Morphology of germanium nanowires grown in presence of B2H6

We study the Au-catalyzed chemical vapor growth of germanium (Ge) nanowires in the presence of di-borane $\left({\mathrm{B}}_2{\mathrm{H}}_6\right)$ , serving as doping precursor. Our experiments reveal that, while undoped Ge nanowires can be grown epitaxially on Si(111) substrates with very long wire lengths, the ${\mathrm{B}}_2{\mathrm{H}}_6$ exposure renders the Ge nanowires significantly tapered. As we describe here, this peculiar morphology stems from the combination of the acicular, one-dimensional nanowire growth and a dramatically enhanced, B-induced conformal Ge deposition. The combination of acicular and conformal Ge growth mechanisms results in cone-shaped Ge nanostructures.