Strategies for controlling Si nanowire formation during Au-assisted electroless etching

Abstract

Dramatic changes in the morphologies of Si nanowires (NWs) ranging from vertical, to hooked, to ground-collapsed shapes are presented depending on the HF/H₂O₂ ratio in the etch solution during Au-assisted electroless etching. It is shown that the deviations from vertical NWs are not due to capillary action but correlated with the development of nanoporous structures, which is attributed to excess hole (h⁺) diffusion toward the NWs' sidewalls during the etching process. The role of hole diffusion is made more evident by observation of NWs with the neck formation when transitioning to different etching regimes during SiNW formation. Furthermore, the hooked and ground-collapsed SiNW can be suppressed at the cost of enhanced tapering by adding a strong electrolyte in solution, which is correlated to reduced etching rate resulting from electric screening of the charged Au surfaces. We provide an important finding that common salts such as NaCl, KCl, and CaCl₂ can be useful retardants for the metal-assisted electroless etching. The new insights into structural transformation presented in this study afford greater control over the structure of SiNWs produced by metal-assisted electroless etching.