Surface and interface structures of epitaxial silicon nitride on Si(111)

Surface and interface structures of an ultrathin silicon nitride film grown epitaxially on the Si(111) surface are investigated by core-level and valence-band photoelectron spectroscopy using synchrotron radiation. The Si $2p$ photoelectron spectra reveal a characteristic series of components for the Si species not only in stoichiometric Si₃N₄ (Si⁴⁺) but also in the intermediate nitridation states with one (Si⁺) or three (Si³⁺) nitrogen nearest neighbors. The Si $2p$ core-level shifts for the Si⁺, Si³⁺, and Si⁴⁺ components are determined to be 0.64, 2.21, and 2.74 eV, respectively. In sharp contrast to the well-known SiO₂/Si(111) case, no trace of the Si²⁺ species is observed, indicating an atomically abrupt and defect-free interface in accordance with a recent interface model of β-Si₃N₄/Si(111). In addition, the origin of the characteristic N $1s$ spectra and a strong surface-state emission observed in the valence-band spectra are discussed.