We report on a growth study of self-catalyzed GaAs nanowires based on time-resolved in situ X-ray structure characterization during molecular-beam-epitaxy in combination with ex situ scanning-electron-microscopy. We reveal the evolution of nanowire radius and polytypism and distinguish radial growth processes responsible for tapering and side-wall growth. We interpret our results using a model for diameter self-stabilization processes during growth of self-catalyzed GaAs nanowires including the shape of the liquid Ga-droplet and its evolution during growth.

Abstract

We report and detail a lithography-free method to pattern Si substrates. In particular, a focused Ga ion beam is used to create regular patterns of holes which serve as a template for the growth of vertically aligned GaAs nanowires (NW)s on Si(111) substrates using self-catalyzed molecular beam epitaxy. We show that the hole diameter plays a crucial role in the growth of the NWs at the drilled holes. The critical parameters defining the width of the holes are: ion dose quantities, wet etching procedures, and high-temperature steps at the process of growth. As a result, we obtained a yield of more than 80% for vertically aligned NW. Compared to other methods of patterning our approach provides the following advantages: (i) it is a lithography-free procedure, (ii) allows for quick patterning process and hole diameter optimization within a small window of trial and error, (iii) and provides potential applicability for different material systems.

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.