Spectra of secondary electrons induced by channeled and nonchanneled ions in Si and Al.

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Energy spectra are measured in the (1--2)-keV range of secondary electrons induced by bombardment of Si(100) and Al(110) with a 1.5-MeV ${\mathrm{He}}^{+}$ beam. The ion beam is either aligned with a major crystallographic direction or incident along a random direction. The shape of the experimental secondary electron spectra are successfully compared with that of spectra calculated with an efficient Monte Carlo model for electron-transport simulation. In addition, the effective layer thickness L for secondary electron generation under channeling incidence conditions is determined. It is found that ${\mathit{L}}_{\mathit{A}}$ for KLL Auger electron generation is equal to the surface peak area in the spectra of the backscattered ions. This similarity is a consequence of the small values of the backscattering collision diameter and the adiabatic radius for K-shell ionization, as compared to the atomic vibration amplitude. In contrast, ${\mathit{L}}_{\mathit{B}}$ for the generation of electrons by direct Coulomb ionization is much larger than that for Auger emission. The large value for ${\mathit{L}}_{\mathit{B}}$---an indication of a reduced channeling effect---is attributed to the relatively large contribution from the moderately localized L shell to the measured spectra. \textcopyright{} 1996 The American Physical Society.