(321b) Geometry-Dependent Formation of Multiple Twins in Si Nanowires

The rational and atomically precise insertion of twin boundaries and stacking faults in semiconductor nanowires would create myriad new opportunities to engineer their electrical, optical, and thermal properties. While these structural motifs are commonplace in group III-V nanowires synthesized via the vapor-liquid-solid (VLS) technique, their presence in group IV nanowires is much less frequent. Here, we report on the appearance of consecutive, geometry-dependent twin boundaries in Si nanowires for the first time. Si nanowires with multiple twins were grown with a Au catalyst via a two-step process in a custom-built ultrahigh vacuum chamber. The first 10 min of nanowire elongation occurred with a substrate temperature of 490 ^oC and Si₂H₆ pressure of 2×10^-4 Torr.  The substrate temperature and Si₂H₆ pressure were then modulated to 410 ^oC and 5×10^-4 Torr, respectively, for another 10 min. The first twin boundary (TB₁) appears at an axial position corresponding to the point where growth conditions were changed and was the same for all nanowires. Importantly, the location of the second twin boundary (TB₂) shows a clear diameter-dependence. High resolution electron microscopy reveals that thin {111} sidewall facets, which extend following TB₁ and distort the triple-phase line, likely underlie the formation of TB₂. We hypothesize that an increased amount of surface hydrogen, present as a result of lowering the substrate temperature and raising the Si₂H₆ pressure, favors the thin {111} facets. These results indicate that surface chemistry can influence defect formation in semiconductor nanowires and constitute an important step toward defect superstructure engineering in this ubiquitous material. [Research supported by ORNL's Shared Research Equipment (ShaRE) User Program, which is sponsored by the Office of Basic Energy Sciences, the U.S. Department of Energy.]