Optical properties of Si- and Mg-doped gallium nitride nanowires grown by plasma-assisted molecular beam epitaxy

The optical properties of GaN nanowires grown by catalyst free plasma-assisted molecular beam epitaxy on Si (111) are investigated by photoluminescence (PL) spectroscopy. The influence of the Si- and Mg-flux as well as the III-V ratio during growth on the PL properties is discussed. The Mg concentration as determined by secondary ion mass spectroscopy ranges from 5x10(18) to 1x10(20) cm(-3). Raman scattering reveals that the nanowires are strain-free, irrespective of Si- or Mg-doping. The near band-edge emission of undoped or slightly Si-doped material is dominated by the narrow D(0)X recombination at 3.4715 eV with a full width at half maximum of 1.5 meV at 4 K. For high Si-fluxes, a blueshift of the D(0)X peak by 1 meV is found, which is attributed to band-filling effects. For moderate Mg-fluxes the acceptor-bound exciton recombination was detected at 3.4665 eV. Point defects due to the N-rich growth conditions are discussed as the origin of the emission band at 3.45 eV. Recombination at coalescence boundaries were identified as the origin of an emission band at 3.21 eV. The luminescence properties below 3.27 eV in highly Mg-doped samples are shown to be affected by the presence of cubic inclusions in the otherwise wurtzite nanowires. (C) 2008 American Institute of Physics. [DOI: 10.1063/1.2980341]