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 power s for axially channeled and semichanneled MeV He ions in Er-sheet-dope d GaAs epitaxial layers grown by molecular-beam epitaxy. Ion channelin g analysis using 2.0 MeV He+ coupled with the observation by transmiss ion-electron microscope has revealed the formation of fine ErAs cluste rs, whose lattice constant shrinks and coincides exactly with that of the GaAs host. Thus the Er atoms take the position corresponding to th e tetrahedral interstitial site of the GaAs lattice. The Er peak energ ies in the backscattering spectra strongly depend on the impact-parame ter dependent stopping powers, in particular for the incidence along [ 110] and the directions slightly tilted from [110] and [100]. We divid e the stopping power into two parts-contributions from outer electrons and from inner electrons of GaAs. The former is calculated from the d ielectric response theory. For the latter, we consider four types of s topping powers dependent upon impact parameter; (1) Dettmann-Robinson theory, (2) the binary-encounter model, (3) the binary encounter combi ned with the local-density approximation, and (4) the extended-local-e lectron-density model. The Er peak energies observed are well reproduc ed employing model (3).