Ag-induced √3 reconstruction on Si(111)/Ge-(5 × 5) and the surfactant behavior of Ag in further growth of Ge

The growth of Ge on Si(111)/Ge-(√3 × √3)Ag substrates was investigated for Ge coverages up to 1 monolayer (ML). The √3Ag substrate was obtained by depositing 0.2 ML Ag on Si(111)/Ge(111)-5 × 5 surfaces. Because of the low Ag coverage, three types of regions - √3Ag 'island', √3Ag 'hole' and exposed Ge(111)-5 × 5 - are produced. This has allowed investigation of Ge growth simultaneously on these three types of surface features by scanning tunneling microscopy. Ge has been found to grow as bilayers almost exclusively on the √3Ag 'hole' regions, with its surface terminated by a √3Ag structure. This indicates that Ag behaves like a surfactant and segregates to the top of the deposited Ge via Ag-Ge exchange. At higher Ge coverages the √3Ag surface undergoes a transformation to a (3 × 1) Ag phase. The deposited Ge hardly grows on the exposed Si/Ge-5 × 5 regions, in contrast to the expected growth behavior on bare Si/Ge-5 × 5 surface.