Vertical semiconductor nanowires (NW) are used as building blocks for the development of new electronic and photonic devices. To achieve a suitable efficiency it is necessary to tune the structural properties of the wires, both individually and for the assembly of NW.

We have conducted a study of an assembly of InAs/InP NW: for these NW grown by Chemical Beam Epitaxy it is possible to separate the diffraction of the wires from that of the substrate and surface overgrowth using grazing incidence diffraction. By measuring several diffraction rods in the direction perpendicular to the surface of the sample it is possible to extract the different contributions to the diffraction, and analyse the wurtzite/zincblende stacking along the wires.[1]

However a complete analysis of NW structure cannot always be obtained by scattering from an assembly of wires, as the distribution of shape and size (diameter & length) will allow access only to the average properties of the wires. It is therefore necessary to conduct the analysis on single nanowires, which can be done using X-rays with the development of micro-focused (1-10 microns) coherent beam.

We will present the results of coherent diffraction on single NW:
- for GaP/GaAs NW the stacking faults induce phase shifts (of the scattered X-ray wave) along the wire for each fault. The resulting scattering can be analysed as the Fourier transform of the stacking faults and can be used to model the stacking along the wire
- for Si nanowires we obtained coherent diffraction images for NW with a diameter smaller than 100 nm. The recorded 3D diffraction exhibits a 6-fold symmetry which can be used to reconstruct the shape of the original wire.

We will discuss how coherent diffraction could be used for the reconstruction of the shape, strain and composition of single NW, particularly for vertical and radial heterostructures.

[1] J. Eymery et al., Nanoletters (submitted).

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1. "Stress in Nanoparticles"