The Surface Hydrogen-Controlled Crystal Structure of Si Nanowires

Semiconducting nanowires promise exciting advances in fields as diverse as photonics, optoelectronics, quantum computation, electrochemistry, and thermophysics. In the ideal situation, the combination of nanowire diameter, lattice structure (e.g. diamond cubic, wurtzite), crystal orientation (e.g. <111> vs. <110>), and sidewall faceting that yields the most robust device performance for a particular application would be known and could be rationally synthesized. Unfortunately, an inadequate understanding of nanowire process-structure and structure-property relationships prevents the accomplishment of this task at the present time. Although hydrogen is prevalent during the hydride-based vapor-liquid-solid (VLS) growth of semiconductor nanowires, its role is largely unknown. To this end, we systematically studied the effect of hydrogen during the growth of Si nanowires. We confirmed its influence on crystal growth orientation and also revealed a common chemical thread that connects multiple unresolved questions in the field for the first time. In-situ transmission infrared (IR) spectroscopy was used to identify the presence and bonding of hydrogen on Si nanowires as a function of growth conditions in the absence of any carrier gas. Si nanowires were grown via a two-step process: (1) brief nucleation followed by (2) elongation under different conditions. Vertically-oriented epitaxial Si nanowires with uniform densities, diameters, and lengths were obtained with this method. Real-time IR measurements reveal the evolution of surface Si-H stretching modes as a function of growth time and conditions. More specifically, the nanowire crystal orientation transitions from <111> to <112> upon reaching a critical hydrogen surface coverage, even for large diameter nanowires and a range of supersaturation conditions. The influence of hydrogen on interface energetics in the three-phase region is significant and opens new avenues to fabricate nanowire superstructures, which will also be presented. The extensive use of hydride chemistries for most group IV and III-V semiconductor nanowire syntheses suggests significant implications for a variety of systems.