Abstractive vs. Dissociative Chemisorption of Molecular Halogens on the Si(111)-7x7 Surface: A Molecular Beam/STM Study

Abstract

The chemisorption of a mono-energetic molecular beams of F2 , Cl2, and Br2 onto a room temperature Si(111)-7x7 surface are examined using scanning tunneling microscopy and the resulting adsorbate structures are studied as a function of translational energy/chemisorption mechanism. For F2 chemisorption on Si(111)-7x7, there is no intrinsic physisorption state and therefore no island formation at low incident translational energy1. Instead at low translational energy (0.03eV), we observe that the dominant adsorption sites are single reacted adatoms (Si-F), while at higher translational energy (0.27eV) dimers/neighboring pairs of reacted adatoms are commonly observed. From previous work by the Ceyer2 and Carter3 groups, it is known that at low translational energy, F2 can adsorb via abstraction ( F2 collides, one F atom chemisorbs, the other is ejected into the gas phase), while at higher translational energies, dissociative chemisorption becomes the predominant adsorption mechanism. Our STM experiments, in conjunction with a simple Monte Carlo model, show that at low translational energy, the abstraction mechanism accounts for nearly all chemisorption of F2 because nearly all adsorption sites are single reacted adatoms. At higher translational energy, the dissociative chemisorption mechanism becomes important and the predominance for single site adsorption is greatly reduced while the occurrence of dimers/pairs of reacted adatoms is increased.