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In situ asymmetric island sidewall growth (AISG) was developed to enhance Ga-face facet growth and improve the crystalline quality of (112̅2) GaN epilayers on m-plane sapphire substrates. In the early growth stage island shaping and sidewall faceting were distinct and controlled by growth design. Using in situ AISG, {0002} instead of {1̅103} sidewall facets were formed on the Ga-rich island surface, which eliminated formation of a {1̅103} phase during subsequent layer growth of semipolar GaN. Enhanced Ga-face sidewall facet growth led to +c regions overlapping −c regions, which reduced defect density. Pure semipolar (112̅2) epilayers with a reduced surface striation density and a basal-plane stacking fault density of 8 × 103 cm−1 were obtained. The observation of a narrow EH2 peak and an intense E1(LO) peak in Raman spectra indicates that almost strain-free high-quality semipolar (112̅2) GaN films were achieved. The photoluminescence emission intensity from the (112̅2) GaN film prepared by in situ AISG was dominated by band-edge emission and enhanced ∼4 times more than that from conventional (112̅2) GaN. [ABSTRACT FROM AUTHOR]