Experimental investigations on growth of GaN-based materials for light emitting applications

Muhammad Ali

    Research output: ThesisDoctoral ThesisCollection of Articles

    Abstract

    This work investigates the fabrication, characterization and application of GaN-based layers in light emitting structures. All the thin films and light emitting diodes (LEDs) discussed in this thesis were grown by metal organic vapor phase epitaxy (MOVPE). The objective of this thesis was to improve the material quality and light extraction of III-N optoelectronic structures. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), x-ray diffraction (XRD), photoluminescence (PL) and electroluminescence (EL) were used to characterize the samples. GaN layers were grown on GaN templates having hexagon shaped openings. Embedded voids were created at the GaN-sapphire interface with a novel process described in this work. A control over the shape of the voids was demonstrated. It was observed that changing the diameter of the hexagonal openings has an impact on the inclination angle of the sidewalls of embedded voids. The GaN layers having the embedded voids with low inclination angle sidewalls showed an improved material quality. Significant threading dislocation (TD) bending was observed near such voids. InGaN/GaN LEDs were grown on GaN layers with embedded voids of different shapes. Improved material quality and more efficient light extraction due to the introduced void geometry enhanced the light output of 60 degrees inclined sidewall embedded void LEDs. Light extraction enhancement was also studied by mask-less chemical roughening of the back side of the sapphire substrate. An optimized roughening process improved the light extraction from the LED structure by more than 20 %. The compositional dependence of indium and aluminum on MOVPE growth conditions in quaternary InAlGaN layers was investigated. InGaN multi-quantum well structures (MQWs) having quaternary barriers with near-UV emission were also studied. It was observed that the internal quantum efficiency (IQE) of InGaN/InAlGaN MQW structure was sensitive to the barrier layer composition. A proof-of-concept LED based augmented reality application was demonstrated. A working InGaN/GaN single micro-pixel light source was integrated into a contact lens. The LED micropixel display was controlled via a radio frequency transmitter in free space and tested on a rabbit.
    Translated title of the contributionExperimental investigations on growth of GaN-based materials for light emitting applications
    Original languageEnglish
    QualificationDoctor's degree
    Awarding Institution
    • Aalto University
    Supervisors/Advisors
    • Sopanen, Markku, Supervising Professor
    • Suihkonen, Sami, Thesis Advisor
    Publisher
    Print ISBNs978-952-60-4845-1
    Electronic ISBNs978-952-60-4846-8
    Publication statusPublished - 2012
    MoE publication typeG5 Doctoral dissertation (article)

    Keywords

    • LED
    • III-nitrides
    • MOVPE
    • light extraction
    • augmented reality

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