Silicon (Si) tip emitter arrays with uniform, smooth, and ultrathin (∼2 nm) amorphous diamond (a-D) apexes were fabricated. Aqueous buffer hydrofluoric acid and H2/Ar plasma have been employed to remove the native oxide layer of Si tips, prior to the a-D film deposition. Scanning electron microscopy study showed that uniform a-D coatings were highly localized on the apex of individual Si tips. Study using high-resolution transmission electron microscopy and x-ray energy dispersive spectroscopy confirmed that the Si/a-D junction is free from the oxide interlayer. Field-emission measurements demonstrated that the removal of the native oxide layer and the a-D apex coating are important to stabilize and enhance the electron emission from Si field emitters.