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Depth-resolved confocal micro-Raman spectroscopy for characterizing GaN-based light emitting diode structures
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View: Figures


Image of FIG. 1.
FIG. 1.

Layer structure of the studied GaN-based LED sample. The total thickness of the MOCVD grown layers is ∼6 μm.

Image of FIG. 2.
FIG. 2.

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.

Image of FIG. 3.
FIG. 3.

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.

Image of FIG. 4.
FIG. 4.

Ray tracing based on Snell's law illustrates that the refraction index mismatch shifts the focusing position, where and denote the depth of the intended and actual focus spot, respectively, for a ray incident at an angle .

Image of FIG. 5.
FIG. 5.

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 A(LO) at 736 cm−1, (II) GaN E(high) at 570 cm−1, (III) a broad double peak feature originated from the MQW active layer, and (IV) sapphire A at 417 cm−1. Note that due to the large intensities of GaN A(LO) and E(high) peaks, all the spectra have been truncated to fit into the displayed area.

Image of FIG. 6.
FIG. 6.

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 A(LO) phonon, (c) GaN E(high) phonon, (d) MQW active layer phonon feature, and (e) sapphire A phonon, respectively.

Image of FIG. 7.
FIG. 7.

The peak frequencies of the GaN A(LO) and E(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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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Depth-resolved confocal micro-Raman spectroscopy for characterizing GaN-based light emitting diode structures