Au/Si(111): Analysis of the (√3 × √3 )R30° and 6×6 structures by in-plane x-ray diffraction

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Abstract
In-plane, fractional-order diffraction-data sets from thin Au layers on Si(111) with (√3 × √3 )R30° and 6×6 structures were measured at the wiggler beamline W1 at the Hamburg synchrotron radiation laboratory. For the √3 × √3 structure the trimer model is confirmed with an Au-Au distance of 2.8 Å. In the √3 × √3 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 6×6 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 √3 × √3 structure or a twin structure where the Au trimers are rotated by 60°. Three of these structural units form the 6×6 unit cell. Model calculations with incoherent superposition of twin domains lead to Patterson maps very similar to the one observed.