Anion F–-Induced Etching of Silicon Nanowires with Diverse Doping Levels, Surface Crystalline Orientations, and Porosity

Abstract

40% aqueous NH4F can passivate Si wafers with H-termination but unexpectedly etch silicon nanowires (SiNWs), attributed to the surface F-termination caused by the nucleophilic attack of F– anions to Si atoms. The grafted Si–F bonds with strong dipole moment significantly polarize and weaken Si back bonds, and the polarized back bonds tend to be vulnerably attacked by F– anions, resulting in the etching of SiNWs. The F-termination is confirmed by ATR-FTIR, XPS, and PL spectroscopy. The etching rate in terms of the NW length is evaluated on SiNWs with diverse doping levels, surface crystalline orientations, and porosity, which could be explained by three factors including the doping level, atomic ratio F1s/Si2p, and the stoichiometric x of the surface termination Si–Fx. NH4F dissolves surface oxides and then commences to etch SiNWs. As comparison, mesoporous SiNWs (mpSiNWs) have porosified surfaces not fully passivated by native oxides, leading to the simultaneous etching of oxides and Si skeletons. This work contributes to understanding the difference in surface reactions between bulky Si and one-dimensional SiNWs, and to providing useful information about surface processing and modification of SiNW-based nanoelectronic devices.