Layer structure of the studied GaN-based LED sample. The total thickness of the MOCVD grown layers is ∼6 μm.
Experimental configuration for performing depth-resolved confocal micro-Raman spectroscopy, where the notations denote: FC-fiber coupler, OF-optical fiber, C-collimator, LLF-laser line filter, HWP-half-wave plate, P-polarizer, DM-dichroic mirror, PZS-piezo stage, LPF-long pass filter, AZ-analyzer, and CP-confocal pinhole.
Axial profile of the laser reflection from a Si sample reveals the axial resolution of our confocal micro-Raman system to be ∼0.64 μm.
Ray tracing based on Snell's law illustrates that the refraction index mismatch shifts the focusing position, where d and D denote the depth of the intended and actual focus spot, respectively, for a ray incident at an angle θ.
Raman Spectra at different depths of the sample from the sample surface (0 μm) toward the sapphire substrate. The corresponding depths of the four Raman spectra are: (A) near sample surface at −0.5 μm, (B) MQW active layer at 0.9 μm, (C) n-GaN at 3.7 μm, and (D) inside the sapphire substrate at 14.8 μm. The Roman numeral labels indicate the corresponding phonon peaks: (I) GaN A1(LO) at 736 cm−1, (II) GaN E2(high) at 570 cm−1, (III) a broad double peak feature originated from the MQW active layer, and (IV) sapphire A1g at 417 cm−1. Note that due to the large intensities of GaN A1(LO) and E2(high) peaks, all the spectra have been truncated to fit into the displayed area.
Axial profile of the phonon peaks marked in Figure 5 . (a) Diagram of the sample layer structure, where the layers from left to right are the p-GaN capping layer, MQW/superlattice active layer, n-GaN, u-GaN, and sapphire substrate, respectively. The axial profiles shown are: (b) GaN A1(LO) phonon, (c) GaN E2(high) phonon, (d) MQW active layer phonon feature, and (e) sapphire A1g phonon, respectively.
The peak frequencies of the GaN A1(LO) and E2(high) phonon modes are plotted as a function of the sample depth from the sample surface toward the sapphire substrate. The peak positions and error bars are determined by least-square curve fitting the Raman peak with single Gaussian function.
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