Engineering the Microstructure of Silicon Nanowires by Controlling the Shape of the Metal Catalyst and Composition of the Etchant in a Two-Step MACE Process: An In-Depth Analysis of the Growth Mechanism

Abstract

In this work, slanted, kinked, and straight silicon nanowires (SiNWs) are fabricated on Si(111) and (100) substrates using a facile two-step metal-assisted chemical etching nanofabrication technique. We systematically investigated the effect of crystallography, morphology of Ag catalyst, and composition of etchant on the etch profile of Ag catalyst on Si(111) and (100) substrates. We found that the movement of AgNPs inside the Si is determined by physiochemical events such as Ag/Ag interaction, Ag/Si contact, and diffusion kinetics. Further, from detailed TEM and micro-Raman spectroscopy analyses, we demonstrate that the metal catalyst moves in the crystallographically preferred etching direction (viz., <100>) only when the interface effect is not predominant. Further, the metal-assisted chemical etching (MACE) system is highly stable at low-concentration plating and etching solutions, but at high concentrations, the system loses its stability and becomes highly random, leading to the movement of Ag catalyst in directions other than ⟨100⟩. In addition, our studies reveal that Ag nanostructures growth on Si(111) and (100) substrates through galvanic displacement is controlled by substrate symmetry and surface bond density. Finally, we demonstrate that by using an optimized balance between the Ag morphology and concentration of the etchant, the angle in slanted SiNWs, kink position in kinked SiNWs, and aspect ratio of straight SiNWs can be controlled judiciously, leading to enhanced optical absorption in the broadband solar spectrum.

Improved omnidirectional polarisation‐insensitive optical absorption and photoelectrochemical water splitting using aperiodic and tapered slanted, kinked and straight silicon nanowires

“…along with the etching conditions affect the porosity gradient in nanowires. This has a significant effect on the reflectance of SiNWs 11 . The kinked, slanted and straight SiNWs substantially reduce the total reflectance ( R T ), diffusive reflectance ( R diff ) and specular ( R spec ) reflectance from visible to the NIR region as shown in Figure 1A–C.…”

“…This has a significant effect on the reflectance of SiNWs. 11 The kinked, slanted and straight SiNWs substantially reduce the total reflectance (R T ), diffusive reflectance (R diff ) and specular (R spec ) reflectance from visible to the NIR region as shown in Figure 1A-C. The R T of the slanted, kinked and straight SiNWs in comparison to bare silicon is shown in Figure 1A.…”

“…Metal‐assisted chemical etching (MACE) is a well‐known top‐down nanofabrication process for forming high aspect ratio SiNWs on silicon substrates. In MACE, the geometry of the SiNWs is controlled by regulating the metal coverage area, substrate orientation and etchant composition 10,11 . Inhomogeneous porosity developed on the SiNWs is a unique feature of the MACE, which increases the surface area and generates a gradient refractive index structure on the nanowires 12 …”

“…In MACE, the geometry of the SiNWs is controlled by regulating the metal coverage area, substrate orientation and etchant composition. 10,11 Inhomogeneous porosity developed on the SiNWs is a unique feature of the MACE, which increases the surface area and generates a gradient refractive index structure on the nanowires. 12 It has been reported that the dense ensembles of the graded refractive index of the SiNWs formed by MACE show an ultra-low reflection over a wide range of angles and wavelength.…”

Controlled Fabrication of Wafer-Scale Zigzag Silicon Nanowire Arrays by Metal-Assisted Chemical Etching through Synergistic Effect of Viscosity and Temperature

“…1,16 Metal-assisted chemical etching of silicon wafers in oxidizing HF solutions or vapors is an emerging low-cost, scalable and versatile top-down fabrication technique capable of producing various silicon microstructures such as nanowires. [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] MACE can be used to fabricate zigzag SiNW arrays on silicon wafers with different orientations. [17][18][19][20][21][22] Chen et al reported the fabrication of three types of SiNWs with different turning angles by MACE using Si(111) at temperature varied from 15 °C to 75 °C.…”