Effect of diborane on the microstructure of boron-doped silicon nanowires

Boron-doped silicon (Si) nanowires, with nominal diameters of 80 nm, were grown via the vapor-liquid-solid (VLS) mechanism using gold (Au) as a catalyst and silane (SiH₄) and diborane (B₂H₆) as precursors. The microstructure of the nanowires was studied by scanning electron microscopy, transmission electron microscopy and electron energy-loss spectroscopy. At lower B₂H₆ partial pressure and thus lower doping levels (≤1×10¹⁸ cm^-3), most of the boron-doped Si nanowires exhibited high crystallinity. At higher B ₂H₆ partial pressure (∼2×10¹⁹ cm ^-3 doping level), the majority of the wires exhibited a core-shell structure with an amorphous Si shell (20-30 nm thick) surrounding a crystalline Si core. Au nanoparticles on the outer surface of the nanowires were also observed in structures grown with high B/Si gas ratios. The structural changes are believed to result from an increase in the rate of Si thin-film deposition on the outer surface of the nanowire at high B₂H₆ partial pressure, which produces the amorphous coating and also causes an instability at the liquid/solid interface resulting in a loss of Au during nanowire growth.