Combined theoretical and experimental investigation of the adsorption geometry of Ga on Si(100) at low coverage

The atomic and electronic structures of Ga chemisorption on the Si(100)-(2×1) at low coverage have been investigated by a combined molecular-cluster total-energy and atomic-force approach and x-ray standing-wave (XSW) measurements. Four possible Ga-dimer adsorption geometries are calculated using cluster models with up to 77 atoms. By comparing the total energies for these optimized geometries, it is found that Ga dimers are centered at valley bridge sites with the Ga-dimer bond oriented parallel to the underlying Si-dimer bonds-a geometry strongly supported by XSW measurements of the Si(100)-(2×2)-Ga surface structure. The calculated Ga-dimer height above the ideal Si(100) is in excellent agreement with the XSW result. The stability of this parallel-dimer model is explained through the bonding between Ga-Si and Si-Si and by analyzing the Si-surface distortion when Ga is present.