THE REACTION OF ATOMIC OXYGEN WITH SI(100) AND SI(111) .1. OXIDE DECOMPOSITION, ACTIVE OXIDATION AND THE TRANSITION TO PASSIVE OXIDATION

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., O(x)(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 O2(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) decompose at higher temperatures than those on Si(111). On Si(100), simple first-order decomposition kinetics is only observed in the monolayer regime, and in the limit of low coverage (theta < 0.3 ML). The implied rate coefficient for desorption in this regime is at least 2 orders of magnitude smaller than that measured by MMBRS at the same temperature. At higher coverages (theta greater-than-or-equal-to 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) approximately 80 kcal mol-1) desorption of SiO, and nearly unactivated nucleation of surface oxide.