Charge states distribution of 0.16-3.3 MeV He ions transmitted through silicon

The equilibrium charge state distribution of He ions transmitted through silicon in a random direction was measured in the energy range 0.16-3.3 MeV. The surface contamination, investigated by back-scattering spectrometry, amounted to a few monolayers. The measured data., integrated with the available literature points, cover a wide range of conditions. At the lower end (velocity similar to 1 a.u.) there is a consistent fraction of neutral He and the process is strongly influenced by solid state effects; at the higher end (velocity similar to 6 a.u.) most of the ions are stripped and the process can be described by individual He-Si collisions. The use of a semi-classical approach, based on the early theory of Bohr, allows for a satisfactory description of the He+/He2+ ratio in the whole energy range. (C) 2002 Elsevier Science B.V. All rights reserved.