STRONG ELEMENT DEPENDENCE OF C 1S AND SI 2P X-RAY PHOTOELECTRON DIFFRACTION PROFILES FOR IDENTICAL C AND SI LOCAL GEOMETRIES IN BETA-SIC

We have measured photoelectron diffraction polar profiles for a beta-SiC film grown epitaxially on Si(001). Tha data reveal dominant single domain growth with a good crystallinity but the C 1s and Si 2p profiles exhibit remarkably strong differences in spite of the identical geometries of the sites occupied by these elements in the ZnS-type lattice. Mast obvious are the large angular shifts and changes in intensity between expected and measured forward scattering peaks. Our results obtained at low angular resolution (similar to 5 degrees) in the high kinetic energy range (similar to 1000 eV) provide a striking example of the limitations of the often invoked forward scattering picture. The measured profiles are rather well reproduced by single scattering cluster simulations. The observed elemental dependence can be traced back to the marked change in complex scattering amplitude between C and Si along with the very general fact, by no means restricted to the SiC(001) case, that a large number of scatterers, in particular out-of-chain atoms with a low scattering angle, make a substantial contribution to the photoelectron wave around forward scattering directions. The: related energy and element dependent interference effects are particularly strong along the low density [001] C chains of the open ZnS-type structure and reflect in a drastic peak splitting due to the strong scattering at lateral Si atoms. In contrast, the [001] Si chains in beta-SiC lead to an essentially structureless forward scattering peak due to the lower scattering amplitude of lateral C atoms.