An epitaxial ZnO heterojunction light-emitting diode with an n-ZnO/MgO/TiN/n+-Si structure is produced by pulsed-laser deposition. By introducing a thin MgO/TiN buffer and a low temperature (LT) ZnO buffer, layer-by-layer growth of high quality ZnO epi-layer on Si(1 1 1) has been realized, which was confirmed by in situ reflection high-energy electron diffraction (RHEED), transmission electron microscopy, high-resolution x-ray diffraction, resonant Raman spectra and photoluminescence spectroscopy. Combining in situ RHEED with Phi-scan XRD analysis, the in-plane epitaxial growth of ZnO[1 1   0]||MgO[1 0  ]||Si[1 0  ] has been demonstrated. The strong room temperature electroluminescence (EL) with a broad emission band ranging from 1.46 to 3.5 eV and centred at 2.31 eV could be observed from the diode under relative low injection current. Furthermore, the EL output light intensity is enhanced obviously by improving the ZnO crystal quality via inserting a ZnO LT buffer layer.