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 confirmed that exfoliation introduced few additional dislocations and that the device structure was maintained.

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.

Herein, the differences in the characteristics of ultraviolet‐B (UV‐B) laser diodes (LDs) are investigated and fabricated on AlGaN templates prepared using two types of fabrication methods: 1) spontaneous nucleation in which AlGaN is grown 3D from spontaneously generated AlN crystal nuclei and 2) AlN nanopillar in which AlGaN is grown on AlN nanopillars. The LD formed on the AlGaN fabricated using the AlN nanopillar method of slope efficiency and maximum peak light output power is improved four and six times, respectively, which are significantly better compared with the spontaneous nucleation method. The analysis also confirms the differences in injection efficiency and internal loss in the semiconductor crystal. Furthermore, detailed analysis of the current–voltage characteristics shows that the AlN nanopillar method has lower reverse leakage current and n‐value than the spontaneous nucleation method. Comparison of these differences with crystallographic properties suggests that these differences in device properties are manifested by differences in V‐shaped pits and dislocation density. The reduction of dislocations and V‐shaped pit density in an AlGaN template is important to realize high‐performance UV‐B LDs, and the AlN nanopillar method is found to be useful for the fabrication of a suitable AlGaN template.

Fabrication techniques for high‐reflectivity (HR) and antireflection (AR) dielectric multilayer mirrors for AlGaN‐based ultraviolet‐B (UV‐B) laser diodes are developed. After depositing several dielectric materials and evaluating their complex refractive indices via ellipsometry, it is determined that SiO2 as a low‐refractive‐index material and Ta2O5 as a high‐refractive‐index material are appropriate material combinations in the UV‐B region at a light wavelength of ≈300 nm due to their low extinction coefficients and large refractive index difference. Based on these results, HR mirror with a reflectance of >99% in the UV‐B region at a center wavelength of 310 nm and an AR mirror with a reflectance of ≈8% in the same wavelength range are demonstrated; a mirror with reflectance that is almost equal to the designed value is demonstrated. Furthermore, these mirrors are coated on the respective edge surfaces of the UV‐B laser diodes. A comparison of the characteristics of the same device before and after edge coating reveals a reduction in the threshold current density of laser oscillation, whereas, simultaneously, an increase in slope efficiency and external differential quantum efficiency is observed. The improvement of these device characteristics, estimated from the above reflectance values, is confirmed to be almost theoretically explainable.