Room-temperature formation of slanted nanobelts and vertical wires on low-doped p-type (100) silicon via metal-assisted chemical etching

Despite its applications, metal-assisted chemical etching (MACE) is still not well understood. To elucidate its underlying mechanism, MACE with micron-sized Au mesh was systematically carried out across a wide range of volume ratios R of HF acid to H_2O_2 solution. Slanted wires were observed at room temperature over a broad range of parameters, which contradicts the well-established viewpoint that slantwise etching only occurs at elevated temperatures. Vertical wires only form in a narrow parameter space featuring a low R. The wide variation in observed slant angles indicates that the associated MACE processes do not exhibit a preferred etching direction. The observed R-dependent change of oxygen content in wires and of bubbling phenomena was explained by two competing mechanisms associated with direct and indirect Si dissolution. The high- and low-R regimes are dominated by direct and indirect mechanisms, respectively, whereas in the intermediate regime both mechanisms play an indispensable role. Formation of slanted wires is caused directly by slantwise mesh movement. Although this movement is closely correlated with the direct mechanism, it cannot be fully explained by the mechanism. We hypothesize that hydrogen bubbling-induced flow has the potential to hydrodynamically drive the mesh slantwise and hence can be directly responsible for slanted wire formation. To the best of our knowledge, this work not only reports a relatively novel phenomenon — slanted wire formation by MACE at room temperature — but also provides, for the first time, a comprehensive understanding of its underlying complex mechanism.