Adatom-dependent diffusion mechanisms on a Ag/Si (111 ) √3×√3 surface

We present first-principles calculations for the diffusion of monovalent alkali and noble metal adatoms on a Ag/Si (111) √3×√3 (√3-Ag) surface, a two-dimensional (2D) electron gas system tunable via the electron doping of adatoms in 2D adatom gas. In this system, a Ag adatom diffuses over the surface through an exchange mechanism in which the adatom and the substrate Ag atoms exchange repeatedly in each diffusion step. The resulting calculated diffusion barrier is only 0.22 eV. Other adatoms (Au, Cu, Li, and Na) are incorporated into the substrate via exchange with a Ag atom followed by the Ag atom diffusion on the surface. The calculated site-projected densities of states show that the adatom and the substrate Ag atoms have a clear difference in their empty states, which can be used to verify an exchanged configuration for the heterogeneous adatoms. On the other hand, a K adatom with a large atomic radius favors a direct diffusion with a barrier of 0.12 eV. Depending on the adatoms on the √3-Ag surface, the various diffusion mechanisms can be traced to the tensile surface stress due to the Ag layer expansion and the surface chemistry related with the heterogeneous (Ag and Si) overlayer, leading to an unusual microscopic mechanism of 2D adatom-gas formation.