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A ZnO-based metal-insulator (HfO2) -semiconductor diode was synthesized on a commercially available n+-GaN/sapphire substrate using a radio-frequency magnetron sputtering system. Electroluminescence measurements revealed that the diode exhibited fairly pure ultraviolet (UV) emission peaking at ~ 370 nm with a line width of less than 8 nm. By choosing a proper thickness of the insulator HfO2 layer, the threshold voltage of the emission could be reduced to 2 V, demonstrating that this ZnO-based fairly pure UV light-emitting diode can be driven by two ordinary dry batteries. The reason for low threshold voltage is proposed in terms of the n+-GaN/sapphire substrate and the high-k insulator HfO2 layer.