Free-standing III-V nanowires and nanotubes are likely to become central components in future electronics and photonics with applications in IT, life-science and energy[1]. A very wide variety of III-V materials and dopants can be self-assembled into complex axial and radial heterostructures - precisely tailoring their structure down to the atomic scale.

  

We have used Scanning Tunneling Microscopy /Spectroscopy (STM/STS) and X-ray PhotoEmission and Low Energy Electron Microscopy (XPEEM/LEEM) to study nanowires and nanowire surfaces. Previously we have developed the means to directly study the interior of III-V semiconductor nanowires by STM[2], and now we have also developed direct methods for studying also the exterior nanowire surfaces to the atomic scale with STM[3].

We describe several significant results on the fundamental limits to atomic scale interface precision in AlGaAs/GaAs nanowires heterostructures, STS measurements on the interior of these wires and new insights into the influence of the growth substrate. We present atomically resolved STM images of the outside surface of InAs nanowires with InP segments along with STS measurements on the clean surfaces. The imaged surfaces of InAs nanowires are quite intriguing as the wires grow in the wurtzite crystal phase, in contrast to the zincblende bulk phase. Further we present STM images and STS measurements of InP nanowire pn-junctions. Using XPEEM and XPS we have characterized III-V nanowire surface chemistry and electronic properties and investigated the influence of various ultra-thin dielectrics to reduce surface band-bending effects.

The structure and morphology of semiconductor nanowire surfaces down to the single atom level are significant in determining both growth and function of the wires. Diffusion and nucleation on the nanowire surfaces will directly influence the final appearance of the wires, and it has been shown that transport and optical properties of semiconductor nanowires are to a considerable extent governed by their surfaces. Scanning Tunneling Microscopy is a great tool for imaging both geometric and electronic structure with high resolution, however direct atomically resolved STM studies of nanowire surfaces is very difficult. Still having overcome these problems (as in the present study) and by also applying synchrotron based spectroscopy/microscopy we obtain fascinating new insights into the interplay between nanowire growth, structure and function.

[1] C.M. Lieber and Z.L. Wang, MRS Bull. 32, (2007) 99

[2] A. Mikkelsen et al, Nature Mater. 3 (2004) 519 ; L. Ouattara et al, Nano Lett. 7 (2007) 2859

[3] E. Hilner, et al, Nano Lett., 8 (2008) 3978