Diffusion inhibition of the oxidant in the oxide, developed from thermal oxidation of the two-dimensional Si planar surface, has been considered to be the mechanism of self-limiting oxidation of Si nanopillars by many groups over a long period. However, in this work, we found that oxide grown thermally on the sidewall of Si nanopillars has lower density compared to that on a two-dimensional planar bottom surface oxidized under the same conditions. The oxidant diffuses faster in lower density SiO2, which means that the oxidant diffusion inhibition model is insufficient for explaining the self-limiting oxidation of Si nanopillars. A simple model is proposed to explain the formation mechanism of low-density oxide grown thermally on the sidewall of Si nanopillars. This work contributes in intrinsically understanding and precisely controlling the oxidation of Si nanopillars for future device fabrications.
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