Unveiling the shape-diversified silicon nanowires made by HF/HNO₃ isotropic etching with the assistance of silver

Abstract

Hydrofluoric (HF)/nitric (HNO3)/acetic (CH3COOH) acid, normally referred to as the HNA method, is a widely utilized technique for performing isotropic etching on silicon (Si) in industrial Si-based processing and device construction. Here, we reported a novel etching strategy based on a HF/HNO3 process with the assistance of silver (Ag) nano-seeds, offering good controllability in preparing diversified Si nanostructure arrays with particularly smooth top surfaces. The involved mechanism was visualized by systematically investigating both the time and temperature dependencies on the etching kinetics with various ratios of HF to HNO3. Moreover, by testing different Ag(+)-ion containing oxidants on Si etching, we have re-examined the state-of-the-art metal-assisted chemical etching (MaCE) using HF/AgNO3 etchants. In contrast with previous reports, we found that the interplay of hole injections from Ag(+) and NO3(-) ions to the valence band of Si collectively contributes to the unidirectional dissolution of Si. Finally, we explored the engineering of the Ag nano-seeds to regularize the orientation of the etched nanowires formed on non-Si (100) wafers, which further provides a reliable pathway for constructing the desired morphologies of one-dimensional Si nanostructures regardless of wafer orientation.

All‐Solution‐Processed Random Si Nanopyramids for Excellent Light Trapping in Ultrathin Solar Cells

“…Figure a,b presents the top‐view and cross‐sectional SEM images of the textured Si surface without the removal of Ag nanoparticles after being etched in alkaline solution for 20 min under the temperature of 65 °C. Distinctly, the Ag nanoparticles are randomly distributed on the top, bottom, and slope facet of the Si pyramids, unlike the case that Ag nanoparticles are embedded at the bottom of Si nanopores in HF‐H 2 O 2 /HNO 3 ‐H 2 O etching system because of their catalytic role . Therefore, in our alkaline etching system, the Ag nanoparticles are not a catalytic agent, or at least they do not locally accelerate the etching.…”

“…Also, the etching rate is greatly increased. However, it should be pointed out here that the Ag nanoparticles do not act as a catalytic agent that accelerates the etching locally but mainly collect electrons from the conduction band, which is different from the case in acid solution . Moreover, we want to clarify that a wetting agent such as isopropyl alcohol (IPA) is needed to obtain sufficient wettability for the Si surface by reducing the Si/electrolyte interfacial energy .…”

Fabrication of periodic arrays of needle-like Si nanowires on (001)Si and their enhanced field emission characteristics

“…The detailed mechanism of the vertical and lateral etching of Si catalyzed by Ag nanoparticles has been reported in several previous studies. [25][26][27] Fig. 4(a)-(f) are the representative SEM images showing the morphological evolution of the Si nanowires with the number of the ACER cycles.…”

Stable Superwetting Surface Prepared with Tilted Silicon Nanowires

“…According to back-bond strength theory, the (100) plane has two bonds symmetrically directed into the reactive solution while those on the (111) plane have three, leading to a geometry that sterically prefers etching silicon atoms along the (100) direction [ 17 , 18 ]. Besides, the pretreatment of HF solution in the Experimental section essentially removed the native oxide of the silicon surface, and further passivated the surface with the formation of Si–H terminations [ 19 ]. The Si–H terminations on the silicon wafer could also induce etching along (100) direction in a HF solution [ 20 , 21 ].…”