Using scanning tunneling microscopy and first-principles density functional calculations, we have investigated systematically the initial stages of single Ge atoms adsorbed on a $\mathrm{Si}\left(111\right)\text{−}(7\times 7)$ surface. When the deposition is at an elevated temperature of $420\mathrm{K}$, single Ge atoms are found to substitute for the Si adatoms randomly. When the deposition is at room temperature, single Ge atoms do not replace the Si adatoms but move frequently within half unit cells. When the room temperature prepared sample is imaged at $78\mathrm{K}$, the Ge atoms are observed to either adsorb at the stable high coordination sites near the corner Si adatoms or hop among the high coordination sites near the center Si adatoms. The adsorption sites of Ge atoms at high coordination positions have been determined by first-principles calculations and comparisons with measured scanning tunneling microscopic images.

• Received 6 February 2006

DOI:https://doi.org/10.1103/PhysRevB.74.125301