Abstract
A microstructure in AlGaN with a whole compositional range on an AlN/sapphire template is systematically investigated. At the interface between AlGaN and AlN, misfit dislocation density increases with Ga composition in AlGaN. However, most misfit dislocations bend owing to the compressive stress and form loops, by which threading dislocation density is markedly reduced if the thickness exceeds several microns. The effective acceptor energy of Mg in Al<sub>0.5</sub>Ga<sub>0.5</sub>N is found to depend on Mg concentration with a negative one-third power. A SiO<sub>2</sub>/AlN dielectric multilayer mirror is very effective for controlling the reflectivity of the Fabry-Perot resonator mirrors at the cleaved edge of the ultraviolet (UV) laser diodes (LDs). A UV LD with an emission wavelength of 358 nm shows a threshold current density as low as 3.9 KA/cm<sup>2</sup> at room temperature.
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