IMPACT-PARAMETER DEPENDENT STOPPING POWERS FOR AXIALLY CHANNELED AND SEMICHANNELED MEV HE IONS IN GAAS-ER

We have investigated the impact-parameter dependence of stopping powers for axially channeled and semichanneled MeV He ions in Er-sheet-doped GaAs epitaxial layers grown by molecular-beam epitaxy. Ion channeling analysis using 2.0 MeV He+ coupled with the observation by transmission-electron microscope has revealed the formation of fine ErAs clusters, whose lattice constant shrinks and coincides exactly with that of the GaAs host. Thus the Er atoms take the position corresponding to the tetrahedral interstitial site of the GaAs lattice. The Er peak energies in the backscattering spectra strongly depend on the impact-parameter dependent stopping powers, in particular for the incidence along [110] and the directions slightly tilted from [110] and [100]. We divide the stopping power into two parts-contributions from outer electrons and from inner electrons of GaAs. The former is calculated from the dielectric response theory. For the latter, we consider four types of stopping powers dependent upon impact parameter; (1) Dettmann-Robinson theory, (2) the binary-encounter model, (3) the binary encounter combined with the local-density approximation, and (4) the extended-local-electron-density model. The Er peak energies observed are well reproduced employing model (3).