Luminescence properties and defects in GaN nanocolumns grown by molecular beam epitaxy

Wurtzite GaN nanocolumns are reproducibly grown by plasma-assisted molecular beam epitaxy on Si(lll) and c-sapphire substrates. The nanocolumns density and diameter (600-1500 Angstrom) are effectively controlled by means of the III/V ratio. The nanocolumns are fully relaxed from lattice and thermal strain, having a very good crystal quality characterized by strong and narrow (2 meV) low-temperature photoluminescence excitonic lines at 3.472-3.478 eV. In addition, the spectra reveal a doublet at 3.452-3.458 eV and a broad line centered at 3.41 eV. This broad emission shows a sample-dependent spectral energy dispersion, from 3.40 to 3.42 eV, explained as due to the effect of strain and/or electric fields associated with extended structural defects located at the nanocolumns bottom interface. From cathodoluminescence data, it is concluded that the doublet emission lines originate at the nanocolumns volume, most probably related to Ga-I defects, given the column growth mode (Ga balling).