6.2.1.4.1 Dipole scattering

This chapter provides information on dipole scattering. In the dipole scattering an inelastic scattering cross section is related to the dielectric response of a system where the dielectric constant characterizes each solid at the frequency ω. The chapter illustrates comparison for the optical constant of the surface-state layer of silicon(111)2 x 1 as derived by EELS with the result of ellipsometry and multiple-internal reflection. The four scattering processes which take an electron from the initial state to the final state near the specular direction are figured out. A useful expression for a loss function was derived for a three layer model description of a crystal. This model can be applied directly to the case of semiconductors. Their surfaces exhibit generally a large density of states in the bulk band gap which cause the pinning of the Fermi level. The mobile carriers will then rearrange themselves in a surface region of the crystal setting up an electrostatic potential which bends the valence and conduction bands making the Fermi level equal everywhere. The effect of the bending may be described by a so-called depletion (or accumulation) layer of thickness W. For dipole scattering only totally symmetric modes are dipole active for metal surfaces. On semiconductors on the other hand both vertical and longitudinal modes may be the dipole active.