Theoretical study on adsorption of Tl atoms on Si(111) surface under periodic boundary condition

The T4site adsorption of group-13 atoms such as Al,Ga,In and Tl on Si(111) surface can be well understood as a result of formation of three bonds between a tri-valent metal atom and three Si dangling bonds, forming (√3× √3) periodicity. However, it is well known that Tl exhibits not only tri-valence but also mono-valence nature caused by relativistic effect. Therefore, there has been a growing interest in the adsorption of Tl on Si(111) surface because Tl has been known to show quite different behavior from other group-13 atoms in (1×1) surface structures at 1.0 ML. We investigated the chemisorption of Tl adatoms on Si(111) by using theoretical calculation under periodic boundary condition. Tl is found to occupy the three fold-filled sites (T4 site) in (√3×√3)-Tl surface at 1/3 ML as to other tri-valent group-13 atoms. Our calculations also show that the Tl adatoms occupy the T4 site (1×1)-Tl surface at 1.0ML.