UHV TRANSMISSION ELECTRON-MICROSCOPY STRUCTURE DETERMINATION OF THE SI(111)-(ROOT-3X-ROOT-3)R30-DEGREES-AU SURFACE

Details of the average atomic structure and the possible nature of the surface domain walls of the Si(111)-root 3 x root 3)R30 degrees Au surfaces are presented. Transmission electron diffraction data supports the missing top layer twisted trimer [Surf Sci. 256 (1991) 135] model in which gold and silicon atoms in the top two layers form sets of like-atom trimers rotated in the same direction about p3 symmetry centers. However, the average degree of trimer relation and the interatomic spacings within the structure vary significantly with the sharpness of the structure's diffraction spots. These variations in structure parameters among data sets as well as large values of the fitted Debye-Waller terms indicate substantial static disorder in the structure and hence any parameters determined can only be averages of locally varying parameters. Silicon displacements from the bulk positions continue through the second silicon double layer. Charge transfer in the structure is evident from a better fit obtained using Au+ scattering factors than with those for neutral gold, confirming a total energy cluster calculation [Surf. Rev. Lett. 1 (1994) 273]. The presence of gold trimers is confirmed by the local symmetry seen in high resolution micrographs which also show surface domain morphology differences between diffuse and sharp diffraction spot regions. These images additionally indicate that the surface domain walls must be either vacancy type, or the gold-to-gold spacing in and near the walls must be close to 3.84 Angstrom. From these observations, models for the Si(111)-(root 3 x root 3)R30 degrees Au domain walls and the Si(111)-(6 x 6) Au structure are proposed.