Doping of a surface band on Si (111) √(3)×√(3)-Ag

A semiconducting surface-state band on Si (111) √(3)×√(3)-Ag is doped by adsorption of additional Ag and Au atoms. Very high levels of doping can be achieved (0.0015 0.086 electrons per 1×1 unit cell), such that the silicon surface degenerates into a metal. The doping alters the band structure of the surface state and causes the rigid-band model to break down. The parabolic-band approximation breaks down as well. These observations shed light on the mechanism of doping at extreme levels.