Assessment of surface potential models by molecular dynamics simulations of atom ejection from (1Â 0Â 0)-Si surfaces

In analytic theories as well as binary collision simulations the attractive forces between the surface atoms and a recoil leaving the target are described by a surface potential. While in an isotropic model the recoil only suffers energy loss, in the more commonly used planar model refraction or reflection of the recoil also occur. In this work I discuss the limitations of these models, including a generalized model containing the isotropic and planar models as special cases, by comparing molecular dynamics (MD) and binary collision (BC) simulations of atom ejection from (1 0 0)-Si surfaces. It is shown that a surface potential close to planar best describes the MD results. However, in the BC simulations (i) emission yields are systematically too high at low ejection energies, (ii) discrepancies with respect to MD are larger for the ejection of atoms from sub-surface layers, and (iii) the emission yield for top-layer atoms does not increase during erosion of the top layer as suggested by MD. Possible reasons for these deficiencies are discussed. It is concluded that BC results are seriously in question when the effect under investigation depends on the fate of low-energy recoils at the surface.