Oxidation of silicon by perchloric acid

Crystalline Si can be oxidized easily at low temperatures by use of perchloric acid (HClO4), i.e., immersion in concentrated perchloric acid at 203°C for ∼40 min forms an ∼8-nm thick silicon oxide layer. The HClO4 solution has high oxidizing activity only at the boiling temperature of 203°C, which probably results from the decomposition of HClO4. X-ray photoelectron spectroscopy measurements show that the oxide layers formed in HClO4 have similar local structure to that of thick thermal oxide layers, containing little amount of suboxide species. The plot of the thickness of the oxide layers formed at 203°C vs. oxidation time consists of two linear lines, the initial oxidation rate being lower. It is suggested that the initial low oxidation rate is due to the presence of oxidation-suppressing species such as Si–H on the surface. On the other hand, the thickness vs. time plots for oxidation at temperatures lower than 195°C are parabolic, indicating that diffusion of the oxidizing species through the oxide layers is the rate-determining step.