Giant focusing peak and potential dependence observed in a transition from axial to planar channeling in Si

A detailed study has been made of energy spectra of 1 MeV He ions, scattered elastically under grazing exit conditions, in silicon at a temperature of 164 K. The spectra, measured during an angular scan through the [110] axis, in a (211) plane, show a very rich structure. Most notable is a very strong focusing peak with a height of 2.5 times the random level, that occurs at a tilt angle of 1-degrees with the [110] axis. The shape and the intensity of each of the spectra were compared with simulations, using two independently developed Monte Carlo simulation programs. The analysis showed that details of the spectra are quite sensitive to the assumed ion-atom potential. Major discrepancies occur when the ZBL potential is used. A considerable improvement is obtained by the use of a Hartree-Fock potential. A few other models of the ion-atom potential were also tried, and the results compared.