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

Wurtzite GaN nanocolumns are reproducibly grown by plasma-assisted molecula r beam epitaxy on Si(lll) and c-sapphire substrates. The nanocolumns densit y and diameter (600-1500 Angstrom) are effectively controlled by means of t he III/V ratio. The nanocolumns are fully relaxed from lattice and thermal strain, having a very good crystal quality characterized by strong and narr ow (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 broa d line centered at 3.41 eV. This broad emission shows a sample-dependent sp ectral energy dispersion, from 3.40 to 3.42 eV, explained as due to the eff ect of strain and/or electric fields associated with extended structural de fects located at the nanocolumns bottom interface. From cathodoluminescence data, it is concluded that the doublet emission lines originate at the nan ocolumns volume, most probably related to Ga-I defects, given the column gr owth mode (Ga balling).