Diffusion and clustering of adatoms on discommensurate surface template: Ge atoms on Si(111)'5 × 5'-Cu reconstruction

摘要

Using scanning tunneling microscopy observations and first-principles total-energy calculations, surface dynamics of Ge adatoms on Si(111)"5 × 5"-Cu reconstructed surface has been studied. The Si(111)"5 × 5"-Cu reconstruction has been proved to be a quasi-periodic discommensurate Cu_2Si mono-layer, which atomic structure is formed via Cu adsorbing in the H_3 sites (Cu(H_3)-atoms) and substituting for Si in the upper half of the Si(111) double layer (Cu(Su)-atoms). Its potential relief shows up as an array of attractive "basins" of the hexagonal shape reflecting the "5 × 5" domain structure. Up to room temperature, each Ge adatom is typically trapped within a single basin, where it hops between three to six adsorption sites atop the Cu(Su) atoms located in the center of the hexagon. This motion is characterized by a diffusion activation energy of 0.29 ± 0.03 eV and an attempt frequency of 10~(9±1) Hz. When two Ge atoms are adsorbed in the same hexagon, they form a kind of a dimer. The most of the dimers have an interatomic spacing, which equals 3a~(1/2), the more seldom are 2a and 7a~(1/2) spacings, while spacings greater than 7a~(1/2), as well as that equal to la are absent. With increasing Ge coverage, the number of Ge atoms occupying hexagons grows until all available adsorption sites become exhausted, that takes place at about 0.1 ML of Ge. As a result, arrays of atomic clusters, including dimers, trimers, tetramers and pen-tamers, are formed at the surface, trimers being the most abundant cluster species at saturation.