X-ray Diffraction Analysis of the Angular Stability of Self-Catalyzed GaAs Nanowires for Future Applications in Solar-Light-Harvesting and Light-Emitting Devices

Semiconductor nanowires are a class of materials that recently have gained increasing interest in solar-cell applications and light-emitting devices. Finding reproducible processing conditions is fundamental for their future mass production. In this work, the stability of individual epitaxial GaAs nanowires (NWs) under molecular beam epitaxy (MBE) processing conditions is studied by means of a time-resolved in situ micro X-ray diffraction (μXRD) method and scanning electron microscopy. Our proposed μXRD method is a nondestructive characterization technique where individual nanoobjects of different dimensions, crystal orientations, and structures are detectable under MBE processing conditions. NWs were grown by self-catalyzed MBE onto prepatterned Si(111) substrate. When exposed to MBE processing conditions at 610 °C without supply of a source material, or with only an arsenic (As) supply, we observe evaporation from the facets with no indication of gallium (Ga) droplet formation. Furthermore, the NWs, which are initially grown perpendicularly to the substrate surface, become angularly unstable; i.e., the NWs tilt and eventually lie down on the substrate surface. Before falling down, our μXRD data evidenced vibration/bending of the NWs. Interestingly, when exposed to the original growth conditions, which include Ga and As supply, the vibrations/bendings are suppressed and the tilting can be reversed. The findings in this paper can also provide insights toward the engineering of self-catalyzed GaAs NW growth by the removal of parasitic growth objects, which inevitably grow together with NWs.