graphite and amorphous Si (a-Si) and SiO 2 films. In contrast, no surface peak was seen for amorphous Ni films deposited on SiO 2 (70 9)/Si(001) substrates. In the case of amorphous NiSi films, strong surface peaks appeared in the backscattering spectra from Si atoms but not from Ni atoms. A nonequilibrium charge exchange process near the surface generates the surface peak, which is reproduced well by two independent fitting parameters, (1) electron capture (or loss) cross-section and (2) outgoing path length in the ion-electron interaction region extended toward the vacuum side. The analysis of the surface peak from Si of the a-SiNi revealed that the Si atoms segregate on the top surface. The dependence of the surface peak profile upon energy, exit angle, and materials of the surface is discussed in detail in the context of the ion-surface interactions.
