A541 is believed to occur at Ni203 point defects, with a 1 and a 2 yielding a combined saturation 02 coverage of 0.11 + 0.03 monolayers. The low temperature "t peak (125 K) induces a small positive Aq~ increase with desorption, suggesting an 02 species with a small positive charge. Possible adsorption sites for oxygen in the y state are step edges a n d / o r domain boundaries. The m a x i m u m 02 coverage of this state is 0.12 + 0.03 monolayers and diminishes with substrate annealing. For all three states, adsorption is not associated with O H or CO 3 contamination, and is independent of oxide epitaxy.
312
Surface Science 256 (1991) 312-316 North-Holland
Cesium and oxygen adsorption on NiO(100) S. Kennou, M. Kamaratos and C.A. Papageorgopoulos Department of Physics, University of loannina, P.O. Box 1186, GR-451 10 loannina, Greece Received 11 February 1991; accepted for publication 2 May 1991 The interaction of Cs and 02 on NiO(100) has been studied by AES, TDS and W F measurements at room temperature. A correlation of the experimental results shows that Cs is initially adsorbed as ionized adatoms which interact strongly with the oxygen of the outmost layer of NiO. The initial dipole m o m e n t of Cs on NiO is 8 D, while the sticking coefficient found to be - 0.7. With increasing coverage Cs remains disordered and forms islands on the surface. Oxygen adsorption on Cs-saturated NiO takes place on top of Cs. The deposited oxygen interacts with Cs and increases the binding energy of the latter.
Surface Science 256 (1991) 317- 343 North-Holland
317
The reaction of atomic oxygen with Si(100) and Si(111) I. Oxide decomposition, active oxidation and the transition to passive oxidation J.R. Engstrom 1, D.J. Bonser, M.M. Nelson and Thomas Engel Department of Chemistry, BG-IO, University of Washington, Seattle, WA 98195, USA Received 3 July 1990; accepted for publication 18 April 1991 The reactions of atomic oxygen with the (100) and (111) surfaces of silicon have been investigated by employing supersonic molecular beam techniques and X-ray photoelectron spectroscopy. The kinetics and mechanism of the active oxidation reaction, i.e., Ox(g ) + Si(s) ---, SiO(g) where x = 1 or 2, has been evaluated by employing modulated molecular beam reactive scattering (MMBRS). On both surfaces, the reaction of atomic oxygen involves the formation of a single stable surface intermediate, which reacts via first-order kinetics to produce SiO(g). The reaction of molecular oxygen, however, involves two stable surface intermediates that are formed sequentially, the second of which is identical to that formed by the reaction with atomic oxygen. We propose that the first intermediate formed in the molecular oxygen reaction is chemisorbed Oz(a), e.g., a peroxy radical or a peroxide bridge. The intermediate formed in the atomic oxygen reaction is assigned to either an isolated oxygen adatom or adsorbed SiO (a surface silanone complex). Oxide decomposition in the mono- and multi-layer regime has been examined with temperature-programmed desorption (TPD). Both increasing oxygen coverages and higher adsorption temperatures lead to higher decomposition temperatures for the oxygen adlayers formed. Above the monolayer regime, adlayers formed on Si(100) d e c o m p o s e at higher temperatures than those on Si(lll). On Si(100), simple first-order decomposition kinetics is only observed in the monolayer regime, and in the limit of low coverage (8 < 0.3 ML). The implied rate coefficient for desorption in this regime is at least 2 orders of magnitude smaller than that measured by M M B R S at the same temperature. At higher coverages (8 > 4 ML) and on both surfaces, the decomposition reaction appears to be heterogeneous, involving void nucleation and growth. The transition between active and passive oxidation has been examined employing in situ, real-time mass spectrometric and XPS measurements. The transition involves a competition between highly activated ( E d - 80 kcal m o l - 1 ) desorption of SiO, and nearly unactivated nucleation of surface oxide.