By growing Al0.6Ga0.4N onto a sapphire substrate using an annealed sputtered AlN (SP-AlN), spontaneous three-dimensional (3D) growth of AlGaN occurs depending on the fabrication conditions. This 3D growth results in threading dislocations, which originate from misfit dislocations, forming at the heterojunction interface between AlN and AlGaN. These dislocations are found to bend and form loops between each other, thus reducing the number of dislocations propagating to the top of the AlGaN. In this study, the dependence of the growth temperature of a homoepitaxial AlN layer (homo-AlN) grown via metalorganic vapor phase epitaxy on SP-AlN was investigated. It was found that the spontaneous nucleation behavior of Al0.6Ga0.4N strongly depends on the fabrication temperature of homo-AlN. The results also suggest that the dominant factor behind this dependence is the increase or decrease in hillock density originated from the SP-AlN-induced screw and mixed dislocations. As a result, a dislocation density as low as 5.8 × 108 cm−2 was successfully obtained for Al0.6Ga0.4N. Furthermore, the low dislocation density was compared with that of an ultraviolet (UV)-B laser diode. In a UV-B laser diode with an oscillation wavelength of 298 nm, the threshold current density required for laser oscillation was found to be reduced by roughly 30% by reducing the AlGaN dislocation density from 8.0 × 108 to 5.8 × 108 cm−2.