Oxidation of Silicon Nanopillars

Systematic investigation of dry oxidation of sub-100 nm diameter Si nanopillars (NPs) of various diameters under varying conditions reveals that at 900 °C, the oxidation involves a deep self-limiting oxidation step where the oxidation nearly stops, and the consumed Si thickness (y) exhibits a first-order-reaction-like relation, y = a1 × (1 - exp (-b1 × t)), under an oxidation time t. At 1000 °C, the high oxidation rate with a slight self-limiting step is observed and small NPs could be entirely oxidized. In this case, the relation changes to y = a2 × t^b2. The above equations are confirmed to be applicable for all reported Si NPs and nanowires. Importantly, the relation y^2 ∝ t that is widely used for Si oxidation in traditional theories can only express oxidation of planar, concave, and large convex surfaces but not for Si NPs and nanowires. Since the oxide around Si NPs is found to grow radially and has a lower density compared to that on the planar surfaces oxidized under the same conditions, oxidation-induced stress in Si and/or tensile stress in oxides is considered to dominate the oxidation of Si NPs via affecting the reaction rate constant instead of oxidant diffusion inhibition. This study contributes to a deeper understanding of the oxidation of nanostructured Si, as well as nanostructured metals. It also supports the precise design/fabrication of Si NP-based sub-10 nm devices such as gate-all-around transistors, optoelectronic devices, and biosensors.