An AlGaN ultraviolet-B laser diode at 298 nm was realized at room temperature using pulse operation. The laser diode has a lattice-relaxed Al0.6Ga0.4N layer from the underlying AlN/sapphire template and a composition-graded p-AlGaN cladding layer. The multimodal laser spectrum with proper polarization properties at 298 nm was obtained over the threshold current at 0.90 A corresponding to 67 kA cm–2. By broadening the width of the p-electrode to 11.5 μm, the threshold current density decreased to 41 kA cm–2.

Aluminum gallium nitride (AlGaN)-based ultraviolet-B band laser diodes (LDs) with a p-type AlGaN cladding layer using polarization doping were fabricated on lattice-relaxed Al0.6Ga0.4N/AlN/sapphire. The threshold current density Jth and lasing wavelength of this LD were 25 kA cm−2 and 298 nm, respectively. The internal loss (αi) was estimated by means of a variable stripe length method using optical excitation. The αi value of this LD was relatively low (i.e. <10 cm−1), thus suggesting that the device is characterized by both, proper light confinement and low internal loss.

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.

Room-temperature pulsed oscillation with a laser wavelength of 290 nm and a threshold current density of 35 kA cm−2 was achieved by fabricating a UV-B laser diode on a thick AlGaN film formed on a 1 μm periodic concavo–convex pattern AlN (PCCP-AlN) on a sapphire substrate. The advantage of this method using PCCP-AlN is that it promotes the nucleation of AlGaN crystals. Planarization of this growth nucleus with AlGaN reduces the threading dislocation density at the top of the AlGaN growth layer while suppressing the formation of giant micrometer-sized hillocks and V-shaped pits that appear irregularly.