Semiconducting nanowires offer the possibility of nearly unlimited complex bottom-up design, which allows for new device concepts. However, essential parameters that determine the electronic quality of the wires, and which have not been controlled yet for the III-V compound semiconductors, are the wire crystal structure and the stacking fault density. In addition, a significant feature would be to have a constant spacing between rotational twins in the wires such that a twinning superlattice is formed, as this is predicted to induce a direct bandgap in normally indirect bandgap semiconductors, such as silicon and gallium phosphide. Optically active versions of these technologically relevant semiconductors could have a significant impact on the electronics and optics industry. Here we show first that we can control the crystal structure of indium phosphide (InP) nanowires by using impurity dopants. We have found that zinc decreases the activation barrier for two-dimensional nucleation growth of zinc-blende InP and therefore promotes crystallization of the InP nanowires in the zinc-blende, instead of the commonly found wurtzite, crystal structure. More importantly, we then demonstrate that we can, once we have enforced the zinc-blende crystal structure, induce twinning superlattices with long-range order in InP nanowires. We can tune the spacing of the superlattices by changing the wire diameter and the zinc concentration, and we present a model based on the distortion of the catalyst droplet in response to the evolution of the cross-sectional shape of the nanowires to quantitatively explain the formation of the periodic twinning.

No abstract

The geometrical and resolution corrections are derived that occur in the measurement of the integrated intensities of surface diffraction rods for the case of a sixcircle diffractometer. Since the six-circle geometry entails as special cases the five-circle and the z-axis diffractometers, the results are valid for these geometries as well. The derivations are valid for any incoming or outgoing angle of the X-ray beam, and are particularly important for measurements at large perpendicular momentum transfer. Expressions are derived for the integrated intensity from rocking scans, from stationary measurements and from reflectivity data. With all correction factors known, it is possible to derive the structure factors with a common scale factor from all these types of scans. It is expected that area detectors combined with stationary measurements will find widespread use in the surface X-ray diffraction community.

A brief description is given of the C‐program ROD with which surface structures can be refined on the basis of X‐ray data. All main features one encounters on surfaces, like roughness, relaxations, reconstructions and multiple domains, are taken into account. The program has proven to be a useful tool over the past ten years.