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.

We report the results of crystal growth of thick AlGaN films on periodical 1 μm-spacing AlN pillar concave-convex patterns and its 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 down to approximately 3.4 × 108 cm−2. A gain-guided UV-B laser diode was fabricated on the optimized AlGaN, demonstrating threshold current density of ~12 kA cm−2.

We have successfully fabricated vertical light-emitting devices by separating a 1 × 1 cm2 wafer composed of deep-ultraviolet light-emitting diode (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.

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This review paper describes the history of development, current issues, and future expectations of UV-B laser diodes, which are expected to be adopted in various applications such as microfabrication and biotechnology in the near future. In order to achieve room temperature operation of this device, there were several challenges are remained, including the development of a crystal growth technique of high crystalline quality AlGaN that enables a laser oscillation with a low excitation carrier density, and the development of a semiconductor layer structure that simultaneously formation of a desirable optical cavity and injection of high density carriers (operation of high current density) to active layer allowing for laser oscillation. These challenges and the technologies that have overcome them are reviewed. Current status of the device characteristics and future challenges are also discussed.