Transfer of Vertically Aligned Silicon Nanowires Array Fabricated Using Metal-assisted Chemical Etching

Abstract

Bulk silicon (Si) possesses an indirect bandgap and low surface area to volume Si ratio. Silicon nanowires (SiNWs), a derived material of Si, overcomes the drawbacks of Si and promises improvement in energy conversion (e.g., solar cell) and storage (e.g., lithium-ion battery) devices, gas sensors, medical diagnostics, drug delivery. The SiNWs-based devices have optical, electronic, and physical properties that can outperform their traditional counterparts in various ways because the SiNWs have a high surface Si area to volume ratio and unique quasi-one-dimensional electronic structure. The metal-assisted chemical etching (MACE) produces the SiNWs using an electrolyte composed of hydrofluoric acid (HF), hydrogen peroxide (H2O2), and a metal salt. Effect of MACE parameters, such as H2O2 concentration (i.e., 0.1 M to 0.3 M), etching time (i.e., 30 minutes to 60 minutes), Si wafer resistivity, HF concentration (i.e., from 0.48M to 9.6M), and etching temperature (i.e., 25℃ to 85℃), on the morphological characteristics (especially length) of SiNWs are compared and thoroughly discussed. Additionally, MACE parameters on the length of SiNWs using Si and porous Si substrates are discussed. The cross-sectional view of FESEM confirms the variation of the length of SiNWs for the variation of MACE parameters. The Raman line broadening and peak shift are due to FANTUM (FANo + quanTUM) effect (i.e., Fano effect and quantum confinement effect), amorphous content (⁓15-20%), and stress in the SiNWs. The tensile strain remains ⁓0.25%, and the crystallinity volume fraction of ⁓80% provides a range of MACE parameter variation to fabricate the SiNWs according to various device applications. The SiNWs, however, need to be transferred to a better substrate for additional flexibility, lesser cost, and transparency compared to Si substrate resulting in improved device functionality. This part explores, optimizes, and compares two techniques to transfer SiNWs to glass: the gluing technique and the two-step electro-assisted technique. The objective is to preserve the length of nanowires on a larger transfer area. Gluing technique spin-coats an adhesive layer made of polyvinyl acetate (PVAc) and methanol solution. The gluing method studies the effect of variation in MACE time on the percentage transfer ratio for the optimized PVAc layer. The electro-assisted technique detaches the vertically aligned SiNWs array with the aid of a sacrificial porous Si layer for variation in anodization time. The yield of the gluing and electro-assisted technique is optimized for MACE and anodization time. The transferred layer is characterized by various parameters, such as the percentage transferred length (%TRL), total transfer area, crystallinity, strain, and morphology of the SiNWs. For optimized values, the gluing method achieved %TRL = 68.2% while transferring 0.95 cm2 of the film area, whereas the electro-assisted technique achieves %TRL = 7.4% for an area of 19 cm2.