Oxidation of clean and H-passivated Si(111) surfaces

The interaction of molecular, unexcited oxygen with clean and with hydrogen passivated Si(111) surfaces was studied by using Auger electron spectroscopy (AES) and low-energy electron diffraction (LEED). On cleaved Si(111)-2 × 1 and Si(111)-7 × 7 surface, the oxygen uptake is observed to occur in two successive stages. The first one is characterized by initial sticking coefficients of S 2 × 1 ≈ 5 × 10 −2 and S 7 × 7 ≈ 0.4 and by saturation coverages of θ 2 × 1 ≈ 0.75ML and θ 7 × 7 ≈ 0.6ML. One monolayer (1 ML) is defined as the total number of sites on a Si(111) plane. In the second adsorption stage, the coverage versus exposure curves are identical for both Si(111)−2 × 1 and Si(111)−7 × 7 surfaces and they are characterized by an initially incr uptake rate which is then followed by a slower quasi-logarithmic growth law. This second process is attributed to electric-field-assisted diffusion, i.e. the Mott-Cabrera mechanism. Hydrogen-passivated Si(111):H−1 × 1 surfaces are prepared by an HF dip thermally oxidized wafers. On such ssurfaces, oxygen uptake was only observed for exposures larger than 1 × 10 26O 2 molecules per cm 2. This corresponds to an initial sticking coefficient of approximatelly 10 −14. Exposure to 10 29 O 2 molecules per cm 2 increases the uptake to θ ≈ 1ML, while after 10 30 cm −2 one observes an uptake equivalent to ≈2.5ML. Then thecoverages are identical for the initially clean Si(111)-2 × 1 −7 × 7 as well as the initially passivated Si(111):H−1 × 1 surfaces.