In-situ investigation of the flux shadowing effect on polytypism and strain evolution in self-catalyzed core and core-shell nanowire systems

Semiconductor nanowires with a quasi-one-dimensional geometry have gained great attention during the past decades. The unique geometry of these objects contributes to their distinctive optical and electrical properties that are promising for novel devices. The configuration of the band gap of the nanowires depends on their crystal properties such as the crystal phase and the strain which in turn are highly controllable. Therefore, the realization and the study of the nanowire crystal are essential for tuning and controlling their properties. In this work we use a molecular beam epitaxy chamber for fabricating GaAs nanowires and we investigate the changes of the nanowire crystal structure during growth as a function of the interspacing between the neighboring nanowires. By means of time-resolved X-ray diffraction technique, monitoring the nanowires during growth at arrays with different densities shows a high dependency of the crystal structure on the nanowire interspacing. This dependency is partially attributed to the shadowing effect of the growth material fluxes which we focus on in the first part of this study. Further, we investigate the strain evolution in the nanowires during an asymmetric growth of a lattice- mismatched InxGa1−xAs shell. The asymmetric growth of the shell materials on the nanowire facets results in an asymmetric strain variation across the nanowire cross section, which induces nanowire bending. The varying strain across the nanowire cross-section can be utilized for engineering the band gap and the optical properties of the nanowire. Therefore, we perform a detailed study of the strain and nanowire bending. The evolution of the strain and nanowire bending during shell growth is observed by means of time-resolved in-situ X-ray diffraction technique on a single nanowire as well as on a nanowire ensemble. This investigation revealed a non-linear dependency of the strain and nanowire bending to the shell growth time, indicating changes of the growth dynamics. Lastly, we exploit the shadowing effect of material fluxes by the neighboring nanowires to control the distribution of the shell material along the nanowire growth axis, which results in a varying strain field along the nanowire growth axis. This method can be employed for strain gradient engineering and nanowire-based
devices with novel geometries.