Stabilization of the asymmetric Ge dimer on Si(100) by charge transfer

The DMol first-principles molecular-cluster total-energy and atomic-force approach is used to study the Ge dimer on the Si(100) surface with clusters containing up to 85 atoms. It is shown that the Ge asymmetric dimer is energetically more stable than the symmetric dimer due to charge transfer from a down dimer atom to an up atom even though this accompanies a weakening of the covalent bonding between the dimer atoms. By calculating the forces on atoms in the first three Si layers and minimizing the total energy of the cluster, the optimized atomic geometry is obtained. The calculated dimer height displacement of this buckled dimer is shown to be 0.60 and the tilt angle is 14.2°. The magnitude of this buckling lies between the range that was calculated for clean Si(100)2×1 and Ge(100)2×1 using the same approach, which agrees with the experimental measurements for those surfaces.