Nearly dislocation-free semipolar AlGaN templates are achieved on c-plane sapphire substrate through controlled nano wire coalescence by selective-area epitaxy. The coalesced Mg-doped AlGaN layers exhibit superior charge-carrier-transport properties. Semipolar-AlGaN ultraviolet light-emitting diodes demonstrate excellent performance. This work establishes the use of engineered nanowire structures as a viable architecture to achieve large-area, dislocation-free planar photonic and electronic devices.