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Ioanna Dimkou

Correlative Microscopy of III-Nitride Nanostructures: Application to Photonics

Published on 1 October 2021
Thesis presented October 01, 2021

Abstract:
The aim of this thesis was to develop a methodology for the characterization of III-nitride nanostructures by correlative microscopy, correlating their structural and chemical features with their optical performance. As a first experiment, we describe the growth by plasma-assisted molecular beam epitaxy of AlGaN/AlN quantum dot (QD) superlattices inserted in self-assembled GaN nanowires (NWs) for application in electron-pumped ultraviolet sources. The optical performance of superlattices on NWs is compared with the emission of multi-quantum wells. The NW ensembles attain internal quantum efficiency (IQE) in excess of 60% at room temperature. The IQE remains stable for high excitation power densities, up to 50 kW/cm2. We demonstrate that the NW superlattice is long enough to collect the electron-hole pairs generated by an electron beam with an acceleration voltage VA = 5 kV. These results are interesting for the application of such nanostructures to fabricate ultraviolet lamps. However, in view of difficulties associated to the sample preparation, we have decided to focus our correlative microscopy efforts on InGaN/GaN Stransky-Krastanov QDs. In this case, microscopy specimens can be prepared by focused ion beam, and they present higher emission efficiency than AlGaN/AlN NWs when optically pumped in situ in an atom probe tomography (APT) system.
Our correlative microscopy study of QDs combined high-resolution chemical and optical characterization by APT, in-situ microphotoluminescence, and ex-situ cathodoluminescence. We demonstrate that the information extracted by these techniques allows the precise modelling of the nanostructures, obtaining excellent agreement with the optical measurements. During the APT experiment, it was possible to resolve single-QD emission lines, which display a spectral shift assigned to the relaxation of elastic strain due to material evaporation. We have also studied the effect of extended and point defects on the luminescence of such QD structures. We observed that emission intensity is higher in the upper QD layers (closer to the surface) and it is not correlated with the dislocation density. This result highlights the relevance of inserting InGaN underlayers to bury non-radiative point defects.

Keywords:
GaN, AlN, nanowire, ultraviolet, InGaN, quantum dots, correlative microscopy, luminescence, atom probe tomography, cathodoluminescence and photoluminescence