Toward High-Performance AlGaN-Based UV-B LEDs: Engineering of the Strain Relaxation Process

Strain management is the key to achieving high-quality aluminum gallium nitride (AlGaN) materials for fabricating AlGaN-based ultraviolet-B (UV-B) light-emitting diodes (LEDs). In this work, a kinetic defect evolution model to capture the strain relaxation process for AlGaN growth of UV-B LEDs was demonstrated. The relaxation started from the inclination of threading dislocations (TDs) dominantly over the coexistence of TD inclination, surface roughening, misfit dislocation (MD) generation, enhanced phase separation, etc. as the thickness of n-AlGaN increased. The critical thickness for strain relaxation by TD inclination, MD generation, and the theoretical MD density were obtained. Finally, flip-chip UV-B LEDs with properly engineered buffer strain were fabricated, which showed a maximum light output power of 49.1 mW under 500 mA and a high external quantum efficiency of 3.00% at 310.2 nm, without a high-reflective Rh electrode or any complex packaging process. This work improves our understanding of the relaxation mechanism and prompts further consideration of the strain management in AlGaN-based UV-B LEDs.