Fabrication of Coaxial Si _1-xGe _x Heterostructure Nanowires by O ₂ Flow-Induced Bifurcate Reactions

We report on bifurcate reactions on the surface of well-aligned Si _1-xGe _x nanowires that enable fabrication of two different coaxial heterostructure nanowires. The Si _1-xGe _x nanowires were grown in a chemical vapor transport process using SiCl ₄ gas and Ge powder as a source. After the growth of nanowires, SiCl ₄ flow was terminated while O ₂ gas flow was introduced under vacuum. On the surface of nanowires was deposited Ge by the vapor from the Ge powder or oxidized into SiO ₂ by the O ₂ gas. The transition from deposition to oxidation occurred abruptly at 2 torr of O ₂ pressure without any intermediate region and enables selectively fabricated Ge/Si _1-xGe _x or SiO ₂/Si _1-xGe _x coaxial heterostructure nanowires. The rate of deposition and oxidation was dominated by interfacial reaction and diffusion of oxygen through the oxide layer, respectively.