In AlGaN, the dominating emission polarization depends on the Al content. Generally speaking, a higher Al content leads to a stronger TM-polarized emission. Normally, the dominating emission polarization of an AlGaN layer changes from the TE polarization into the TM polarization when the emission wavelength is shorter than 300 nm. Because a TM-polarized photon propagate along the lateral dimension of a c-axis grown LED sample, its light extraction efficiency is lower, when compared with a TE-polarized photon. In this study, the material characterization techniques of transmission electron microscopy observation, reciprocal space mapping and omega-2theta scan in X-ray diffraction measurement, and geometric phase analysis are used for first identifying the existence of the high-Al layers (HALs) on both sides of a quantum well (QW) in three 3-period AlGaN QW structures of different deep-UV emission wavelengths. Then, optical analyses, including transmission and photoluminescence (PL) measurements, particularly the PL measurements under an applied stress along the sample c-axis, are undertaken for understanding the effects of such HALs on the band structures and hence the polarized emission behaviors of the samples. Simulation studies are also performed for providing the favorable comparisons with the experimental data. Basically, the HALs produce an extra compressive strain in the c-plane for lowering the heavy-hole (HH) band edge (lower than the edge of the split-off band) such that the TE-polarized emission through the electron transition between the conduction and HH band becomes dominating. In this situation, the light extraction efficiency of such a deep-UV light-emitting diode can be enhanced.
Chia-Ying Su, Meng-Che Tsai, Keng-Ping Chou, Huang-Hui Lin, Ming-Yen Su, Hsin-Chun Chiang, Yuh-Renn Wu, Yean-Woei Kiang, and Chih-Chung Yang, "Unintentionally formed thin barriers of elevated Al contents in a deep-UV AlGaN quantum well for generating favored compressive strain (Conference Presentation)," Proc. SPIE 10532, Gallium Nitride Materials and Devices XIII, 1053223 (Presented at SPIE OPTO: February 02, 2018; Published: 14 March 2018); https://doi.org/10.1117/12.2287333.5751529788001.
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