Four structures corresponding to various coverages of Bi adatoms on Si(111) and Ge(111) surfaces are studied using ab initio methods: the $T4\mathrm{}$ of $\frac{1}{3}\hspace{0.5em}\mathrm{ML}$, the honeycomb of $$$\frac{2}{3}$ ML, the $1\times 1$ of 1 ML, and the milkstool of 1 ML. It is shown that when $$$\frac{1}{3}\hspace{0.5em}\mathrm{ML}$ of Bi adatoms is adsorbed on the Si(111) surface, the $T4\mathrm{}$ structure is favored against the phase separation into an adatom structure of higher coverage and a clean reconstructed surface without adatoms. In the case of the Ge(111) surface, on the contrary, the energetics suggests the coexistence of the $T4\mathrm{}$ structure with the milkstool structure and with a clean reconstructed surface. For higher coverages of Bi adatoms the energetic considerations imply that on both Si(111) and Ge(111) surfaces, the preferred adatom structure is the milkstool rather than the honeycomb. The $1\times 1$ structure is energetically unfavorable in all the cases studied.

• Received 12 May 1997

DOI:https://doi.org/10.1103/PhysRevB.56.10283