XPS study of the oxidation process of Si(111) via photochemical decomposition of N 2O by an UV excimer laser

Abstract

Early stages of Si(111) oxidation by N 2O via the dissociative adsorption at room temperature as well as via the photochemical decomposition have been investigated with X-ray photoelectron spectroscopy (XPS). Monolayer coverage was accomplished by 10 4 L N 2O exposure, and further oxidation did not take place by N 2O alone. The asymmetric O(1s) spectra were deconvoluted into two bands. The higher binding energy component was ascribed to the oxygen in the on-top sites and the lower binding energy one to the oxygen in the bridge sites in accordance with the assignment by Hollinger et al. [Surface Sci. 168 (1986) 609]. The ratio of the intensity of these two components was almost the same as that observed in the oxide formed by O 2, suggesting that the adsorbed oxygens for N 2O at monolayer coverage are incorporated into the same sites as those for O 2. Radical oxygen produced by an UV laser (193 nm) via photochemical decomposition of N 2O could enhance the further oxidation. In the plots of the nominal thickness ( d OX ) of the oxide layer versus the yield of the oxygens incorporated into the Si surface, there appeared a distinct inflection region around d OX ≈5 A ̊ , where the rate of growth of the oxide layer seems to become slow in spite of the net uptake of oxygen. This suggests that the radical oxygen atoms supplied make bonds preferentially with the topmost silicon atoms till the SiO 4 are completed. The simulation of the oxidation step based on the layer-by-layer random bonding model can explain the evolution of the intermediary suboxide species in the early stages of oxidation and the appearance of such inflection region. It is suggested that the interface structure between c-Si and a-SiO, may be abrupt.