[100]AXIAL ION CHANNELING IN FE SINGLE-CRYSTALS - FLUX RELATED PHENOMENA IN THE NEAR-SURFACE REGION

The angular dependent Rutherford backscattering yield of 1.0 MeV He+ i ncident on Fe single crystals along the [100] direction is compared wi th Monte Carlo simulations of channeling scans. Different depth region s starting at the surface with widths ranging from 30 to 1600 Angstrom are analyzed. If the amorphous native surface oxide layer (typically: 20-40 Angstrom thickness) is included in the simulations, the agreeme nt between simulated and measured channeling dips is very good. Discre pancies can be explained by imperfections of the crystals, mainly in t he surface region, which cause an enhanced multiple scattering of the ions. According to the simulations, the strongest channeling effect oc curs for a depth of about 200 Angstrom. Further decreasing the depth r egion width results in a drastic reduction of the compensation shoulde rs and, depending on the exact location of the tilt plane, of psi(1/2) . This behavior is correlated to a change of the flux in the transvers e plane from a ''shadow cone'' regime, for the first 100 Angstrom, to a flux centered in the channel after about 200 Angstrom. Depending on the tilt plane, the psi(1/2)-values determined by the simulation disag ree quite substantially from the values calculated with the semiempiri cal Barrett expression. (C) 1998 Elsevier Science B.V.