ON THE UNCOUPLING OF SURFACE SUPERLATTICE REFLECTIONS IN TED ANALYSIS OF RECONSTRUCTED SURFACES

A Bloch-wave analysis is made of the problem of uncoupling surface superlattice reflections from fundamental reflections in transmission electron diffraction (TED) analysis of reconstructed surfaces. This uncoupling problem is proved to be of crucial importance in determining the structure of reconstructed surfaces, for example the Si(111) 7 X 7 surface [Takayanagi, Tanishiro, Takahashi & Takahashi (1985). Vac. Sci. Technol. A3, 1502-1506; (1985). Surf. Sci. 164, 367-392]. It is found that a complete uncoupling, weak coupling and sometimes strong coupling between the bulk scattering and surface superlattice scattering are all possible depending on the diffraction conditions. For a kinematical analysis of reconstructed surfaces to be valid, a weak coupling or a complete uncoupling condition must be realized. General rules for choosing the appropriate diffraction conditions are given.