The Non-adiabatic Dynamics In Energetic Negative Ions Scattering From Water Covered Silicon Surfaces

Ion-surface interactions play an important role in many fields,such as plasma physics,surface analysis,semiconductor manufacturing,astrophysics and so on.It generally includes the charge exchange,electron excitation and energy loss processes between incident ions and the surface atoms.The charge transfer can not only probe the surface electron structure,but also decide/determine the charge state distribution of scattered ions.Ion surface scattering is the most direct way to probe the charge exchange process.A large number of studies that keV energies atoms and ions scattering on metal surfaces have shown that,the resonance charge transfer(RCT)through electron tunneling between incident particles and the surface atoms is a dominant process.Semiconductor surface has a specific localized dangling-bond surface states and a narrow band gap,which can significantly affect charge exchange in the ionsurface interactions.Semiconductor plays an important role in manufacturing.In addition,the surface doping has prominently affected the characteristic and performance of semiconductor electronic devices.In this work,we mainly studied the charge exchange process of low energy negative ions scattering on water covered silicon surfaces.Experimentally,we measured the dependence of scattered charge state fraction on incident energy and angle using one dimensional position sensitive detection technology with energies from 8.5 to 22.5 keV and the scattering angle at 38°.We found that,the positive and negative ion fractions increase monotonically with incident velocity for specular scattering.For a given incident energy,positive ion fraction slightly decreases with incident angle,while the variation of the negative ion fraction with incident angle is bell shaped.Traditional physical picture(Jellium model on metal surface)cannot explain our measured results,and hence we have to take into account the incident-velocity effect at short distances where the yield of negative ions depends on the number of initial neutral atoms.It is a dynamical equilibrium population never achieved at that moment.After solving the modified rate equations,the better agreement between experiment and calculation is obtained.These results strongly indicates that the ion-surface scattering presents non-adiabatic feature in this relatively high energy range.Moreover,we also carried out the experiments for charge exchange after negative ions scattering on the surfaces with different doping concentrations.The experimental results have shown that,the surface doping has no influence on negative-ion formation for fast collisions in 8.5-22.5keV energy range.Furthermore,we build a complete theory picture including the contribution from positive ions to treat negative ion fraction.These directly declare that the charge transfer in this high energy regime is non-adiabatic.