The initial growth stages of SiO2 on clean reconstructed Si (001) surfaces have been studied by X-ray and uv photoelectron spectroscopy (XPS and UPS) in large substrate temperature (Ts) and O2 pressure ranges. At Ts > 600°C the initial oxidation is understood as a heterogeneous growth juxtaposing SiO2-like regions and bare silicon terraces. In contrast, the RT oxidation is explained by a more random distribution of O-chemisorption with intermediate local bondings (Si-SixO4-x, X > 0), and presenting a full surface state quenching for mean coverages lower than at high Ts where the growth mode agrees with a nucleation process, essentially promoted by pressure increases. In order to gain information on the relevant nucleation sites we compared afterwards the initial oxidation stages of vicinal and non-vicinal (001) surfaces by angle resolved O (1s) core level XPS (ARXPS). The initial O (1s) emission on stepped surfaces is found to be strongly anisotropic: the relevant angular changes, which are critically dependent on the relative position of the step-axis and the analysing direction, can only be explained in terms of surface roughness effects; more precisely, the initial oxide preceding the terrace oxidation is nucleated along the [110 step-axis. © 1990.
