Examining the three-dimensional structure of an AlGaAs shell on GaAs nanowires
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Semiconducting direct-bandgap nanowires (NWs) are a promising material system for future optoelectronic devices. In this work, the effect of shell growth temperature has been examined on the structural and optical properties of core-shell self-catalyzed GaAs/AlGaAs NWs. A correlated study has been performed on 35 NWs from two different growth batches. The wires have been examined using micro-photoluminescence, conventional (mainly dark-field) transmission electron microscopy, high-angle annular dark field scanning transmission electron microscopy (HAADF STEM) and energy dispersive x-ray spectroscopy. In addition to characterizing the morphology, crystallography and photoluminescence response of the nanowires, a novel HAADF STEM approach has been developed to allow for observation of composition variation in the AlGaAs shell without the use of cross-section samples prepared by focused ion beam. The lower growth temperature wires are found to perform better optically, with stronger and more clear emission spectra. This is believed to be partly caused by oxidation due to an insufficient protective GaAs surface layer in the high-temperature sample, possibly provoked by insufficient droplet consumption. The high-temperature sample also displays high-energy emission above 1.51 eV. Different structural and compositional regimes are found in the shell of the two samples, and a explanation for the mechanism behind high-energy emission creation is pointed to. The high-energy emission is hypothesized to be either due to radially varying Al content in the shell when grown at higher temperature or due to undiscovered variations in the tip.