Position controlled Growth of GaAs/AlGaAs core-shell Nanowires - more uniform in their structural and optical Properties?
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Semiconducting nanowires (NWs), especially those with a direct band gap, could be promising building blocks in future optoelectronic devices. Position controlled, self catalyzed GaAs/AlGaAs core-shell NWs with an AlGaAs axial insert, grown using molecular beam epitaxy, are studied structurally and optically, and compared to similar, but random grown, core-shell NWs as well as position controlled bare-core NWs. The characterization tools used include transmission electron microscopy, photoluminescence, scanning (transmission) electron microscopy, high angle annular dark field scanning transmission electron microscopy and energy dispersive spectrometry. In addition to the effect of growing NWs in regular, predefined patterns, the structure and morphology of the NWs are explained through the growth parameters, and the optical properties studied on the exact same NWs are related to their structure. Position controlled NWs are found to be highly uniform both structurally and optically. The NWs show excitonic emission related to a pure, defect free zinc blende segments. Small, defect related, structural variations seen in the tip and bottom, induced by the growth and consumption of the Ga catalyst droplet, are believed to cause variations in the emission below the free exciton energy. Shell growth is found to cause variations in morphology as well as a varying high energy emission in the range 1.6 - 1.8 eV, which is hypothesized to be due to confined GaAs in the AlGaAs shell.