GaN nanorods grown on Si(111) substrates by plasma-assisted molecular beam epitaxy

Various GaN nanorod structures grown on Si(111) substrate are realized by plasma-assisted molecular-beam epitaxy. Evolution of the nanorod structure from isolated regular nanorods, to isolated non-regular nanorods, and to dense nanorods is well controlled by the GaN buffer structure. Adding the parameter of beam-equivalent pressure of N/Ga ratio to the nanorod growth, the density of the regular nanorod becomes also a controllable item. There are several combinations of rod density and diameter in the nanorod growth. High density-small diameter and low density-large diameter can be grown directly on the surface without buffer layer. Low density-small diameter, low density-large diameter, and high density-large diameter can be achieved by inserting a GaN buffer. Nanorod of single crystal wurtzite structure without dislocation was characterized by high-resolution transmission electron microscopy. Only Ga and N signals were detected by energy-dispersion x-ray spectroscopy analysis. Single freestanding nanorod was prepared to perform micro-Raman spectroscopy. Wurtzite-type Raman modes at different scattering configurations have small line width and indicate the high crystalline quality of the nanorod. Fr\"{o}lich interaction and the surface vibrational modes are observed at high laser power densities which will be discussed in detail.