In this paper, we investigated the dependence of threshold power density on the Al0.55Ga0.45N underlying layer film thickness in ultraviolet-B band (UV-B) lasers on various AlN wafers (four types). We also prepared and compared AlN templates for AlN freestanding substrates, AlN films fabricated by metalorganic vapor phase epitaxy, and annealed sputtered AlN templates at high temperature. The initial growth of AlGaN became three-dimensional by inserting a homoepitaxial Ga-doped AlN layer between the AlN template and Al0.55Ga0.45N, before it shifted to two-dimensional growth. It is possible to reduce the dislocation in Al0.55Ga0.45N using this mode. The dependence of AlGaN film thickness and that of the AlN template on samples with an inserted homoepitaxial Ga-doped AlN layer were studied. Compared with Al0.55Ga0.45N having a thickness of 5 μm, there was almost no noticeable difference between the dark spot density characterized by cathodoluminescence and the threshold power density in UV-B lasers for the AlN template. Besides, the characteristics were noticeably different for the film thickness of Al0.55Ga0.45N. The threshold power density in UV-B laser and dark spot density were reduced by increasing the film thickness. Through the optimization of the crystal growth condition, the threshold power density in UV-B laser and dark spot density were reduced to 36 kW cm−2 and 7.5 × 108 cm−2, respectively.

In this study, we investigated a method for growing AlGaN using an intermediate AlN molar fraction on a sputtered AlN template with high-temperature annealing via an AlN homoepitaxial layer. We examined in detail the growth mode dependence of AlGaN on the amount of Ga doping in the AlN homoepitaxial layer.As a result, homoepitaxial growth of AlN on sputtered AlN improved the surface flatness. In addition, it was found that the macroscopic flatness is improved by increasing the amount of Ga doping, but this decreases the microscopic flatness. Moreover, when Al 0.60 Ga 0.40 N was grown on homoepitaxial AlN layers with different amounts of Ga doping, a difference in the growth mode was observed, and this revealed a remarkable difference in the dislocation density of Al 0.60 Ga 0.40 N. We also investigated the dislocation density dependence of the lasing threshold power density in a UV-B optical pumped laser. As a result, the lasing threshold power density was reduced from approximately 220 to 50 kW cm −2 by reducing the dislocation density from 1.6 × 10 9 to 8.0 × 10 8 cm −2 . It was confirmed that it is, therefore, essential to reduce the dislocation density to reduce the threshold power density of the UV-B laser.

We investigated the etching rate of the m-plane of AlGaN by wet etching with tetramethylammonium hydroxide aqueous solutions (25 wt%, 85 °C). After dry etching was performed along the m-plane of AlGaN, wet etching was performed to stably form the m-plane facet of AlGaN. Also, the etching rate increases as the increased AlN molar fraction. In the case of forming a heterojunction such as a UV light-emitting diode, by performing wet etching for 5 min, the flat m-plane facets were formed even though there was a large dependence in AlN molar fraction. These facets were almost vertical and flat with respect to the c-plane, so it has the potential for the use as laser mirror. Also, no change in current density-voltage characteristics was confirmed after the wet etching. Therefore, this method is effective for deep UV laser diode fabrication technology on sapphire substrate.