A theoretical investigation of adsorbate-induced surface relaxation effects using cluster models: Al on Si(111)

Differently sized cluster calculations are used to investigate theoretically the preferred adsorption site for an Al adatom on the Si(111) surface. By performing partial geometry optimizations at the Hartree-Fock level on AlSi_n subclusters around the site of interest we find significant Al adatom-induced surface relaxation effects distorting the Si atoms from their bulk lattice positions. The largest relaxation effects take place at the T₄ site resulting in Al adsorption at the T₄ site to be 5 kcal/mol more stable than at the H₃ site and considerably more stable than adsorption at the T₁ site. However, we only have confidence in this result after performing for the T₄ site a partial geometry optimization on the AlSi₅ subcluster in the AlSi₂₆H₂₄ cluster and by including appropriate correlation corrections.