Formation Mechanism of GaN Nanowires with Various Shapes on Si(111)

We discuss the formation mechanism of GaN nanowires (NWs) with various shapes grown on Si(111) by using a plasma-assisted molecular-beam epitaxy. The GaN NWs have not only symmetrical structures but also various features such as reverse-mesa and reverse-funnel shapes. To manipulate the shape of GaN NWs, we controlled the growth kinetics of gallium (Ga) atoms by varying V/III ratio, defined as the ratio of the nitrogen (N) flux to the Ga flux, and the growth time. Narrow linewidth broadenings of the x-ray diffraction rocking curves and the coherent crystal structure, which are confirmed by using Cs-corrected transmission electron microscope images, indicate the formation of highly crystalline GaN NWs. The formation of GaN NWs, particularly reverse-funnel shaped NWs (RFS-NWs), can be explained by using the so-called wedding-cake growth mechanism, which is related to a reduction in the number of Ga atoms reaching the top surfaces of the GaN NWs from the surface of SiN_x/Si(111) as the height of the GaN NWs increases. Photoluminescence spectra from the GaN RFS-NWs show a double-peak feature at wavelengths of 361.79 and 373.58 nm, which is attributed to two different widths of the GaN NWs in the vertical direction.