The potential for the metal nanocatalyst to contaminate vapour-liquid-solid grown semiconductor nanowires has been a long-standing concern, because the most common catalyst material, Au, is highly detrimental to the performance of minority carrier electronic devices. We have detected single Au atoms in Si nanowires grown using Au nanocatalyst particles in a vapour-liquid-solid process. Using high-angle annular dark-field scanning transmission electron microscopy, Au atoms were observed in higher numbers than expected from a simple extrapolation of the bulk solubility to the low growth temperature. Direct measurements of the minority carrier diffusion length versus nanowire diameter, however, demonstrate that surface recombination controls minority carrier transport in as-grown n-type nanowires; the influence of Au is negligible. These results advance the quantitative correlation of atomic-scale structure with the properties of nanomaterials and can provide essential guidance to the development of nanowire-based device technologies.

Semiconductor nanowires show promise for many device applications, but controlled doping with electronic and magnetic impurities remains an important challenge. Limitations on dopant incorporation have been identified in nanocrystals, raising concerns about the prospects for doping nanostructures. Progress has been hindered by the lack of a method to quantify the dopant distribution in single nanostructures. Recently, we showed that atom probe tomography can be used to determine the composition of isolated nanowires. Here, we report the first direct measurements of dopant concentrations in arbitrary regions of individual nanowires. We find that differences in precursor decomposition rates between the liquid catalyst and solid nanowire surface give rise to a heavily doped shell surrounding an underdoped core. We also present a thermodynamic model that relates liquid and solid compositions to dopant fluxes.

Correlated Raman microscopy and transmission electron microscopy were used to study the ordering of {111} planar defects in individual silicon nanowires. Detailed electron diffraction and polarization-dependent Raman analysis of individual nanowires enabled assessments of the stacking fault distribution, which varied from random to periodic, with the latter giving rise to local domains of 2H and 9R polytypes rather than the 3C diamond cubic structure. Some controversies and inconsistencies concerning earlier reports of polytypes in Si nanowires were resolved.

Scanning photocurrent microscopy (SPCM) is used in semiconductor nanowire devices to establish quantitative potential profiles correlated with nonuniformities in electrical resistivity. Surface doping leads to a nonuniform axial photocurrent (a). Surface etching improves the uniformity of the local photocurrent (b) and reduces the radial and axial carrier concentration gradients (c, blue curve after etching).

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