Fabrication and analysis of GaN nanorods grown by MBE

Abstract

GaN nanorods were grown on c-plane sapphire substrates under N-rich conditions by plasma-assisted molecular-beam epitaxy. Scanning electron microsopy revealed densely packed nanorods of hexagonal cross section with diameters ranging from roughly 40 to 100 nm. Atomic force microscopy indicated that the rods protruded 50 to 75 nm above the average height of the surface. Transmission electron microscopy (TEM) showed that the nanorods were approximately 1.4 micrometers in length but an accurate measurement of the rod separation was difficult to assess. Contrary to expectations for GaN grown under N-rich conditions, a high density of basal plane stacking faults were not revealed in TEM under typical imaging conditions. X-ray diffraction using the (0002), (0004), (0006), (104), and (105) reflections yielded c = 0.5188 ± 0.0002 nm and a = 0.3188 ± 0.0004 nm. Low temperature photoluminescence and cathodoluminescence showed broad near-bandgap emission around 3.4 eV that shifted to the blue with reduced temperature in the usual manner, and the presence of a similarly blue-shifting peak near 3.2 eV. The spectra were deconvolved using nine lineshape functions revealing 2 phonon replicas asssociated with the peak near 3.2 eV. Room temperature spectroscopic reflection fited to the standard Aspnes third-derivative lineshape function yielded a transition energy of 3.407 eV for the A exciton and 3.490 eV for the B+C excitons (not spectrally resolved). Both the x-ray and photoreflectance results indicate that the nanorods are fairly relaxed. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)