For the first time, an ultrahigh degree of optical polarization (DOP) of 81.8% in AlGaN-based deep ultraviolet LED (DUV-LED) operated at 286 nm has been experimentally demonstrated. The very high DOP was obtained by introducing the novel moth-eye microstructure fabricated on the backside of a sapphire substrate. Compared with conventional DUV-LED with a DOP of 64.7%, a significant 1.26-fold enhancement was obtained. It was worth mentioning that the DOP was accurately measured via self-built full spatial transverse electric (TE) and transverse magnetic (TM) mode light intensity test system, which was mainly composed of angle resolution bracket, Glan-Taylor prism, and spectrometer. For both TE and TM mode light, the extraction angle inside the semiconductor was extended from conventional (−26°, 26°), (−52°, −41°), (41°, 52°) to (−52°, 52°). Combined with finite difference time domain simulation, it was further confirmed that the novel moth-eye microstructure could notably weaken the total internal reflection at the sapphire/air interface and enlarge the light extraction angle. As a result, compared with the conventional one, DUV-LED with a moth-eye microstructure approximately doubled the light extraction efficiency.

In this letter, we demonstrate a crack and strain free AlN epilayer with a thickness of 10.6 μm grown on a pyramidal patterned sapphire substrate by metalorganic chemical vapor deposition. The full width at half maximum of the X-ray rocking curve was 165/185 arcsec for (002)/(102) planes, respectively. The total threading dislocation density was less than 3 × 108 cm−2. The dislocation evolution and the coalescence process were probed by transmission electron microscopy and scanning electron microscopy. A dual coalescence of the AlN epilayer was observed, which can effectively relax strain during the heteroepitaxy process. Owing to the approximately entire strain relaxation demonstrated by reciprocal space mapping and Raman shift, the surface morphology was crack-free and atomically smooth with a root-mean-square roughness of 0.14 nm. Temperature dependent Raman spectra showed the Raman linewidth of 4.3 cm−1 at 300 K which was comparable to that of bulk AlN; it also demonstrated good crystalline quality of the AlN epilayer.

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