Wetting layer: The key player in plasma-assisted silicon nanowire growth mediated by tin

Unidirectional growth of silicon nanowires (SiNWs) can be achieved via a vapor-liquid-solid (VLS) mechanism. Interestingly, when adopting low surface tension metals such as tin or indium to mediate the growth of SiNWs in a plasma-assisted VLS process, the standard scenario is challenged by the fact that low surface tension metals tend to coat the high energy sidewalls of SiNWs. Moreover, we show that tin-assisted SiNW growth can continue without any apparent metal droplet on top. To address these issues, we investigate the time evolution of tin-assisted SiNW growth. We suggest that, during the initial growth phase, an ultrathin metal layer wetting the SiNW sidewalls allows silicon adatoms to diffuse toward the top of the nanowires, and thus enhances the axial growth while suppressing the radial one (in a way similar to molecular beam epitaxy growth of SiNWs). Later on, the wetting layer can assist the growth even when the tin droplets are exhausted from the top of SiNWs. We propose that this phenomenon may hold for all low surface tension metal mediated VLS growth of SiNWs and could be helpful to tailor them for specific device applications.