CLUSTER MODEL STUDY ON GAAS EPITAXIAL CRYSTAL-GROWTH BY ARSENIC MOLECULAR-BEAM .2. AS2 ADSORPTION ON A GAAS SURFACE

We study the chemisorption of an As2 cluster on a flat GaAs(100) surfa ce and at the step site of this surface with the Hartree-Fock geometry optimization method followed by the energy calculation with the secon d-order Moller-Plesset perturbation method. On the flat surface, the a ctivation energies for both molecular and dissociative adsorption are high and the molecular adsorption is more favorable than the dissociat ive one, so that the As2 Cluster is hardly adsorbed and dissociated. A t the step site, on the other hand, the dissociative adsorption occurs smoothly: the dissociative adsorption becomes more favorable than the molecular one and the activation energy for the molecular adsorption is only 9.6 kcal/mol. Therefore, the As2 cluster arriving at the step site is easily adsorbed and dissociated and thus one As layer is added on a Ga surface. We explain the mechanism of this reaction and the di fference in the reactivity between the flat surface and the step site.