First-principles calculations of Ga adatom structures for Ge(111) and Si(111) surfaces

The (3×3)R30° and (1 × 1) structures of Ga adatoms on Ge(111) and Si(111) surfaces are studied using first-principles calculations. Like on the Si(111) surface, the T4 structure is found to be more stable than the H3 structure on the Ge(111) surfaces within the (3×3)R30° translational symmetry. Also, on both Ge(111) and the Si(111) surfaces, the T4 structure is energetically preferred over the (1 × 1) structure. However, the energy difference between the T4 and (1 × 1) structures for the Ge(111) surface (0.5 eV/Ga) is less than half of that for the Si(111) surface (1.3 eV/Ga). These results are corroborated by the corresponding redistributions of the electronic charge densities.