Multiple scattering of MeV light ions through thin amorphous anodic SiO2layers formed on silicon single crystals

The multiple-scattering angular distributions of protons, helium, and nitrogen ions between 500 keV and 2 MeV which have passed through anodic Si${\mathrm{O}}_2$ films (in the 100-1500-Å thickness range) formed on the top of [110] Si single crystals have been determined. They were obtained by comparing, at different penetration depths, the backscattering yields in Si crystals covered by Si${\mathrm{O}}_2$ layers to the azimuthally averaged yields in a bare crystal at various angles of incidence with respect to the [110] axis. The precision and limits of the method used are discussed. The detailed and precise experimental data on channeling in silicon which are required are reported. The experimental distributions are compared to the theoretical predictions obtained by extension of the Bothe's formula to diatomic targets and using a scattering cross section corresponding to the Thomas-Fermi potential. In the particular case of Si${\mathrm{O}}_2$, the theoretical distributions are shown to be nearly identical to the distributions calculated for a monoatomic target of atomic number $Z=10\mathrm{}$. Very good agreement is observed, for protons and helium, between the experimental and the theoretical distributions when the Thomas-Fermi screening radius of the target atoms is used. This screening radius is well adapted to our experimental cases since, in our energy domain, protons and helium have a very high probability to be completely stripped in matter.