Angular Dependence of Surface Scattering and Surface Mobility Cusp

Recent theories of the surface mobility of semiconductors predict the existence of a cusp in the surface mobility as a function of surface-potential excess, if the surface is strongly scattering, and if the bulk mean free path is larger than the Debye screening length. Recent experiments on InSb surfaces, however, show no cusp. This disagreement is resolved by including an angular dependence in the Fuchs' reflectivity parameter $p$, which describes the surface scattering. It is concluded that: (a) a surface mobility cusp should occur if and only if electrons incident at grazing angles are strongly scattered, (b) observation of the existence or nonexistence of a surface mobility cusp on a particular surface provides insight into the scattering mechanisms dominant on the surface.