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In this study, AlGaN-based ultraviolet-B band laser diodes with 150-mW peak output power in pulsed operation were demonstrated at room temperature. The oscillation wavelength, differential quantum efficiency, and slope efficiency of a laser diode were 300 nm, 3.6%, and 0.15 W/A, respectively. These results were obtained by increasing the injection efficiency and decreasing the positive fixed polarization charge formed at the interface between a p-side waveguide layer and an electron blocking layer when polarization doping is formed in a p-AlGaN cladding layer.

Thin films of AlN, AlGaN, and AlGaN-based device structures of approximately 1 cm2 formed on a sapphire substrate were successfully exfoliated from the substrate by immersion in heated (115 °C) and pressurized (170 kPa) water. These thin films were crystalline, grown on periodically formed AlN nanopillars. The water was permeated through intentional voids formed in the AlGaN or AlN crystalline layers by using periodic AlN nanopillars. The exfoliated AlGaN exhibited clear X-ray diffraction peaks from its (0002) plane diffraction. Transmission electron microscopy (TEM) confirmed that exfoliation introduced few additional dislocations and that the device structure was maintained.

We have successfully fabricated vertical LEDs by separating a 1 × 1 cm2 wafer composed of deep-ultraviolet LED on a sapphire substrate from the substrate using a laser liftoff (LLO) method. Reproducible substrate separation was achieved by the LLO method using an Al0.68Ga0.32N underlayer film on an AlN template with periodic pillars. The fabricated vertical LED successfully demonstrated notable luminescence (peak wavelength: 298 nm) characteristics up to a current density of ∼43 kA cm−2 at room temperature and in pulsed drive, which is expected to be used in high-power LEDs and laser diodes.

This review paper describes the historical development, current issues, and future expectations of UV-B laser diodes, which are expected to be adopted in various applications such as in microfabrication and biotechnology in the near future. To achieve RT operation of UV-B laser diodes, several challenges need to be addressed, including the development of a crystal growth technique for high-crystalline-quality AlGaN films that enables laser oscillation with a low excitation carrier density, and the development of a semiconductor layer structure for simultaneous formation of a desirable optical cavity and injection of high-density carriers (operation of high current density), allowing laser oscillation in the active layer. These challenges and the corresponding technologies that have overcome them are reviewed. The current status of device characteristics and future challenges are also discussed.

We report the results of crystal growth of thick AlGaN films on periodical 1 μm spacing AlN pillar concave-convex patterns and their impact on the performance of UV-B laser diodes. The formation of voids in the AlGaN film by increasing the AlN pillar height and the use of high-quality AlN templates were effective in improving the quality of AlGaN, and the dislocation density in the AlGaN film was reduced to approximately 3.4 × 108 cm−2. A UV-B laser diode was fabricated on the optimized AlGaN, demonstrating a threshold current density of ∼12 kA cm−2 and a high yield rate of over 90%.

To reduce the threshold current density (Jth) of ultraviolet (UV)-B AlGaN-based laser diodes, we investigated the critical parameters aiming to increase the injection efficiency ηi and the optical confinement factor Γ. Optimization of the thickness of the waveguide layer, the average Al content of the p-type AlGaN cladding layer, and the film thickness of the cladding layer demonstrated that the device characteristics can be improved. This optimization achieved a reduction in Jth to 13.3 kA cm−2 at a lasing wavelength of 300 nm, thus offering the lowest Jth value yet achieved for a UV-B laser diode.