Polarity Control and Nanoscale Optical Characterization of AlGaN-Based Multiple-Quantum-Wells for Ultraviolet C Emitters

Optical properties of AlGaN UVC multiple-quantum-wells (MQWs) with nanoscale inverted polarity domains are strongly related to polar surfaces and nanoscale structures. In this work, the impact of pregrowth nitridation of the sapphire substrate on the polarity control of UVC MQW is highlighted, and the optical properties of III- and N-polar domains were distinguished. Nanoscale cathodoluminescence peak separation of more than 30 nm is observed in lateral-polarity-structure (LPS) UVC MQWs, which is ascribed to the potential minima induced by the local variation of QW thickness and Ga enrichment inside N-polar domains. After an AlGaN/AlN superlattice is inserted and the V/III ratio is enhanced during growth, the surface morphology of the N-polar domain is greatly improved, leading to a single-peak emission at a wavelength of 275 nm in both the III- and N-polar domains, and a 10-fold stronger peak intensity at the inversion domain boundary. Such understandings on the polar surface optimization and underlying reasons of peak separation enable rational design for efficient UVC emitters with improved performance.