THE REACTION OF ATOMIC OXYGEN WITH SI(100) AND SI(111) .2. ADSORPTION, PASSIVE OXIDATION AND THE EFFECT OF COINCIDENT ION-BOMBARDMENT

The reactions of atomic oxygen with the (100) and (111) surfaces of silicon have been investigated by employing supersonic molecular beam techniques, X-ray photoelectron spectroscopy (XPS), and low-energy ion scattering spectroscopy (ISS). Atomic oxygen adsorbs with unit probability on the clean silicon surface, independent of substrate temperature (110-800 K) and incident mean translational energy (3-16 kcal mol-1). Oxidation of clean silicon with an oxygen atom beam is characterized by two stages: a "fast" stage that corresponds to oxygen chemisorption in the topmost 2-3 silicon layers; and a "slow" stage that corresponds to oxygen incorporation and oxide film growth. The chemisorption stage is described by first-order Langmuirian kinetics with an apparent saturation coverage of approximately 4 ML O(a), the oxide growth stage by "direct" logarithmic kinetics, where dx/dt = alpha exp(-x/L), where x is the oxide thickness. Observation of significant oxidation at substrate temperatures of 110 K suggests that oxide growth in the slow stage may occur by a field-assisted mechanism, where an internal electric field aids transport of oxygen to the underlying silicon substrate layers. XPS and ISS results support a two-dimensional layer-by-layer growth mechanism for oxidation at substrate temperatures below 900 K. At higher temperatures, T(s) greater-than-or-equal-to 1050 K, oxide growth is three-dimensional involving nucleation and growth of bulk-like oxide islands even for mean coverages as low as 3 ML O(a). ISS results lend support to the formation of "on-top" adsorbed oxygen atoms that cap silicon dangling bonds at the oxide/gas interface. Coincident bombardment of the silicon substrate with an Ar+ ion beam leads to an enhanced rate of oxidation. The enhancement can be understood in terms of a model involving secondary implantation of adsorbed oxygen atoms, coupled with the simultaneous formation of reactive sites (e.g., dangling bonds, vacancies) for oxygen chemisorption. The effect of coincident ion bombardment is reduced at elevated substrate temperatures (approximately 800 K), since the resulting increased propensity for adlayer rearrangement leads to a decrease in the number of active sites for oxygen chemisorption.