THE METAL ADATOM ON GAAS(110) - A SURFACE NEGATIVE U CENTER

Metallic atom adsorption on the GaAs(110) surface is studied in the tight-binding approximation. By comparison with the experimental results and the lack of As localized states in the band gap, we first show that adsorption cannot occur on a completely unrelaxed surface. The other point concerns the Fermi level position which is located in a minimum density of states. We show that adatom relaxation is quite important and must be considered as it strongly modifies the energy to add or to remove an electron from the crystal. We show that a possible explanation of the observed adatom level energies is the existence of a negative U center.