Backscattering of 5-15 keV Light Ions from Silicon

Abstract

<p>In this thesis the scattering of hydrogen and helium ions, in the incident particle energy range 5-15 keV, from silicon targets is studied both theoretically and experimentally. In the theoretical treatment, electronic and nuclear energy losses are included in a computer simulation model. The computer simulation technique has been developed to study the interaction of the incident ions and target atoms for an incident reduced energy range 0.05<ε₀<20 of interest in fusion research. The theoretical model is based on a Thomas-Fermi interatomic potential and experimental values for the electronic energy loss coefficient. The backscattered energy and angular distributions are calculated by following the histories of 5,000 to 10,000 particles. An experimental system using the time-of-flight technique has been developed to measure the energy spectra of light ions and neutrals backscattered from solid surfaces thus permitting a comparison of the theory with experimental data for a well characterised target. To test the time of-flight technique, ions and neutrals, were scattered from thin silver films (20-40 Å) evaporated on silicon substrates. From the measurements it was found that the sensitivity of the technique is better than 1/20 of a monolayer for silver on silicon. The energy resolution for helium scattered from these targets was found to be 10-15% which is consistent with the time-of-flight resolution and the electronic-loss energy broadening. To measure the energy of the backscattered particles, time-of-flight spectra for hydrogen and helium scattered from silicon were recorded. These spectra were then transformed to energy spectra. The comparison between the experimental and theoretical results showed good agreement down to a backscattered energy of 500 eV (cut-off energy due to detector calibration limits). A number of experiments were done to measure the charge fractions of the backscattered H and He particles from a silicon target. For hydrogen scattering no surface peak were observed and also there was no penetration effect on the charge fractions. This is an indication that hydrogen neutralization occurs at the surface. On the other hand, in the helium case peaks were observed in the charge fractions corresponding to surface scattering. This indicates that helium bound states occur inside the target. Finally, the effect of surface cleaning was studied. The results showed that a factor of 2 increase in ion yield was observed after cleaning the surface by 5 keV neon bombardment. The reduction in the charge fraction for the practical surface was mainly due to carbon and oxygen impurities on the surface.</p>