Optical properties of AlGaN UVC multiple-quantum-wells (MQWs) with nanoscale inverted polarity domains are strongly related to the polar surfaces and nanoscale structures. In this work, the impact of pre-growth 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 local variation of QW thickness and Ga enrichment inside N-polar domains. After inserting an AlGaN/AlN superlattice and enhancing V/III ratio during growth, the surface morphology of N-polar domain is greatly improved, leading to a single peak emission at wavelength of 275 nm in both III- and N-polar domains, and 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.