Visualising highly localised luminescence in GaN/AlN heterostructures in nanowires

TL;DR

This study uses nanoscale cathodoluminescence imaging to visualize highly localized luminescence in GaN/AlN nanowire heterostructures, revealing unintentional quantum rods and their optical properties.

Contribution

It demonstrates nanoscale spatial resolution of luminescence in GaN/AlN nanowires and identifies unintentional quantum rods using combined STEM and nanoCL techniques.

Findings

Luminescence localized within 5 nm regions in QDiscs.

Detection of unintentional quantum rods on nanowire sidewalls.

Quantum rods emit at similar but shorter wavelengths than QDiscs.

Abstract

The optical properties of a stack of GaN/AlN quantum discs (QDiscs) in a GaN nanowire have been studied by spatially resolved cathodoluminescence (CL) at the nanoscale (nanoCL) using a Scanning Transmission Electron Microscope (STEM) operating in spectrum imaging mode. For the electron beam excitation in the QDisc region, the luminescence signal is highly localized with spatial extension as low as 5 nm due to the high band gap difference between GaN and AlN. This allows for the discrimination between the emission of neighbouring QDiscs and for evidencing the presence of lateral inclusions, about 3 nm thick and 20 nm long rods (quantum rods, QRods), grown unintentionally on the nanowire sidewalls. These structures, also observed by STEM dark-field imaging, are proven to be optically active in nanoCL, emitting at similar, but usually shorter, wavelengths with respect to most QDiscs.