  The luminescence and inner structure of GaAs-AlGaAs quantum well tube (QWT)
nanowires were studied using low-temperature cathodoluminescence (CL)
spectroscopic imaging, in combination with scanning transmission electron
microscopy (STEM) tomography, allowing for the first time a robust correlation
between the luminescence properties of these nanowires and their size and inner
3D structure down to the nanoscale. Besides the core luminescence and minor
defects-related contributions, each nanowire showed one or more QWT peaks
associated with nanowire regions of different diameters. The values of the GaAs
shell thickness corresponding to each QWT peak were then determined from the
nanowire diameters by employing a multishell growth model upon validation
against experimental data (core diameter and GaAs and AlGaAs shell thickness)
obtained from the analysis of the 3D reconstructed STEM tomogram of a
GaAs-AlGaAs QWT nanowire. We found that QWT peak energies as a function of
thus-estimated (3-7 nm) GaAs shell thickness are 40-120 meV below the
theoretical values of exciton recombination for uniform QWTs symmetrically
wrapped around a central core. However, the analysis of the 3D tomogram further
evidenced azimuthal asymmetries as well as (azimuthal and axial) random
fluctuations of the GaAs shell thickness, suggesting that the red-shift of QWT
emissions is prominently due to carrier localization. The CL mapping of QWT
emission intensities along the nanowire axis allowed to directly image the
nanoscale localization of the emission, supporting the above picture. Our
findings contribute to a deeper understanding of the luminescence-structure
relationship in QWT nanowires and will foster their applications as efficient
nanolaser sources for future monolithic integration onto silicon.
