Site-Specific Design of Cone-Shaped Si Nanowires by Exploiting Nanoscale Surface Diffusion for Optimal Photoabsorption

Abstract

In this study, we introduce a new strategy to control silicon nanowire (SiNW) morphology (cone shape and cylindrical shape) by exploiting adatom diffusion at the nanostructured surface. In our approach, by using chemical treatment with a mixture of HF and H2SO4 acids, ordinary flat Si substrates were changed to nanoscale-faceted, corrugated surfaces, accompanied by a transition from a hydrophobic to hydrophilic surface. These nanostructured surfaces enhanced the surface diffusion, which eventually stimulated radial growth to change the NW morphology from a cylindrical shape to a cone shape with a sharp tip. By using site-specific chemical treatment, both cone-shaped and cylindrical-shaped SiNWs could be achieved selectively on the same chip under the same conditions. Furthermore, these shape-controlled SiNWs demonstrated an outstanding antireflection effect over a broad range of wavelengths, as determined by optical measurements and finite-difference time-domain modeling.