edfas.org 17 ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 27 NO. 3 identified in the full wafer PL maps using the LabRAM Odyssey system with a spatial resolution down to 200 nm. Emission wavelength shifts within the defect regions were analyzed to understand the nature of the structural imperfections. Time-resolved photoluminescence measurements were performed at both defect and neighboring nondefective regions using a pulsed laser excitation source to investigate the impact of defects on minority carrier lifetimes. The decay curves were analyzed to determine the radiative and nonradiative recombination rates. Raman spectroscopy was employed to analyze the residual stress in the vicinity of specific surface defects, such as micro-pits and micro-cracks. Shifts in the E2 phonon mode of GaN were mapped across these defect regions using the LabRAM Odyssey system. Finally, large-area PL mapping was performed on a commercially available LED display panel using the HORIBA SMS system to identify defective pixels and assess brightness uniformity across the panel. High-resolution PL mapping with a 5 µm step size was subsequently conducted on selected defective and normal pixels to investigate local intensity variations. DATA AND RESULTS FULL WAFER PHOTOLUMINESCENCE MAPPING Full wafer PL intensity maps (Figs. 2a and b) of both blue and green emitting epi-wafers revealed significant nonuniformity across the wafer surface. The green LED wafer exhibited a higher degree of nonuniformity, particularly at the edges, and showed evidence of bowing. Both wafers displayed a noticeable PL emission wavelength shift as measurements progressed from the center toward the edge (Figs. 2e and f), indicating the presence of structural defects or variations in the epitaxial layer composition. Notably, the PL map of the 532 nm (green) µLED wafer (Fig. 2b) showed distinct low-intensity regions, or “cold spots,” suggesting localized defects. HIGH-RESOLUTION PHOTOLUMINESCENCE OF COLD SPOTS Optical microscopy of the cold spot regions revealed clusters of defects on the wafer surface. High-resolution PL mapping (200 nm step size) of these de- fect regions demonstrated a clear shift in the PL emission frequencies compared to the surrounding nondefective areas. This spectral shift indicates the presence of point defects or local strain variations within the epitaxial structure (Fig. 3b). TIME-RESOLVED PHOTOLUMINESCENCE ANALYSIS TRPL spectra acquired at a cold spot defect and a neighboring nondefective region (Fig. 3c) showed a significant reduction in the carrier lifetime at the defect site. This observation suggests that the defects act as nonradiative recombination centers, trapping carriers and reducing the efficiency of radiative emission, ultimately leading to lower PL intensity. Fig. 2 (a) and (b) Full wafer PL maps, showing 450 and 532 nm peak intensity variations. (c) and (d) Variation in emission wavelengths. (e) and (f) PL point spectra extracted from the four points marked on (c) and (d). Both wafers showed a significant shift in the PL emission, indicating structural defects. (a) (b) (c) (d) (e) (f)
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