First-principles study of Ag adatoms on the Si(111)-√3 x √3-Ag surface

We have performed first-principles calculations for a model system with the root 3x root 3 periodicity in which a Ag adatom per unit cell is adsorbed upon the Si(111)-root 3x root 3-Ag surface, whereby discussing the atomic and electronic structures of the Si(111)-root 21x root 21-(Ag+Ag) and -(Ag+Au) surfaces. It has been found that the adatom prefers to be located upon Ag triangles rather than upon Si trimers of the underlying surface. The band structure calculated for the most favorable atomic arrangement exhibits the two surface states characteristic for the root 21x root 21 surfaces. By performing population analyses for these states it is shown that the lower state stems from the so-called Si surface state of the underlying surface, while the upper state is an adatom-induced state, at variance with previous assignments. The electronic structure of the Si(111)-root 3x root 3-Ag surface is also discussed, and a picture concerning the so-called SI surface state is presented. [S0163-1829(99)00216-7].