Electroless HF/AgNO3 Etching of n‑Si(100) Wafers: High‑Density Silicon Nanowire Arrays with Low Reflectance and Ohmic Conductance

Background

Nanostructured materials display size‑dependent electronic and optical properties distinct from their bulk counterparts. In silicon, the indirect band‑gap hampers efficient light emission, whereas one‑dimensional structures such as SiNWs enable enhanced radiative recombination through quantum confinement. SiNWs also exhibit superior charge transport and mechanical robustness, making them attractive for nanoelectronics and optoelectronics. Numerous fabrication routes exist—chemical vapor deposition, laser ablation, thermal evaporation, and molecular beam epitaxy—but these bottom‑up methods demand high temperatures, vacuum, and costly precursors, limiting large‑area production. Electroless etching, a simple solution‑based technique, offers a low‑cost alternative that yields high‑density, well‑aligned nanowire arrays on Si(100) substrates. This study focuses on optimizing the electroless HF/AgNO₃ process to produce SiNWs with controlled morphology and minimal surface contamination, followed by a comprehensive microstructural, optical, and electrical characterization.

Methods

Fabrication of SiNWs

n‑type Si(100) wafers (resistivity 0.75–1.25 Ω cm, thickness 500–550 µm) were sequentially cleaned in acetone, ethanol, DI water, and boiling piranha solution (4:1 H₂SO₄–H₂O₂) for 30 min, followed by HF (48 % CMOS grade) dip to remove native oxide. The etchant comprised 5 M HF mixed with 0.01 M AgNO₃ (AgNO₃ >99 % purity). Wafers were immersed at 60 °C for 60 min in a sealed Teflon vessel. Post‑etch, samples were rinsed, then ultrasonic‑cleaned in 3 mol L^–1 aqua regia (HNO₃:HCl 1:3) for 15 min to remove residual Ag, followed by DI water rinse and drying.

Characterizations

Microstructure and composition were examined by FESEM (Zeiss Supra 35 VP) with EDX and TEM (Philips CM12). Reflectance (200–1100 nm) was measured using a PerkinElmer Lambda 35 UV–Vis spectrophotometer. Electrical and topographic mapping employed a conductive AFM (Seiko SPI 3800N, gold‑coated tip, ~10 nm radius) in contact mode, sweeping 0–2 V bias. I–V curves were extracted from the current maps, and resistivity was calculated via ρ = RA/L.

Results and Discussion

Microstructures

FESEM images confirm the transition from a smooth Si surface (Fig. 1a) to mesoporous, vertically aligned nanowire arrays after HF/AgNO₃ etching (Fig. 1b). The nanowires exhibit average lengths of ~20 µm and diameters ranging from 20 to 300 nm. Prior to aqua regia cleaning, EDX reveals ~11 at % Ag; post‑cleaning, the surface is essentially pure Si, confirming effective silver removal.

Elemental Composition

EDX spectra before and after cleaning (Fig. 4) show complete elimination of Ag, validating the post‑etch purification step critical for accurate optical and electrical measurements.

Size and Shape of SiNWs

TEM imaging (Fig. 5) demonstrates a distribution of diameters (20–200 nm) and morphologies (round, rectangular, triangular), attributed to stochastic Ag nucleation on the Si surface. Controlled templating could further refine uniformity.

Optical Properties

Reflectance spectra (Fig. 6) show the SiNWs achieving a maximum of 19.2 % versus 65.1 % for the bare wafer. The average reflectance drops to 12 % (visible) and 10 % (near‑IR), with minima of 3.5 % (UV) and 9.8 % (visible–IR). These results confirm the nanowire arrays as effective broadband antireflection structures. Band‑gap analysis via the Kubelka–Munk method yields E_g ≈ 1.20 eV for the nanowires, slightly higher than the 1.15 eV of bulk Si, consistent with quantum confinement predictions.

Electrical Properties

Conductive AFM I–V curves (Fig. 10) exhibit linear, ohmic behavior up to 2 V, confirming the nanowires’ suitability for electronic applications. Using the measured tip radius (~10 nm) and wire length (~722.7 nm), the calculated resistivity is 33.94 Ω cm, higher than literature values for MBE‑grown wires but indicative of the intrinsic material quality after electroless etching.

Conclusions

Electroless HF/AgNO₃ etching of n‑Si(100) wafers yields high‑density, vertically aligned SiNWs (20–200 nm) with sub‑10 % reflectance across the visible–near‑IR spectrum and a band‑gap shift to 1.20 eV. The nanowires display linear ohmic I–V behavior with an average resistivity of 33.94 Ω cm, confirming their potential for low‑cost antireflection coatings and nano‑electronic devices.