ZnO-based light emitting diodes were fabricated on c-plane sapphire using ZnO:P∕Zn0.9Mg0.1O∕ZnO∕Zn0.9Mg0.1O∕ZnO:Ga p-i-n heterostructures. The p-i-n heterojunction diodes are rectifying and show light emission under forward bias. The electroluminescence spectra shows deep level emission at low bias, but near band edge ultraviolet emission at high voltage bias. A decrease in leakage currents in as-fabricated structures was achieved via low temperature oxygen annealing.