(514d) Elucidating the Chemical Origin Underlying Stable Semiconductor Nanowire Growth

Semiconductor nanowires are promising building blocks in the areas of energy conversion, electronics, and photonics due to their unique and tunable optoelectronic properties. Yet, significant challenges in heterostructure formation, uniform doping, and even simple axial growth remain, and stem from a poor understanding of the chemical phenomena governing bottom-up nanowire synthesis. Here, we use in situ infrared spectroscopy to quantitatively probe, for the first time, the chemical bonding underlying the vapor-liquid-solid mechanism and show that surface hydrogen atoms are vital for stable nanowire growth. We employ a novel method using infrared absorption spectra to extract the coverage of surface hydrogen, a reactive intermediate species, for Ge nanowires grown with Ge₂H₆ at a range of process conditions (260 − 330 °C, 0.5 − 1.5 × 10^-4 Torr). Surface hydrogen coverage, which dictates the nanowire sidewall energy, is not a static value as commonly presumed, but instead varies dramatically over a narrow temperature and pressure range. Below a threshold coverage, the catalyst droplet unpins from atop the nanowire and growth abruptly terminates. To underscore the significance of the sidewall energy on vertical nanowire growth, we adsorb –CH₃ groupsonto the sidewall which prevents droplet destabilization and enables growth beyond the process window of the Ge₂H₆ precursor. Our experiments elucidate a key chemical mechanism underlying nanowire growth and identify surface chemistry as a route to robustly control dopant profile and heterostructure formation, as well as synthesize previously inaccessible nanowire morphologies.