Motoaki Iwaya

The effect of wet etching on the characteristics of ultraviolet‐B (UV‐B) lasers fabricated on AlGaN templates grown as crystals or wet‐etched AlN nanopillars is compared in detail. Specifically, a tetramethylammonium hydroxide (TMAH) solution is used to form templates with AlN nanopillars of different diameters, followed by determining the dependence of the dislocation density in AlGaN crystals grown on the templates, the threshold power density ( P th ) of an optically pumped UV‐B laser, and the threshold current density ( J th ) of a UV‐B laser diode (LD) on the extent of etching. It is observed that the pillar diameter decreases almost linearly as the etching time with the TMAH solution increases. The dislocation density of AlGaN crystals grown on AlGaN templates is almost independent of the etching time. Meanwhile, P th of the optically pumped UV‐B laser is almost halved upon etching with TMAH. Finally, the J th of the UV‐B LD tends to decrease by 10%–20% upon etching. The optical gain is estimated by optical excitation; wet etching of AlN nanopillars with TMAH solution results in a larger optical gain at the same excitation intensity, indicating that nonradiative recombination centers, such as point defects, are reduced.

In this study, the effect of polarization modulation on the carrier‐injection efficiency of AlGaN‐based ultraviolet‐B laser diodes (LDs) using polarization doping in the p‐type AlGaN cladding layer is discussed. The structure of an intentional Al‐content drop at the interface between the p‐type AlGaN electron‐blocking layer and the p‐type AlGaN cladding layer is investigated. First, the usefulness of this structure using a device simulator is confirmed. The optimized structure is also verified by fabricating devices. The fabricated devices are evaluated at room temperature and under pulsed operation. The carrier injection efficiency of the ultraviolet‐B LD is confirmed to exceed 10%. By utilizing a structure with Al‐content drops in the AlGaN, it is found that the peak optical output power is improved by a factor of 6.2 and the carrier‐injection efficiency by a factor of 3.3, compared to a case without this structure.