Electrically conductive and optically active porous silicon nanowires

Abstract

We report the synthesis of vertical silicon nanowire array through a two-step metal-assisted chemical etching of highly doped n-type silicon (100) wafers in a solution of hydrofluoric acid and hydrogen peroxide. The morphology of the as-grown silicon nanowires is tunable from solid nonporous nanowires, nonporous/nanoporous core/shell nanowires, to entirely nanoporous nanowires by controlling the hydrogen peroxide concentration in the etching solution. The porous silicon nanowires retain the single crystalline structure and crystallographic orientation of the starting silicon wafer and are electrically conductive and optically active with visible photoluminescence. The combination of electronic and optical properties in the porous silicon nanowires may provide a platform for novel optoelectronic devices for energy harvesting, conversion, and biosensing.

Figure 1

SEM and TEM images show the evolution of silicon nanowire morphology from n-Si(100) with 0.008–0.02 Ω·cm resistivity in etchant solutions composed of 4.8 M HF and variable concentrations of H₂O₂ through a two-step reaction. (A) and (B): 0.1 M H₂O₂ for 30 min; (C) and (D): 0.1 M H₂O₂ for 60 min; (E) and (F): 0.15 M H₂O₂ for 30 min; (G) and (H): 0.15 M H₂O₂ for 60 min; (I) and (J): 0.2 M H₂O₂ for 30 min; (K) and (L): 0.2 M H₂O₂ for 60 min; (M) and (N): 0.3 M H₂O₂ for 30 min; and (O) and (P): 0.3 M H₂O₂ for 60 min. The scale bars for all SEM and TEM images is 10 µm and 60 nm, respectively.

Figure 2

(A) High resolution TEM images with SEAD patterns of nonporous and porous silicon nanowires. (A) Nanowires obtained from the reaction in 0.1 M H₂O₂ and 4.8 M HF for 30 min; (B) Nanowires obtained from the reaction in 0.2 M H₂O₂ and 4.8 M HF for 30 min; and (C) Nanowires obtained from the reaction in 0.3 M H₂O₂ and 4.8 M HF for 60 min. All the scale bars are 5 nm.

Figure 3

(A) SEM image of a porous silicon nanowire device. The inset shows a higher resolution SEM image highlighting the porous structure. The scale bar in inset is 100 nm. (B) Drain current (I_d) versus drain voltage (V_d) at variable gate voltages.

Figure 4

(A) Photoluminescence spectrum of porous silicon nanowires obtained with 60 min etching in a solution with 0.30 M H₂O₂; (B) Optical micrograph of a single porous silicon nanowires; and (C) Confocal photoluminescence image of the same single porous silicon nanowire. The scale bar in (B) and (C) is 3 µm.