Surface-stale electrical conduction on the Si(111)-root 3 x root 3-Ag surface with noble-metal adatoms

The Si(111)-root 21 x root 21-(Ag + Cu) superstructure was found to be indu ced by adsorption of about 0.14 monolayers (ML) of copper atoms on the Si(1 11)-root 3 x root 3-Ag surface at room temperature (RT), which was quite si milar to the cases of the root 21 x root 21 superstructures induced by gold adsorption at RT and by silver adsorption at low temperature on the root 3 x root 3-Ag surface. Photoemission spectroscopies showed that the high ele ctrical 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 simi lar to the gold-induced root 21 x root 21 surface. Each noble-metal adatom (gold, silver, copper) of less than ca. 0.1 ML coverage on the root 3 x roo t 3-Ag surface was found to exist commonly as two-dimensional adatom gas (2 DAG) phase before they turned into the root 21 x root 21 structure. The enh ancement in surface conductance by the 2DAG phase was concluded to result f rom the donation of the adatoms' valence electrons into the surface-state b and of the root 3 x root 3-Ag substrate. The difference in stability of the root 21 x root 21 superstructures among gold-, silver- and copper-induced ones is discussed in terms of the first atomic ionization energy of the ada toms. (C) 2000 Elsevier Science B.V. All rights reserved.