3D Composition Profiling at the Nanoscale: Doping Limits in Semiconducting Nanowires
The vapor-liquid-solid (VLS) mechanism of semiconductor nanowire growth provides a means to fabricate one-dimensional structures with control over doping and aspect ratio provided in situ during growth. Developing deep understanding and precise control of the structure and chemical composition of VLS-grown nanowires is crucial, as any unintended gradients in dopant concentration can severely degrade the ultimate device performance. This is particularly important for optoelectronic applications such as solar cells and LEDs, where broadened axial and/or radial doping junctions lower efficiencies.
Contrary to the traditional model of VLS growth, where dopant species are assumed to incorporate uniformly across a planar liquid-solid interface, we demonstrate that VLS-mediated doping is highly radially anisotropic, with dopant concentration variations across the nanowire diameter of as much as two orders of magnitude. Finite element modeling of the doping process, coupled with recent in situ TEM observations reported in the literature, suggest that this radially inhomogeneous dopant distribution is a direct consequence of growth from a faceted liquid-solid interface, rather than the commonly assumed planar interface.
These observations suggest that this doping inhomogeneity is general to all nanowire systems, motivating the search for novel catalysts for nanowire growth that can alleviate or eliminate this side faceting behavior. Using both aqueous solution and e-beam lithographic techniques, we are able to fabricate composition-controlled Au-Cu alloy catalysts for nanowire growth, providing a platform on which to study the limits to which varying catalyst phase and chemistry can be used to control doping in VLS nanowire growth.