Au/Si(111): Analysis of the (3 3 )R30 and 66 structures by in-plane x-ray diffraction
In-plane, fractional-order diffraction-data sets from thin Au layers on Si(111) with (\u221a3 \xd7 \u221a3 )R30\xb0 and 6\xd76 structures were measured at the wiggler beamline W1 at the Hamburg synchrotron radiation laboratory. For the \u221a3 \xd7 \u221a3 structure the trimer model is confirmed with an Au-Au distance of 2.8 \xc5. In the \u221a3 \xd7 \u221a3 unit cell, two additional sites beside the Au trimer were found which can be identified with distorted substrate layers or additional partially occupied Au sites. The 6\xd76 structure is a sixfold twinned structure. The observed Patterson function clearly indicates the main features of the structural units. Each consists of three trimer clusters of Au atoms, forming a nearly equilateral triangle. The local structure of each trimer is either the original \u221a3 \xd7 \u221a3 structure or a twin structure where the Au trimers are rotated by 60\xb0. Three of these structural units form the 6\xd76 unit cell. Model calculations with incoherent superposition of twin domains lead to Patterson maps very similar to the one observed.