Cu induced morphology changes at Pb/Si (1 1 1) interface: Separation of “5 × 5”-Cu structure into individual domains

The adsorption of small amount of Cu onto 1 × 1-Pb/Si (1 1 1) and √3 × √3R30°-Pb/Si (1 1 1) coexisting phases was studied with variable temperature scanning tunnelling microscopy and spectroscopy. The most significant features connected with the deposition of copper at room temperature appeared as tiny clusters randomly scattered over smooth 1 × 1-Pb islands. Behaviour of the clusters at room at low temperature assumed that Cu penetrates through inert 1 × 1-Pb layer and mixes up with silicon. Moderate thermal treatment of (Cu, Pb)/Si (1 1 1) system induced the formation of shallow depressions of variable shape and size that are embedded into 1 × 1-Pb phase. Tunnelling spectra taken over a particular depression showed the peaks typical for the copper silicides. Moreover, the temperature rise causes the formation of large objects of distinctive hexagonal shape on neighbouring √3 × √3R30°-Pb/Si (1 1 1) structure. Filled and empty state STM images of the hexagons differ strongly suggesting covalent bonding between constituent atoms. Detailed analysis of the hexagonal objects (and their agglomerates) reveals the specific features of quasi-commensurate 5 × 5-Cu structure.