Paper EN+EM+NS-MoA7
Using Surface Chemistry to Modulate the Bandgap of Ge Nanowires

Monday, October 31, 2011, 4:00 pm, Room 103

Small-diameter semiconductor nanowires are highly attractive building blocks for next generation photovoltaic devices because they exhibit highly tunable optoelectronic properties as a result of quantum confinement. Bottom up approaches, such as the vapor-liquid-solid (VLS) growth mechanism, are controllable down to ~20 nanometers but significant challenges exist at smaller length scales where property tuning is maximized. Furthermore, it is expected that the optoelectronic properties of quantum-confined nanowires will be heavily dependent on surface chemistry, yet there are no experimental studies that fundamentally probe this relationship. Germanium is an ideal system to study because of its large Bohr exciton radius, low rate of oxidation, and chemical similarity to silicon. In this work we controllably synthesize germanium nanowires with diameters from 5 to 20 nanometers using gold catalyst particles with a narrow size distribution. To obtain epitaxial Ge nanowires with uniform diameters and lengths, a two-step growth process is employed that includes a brief, high-temperature nucleation (390^oC) above the Au-Ge eutectic point, followed by elongation at various process conditions, generally below the eutectic point (280 – 340^oC, 1x10^-7 – 1x10^-4 Torr). To limit catalyst diffusion and formation of the √3 x √3 Au/Si reconstruction on the Si(111) surface, a key problem for controllable sub-20 nm growth, the substrate is passivated with -CH₃ groups prior to nanowire synthesis by introducing small quantities of methylgermane. Nanowire surface chemistry is probed with in-situ transmission infrared (IR) spectroscopy both during and after growth. We introduced different adsorbates (-H and -CH₃) and monitored their influence on the band gap as a function of surface coverage. We show that the band gap of Ge nanowires blue shifts with H passivation and red shifts with CH₃ termination.