Phase Transformation in Radially Merged Wurtzite GaAs Nanowires

Abstract

III–V Nanowires (NWs) grown with metal–organic chemical vapor deposition commonly show a polytypic crystal structure, allowing growth of structures not found in the bulk counterpart. In this paper we studied the radial overgrowth of pure wurtzite (WZ) GaAs nanowires and characterized the samples with high resolution X-ray diffraction (XRD) to reveal the crystal structure of the grown material. In particular, we investigated what happens when adjacent WZ NWs radially merge with each other by analyzing the evolution of XRD peaks for different amounts of radial overgrowth and merging. By preparing cross-sectional lamella samples we also analyzed the local crystal structure of partly merged NWs by transmission electron microscopy. Once individual NWs start to merge, the crystal structure of the merged segments is transformed progressively from initial pure WZ to a mixed WZ/ZB structure. The merging process is then modeled using a simple combinatorial approach, which predicts that merging of two or more WZ NWs will result in a mixed crystal structure containing WZ, ZB, and 4H. The existence large and relaxed segments of 4H structure within the merged NWs was confirmed by XRD, allowing us to accurately determine the lattice parameters of GaAs 4H. We compare the measured WZ and 4H unit cells with an ideal tetrahedron and find that both the polytypes are elongated in the c-axis and compressed in the a-axis compared to the geometrically converted cubic ZB unit cell.

Surface effects of vapour-liquid-solid driven Bi surface droplets formed during molecular-beam-epitaxy of GaAsBi

“…III-V epitaxial nanostructures typically form in a cubic zincblende (ZB) crystal structure, though can preferentially adopt the hexagonal wurtzite (WZ) phase5152 in cases of relatively small dimensional nanowires (tens of nm diameter)5354. Size is not the only factor that allows such a structural transition, because the crystalline polytype may also be tuned by the use of appropriate growth conditions.…”

“…Consequently, we determine a pure ZB phase to persist throughout our (001) GaAs 1− x Bi x /GaAs epitaxial structure. While it is possible a WZ phase will form early on during nanowire growth, temporal changes to growth dynamics, namely, epitaxial burying5458 and/or nanowire merging53, will see any embedded WZ crystal rapidly transition into the bulk ZB phase.…”

X-ray diffraction reveals the amount of strain and homogeneity of extremely bent single nanowires

“…3(c) in the q (z,y) and q (z,x) projections. The width of the NW peak along the radial q direction is large and lies between the known peak positions of wurtzite and zinc-blende crystalline structures, which can be present in the NW at the same time (Jacobsson et al, 2015). This hinders our study of the NW in terms of crystalline phase distribution along the NW growth axis from these data.…”

Threefold rotational symmetry in hexagonally shaped core–shell (In,Ga)As/GaAs nanowires revealed by coherent X-ray diffraction imaging

“…However, it is well known that GaAs NWs exhibit a variety of crystal phases formed within a single NW. Next to the favoured ZB phase additional polytypes are the WZ phase and twinned zincblende (Biermanns et al, 2012) or even a more complicated hexagonal type of structure (Jacobsson et al, 2015). This multiphase behaviour was considered in our FEM calculation by interpolating the deformation fields created by the different phases at atomic positions.…”