edfas.org 19 ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 27 NO. 3 revealed local variations in PL intensity, even within seemingly functional pixels, highlighting potential sub-pixel nonuniformities. DISCUSSION The results clearly demonstrate the power of multimodal spectroscopy in characterizing defects in µLED epi-wafers and fabricated display panels. Full wafer PL mapping provides a rapid and nondestructive method for identifying regions of nonuniformity and potential defect clusters, enabling early-stage quality assessment and process optimization. The observed PL emission shifts across the wafers suggest variations in the epitaxial layer quality or residual stress gradients introduced during the growth process. High-resolution PL and TRPL analysis offer deeper insights into the nature and impact of microscopic defects. The spectral shifts observed in the vicinity of cold spots indicate the presence of point defects or localized strain, while the significant reduction in carrier lifetime at these defect sites confirms their role as nonradiative recombination centers, directly contributing to reduced luminous efficiency and potentially dead pixels in the final display. Raman spectroscopy provides complementary information regarding the structural quality and stress state of the epitaxial layers. The tensile stress identified near micro-pits and micro-cracks, often associated with threading dislocations, further emphasizes the importance of optimizing epitaxial growth conditions to minimize the formation of these yield-limiting defects. The successful application of PL mapping to a fabricated LED display panel demonstrates its potential for quality control at the final product stage, enabling the identification of defective pixels and assessment of brightness uniformity. This is particularly crucial as µLED dimensions decrease, making traditional electroluminescence-based inspection methods more challenging due to the difficulty in making electrical contacts to individual micro-emitters. The ability of HORIBA’s LabRAM Odyssey and SMS320 systems to accommodate large wafer and panel sizes (up to 300 mm) underscores their suitability for industrialscale µLED manufacturing metrology. The integration of multiple spectroscopic modalities in a single platform provides a comprehensive approach to defect characterization, facilitating a deeper understanding of defect origins and their impact on device performance. CONCLUSION This study highlights the critical role of multimodal spectroscopic techniques in the characterization and detection of yield-killing defects in µLED epi-wafers and fabricated display panels. Photoluminescence mapping effectively identifies wafer nonuniformity and localized defects. High-resolution PL and TRPL provide insights into the electronic properties of these defects, revealing their impact on carrier lifetimes. Raman spectroscopy elucidates the residual stress associated with structural imperfections. The application of these techniques offers valuable feedback for optimizing epitaxial growth processes, improving wafer quality, and ultimately enhancing the production yield of high-performance µLED displays. The ability to perform these analyses on large-area wafers and panels makes these spectroscopic tools indispensable for both µLED research and development and for quality control in the manufacturing environment. REFERENCES 1. “Micro-LED Market Size, Share, Industry Growth, Trends & Analysis by Application, Vertical, and Region 2027,” Markets and Markets, November 2024. 2. “The MicroLED Yield Challenge, and Strategies to Overcome,” MicroLED Industry Association, February 2023, www.microledassociation.com/wp-content/uploads/2023/02/MicroLED-yields-2023-02. pdf. 3. H. Sasaki, et al.: “Investigation of Surface Defect Structure Originating in Dislocations in AlGaN/GaN Epitaxial Layer Grown on a Si Substrate,” Journal of Crystal Growth, 2007, 298, p. 305-309. ABOUT THE AUTHOR Praveena Manimunda is an applications scientist II at HORIBA, where he specializes in developing advanced metrology methods utilizing photoluminescence and Raman spectroscopy. With a Ph.D. from the Indian Institute of Science (IISc), Bangalore, and postdoctoral research experience at McGill University in Canada, Manimunda brings over a decade of industry expertise in materials science and semiconductor analysis. He has authored and co-authored more than 30 scientific publications, contributing significantly to the advancement of optical spectroscopy techniques in cutting-edge research and industrial applications.
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