Luminescence of GaAs/AlGaAs core-shell nanowires grown by MOVPE using tertiarybutylarsine

The luminescence properties of GaAs/AlGaAs core–shell nanowires grown by metalorganic vapor phase epitaxy (MOVPE) on (1¯ 1¯ 1¯ )B GaAs using tertiarybutylarsine, trimethylgallium and trimethylaluminium are reported. Untapered kink-free GaAs nanowires with average diameters around 50–70nm were grown at 400°C by the vapour–liquid–solid method; to this purpose, colloidal Au nanoparticles were used as metal catalyst. Al0.33Ga0.67As shells were grown at 650°C around GaAs nanowires by conventional MOVPE, with thickness ranging in the 70–160 nm interval. Low-temperature photoluminescence (PL) and high spatial resolution cathodoluminescence (CL) measurements were performed, respectively, on dense ensembles of core–shell nanowires (still on their original substrates) and single nanowires; comparison between secondary electron and monochromatic CL images of single nanowires led to spatially resolve the major CL emissions. The low-temperature luminescence of nanowires above the GaAs band-gap energy consists of three contributions: (i) the 1.997 eV band-edge (excitonic) emission of the Al0.33Ga0.67As shell, followed (in CL spectra) by a more intense GaAs-like LOphonon replica; (ii) a broad weaker band at 1.90 eV, ascribed to a donor–acceptor pair recombination associated to residual Si donors in the AlGaAs; and (iii) a dominant and very broad band at 1.67 eV, due to the spatially indirect recombination between electrons in the core and holes in the shell. Comparison between CL and PL spectra suggests that each nanowire has a slightly different GaAs core emission, its peak energy varying in the 1.46–1.49 eV interval.