AVS 58th Annual International Symposium and Exhibition
    Nanometer-scale Science and Technology Division Tuesday Sessions
       Session NS-TuM

Paper NS-TuM9
The Surface Hydrogen-Controlled Crystal Structure of Group IV Nanowires

Tuesday, November 1, 2011, 10:40 am, Room 203

Session: Nanowires and Nanoparticles II: Characterization and Synthesis
Presenter: Naechul Shin, Georgia Institute of Technology
Authors: N. Shin, Georgia Institute of Technology
M.A. Filler, Georgia Institute of Technology
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Semiconductor nanowires offer exciting opportunities to fabricate high performance devices for energy conversion, photonics, and quantum computation. The precise control of crystal structure and geometry is required to achieve a desired behavior, especially in highly confined nanoscale systems. Unfortunately, a fundamental understanding of the surface chemistry that controls surface energetics is currently lacking, despite its critical importance for robust synthesis. Although hydrogen is prevalent during the hydride-based vapor-liquid-solid 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 and confirmed its influence on crystal growth direction, catalyst ripening, and sidewall faceting 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. Si nanowires were grown via a two-step process: (1) brief nucleation at high temperature (550oC) and low pressure (5x10-5 Torr) followed by (2) elongation under different conditions (400 – 500oC, 5x10-5 – 5x10-3 Torr). Vertically-oriented epitaxial Si nanowires with uniform densities, diameters, and lengths were obtained with this method. In-situ IR data recorded in real-time reveals the evolution of surface Si-H stretching modes near 2090 cm-1 and 2075 cm-1. Our data indicates that surface-bound hydrogen is responsible for changes in crystal orientation even for relatively large diameter nanowires. More specifically, the surface energy of the nuclei-vapor interface near the triple-phase-boundary is dramatically reduced by hydrogen adsorption and drives a transition from <111> to <112> oriented growth. We propose a simple nucleation model that explains this observation. This knowledge is then applied to rationally fabricate nanowire superstructures through the judicious incorporation of small quantities of Ge during growth. This work demonstrates the important role that surface chemistry plays in the growth of semiconductor nanowires, and the extensive use of hydride chemistries for most group IV and III-V semiconductor nanowire syntheses suggests significant implications for many materials systems.