Unambiguously Enhanced Ultraviolet Luminescence of AlGaN Wavy Quantum Well Structures Grown on Large Misoriented Sapphire Substrate

High-quality epitaxy consisting of Al1-xGaxN/Al(1-y)GayN multiple quantum wells (MQWs) with sharp interfaces and emitting at approximate to 280 nm is successfully grown on sapphire with a misorientation angle as large as 4 degrees. Wavy MQWs are observed due to step bunching formed at the step edges. A thicker QW width accompanied by a greater accumulation of gallium near the macrostep edge than that on the flat-terrace is observed on 4 degrees misoriented sapphire, leading to the generation of potential minima with respect to their neighboring QWs. Consequently, a significantly enhanced photoluminescence intensity (at least ten times higher), improved internal quantum efficiency (six times higher at low excitation laser power), and a much longer carrier lifetime are achieved. Importantly, the wafer-level output-power of the ultraviolet light emitting diodes on 4 degrees misoriented substrate is nearly increased by 2-3 times. This gain is attributed to the introduction of compositional inhomogeneities in AlGaN alloys induced by gallium accumulation at the step-bunched region thus forming a lateral potential well for carrier localization. The experimental results are further confirmed by a numerical modeling in which a 3D carrier confinement mechanism is proposed. Herein, the compositional modulation in active region arising from the substrate misorientation provides a promising approach in the pursuit of high-efficient ultraviolet emitters.