UHV transmission electron microscopy structure determination of the Si( 111)-(v x V )R30°Au surface

Details of the average atomic structure and the possible nature of the surface domain walls of the Si( 111 )-x/3 x V~)R301Au 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 rotation and the interatomic spacings within the structure vary significafitly with the sharpness of the structure's diffraction spots. These variations in structure parameters among data sets as well as large ~alues 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 sil!con double layer. Charge transfer in the structure is evident from a better fit obtained using Au + scattering factors than with thos~ for neutral gold, confirming a total energy cluster calculation I-Surf. Rev. Lett. 1 (1994) 273]. The presence of gold trimers is confiCmed by the local symmetry seen in high resolution micrographs which also show surface domain morphology differences between i diffuse and sharp diffraction spot regions, These images additionally indicate that the surface domain walls must be either vacancy itype, or the gold-to-gold spacing in and near the walls must be close to 3,84 A,. From these observations, models for the Si( 111 )-(x/3 ~ V~)R30°Au domain walls and the Si(l l l )-(6 x 6) Au structure are proposed.