ADSORPTION DYNAMICS OF MONOENERGETIC IODINE MONOBROMIDE (IBR) ON THE SI(111)-7X7 SURFACE

The adsorption of monoenergetic IBr molecules on the Si(111)-7X7 surfa ce has been studied using scanning tunneling microscopy, mass spectrom etry, Auger electron spectroscopy, and supersonic molecular beam techn iques. The adsorption proceeds predominantly via the direct abstractiv e adsorption mechanism and preferentially occurs at the center Si adat oms. The IBr abstraction probabilities at the incident energies of 0.1 5 and 0.82 eV have been determined to be 0.90+/-0.03 and 0.77+/-0.03, respectively. The minor dissociative adsorption channel of IBr can be enhanced at the expense of the abstractive adsorption channels by rais ing the incident energy. Most importantly, no atomic selectivity for i odine or bromine was observed. A reaction mechanism involving two type s of transition states, Si ... I ... Br-(s) and Si ... Br ... I-(s), h as been proposed to interpret the experimental observations. The attra ctive interaction between the nearly symmetric highest occupied molecu lar orbitals (HOMO, pi antibond) of IBr and the partially-filled Si a datom dangling bonds governs the surface site selectivity and the atom ic selectivity of IBr adsorption on Si(lll). Comparison with the adsor ption of ICl on the surface has also been made to clarify the role of the asymmetric molecular bonding in adsorption dynamics. (C) 1998 Amer ican Institute of Physics.