(457g) Solid-Liquid-Vapor Anisotropic Etching of Semiconductor Nanowires
Semiconductor nanowires synthesized by bottom-up techniques are promising materials for next generation electronic, photonic, and energy conversion devices because one can add multiple functionalities along the nanowire length. To achieve this, however, temperature and gas phase composition must be modulated multiple times during growth. These changes frequently result in graded composition profiles that are undesirable in many applications. Here, we demonstrate anisotropic in situ etching of semiconductor nanowires, a process that permits the removal of these transition regions. Ge nanowires are first synthesized via the vapor-liquid-solid technique with a eutectic Au-Ge catalyst at 370 oC and a GeH4 partial pressure of 0.27 Torr. An intentional diameter expansion near the nanowire base serves as an internal reference point to accurately measure etch rate. The introduction of 1,3 butanedione (BD) at a partial pressure of 0.0035 Torr initiates etching selectively from the eutectic catalyst. We find that the atomic etch rate is constant regardless of sidewall taper. Furthermore, we study this etching behavior as a function of temperature, BD partial pressure, and nanowire diameter. Our data indicate a solid-liquid-vapor etching mechanism, where Ge atoms initially dissolve from the nanowire into the eutectic catalyst through the solid-liquid interface prior to reaction with and removal by BD at the liquid-vapor interface. This etching technique, which effectively reverses vapor-liquid-solid growth, opens new opportunities to precisely control nanowire structure.