Monolithic axial and radial metal-semiconductor nanowire heterostructures.

The electrical and optical properties of low dimensional nanostructures depend critically on size and geometry and may differ distinctly from those of their bulk counterparts. In particular, ultra-thin semiconducting layers as well as nanowires have already proven the feasibility to realize and study quantum size effects enabling novel ultra-scaled devices. Further, plasmonic metal nanostructures attracted recently a lot of attention because of appealing near-field mediated enhancement effects. Thus, combining metal and semiconducting constituents in quasi 1D heterostructures will pave the way for ultra-scaled systems and high-performance devices with exceptional electrical, optical and plasmonic functionality. This paper reports on the sophisticated fabrication and structural properties of axial and radial, Al-Ge and Al-Si nanowire heterostructures, synthesized by a thermally induced exchange reaction of single-crystalline Ge-Si core-shell nanowires and Al pads. This enables a self-aligned metallic contact formation to Ge segments beyond lithographic limitations as well as ultra-thin semiconducting layers wrapped around monocrystalline Al core nanowires. High-resolution transmission electron microscopy, energy dispersive X-ray spectroscopy and µ-Raman measurements proved the composition and perfect crystallinity of these metal-semiconductor nanowire heterostructures. This exemplary selective replacement of Ge by Al represents a general approach for the elaboration of radial and axial metal-semiconductor heterostructures in various Ge-semiconductor heterostructures.