Synchrotron X-ray diffraction on ensemble and individual GaAs/In_xGa_1-xAs core/shell nanowires at beamline P08 – PETRA III (DESY)

Chemical composition, strain, structural polytypism and stacking faults in semiconductor nanostructures can be described quantitatively by high-resolution X-ray diffraction. It is a non-destructive technique that is suitable for the characterization of epitaxial nanostructures on their original substrates. The extracted information can be a valuable contribution to the understanding of the growth and strain relaxation mechanisms, which in turn are essential elements for tailoring the electronic properties in functional devices. Beamline P08 at PETRA III, Hamburg, offers a well-functioning high-resolution X-ray diffraction setup for the characterization of nanowire ensembles. Moreover, a newly developed configuration with a nano-focused beam is operational and can be used for the investigation of individual nanostructures.
The strengths of our setup have been tested in the characterization of GaAs/In_xGa_1-xAs core/shell nanowires. The nanowires were grown vertically on Si(111) substrates by molecular beam epitaxy (at HZDR). A set of samples with different shell thicknesses (5-80 nm) but the same In concentration (x≈0.20) and the same core diameter (25 nm) has been characterized. The measured in-plane and out-of-plane lattice constants as a function of the shell thickness suggest that the shell grew coherently around the core even for the thickest shell, the thickness of which is well beyond the critical value for planar In_0.2Ga_0.8As layers on GaAs. Furthermore, the tensile strain of the core due to the lattice mismatch with the shell increases with increasing the shell thickness up to 40nm, whereas the corresponding compressive strain of the shell decreases gradually to zero. All aforementioned results demonstrate the unique possibilities for strain engineering in core/shell nanowires.