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.
Highlights
Over the past few years, semiconductor nanowires (NWs) have become a large research field due to their interesting electrical and optical properties due to quantum effects, large surface to volume ratio, and capability for bottom-up assembly
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
The samples were reintroduced into the metal−organic chemical vapor deposition (MOCVD) system and annealed under a H2/ AsH3 environment at 650 °C for 10 min to remove surface oxides formed as the NWs were exposed to air
Summary
Over the past few years, semiconductor nanowires (NWs) have become a large research field due to their interesting electrical and optical properties due to quantum effects, large surface to volume ratio, and capability for bottom-up assembly. The crystal structure of samples with different amounts of radial overgrowth and merging was investigated. We use a rather simple combinatorial approach to explain the main features in the resulting crystal structure of radially merged NWs. Comparing energy contributions from differences in cohesive energy, surface energies and dislocation line energy, only a small energy difference for transforming 2H compared to 3C is found, which justifies the use of a combinatorial model. Removing the Au seed particles is essential to limit axial elongation of the NWs during subsequent radial overgrowth For this step, the samples were reintroduced into the MOCVD system and annealed under a H2/ AsH3 environment at 650 °C for 10 min to remove surface oxides formed as the NWs were exposed to air. NWs in the same azimuth as the 3C substrate (2̅2̅4̅) is due to the 180°
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