Low-threshold 303 nm lasing in AlGaN-based multiple-quantum well structures with an asymmetric waveguide grown by plasma-assisted molecular beam epitaxy on c-sapphire

We report on AlGaN multiple-quantum-well separate confinement laser heterostructures grown by plasma-assisted molecular-beam epitaxy directly on c-sapphire at low temperatures (< 800 degrees C). Threading dislocation density was reduced down to 10(9)-10(10) cm(-2) owing to both intentionally introduced strained AlGaN/AlN superlattices and self-organized blocking structures in the AlGaN step-graded buffer layers. The quantum wells were fabricated by a submonolayer digital alloying technique. Calculations of the optical gain and confinement in the optically pumped laser structures yielded its optimum design comprising an asymmetric waveguide. Lasing at 303 nm with the relatively low threshold excitation density of 0.8 MW/cm(2) at 295K has been achieved.