Surface-state electrical conduction on the Si(111)-3×3-Ag surface with noble-metal adatoms

The Si(111)-21×21-(Ag+Cu) superstructure was found to be induced by adsorption of about 0.14 monolayers (ML) of copper atoms on the Si(111)-3×3-Ag surface at room temperature (RT), which was quite similar to the cases of the 21×21 superstructures induced by gold adsorption at RT and by silver adsorption at low temperature on the 3×3-Ag surface. Photoemission spectroscopies showed that the high electrical conductance observed for this copper-induced superstructure was due to a two-dimensional intrinsic dispersive surface-state band crossing the Fermi level, not due to the surface space-charge layer, which is again similar to the gold-induced 21×21 surface. Each noble-metal adatom (gold, silver, copper) of less than ca. 0.1 ML coverage on the 3×3-Ag surface was found to exist commonly as two-dimensional adatom gas (2DAG) phase before they turned into the 21×21 structure. The enhancement in surface conductance by the 2DAG phase was concluded to result from the donation of the adatomsʹ valence electrons into the surface-state band of the 3×3-Ag substrate. The difference in stability of the 21×21 superstructures among gold-, silver- and copper-induced ones is discussed in terms of the first atomic ionization energy of the adatoms.