Structural and morphological evolution of gallium nitride nanorods grown by chemical beam epitaxy

The morphological and structural evolution is presented for GaN nanorods grown by chemical beam epitaxy on (0001) Al₂ O₃ substrates. Their structural and optical properties are investigated by x-ray diffraction, scanning and transmission electron microscopy, and temperature-dependent photoluminescence measurements. While increasing the growth temperature and the flow rate of radio-frequency nitrogen radical, the three-dimensional growth mode will be enhanced to form one-dimensional nanostructures. The high density of well-aligned nanorods with a diameter of 30-50 nm formed uniformly over the entire sapphire substrate. The x-ray diffraction patterns and transmission electron microscopic images indicate that the self-assembled GaN nanorods are a pure single crystal and preferentially oriented in the c -axis direction. Particularly, the "S-shape" behavior with localization of ∼10 meV observed in the temperature-dependent photoluminescence might be ascribed to the fluctuation in crystallographic defects and composition.