edfas.org ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 27 NO. 3 18 RAMAN SPECTROSCOPY OF SURFACE DEFECTS Optical microscopy of larger cold spots revealed structures consistent with micro-pits, often surrounded by Fig. 3 Effect of yield-killing structural defects on emission wavelength and carrier dynamics. (a) Optical image of the defect region. (b) High-resolution PL map (200 nm, step size) showing changes in the emission wavelength. (c) TRPL spectra recorded at defect and clean region, showing drastic changes in the carrier lifetime. (a) (b) (c) Fig. 4 (a) Optical micrograph of the surface defect and micro-cracks. (b) Raman map, showing GaN, peak intensity variation near the defect region and (c) stress-induced spectral changes near the defect region. (a) (b) (c) Fig. 5 (a) Large area PL map of a commercially available LED display panel, showing defective pixels. (b) and (c) High-resolution PL maps of normal and defective pixels showing brightness variations. (a) (b) (c) micro-cracks (Fig. 4a). Raman mapping of the E2 phonon mode of GaN in the vicinity of these micro-pits (Fig. 5b) and micro-cracks (Fig. 5c) showed a redshift compared to the surrounding defect-free regions (Fig. 4b). This redshift indicates the presence of tensile stress associated with these structural defects. Micropits are often linked to threading dislocations originating during the epitaxial growth process,[3] which can introduce strain fields in the surrounding lattice. PHOTOLUMINESCENCE MAPPING OF LED DISPLAY PANEL Large-area PL mapping of a commercially available LED display panel (Fig. 5a) success- fully identified individual defective pixels exhibiting low brightness or no emission. Subsequent high-resolution PL mapping (5 µm step size) of both defective and normal pixels (Fig. 5b)
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