X-ray diffraction from single GaAs nanowires

In recent years, developments in x-ray focussing optics have allowed to produce highly intense, coherent x-ray beams with spot sizes in the range of 100nm and below. Together with the development of new experimental stations, x-ray diffraction techniques can now be applied to study single nanometer-sized objects.

In the present work, x-ray diffraction is applied to study different aspects of the epitaxial growth of GaAs nanowires. Besides conventional diffraction methods, which employ x-ray beams with dimensions of several tens of µm, special emphasis lies on the use of nanodiffraction methods which allow to study single nanowires in their as-grown state without further preparation.

In particular, coherent x-ray diffraction is applied to measure simultaneously the 3-dimensional shape and lattice parameters of GaAs nanowires grown by metal-organic vapor phase epitaxy. It is observed that due to a high density of zinc-blende rotational twins within the nanowires, their lattice parameter deviates systematically from the bulk zinc-blende phase.

In a second step, the initial stage in the growth of GaAs nanowires on Si (1 1 1) surfaces is studied. This nanowires, obtained by Ga-assisted growth in molecular beam epitaxy, grow predominantly in the cubic zinc-blende structure, but contain inclusions of the hexagonal wurtzite phase close to their bottom interface. Using nanodiffraction methods, the position of the different structural units along the growth axis is determined. Because the GaAs lattice is 4% larger than silicon, these nanowires release their lattice mismatch by the inclusion of dislocations at the interface. Whereas NWs with diameters below 50nm are free of strain, a rough interface structure in nanowires with diameters above 100nm prevents a complete plastic relaxation, leading to a residual strain at the interface that decays elastically along the growth direction.

Finally, measurements on GaAs-core / InAs-shell nanowire heterostructures are presented. In this system, a saturation of the dislocation density at the core-shell interface causes residual stresses at the heterojunction and significant strain in the GaAs core, increasing with the thickness of the InAs shell.