Electronic structure of the Si(111)-21×21-(Ag+Au) surface

Angle-resolved ultraviolet and x-ray photoelectron spectroscopies were used to analyze the electronic band structure of the Si(111)-21×21(R+/-10.89°)-(Ag+Au) surface that was induced by Au adsorption of 0.19 atomic layer onto the Si(111)-3×3-Ag surface at room temperature. We found two intrinsic dispersive surface-state bands crossing the Fermi level, which were considered to originate from an antibonding surface state of the initial 3×3-Ag structure. The electrons accumulated in these bands were found to be donated by Au adatoms. The electron transfer from Au adatoms into the substrate bulk (surface space-charge layer) was also confirmed by measuring the changes in band bending. The results seemed to be consistent with an atomic model in which the Au adatoms sit atop the Ag trimers of the 3×3-Ag framework. We proposed a kind of atomic bonding mechanism on this surface, referred to as ``parasitic surface bonding,'' where Au adatoms make metallic bonds via a surface-state band of the substrate surface.