Nanoscale optoelectronic properties in traditional and emerging materials for light-emitting diodes

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2019Cataloged from PDF version of thesis.Includes bibliographical references (pages 137-159).Although InGaN/GaN-based quantum well (QW) heterostructures continue to set the industry standard for inorganic blue and green light emitting diodes (LEDs), these devices suffer from efficiency droop at high current densities and material quality degradation at longer emission wavelengths. Establishing rational process design principles to address such issues remains inhibited by ongoing controversy surrounding the impact of commonly observed defects such as well-width fluctuations or V-pit defects on carrier recombination. Organic-inorganic perovskites have begun to attract attention as a potential next-generation LED material, but these nascent materials suffer from rapid material degradation under device operating conditions. Understanding structure-property correlations will be necessary to improve incumbent InGaN/GaN technologies and evaluate the potential of organic-inorganic perovskites.In InGaN/GaN QW heterostructures, we first employ aberration-corrected scanning transmission electron microscopy (STEM) to examine the impact of well-width fluctuations and QW period on measured EQE and find no significant correlation. Next, we observe time-delayed cathodoluminescence (CL) rise dynamics in droop-mitigating QW designs and propose a model linking rise behavior to carrier transport and deep level defects. Finally, we use CL-STEM to map radiative recombination around commonly observed V-pit defects with nanoscale spatial and spectral resolution. Furthermore, dark field diffraction contrast imaging elucidates the relationship between V-pit optical emission and threading dislocation character. These results provide a platform for evaluating the impacts of microstructural defects on LED device performance. In methylammonium lead iodide, we use STEM imaging to establish a direct correlation between local stoichiometry and CL intensity.We demonstrate that areas of high CL intensity correspond to regions which are enriched in iodide content relative to lead. Furthermore, CL-STEM imaging reveals the presence of localized high-energy emissions which we attribute to beam-induced ion migration. The continuous evolution of such high-energy emissions under electron beam irradiation suggests these local spectral heterogeneities could reflect material evolution during device degradation.In summary, the current work demonstrates novel insights gained by the application of advanced electron imaging techniques to two vastly different materials systems. Our findings suggest that continued improvements in process design will hinge on controlling the distribution of structural defects in order to minimize undesirable recombination pathways.by Zhibo Zhao.Ph. D.Ph.D. Massachusetts Institute of Technology, Department of Materials Science and Engineerin