We show by scanning tunneling microscopy (STM) imaging that native oxide growth in moist air on hydrogen terminated Si(111) 1×1 surfaces begins by continuing formation of small oxide nuclei, 10–20 Å in diameter, in the topmost Si layer. Their statistical distribution on the flat terraces points to a homogeneous nucleation process. Oxidation is extremely slow; after about 800 h only one complete monolayer is oxidized. In addition, a small number of three-dimensional oxide nuclei, several layers deep and 50–100 Å in width, are formed at step edges as a minority species, which may be related to surface defects or contaminations.
The initial stages of oxide formation on atomically flat, monohydride terminated Si(111) surfaces by oxidation in 30% hydrogen peroxide solution (H2O2) were investigated by scanning tunneling microscopy and x-ray photoelectron spectroscopy. The reaction proceeds via homogeneous nucleation of small oxide clusters on the surface and subsequent lateral growth of these clusters within the surface bilayer. The oxidation of the topmost Si(111) bilayer in H2O2 solution is completed after 30 min, leading to a SiO1.2 average layer composition. For the next bilayer the oxidation rate decreases drastically—after 2 months only 60% of the second bilayer are oxidized. An inverse logarithmic rate of the second bilayer oxidation is consistent with a field assisted growth mechanism. The significant differences in the oxidation rates between the first and the second bilayer allow to produce well defined oxide layers of about 5 Å thickness.
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