Co-adsorption of Cu and Pb on the Si(1 1 1)–(7 × 7) surface: interface formation

The adsorption of Cu and Pb atoms on a Si(1 1 1)–7 × 7 surface was studied by means of scanning tunnelling microscopy (STM). After deposition of ⩽0.1 monolayer (ML) of copper on the Pb √3×√3 mosaic phase and a subsequent annealing up to ∼373 K, new objects of a hexagonal shape appear on a surface. They are scattered over the surface and localised next to the mosaic phase islands. The difference between atomically resolved images of filled and empty states suggests strong covalent bonding within a hexagon. Increasing the concentration of Cu atoms leads to an increasing number of hexagons and their agglomeration. Annealing of this surface at a higher temperature (>470 K) leads to the transformation of these hexagonal-like objects and their agglomeration into the pseudo-“5 × 5” structure, commonly observed for the Cu/Si(1 1 1)system. The absence of hexagons at very low Cu concentration demonstrates the presence of a long-range, attractive interaction among Cu atoms and their strong diffusion just above room temperature (RT).