We report on the electro-optical properties of ZnO-based heterojunction with i-ZnO layer. The structure was grown on (111) p-Si by plasma-assisted molecular beam epitaxy at 570°C. Quality of the obtained epilayers was verified by X-ray diffraction. Optical properties of the heterostructures were studied by photo- and cathodoluminescence. Photoluminescence spectra were measured within temperature range of 5.4–300K and cathodoluminescence measurements were performed at room temperature. A defect-related radiative recombination was observed. In order to recognize the origin of the defects, responsible for the radiative recombination process, electrical measurements were performed on a diode of ZnMgO:N/i-ZnO/n-ZnMgO. Rectifying properties of the junction were confirmed by the current–voltage (I–V) and capacitance–voltage (C–V) measurements. Metastability of the I–V characteristics as well as persistent photocapacitance observed within a temperature range of 40–280K were explained by the presence of metastable defects, presumably related oxygen vacancies, and slow interface states.