Porosification-induced back-bond weakening in chemical etching of n-Si(111)

It is imperative to control the etching direction in metal-assisted chemical etching (MACE) to engineer the surface crystalline orientation of as-generated silicon nanowires (SiNWs). At room temperature, MACE of n-Si(111) carries out along the intrinsic back-bond etching direction of [111] under [HF]/[AgNO₃] ≥ 50. When n-Si(111) is heavily doped, MACE generates mesoporous SiNWs (mp-SiNWs) standing on a mesoporous silicon (mpSi) layer. The porosification substantially weakens the back bonds underneath the sinking metal particles, leading to a deviation of etching from [111]. The selection of etching direction is governed by an angle of the selected direction to [111], and small angle is preferential. Due to the thermodynamic preference of the intrinsic back-bond etching, the etching along [111] is retained at ≥30 C. Zigzag mp-SiNWs are first created under high [HF] and solution stirring, attributed to the HF-induced heating and stirring-stimulated H₂ evaporation. This work would potentially pave a way to employ mp-SiNWs with controllable surface orientations in catalysis, electronics, optoelectronics, sensors, and surface functionalization.