The self-assembly approach based on the catalytic action of a metallic seed particle is a widespread method to synthesize semiconductor nanowires. A wide variety of structures involving axial and radial (i.e. core-shell) heterostructures have been obtained using this bottom-up approach, predominantly via chemical vapour deposition (CVD) techniques. Comparatively, nanowire growth by molecular beam epitaxy (MBE) remains at an earlier stage. While tremendous advances have been accomplished recently in determining the mechanisms involved during the growth of the nanowires by MBE, many aspects are still not clearly understood. To fully benefit from the potential offered by these structures, it is essential to master the fabrication parameters that will allow control of the end-product.
We report on the results of a systematic investigation performed on Au-catalyzed GaAs nanowires on GaAs (111)B substrates grown by MBE. The morphology and crystalline structure of the wires at various stages was studied by varying the growth time. Nanowires grown for 3, 10, 30, and 40 minutes (all at a temperature of 550°C) were analyzed by both scanning (SEM) and transmission electron microscopy (TEM). Our study revealed that radial growth occurs early in the process, as wire tapering was dominant only after 3 minutes of growth. At this stage no clear facet of the wire sidewall was noticed. For wires that grew longer (30 and 40 minutes), only the tip is tapered, giving the wire a "pencil" shape. Rotation of the sidewall facets was observed between the tapered segment and the base of the wire, with the former having nominally {-2 1 1}-oriented facets and the latter having {-1 1 0} facets. We explain this transition in terms of an epitaxial layer-by-layer growth mode on the sidewall and minimization of surface energy. Electron diffraction measurements revealed that the crystals grew with a wurtzite structure, a phase not typical in bulk GaAs. This could open the door to the production of wurtzite quantum dots (grown in the wire) with strong built-in piezoelectric fields [1].
[1] Phys. Rev. B 68 (2003) 155331.
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