n‐type AlN/AlGaN Superlattice Cladding Layer for Ultraviolet Laser Diodes

An AlGaN laser diode (LD) on AlN substrate using an n‐type AlN/Al_0.63Ga_0.37N superlattice (SL) cladding layer is compared to that using a conventional n‐type Al_0.7Ga_0.3N alloy. An n‐type AlN/Al_0.63Ga_0.37N SL cladding layer with a thickness of more than 1 μm can be pseudomorphically grown on the AlN substrate, while the Al_0.7Ga_0.3N alloy cladding layer should be grown to a thickness of less than around 350 nm to prevent lattice relaxation. The series resistances of the LD structures are estimated to be 18 Ω for the AlN/Al_0.63Ga_0.37N SLs (1.5 μm) and 38 Ω for the Al_0.7Ga_0.3N alloy (350 nm). Because the SLs allow to pseudomorphically grow a thicker n‐type cladding layer, they are effective for reducing the lateral resistance of n‐type cladding layers and thereby the series resistance of the LD. In the electroluminescence measurement of the LD structure with SLs, the integrated emission intensity superlinearly increases and a sharp peak appears at 289.1 nm at high current injections above around 11 kA cm⁻², indicating the emergence of stimulated emission. The LD structure with the SL cladding layer exhibits lower series resistance, leading to lower current density for stimulated emission than that with the alloy cladding layer.