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 x 1 and Si(111)-7 x 7 surfaces, the oxygen uptake is observed to occur in two successive stages. The first one is characterized by initial sticking coefficients of S2 x 1 almost-equal-to 5 x 10(-2) and S7 x 7 almost-equal-to 0.4 and by saturation coverages of theta(2 x 1) almost-equal-to 0.75 ML and theta(7 x 7) almost-equal-to 0.6 ML. 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 x 1 and Si(111)-7 x 7 surfaces, and they are characterized by an initially increased 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 x 1 surfaces are prepared by an HF dip of thermally oxidized wafers. On such surfaces, oxygen uptake was only observed for exposures larger than 1 x 10(26) O2 molecules per cm2. This corresponds to an initial sticking coefficient of approximately 10(-14). Exposure to 10(29) O2 molecules per cm2 increases the uptake to theta almost-equal-to 1 ML, while after 10(30) cm-2 one observes an uptake equivalent to approximately 2.5 ML. Then the coverages are identical for the initially clean Si(111)-2 x 1 and -7 x 7 as well as the initially passivated Si(111):H-1 x 1 surfaces.