AlGaN-based ultraviolet light-emitting diodes (UV LEDs) are promising next-generation UV sources for a wide variety of applications. The state-of-the-art AlGaN-based UV LEDs exhibit much lower output power and external quantum efficiency than highly commercialized GaN visible LEDs. One key issue for UV LEDs is the poor light-extraction efficiency. We have reviewed the recent progress in the light extraction approaches for AlGaN-based UV LEDs, including the highly reflective techniques, and the surface/interface modification for total internal reflection mitigating. Moreover, AlGaN-based UV LEDs in the nanoscale structures, such as nanopillar, nanorod, and nanowire structures, are also discussed.
We report the regrown Al-rich of n-AlGaN material with improved crystalline quality and reduced stress on nanoporous AlGaN template, which was prepared by the electrochemical etching (EC). First, the EC of Al-rich AlxGa1-xN (x > 50 % ) material was investigated to get suitable nanoporous template. Various anodizing voltage and anodizing time were applied to fabricate the nanoporous AlGaN template. The nanopore size and density were found to increase as the anodizing voltage and the anodizing time increase. Moreover, branching pores and vertical pores were apt to be formed at low and high voltages, respectively. Photoluminescence (PL) measurement and Raman spectra indicate that the nanoporous AlGaN materials exhibit higher PL intensity and dramatical release of stress compared to the as-grown AlGaN films due to the presence of nanopores. Furthermore, the nearly stress-free regrown n-AlGaN with high quality using optimized nanoporous AlGaN material as the template was also obtained, which demonstrates that the nanoporous AlGaN template could potentially be applied to heteroepitaxy of efficient AlGaN-based ultraviolet optoelectronic.
Al<sub>x</sub>Ga<sub>1-x</sub>N layer was grown on sapphire substrate with GaN template by Metal Organic Chemical Vapor Deposition
system (MOCVD). High temperature AlN (HT-AlN) interlayer was inserted between Al<sub>x</sub>Ga<sub>1-x</sub>N layer and GaN template
to solve the cracking problem that often appears on Al<sub>x</sub>Ga<sub>1-x</sub>N surface when directly grown on high temperature GaN
template. Optical microscope, scanning electron microscopy (SEM), atomic force microscope (AFM), high resolution x-ray
diffraction (HRXRD) and cathodoluminescence (CL) were used for characterization. It was found that the cracking
was successfully eliminated. Furthermore, the crystalline quality of Al<sub>x</sub>Ga<sub>1-x</sub>N layer with HT-AlN interlayer was much
improved. Interference fringes were found in the HRXRD images. CL test showed that yellow emission was much
reduced for AlGaN layer with HT-AlN interlayer.