Substrate temperature-induced changeover from homogeneous to step nucleated growth modes for the initial thermal oxidation of Si(001) by O2

The initial growth stages of SiO2 on clean reconstructed Si(001) surfaces have 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.