Substrate temperature dependence of the initial growth mode of SiO2 on Si(100)-(2×1) exposed to O2: A photoemission study

Article abstract of DOI:10.1103/PhysRevB.40.10356

X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS) are used to probe the initial states of oxidation of Si(100)-(2×1) surface, systematically as a function of substrate temperature (from RT to Ts<800°C) and O2 pressure [from UHV to P(O2)<10-5 mbar] and partly as a function of oxidation time. The critical temperature Tc (pressure Pc) for a given pressure (temperature Tc) separating the desorption or combustion oxidation mode from the passivating one is clearly shown. By connecting the UPS-deduced surface-state disappearance and local Si 2p bonding changes with oxygen coverage at various Ts, the following pictures can be given in the monolayer regime. At high Ts (Ts>600°C) and low pressure, in the desorption-governed region, the initially oxidized surface is understood as a juxtaposition of SiO2-like regions (Si-O4 bondings) and bare silicon terraces still presenting the dimer reconstruction. In contrast to this nonuniform growth, the RT oxidation is explained by a more random distribution of O chemisorption with more intermediate local bondings located near the overlayer and a full surface-state quenching for mean coverages lower than at high Ts. The study also focuses on the initial growth-rate differences according to whether exposure changes result from time or pressure variations at high Ts. They agree with a seeding process, for the islandlike growth, essentially promoted by pressure increases and concomitant arrival of O2 molecules on a seeding site, in contrast with time increases leading to a more sequential arrival which favors the desorption. This point of view explains the rapid switch, as a function of increasing pressure, from the desorbing to the passivation mode at high Ts.