Fabricating vertically aligned sub-20 nm Si nanowire arrays by chemical etching and thermal oxidation.

Silicon nanowires (SiNWs) are appealing building blocks in various applications, including photovoltaics, photonics, and sensors. Fabricating SiNW arrays with diameters <100 nm remains challenging through conventional top-down approaches. In this work, chemical etching and thermal oxidation are combined to fabricate vertically aligned, sub-20 nm SiNW arrays. Defect-free SiNWs with diameters between 95 and 200 nm are first fabricated by nanosphere (NS) lithography and chemical etching. The key aspects for defect-free SiNW fabrication are identified as: (1) achieving a high etching selectivity during NS size reduction; (2) retaining the circular NS shape with smooth sidewalls; and (3) using a directional metal deposition technique. SiNWs with identical spacing but variable diameters are demonstrated by changing the reactive ion etching power. The diameter of the SiNWs is reduced by thermal oxidation, where self-limiting oxidation is encountered after oxidizing the SiNWs at 950 °C for 1 h. A second oxidation is performed to achieve vertically aligned, sub-20 nm SiNW arrays. Si/SiO2 core/shell NWs are obtained before removing the oxidized shell. HRTEM imaging shows that the SiNWs have excellent crystallinity.