Substrate temperature dependence of the initial growth mode ofSiO2on Si(100)-(2×1) exposed toO2: A photoemission study

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 $T_s$<800 °C) and ${\mathrm{O}}_2$ pressure [from UHV to P(${\mathrm{O}}_2$ ${)<\mathrm{}10\mathrm{}}^{\mathrm{-}5}$ mbar] and partly as a function of oxidation time. The critical temperature $T_c$ (pressure $P_c$) for a given pressure (temperature $T_c$) 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 $T_s$, the following pictures can be given in the monolayer regime. At high ${\mathit{T}}_{\mathit{s}}$ (${\mathrm{T}}_{\mathrm{s}}$>600 °C) and low pressure, in the desorption-governed region, the initially oxidized surface is understood as a juxtaposition of ${\mathrm{SiO}}_2$-like regions (Si-${\mathrm{O}}_4$ 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 $T_s$. The study also focuses on the initial growth-rate differences according to whether exposure changes result from time or pressure variations at high $T_s$. They agree with a seeding process, for the islandlike growth, essentially promoted by pressure increases and concomitant arrival of ${\mathrm{O}}_2$ 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 $T_s$.