Nanoscale spectroscopic imaging of GaAs-AlGaAs quantum well tube nanowires: correlating luminescence with nanowire size and inner multishell structure

TL;DR

This study combines spectroscopic imaging and electron microscopy to correlate luminescence properties of GaAs-AlGaAs quantum well tube nanowires with their nanoscale size and 3D inner structure, revealing carrier localization effects.

Contribution

It provides the first detailed correlation between nanowire luminescence and 3D structural features, including shell thickness variations and asymmetries, using combined CL and STEM tomography.

Findings

QWT peak energies are 40-120 meV below theoretical values.

Shell thickness fluctuations cause carrier localization and emission red-shift.

Nanoscale emission localization is directly imaged along the nanowire axis.

Abstract

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.