Growth habits and defects in ZnO nanowires grown on GaN/sapphire substrates

Growth habits and defects in epitaxial ZnO nanowires grown from Au catalyst on (00.1) GaN/sapphire substrate using the vapor-liquid-solid (VLS) technique were studied using electron microscopy and x-ray diffraction. The results revealed presence of both horizontal (crawling-like) and vertical nanowires having similar orientation relationship to the substrate (00.1)ZnO‖(00.1)GaN, [11.0]ZnO‖[11.0]GaN. The crawling-like growth precedes the vertical growth, and the coalescence and overgrowth of the crawling nanowires produce a highly defective layer which separates the substrate and vertical nanorods. Transmission electron microscopy revealed a high density of planar defects in this interfacial layer. A significant density of stacking faults residing on the (0001) planes was also observed in the shorter vertical nanorods. The crawling nanowires are under residual compressive strain, whereas the vertical nanorods grow strain-free.