Crystal Structure Engineering of Semiconductor Nanowires

Semiconductor nanowires provide a promising route to achieve next generation photonic, electronic, and energy conversion devices. Control of nanowire crystal structure (e.g. lattice, orientation, faceting) is critical to enable the appropriate function for a particular application. Unfortunately, this remains a challenging task with bottom-up syntheses. To this end, we demonstrate the ability to precisely engineer nanowire interface energetics via the addition of bifunctional precursors and, in doing so, rationally control Ge nanowire crystal growth orientation for the first time. More specifically, Ge nanowires are grown using the vapor-liquid-solid (VLS) technique with germane and a series of bifunctional alkylgermane species, including methylgermane, ethylgermane, and tert-butylgermane. Scanning electron microscopy (SEM) reveals dramatics change in growth direction as well as a reduction in tapering. In the case of methylgermane, transmission electron microscopy (TEM) confirms a transition from <111> to <110> oriented growth while maintaining single crystallinity throughout. Infrared and X-ray photoelectron spectroscopy (XPS) indicate that the nanowire surfaces are alkyl terminated. In addition to reducing the rate of oxidation, these surface species adjust the solid-vapor interface energy near the three-phase boundary, thereby influencing nucleation events and enabling changes in crystal orientation. The control of nuclei surface chemistry demonstrated by this work provides an important new handle for controlling nanowire properties for different applications. Furthermore the ability to effectively passivate sidewalls during growth is expected to enable more robust doping profiles by only permitting precursor incorporation through the catalyst tip.