Thesis

Multimicroscopy characterisation of AlGaN-based deep ultraviolet light emitting diodes

Creator
Rights statement
Awarding institution
  • University of Strathclyde
Date of award
  • 2025
Thesis identifier
  • T17514
Person Identifier (Local)
  • 202457759
Qualification Level
Qualification Name
Department, School or Faculty
Abstract
  • This thesis describes the characterization of AlGaN light-emitting diodes (LEDs) designed to operate at 265 nm emission wavelength. AlGaN LEDs emitting in the deep ultraviolet (DUV) spectrum are of great interest for medical applications, sterilization, gas sensing, and photolithography, but their performance is limited by defects and doping challenges. A thorough analysis of micron and sub micron-scale features is essential to identify sources that degrade desirable LED properties, such as efficiency, reliability, and monochromacity. In this thesis, a multimicroscopy approach is employed, combining secondary electron, backscattered-electron, and atomic force microscopies to analyse surface morphologies; wavelength-dispersive x-ray spectroscopy and back-scattered electron microscopy to assess compositional variations; and hyperspectral, spatially resolved cathodoluminescence together with time-resolved cathodoluminescence to characterize optoelectronic properties of state-of-art DUV LED samples and charge carrier dynamics. Two series of three samples each, provided by researchers from Prof. Andreas Waag’s group at the Technische Universität Braunschweig, were examined: incomplete LED structures terminated after the electron-blocking layer with varying parameters of the AlN/AlGaN superlattice transition under the active region (Series A) to characterize the quantum-well region; and complete LED structures with varying Mg-doping levels in the Mg-doped AlGaN and graded AlGaN layers (Series B) to investigate the impact of Mg doping in the p-AlGaN layers on the top p-GaN layer. Measurements of Series A surface morphologies revealed micron-sized hillocks, tens to hundreds of nanometres in height, as well as round pits. Spatially-resolved compositional analysis linked variations in AlN and GaN fractions to these defects, which in turn were found to a"ect the quantum-well emission wavelengths. Edges of hillocks contained increased GaN fractions, reducing the quantum-well emission energy and leading to splitting of the single quantum-well emission peak into two peaks with different carrier lifetimes, as revealed by time-resolved cathodoluminescence. Furthermore, regions of increased AlN fraction were found to exhibit distinct defect-luminescence profiles, indicating changes in radiative defect populations. Series B samples revealed smoother overall surfaces compared to Series A, but contained large hillocks up to →30 µm across and →150 nm in height. These hillocks displayed a ring-like composition profile of increased GaN fraction on the hillock except for an AlN-rich centre, suggesting a link to the buried hillocks in the AlGaN layers. Cathodoluminescence measurements revealed variations in the yellow, blue, and ultraviolet luminescence bands around hillocks, consistent with differing defect populations and possible Mg compositional variations. The surface morphology measurements also revealed the presence of irregularly shaped pits that would expose lower layers within the heterostructure with increased AlN fraction. Compositional measurements around these pits suggested a link to hillocks in the buried AlGaN layer. Wavelength-dispersive x-ray spectroscopy was shown to be sensitive enough to detect Mg concentrations on the order of 1019 cm→3. The results and methodologies presented in this thesis provide critical insights into the role of micron- and sub micron-scale features in degrading LED performance, thereby suggesting key pathways for improving the efficiency, reliability, and overall performance of next-generation UV-LEDs.
Advisor / supervisor
  • Martin, Robert
Resource Type
DOI

Relations

Items

Thumbnail Title Date Uploaded Visibility Actions
file details File 2025-11-07 University of Strathclyde