Strain Mapping of Single Nanowires using Nano X-ray Diffraction

Sammanfattning: Nanowires are explored as basic components for a large range of electronic devices. The nanowire format offersseveral benefits, including reduced material consumption and increased potential for combining materials to formnew novel heterostructures. Several factors, such as mechanical stress from contacting or a lattice mismatch in aheterostructure, can strain and change the lattice tilt. The strain is often intertwined with small gradients ofcomposition. The strain relaxation can differ significantly from bulk due to the small diameters, but the mechanismsare not fully comprehended. X-rays have a penetrating power that makes it possible to investigate embeddedsamples without preparation or slicing. The high flux of coherent X-ray beams from synchrotron radiation facilities,combined with the nano-focus capabilities developed in recent years, have made it possible to probe nano-crystals.The 4th generation of synchrotrons, including MAX IV in Lund, Sweden, has even higher brilliance than previoussources. Diffraction imaging techniques using synchrotron radiation can reveal small strains down to 10-4-10-5. Thefield of coherent imaging pushes the limits of resolutions below the size of the focus. With Bragg ptychography, thedisplacement field in a crystal can be probed with resolution beyond the probe focus by numerically reconstructingthe phase.This thesis includes the development of X-ray nano-diffraction methods for the characterizing of nanowires, includingGaInP/InP barcode nanowires, p-i-n InP nanowire devices and metal halide perovskite CsPbBr3 nanowires. Itincludes a theoretical background of the scattering mechanisms in Thomson scattering in nano-crystals, goesthrough the formalism for coherent diffraction imaging, crystal structure and deformation in nanoobjects and thetechnical aspects of the experimental setup and measurement. Moreover, theoretical modelling of elastic strainrelaxation in these nanowires was performed with finite element modelling.Single III-V nanowire heterostructures and III-V nanowire devices were probed with scanning XRD and Braggprojection ptychography (BPP). How the techniques compare to each other and how the results are affected by thedifferent approximations that are made in the respective technique was explored. Finite element simulationscombined with nano-diffraction revealed that the lattice mismatch of 1.5% could be relaxed elastically for thediameter of 180 nm. From the strain mapping of the nanowire device, we found how the contacting of the nanowirebends the nanowire resulting in a tilt normal to the substrate.Single perovskite metal-halide perovskite CsPb(Br(1-x)Clx)3 nanowire heterostructures were characterized withscanning nano-XRD and XRF, which showed that the lattice spacing was affected by composition and strain.Composition gradients revealed that Cl diffusion had taken place within the heterostructure. Furthermore, extractingthe lattice tilts from shifts of the Bragg peak revealed a ferroelastic domain structure with simultaneously existinglattice tilts. These findings are beneficial for the further development of MHP nanowires devices.