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 50–70 nm 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 LO-phonon 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.